none on the variation of species with especial reference to the insecta; followed by an inquiry into the nature of genera. by t. vernon wollaston, m.a., f.l.s. "no compound of this earthly ball is like another, all in all." tennyson. london: john van voorst, paternoster row. . "i do not enter so far into the province of the logicians as to take notice of the difference there is between the _analytic_ and _synthetic_ methods of coming at truth, or proving it;--whether it is better to begin the disquisition from the subject, or from the attribute. if by the use of _proper media_ anything can be showed to be, or not to be, i care not from what term the demonstration or argument takes its rise. either way propositions may beget their like, and more truth be brought into the world."--_religion of nature delineated_, p. (a.d. ). printed by taylor and francis, red lion court, fleet street. to charles darwin, esq., m.a., v.p.r.s., whose researches, in various parts of the world, have added so much to our knowledge of zoological geography, this short treatise is dedicated. preface. to make a dry subject entertaining, is impossible; but to render it, at any rate, readable, has been my endeavour in the following pages. how far i have succeeded in the experiment, it is not for me to decide. it having been suggested, by several of my friends, that it might be desirable to bring together into a small compass some of the evidence on insect variation (with reference to external disturbing causes) which my researches in the madeira islands have supplied me with, i have been encouraged to do so: and i have added numerous conclusions from other data also, which have from time to time fallen in my way,--so as to confer on the volume a more practical interest, for the general naturalist. one of my main objects, however, has been to call attention to the fact, that the annulosa have not been hitherto sufficiently considered, in the great questions arising out of the distribution of animals and plants; hoping that, by so doing, some few of our british entomologists, who have not looked into this branch of their science, may be induced to enlist themselves in the cause of insect geography. if such a result be brought about; or if i be fortunate enough to open for discussion any of the topics which have been touched upon, and so lead to a more perfect solution of the problems which i have attempted to explain, i shall consider myself more than repaid. hereford street, park lane, london. may th, . contents. chapter i. introductory remarks chapter ii. fact of variation as a matter of experience as probable from analogy chapter iii. causes of variation § . climatal causes generally (whether dependent upon latitude or upon altitude) § . temporary heat or cold, of an unusual degree § . nature of the country, and of the soil § . isolation; and exposure to a stormy atmosphere chapter iv. organs and characters of variation chapter v. geological reflections chapter vi. the generic theory chapter vii. conclusion corrigendum. page , for _pecteropus maderensis_ read _pecteropus rostratus_. specific variation in the insecta. chapter i. introductory remarks. a very small amount of information gained by the student in the field of nature is sufficient to kindle the desire to increase it. the more we know, the more we are anxious to know; though the less we seem to know. it is one of the distinctive privileges of the naturalist that he has to labour in a mine which is inexhaustible: the deeper he digs beneath the surface, the richer is the vein for excavation, and the more interesting are the facts which he brings successively to light. dive he ever so deep, truth, "at the bottom of the well," is assuredly present, under some form or other, to reward him still; nor will she even for once elude his grasp, provided he be content to receive her as she is, instead of endeavouring to mould her to his preconceived ideas of what she ought to be. in these times of patient research, when the microscope is disclosing, day by day, fresh wonders to our view, and new lines of speculation are springing out, as it were spontaneously, from the regions of thought, it is remarkable that many of the commoner questions relating to the members of the external world around us have remained comparatively unsolved; nor indeed have some of them ever been discussed at all, except in a desultory manner and with insufficient data to reason from. foremost amongst these, numerous problems affecting the distinction between "varieties" and "species" (as usually accepted) of the animal kingdom stand pre-eminent,--especially in the annulose orders, in which those distinctions are less easy, _à priori_, to pronounce upon. the descriptive naturalist, whose primary object it is to register what he sees (apart from the obscurer phænomena which come within the province of the more philosophical inquirer), can have scarcely failed to remark the variation to which certain insects are at times liable from the external agencies to which they have been exposed: and yet, in spite of this, it is but too true that even physiologists have frequently shunned the investigation of the _circumstances_ on which such variations do manifestly in a great measure depend, as though they were in no degree accountable for the changes in question, and did not indeed so much as exist except in theory. in the following pages i purpose, _inter alia_, to throw out a few general hints; first, on the fact of aberration, as a mere matter of experience; and, secondly, on some of the _causes_ to which the physiologist would, in many instances, endeavour to refer it. the _former_ of these considerations (namely, the _fact_ of specific instability as ordinarily noticed) nobody will be inclined to dispute: and yet it is abundantly evident that it cannot be taken into account, at any rate satisfactorily, without involving the _latter_ also,--it being scarcely possible to attach the proper value to an effect without first investigating its cause. the importance of assigning its legitimate weight (and that only) to a variety, is perhaps the most difficult task which the natural historian has to accomplish; since on it depends the acknowledgment of the specific identity of one object with another,--whilst, to draw the line of separation between varieties and species is indeed a gordian knot which generations have proved inadequate to untie. now it is not the object of this publication to attempt to throw positively new light upon a subject which has ever been one of the main stumbling-blocks in the lower sciences, and which is perhaps destined to be so to the end; still less would i wish to imply that the causes of variation _are_ altogether overlooked in these days of accurate inquiry,--when thousands are accumulating data, in all parts of europe, destined to be wielded by the master's hand whensoever the harvest-time shall have arrived: but i do, nevertheless, believe that there exists a growing tendency, especially in some portions of the continent, to regard every difference (if at all permanent) as a specific one; and hence i gather the information that a reviewal of our first principles is occasionally necessary, if we would not restrict (however gradual and imperceptibly) that legitimate freedom which nature has had chalked out for her to sport in, or strive to impose laws of limitation in one department which we do not admit to be coercive in another. perhaps, however, before entering on the subject-matter of this treatise, my definition of the terms "species" and "variety,"--so far at least as such is practicable,--will be expected of me. i may state, therefore, that i consider the _former_ to involve that ideal _relationship amongst all its members_ which the descent from a common parent can alone convey: whilst the _latter_ should be restricted, unless i am mistaken, to those various aberrations from their peculiar type which are sufficiently constant and isolated in their general character to _appear_, at first sight, to be distinct from it. the _first_ of these enunciations, it will be perceived, takes for granted the acceptance of a dogma which i am fully aware is open to much controversy and doubt,--namely, that of "specific centres of creation." without, therefore, examining the evidences of that theory which would be out of place in these pages (and which has been so ably done already by the late professor edward forbes), i would merely suggest that the admission of it is almost necessary, in order to convey to our minds any definite notion of the word "species" at all: and that, hence, whilst i would not wish to reject the hypothesis as involving an absurdity (which i believe to be the exact opposite of the truth), i would, in the present state of our knowledge, desire rather to regard it as a _postulate, assumed to illustrate the doctrine of species_, than as a problem capable of satisfactory demonstration. the _second_ of the above definitions may likewise require briefly commenting upon; for i have frequently heard it asserted that everything is to be regarded as a "variety" which has wandered in the smallest degree from its normal state. now this i contend is essentially an error; for a "variety," to be technically such, must have in it the _primâ-facie_ elements of stability,--and to an extent moreover that, without the intermediate links (which, although rarer than the variety itself, _must nevertheless exist_) to connect it with its parent stock, its condition is such that it might be registered as specifically distinct therefrom. thus, to take an example for illustration, there are many darkly coloured insects which, as every entomologist knows, vary, by slow and regular gradations, into a pallid hue, sometimes into almost white. it also most frequently happens, in such instances, that the _extreme_ aberration is of more common occurrence than the intermediate ones. here then is a case in point: there is but a _single_ variety involved, namely a pale one,--the gradually progressive shades which imperceptibly affiliate it with its type not being regarded in themselves as "varieties" at all. if this indeed were not so, then would our position be far from pleasant, since we should be compelled to record, as a variety, _every_ separate degree of colour which could possibly be found between the outer limits,--seeing that (increasing, as they did, in an even ratio) no _one_ could be tabulated in preference to another. this however is an example in which the rate of alteration (so far as colour is concerned) is _equal_; and one therefore in which the extreme end of the series can be alone singled out as _the_ aberration to be specially noticed. it sometimes occurs that, between the two extremes, there are several nuclei, or centres of radiation, to which the name of varieties may be legitimately applied,--inasmuch as they may possess a series of characters which do not, all, in combination, progress evenly; and which consequently stand out as it were, to as certain extent isolated, from the remainder. as a corollary arising out of these remarks, it would seem to follow that even small differences _should be regarded as specific ones_ so long as the intermediate links have not been detected which may enable us to refer them to their nearest types. in a general sense, i believe that it would be proper to do so: nevertheless there are instances, the results, for example, of isolation, in which _abrupt_ modifications may be _à priori_ looked for; and in which our judgment must be regulated by our knowledge of the local circumstances which may be reasonably presumed to have had some influence in producing them. the consideration of these, however, and other kindred questions, must be deferred to a subsequent chapter of this work. chapter ii. fact of variation. it is scarcely possible to survey the members of the external world around us without being struck with the instability with which everything is impressed. the very shadows, as they pass, leave a moral lesson behind them on the mountain-slope, which the student of nature would do well to contemplate. whatever be our preconceived ideas of the "immutability of the universe," from first to last the same truth is re-echoed to our mind,--that here all is change. organic and inorganic matter are alike subjected to renovation and decay; and, dependent on that general law, _variability_ within specific limits would seem to be an almost necessary consequence. in the animal and vegetable kingdoms, this principle of fluctuation is peculiarly apparent; and not more surely do the winds of heaven ruffle the forests over which they rage, than does the ebb and flow which is perpetually going on amongst created things mar their boasted constancy. the _fact_ of aberration, to which we would briefly allude in this chapter, requires but little comment; it is patent _à priori_. as a matter of experience, every observer who has spent a week in the field of nature knows it to exist. however difficult it may be, in some instances, to distinguish aright between species and varieties, as rigidly defined, there is an instinct within us which often recognizes the _latter_, even at first sight, as unmistakeably such: and in these cases, a well-educated eye, although of course occasionally deceived, will not often be found to err. in the vegetable world this proneness to variation is self-evident; and botanists innumerable, who have investigated the _causes_ on which the modifications of certain plants have been presumed to depend, have not been behindhand in acknowledging it. soil, climate, altitude, and a combination of other circumstances and conditions, have been successively taken into account, and to each an amount of disturbing influence (more or less, as the case may be) has been conceded. "the more powerful agents," writes professor henfrey, "enforce their general laws, but every little local action asserts its qualifying voice; and we see that all these irregularities and uncertainties (as we in our ignorance call them, and complain of) are necessary and important parts of a great whole,--are but isolated features of a comprehensive plan, in accordance with which all work in concert to bring about that _change_ absolutely indispensable to the existence of animal and vegetable life upon the earth's surface, and that _variety of conditions_ by which is ensured a fitting abode for each kind of its multifarious and diversified inhabitants." whilst exploring the barren moor, or bleak upland heights, the botanist would as assuredly look for a change in the outward configuration of certain species, which colonize equally the rich meadows and teeming ravines, as a geographical difference is _à priori_ anticipated between the hard, sturdy mountaineer and the more enervated denizen of the plain. a daisy, gathered on the cultivated lawn, has usually attained a greater degree of perfection and luxuriance than its companion from the sterile heath; and the bramble which chokes up the ditches of the sheltered hedgerow, wears a very different aspect from its stunted brother of the hills. nor is this dependency on external circumstances less apparent in the animal kingdom also,--the domesticated races of which every agriculturist is aware are capable of modification, artificially, to an almost unlimited extent; and which exhibit, when even in a state of nature, nearly as great a variety, from purely natural causes, as they have been proved to do when subjected to the laws and routine of agrarian science. take the sheep, for example, of dartmoor or wales, and compare them with those from the wolds of lincolnshire and the downs of kent; or contrast the hereford oxen with those of the midland counties, or of the caledonian breed, still extant in cadzow forest, and it will require but little argument to convince us how important is the operation of local circumstances in regulating the outward contour of these higher creatures. if therefore this general obedience to influences from without be self-evident in the vegetable world, and equally traceable amongst the mammalia, why, we may ask, are the lower members of the animal creation to be denied analogous effects from the same causes? we are often told that the annulosa present so many anomalies in their organization, that we cannot apply the argument of analogy, when reasoning on their structure and attributes; and that we must consequently be content to leave it an open question, as to whether or not they possess anything in common with the vertebrata, or can be presumed to be acted upon, by external agencies, in at all a similar manner. now, whilst there is clearly some truth in this assertion (especially as regards the _senses_ of insects, which must ever remain a subject of obscurity), i contend that to accept it in all its fullness would be in the highest degree unphilosophical; whilst, to endorse it to the extent which even its partial advocates do insist upon, would at once involve us in a host of difficulties (affecting other departments of natural science), the very existence of which they have themselves tacitly repudiated. "creation," says one of our most intelligent writers of modern times, "_is full of analogies_, pointing to one general originator, and linking all sentient things into one great family of related fellow-creatures:"--and there is an amount of sagacity in the remark which it would be wise for us to digest. throughout the whole of animated nature, it is impossible not to perceive that certain circumstances do, in the main, produce certain results. they may often fail to produce them, and the results themselves may frequently be modified (or, apparently, even reversed), from counter influences of divers kinds. this touches not, however, the existence of the law; and the effect is not the less specifically dependent on its own peculiar cause, because those "counter influences" prevail,--and because _different_ effects may chance, therefore, to be occasionally brought about by causes which may possibly _seem_ to be identical. we should, rather, bear in mind that the agents which operate in moulding the outward contour of organic beings are various, and capable _inter se_ of permutations innumerable; so that it is only on a broad scale that parallel results can be looked for in creatures severally exposed to the action of elements, which are _liable_ to be differently compounded from what may _primâ facie_ appear to be the case: and that, consequently, where opposite phænomena are displayed under circumstances seemingly coincident, our first object should be (_not_ to regard the phænomena as indicative, that no constant result can be anticipated from causes which are similar, but), to inquire whether the circumstances in question _are_ really coincident or not,--seeing that some counteracting stimulus may have been, here or there, unexpectedly at work, which shall enable us, so soon as it is detected, to account for the discrepancy. it is by this process alone that we can hope to make real use of analogy, without abusing it: for whilst there is danger, on the one hand, of needlessly rejecting the argument which it suggests to us, through opposite effects being observed (amongst the members of the organic world) from conditions which _we assume to be_ co-ordinate, but which in fact are not so; we may, on the other, run a similar risk (and thus fail to discern a _corresponding modus operandi_ in the maturation of like results), from a mere _à priori_ belief that the lower animals cannot be acted upon, by external influences, in a manner at all equivalent to that which is self-evident in the higher ones. "to make a perfect observer in any department of science," writes sir john herschel, "an extensive acquaintance is requisite, not only with the particular science to which his observations relate, but with every branch of knowledge which may enable him to appreciate and neutralize _the effect of extraneous disturbing causes_. thus furnished, he will be prepared to seize on any of those minute indications which often connect phænomena which seem quite remote from each other. he will have his eyes as it were opened, that they may be struck at once with any occurrence which, according to received theories, ought _not_ to happen; for these are the facts which serve as clews to new discoveries[ ]." there can be no doubt that amongst a large proportion of our naturalists, _differences_, as such, are too exclusively studied. essential as their investigation is (for we could not progress a step without some presumptive notion as to the specific identity, or not, of the objects about which we have to treat), we should not forget that there are other questions, likewise, which ought to occupy our attention in, at any rate, an almost equal degree,--as being of eminent significance in guiding us to a correct interpretation of the phænomena with which we have to deal. such are, more especially, similitudes and analogies, in their widest sense,--which are too often neglected, even by those who admit the necessity of recognizing them where they may be shown to exist. lord bacon, in referring to a similar tendency amongst a certain section of the naturalists of his day, remarks (though perhaps his love of analogies may have led him to somewhat overrate their importance): "up to this time the industry of men has been great, and very curious in marking the variety of things, and explaining the accurate differences of animals, herbs, and fossils,--the _chief part of which_ are the mere sport of nature, rather than serious and of use toward the sciences. such things tend to our enjoyment, and sometimes to even practical use; but little or nothing towards an insight into nature. and so our labour is to be turned to inquiry into, and notice of, similitudes and analogies, both in the whole and in the parts of things: for these are they which unite nature, and begin to establish sciences[ ]." i believe that, if analogies were more carefully studied in the lower departments of the animal kingdom, we should be less inclined to deny some sort of uniformity to the action of elements and conditions which, by a law of nature, must at times operate equally upon the various and dissimilar members of the organic creation. amongst the insecta, where the individuals exist in such multitudes that accuracy in generalizations concerning them, becomes, as it were, peculiarly within our reach, this doctrine cannot be too rigidly insisted upon; and it is not difficult to foresee that, should the principle of external disturbing influences ever be admitted by entomologists to the extent which it has been accepted by the students of the vertebrata, our so-called "species" will have to submit to a process of elimination and inquiry, which at present would be well nigh incredible. the time for such a step is yet far off: perhaps indeed, considering the innovations of nomenclature which it would necessitate, it will never arrive at all; yet the fact remains the same, that, _if_ analogy with creatures of a more perfect development be not altogether disallowed us, during our researches into the insect tribes, or _if_ similar causes may be presumed to have somewhat similar effects in opposite sections of the animate world, an enlargement of our prescribed limits, for specific variation, ought in reality to follow (sooner or later) as an inevitable consequence. in whichever light, therefore, insect aberration is viewed by us,--whether as a matter of experience (which, being self-evident, will satisfy the practical observer), or as probable from analogy (which will hardly be denied, at any rate to a certain extent, by even the most theoretical),--we affirm that _it does, ipso facto, exist_. "there is no similitude in nature that owneth not _also to a difference_;" let this be constantly borne in mind, for it is a truism almost beyond controversy, and one which, to a reflective mind, will scarcely admit of a doubt. it will be perceived, from the above remarks, that i draw a distinction between insects which simply vary (that is to say, which aberr from their normal state), and those which afford (in the sense as enunciated in the last chapter) one or more actual "varieties,"--technically so called and it will be further gathered, that, whilst i regard the former as universally to be met with, the latter are, on the contrary, of only occasional occurrence. that positive and well-defined varieties, or races, should be confined to certain species, is not remarkable; but that every individual insect should differ, however slightly, from its nearest relation and ally, may perhaps require some few words of explanation, even to a naturalist. it is not essential however to our present subject (which is merely a plea for specific variation generally, as commonly understood) that any such dogma should be propounded; nevertheless, since all analogy teaches us to anticipate it, and observation tends more and more, as our knowledge advances, to corroborate the fact, i shall be pardoned for venturing a passing thought upon a question even thus difficult of demonstration. perhaps we are too prone to regard those specific characters, which are so subtle that they cannot be grasped by our clumsy faculties except in their broadest and plainest features, as incapable of fluctuation. yet a practised eye can detect discrepancies innumerable in specimens which appear absolutely alike to one that is uneducated; whilst a third person, better qualified still, will trace out other and more delicate distinctions, with even greater precision. and thus it is that we rise, step by step, even amongst the humbler representatives of the animal kingdom, to the comprehension of that great truth which is so conspicuous in the nobler ones, and which we have already summoned to our aid, that "there is no similitude in nature which owneth not also to a difference." let us not forget that the sphere of our senses is limited; and that, although tuition will do much to enlarge their capacity for perception, we are at the best but a dim-sighted race: hence, we should be careful to avoid conclusions which are not warranted by analogy, and which our understanding, as it becomes gradually brighter, no less assuredly condemns. true it is, that we may not be able, as in the higher animals, to appreciate the differences between individuals without a rigid inspection, and that sometimes we may fail to do so even when the objects are critically examined; yet the fact that new peculiarities do unquestionably open out upon us, as we become more and more trained for the recognition of them, ought to warn us that others _may_ exist likewise, despite our _primâ-facie_ conclusions; whilst analogy with what we know to be the case in other departments of the organic world should suggest, unless indeed there is presumptive evidence to the contrary, that they in all probability _do_. the alpine range, when seen from afar, appears a monotonous mass of a dull uniform hue; and nothing, of all the wondrous details which it includes, can be distinguished, except perchance the outline of its jagged peaks projected in faint relief against the distant sky. one by one, however, as we approach it, inequalities present themselves; the surface which lately seemed so uniform and grey that it could be compared only to a cloud, is found to be cleft by ravines; and valleys, in all their magnificence and breadth, expand slowly to our view. yet, marvellous as is the change, this is not all: wood and water, without which the landscape would be barren, are in turn revealed; whilst the play of light and shade upon the mountain-slopes proclaims at length that the picture is well nigh complete. still more to be disclosed does in reality remain; and we must advance nearer yet if we would either fully realise the whole, or enter into the surprising minutiæ of each of its component parts. and so it is with the objects which we have been just discussing. when contemplated in a mass, and by an uneducated eye, hosts of them may appear to be identical; but as our vision becomes clearer and more acute, differences, formerly inappreciable, are gradually made manifest,--until at last we can detect modifications innumerable, throughout the entire length of the living panorama; and are enabled to endorse the belief (repugnant _à priori_ though it be), that _individual variations_, even to the extent which i have ventured to suggest, are not incompatible with _specific similitudes_. footnotes: [ ] preliminary discourse on the study of natural philosophy (london, ), p. . [ ] "magna enim hucusque atque adeo curiosa fuit hominum industria, in notanda rerum varietate, atque explicandis accuratis animalium, herbarum, et fossilium differentiis; quarum pleræque magis sunt lusus naturæ, quam seriæ alicujus utilitatis versus scientias. faciunt certe hujusmodi res ad delectationem, atque etiam quandoque ad praxin; verum ad introspiciendam naturam parum aut nihil. itaque convertenda plane est opera ad inquirendas et notandas rerum similitudines et analoga, tam in integralibus, quam partibus: illæ enim sunt, quæ naturam uniunt, et constituere scientias incipiunt."--_novum organum_, lib. ii. . chapter iii. causes of variation. "it is not impossible," says a writer of the last century, "that such laws of nature, and such a series of causes and effects, may have been originally designed, that not only general provisions may have been made for the several species of beings, but that even _particular cases_ (at least many of them) may have been provided for without innovations in the course of nature[ ]." and let us not suppose that this is a mere, wanton speculation, unsupported by evidence (if not actually circumstantial, at least) strongly presumptive; since the further we penetrate into the ramifications of the organic world, the less are we inclined to ignore the operation of those various modifying influences which our understanding tells us do everywhere exist. to investigate the causes of things, and to endeavour to trace out by slow, inductive processes those secondary agents, by the assistance of which a large proportion of the phænomena around us are gradually matured, is no insignificant task; yet how much animadversion from without have the students in such fields of research frequently to endure! a fact many times repeated, and which comes within our daily experience, is too often looked upon as a matter of course, and as therefore beneath the notice of an intelligent mind; yet the man who regards _truth_ as valuable, for its own sake, under whatever aspect it may come, and who can rise to the appreciation of _results_, whether they be of rare or constant occurrence, will have learnt to pronounce nothing as unimportant which may supply a single link in that chain of knowledge which would be broken and imperfect without it. a spirit of inquiry, however, is becoming, year by year, more evident; and we may confidently anticipate the period when such reproaches will have for ever died away. natural history, in all its branches, will then advance more rapidly than heretofore, and each separate labourer, in his own peculiar province, will breathe a more genial atmosphere; whilst observation and reason, mutually dependent on each other, will work in concert more effectually. "reason without _observation_," writes the author above quoted, "wants matter to act upon; and observations are neither to be justly made by ourselves, nor to be rightly chosen out of those collected by others, without the assistance of _reason_. both together may support opinion and practice, in the absence of knowledge and certainty." in the last chapter we offered a few passing remarks on insect-aberration generally, whether regarded as a _universal fact_ (which, however, even supposing such to be true, it is not the object of the present treatise to substantiate), or as an _occasional_ one,--that is to say, as existing at all times to that extent (as an hereditary principle), that it is _liable_ to manifest itself, or not, according as external agencies may favour or oppose its occurrence. in the latter case, which alone i propose to consider, this inherent tendency may be displayed, either through the expression of "varieties" well defined, or by a mere proneness to wander, irregularly and at large, from an assumed diagnostic type. in the following pages, the _former_ of these resultant conditions (namely, that in which "varieties," technically so called, though _more or less_ isolated in their character, are apparent) will be especially discussed; since my principal desire is, to point out the influence of _local disturbing causes_ in regulating, to a greater or less extent, though of course within certain specific limits, the outward contour of the insect tribes,--and it requires no argument to prove that, where those local elements (whatsoever they may be) prevail, the _same_ effects will, for the most part (in the same species), be produced; and that, therefore, modifications which are characteristic of countries and regions far removed from each other have an _à priori_ claim for stability, above those which circumstances less important than geographical ones, and which are consequently more fluctuating in their combinations, may from time to time (as it were, accidentally) shape out. having then examined our premises, and prepared ourselves, with an unbiassed mind, for the reception of phænomena which should be constant (and in some instances, also, conspicuous) _in proportion as_ the conditions which unite in bringing them about are significant; let us advert to a few of the more prominent cases in which our instinct would seem to warrant the belief that aberrations are to be usually anticipated. and since it will hardly be denied that, like the representatives of other departments of the animate world, insects _may_, in their outward configuration and development, be in some measure under the control of the external influences to which they are immediately exposed, we will take a rapid glance at a few of the circumstances and conditions which are known to have more or less of a qualifying effect on the members of large and opposite sections of the organic creation; and then see how far we are enabled, by means of facts, to trace out results for the insecta, corresponding to those which are admitted to obtain in the other groups. and, since the existence of analogous results infers, to a certain extent, the similarity of the agents which have brought them about, our "causes of variation" (provided the effects can be shown) may be in reality almost demonstrated. amongst the numerous influences and conditions, in obedience to which the members of a large proportion of the animate world would appear, at times, in their outward aspect to be modified or fashioned, the following may be selected as perhaps of primary importance:-- . climatal causes _generally_ (whether dependent on latitude or upon altitude). . temporary heat or cold, of an unusual degree. . nature of the country and of the soil. . isolation, and exposure to a stormy atmosphere. § i. _climatal causes generally, whether dependent on latitude or altitude._ perhaps, judging superficially, climatal causes generally would appear to have more effect on insect development than any with which we are acquainted; yet, powerful as they unquestionably are, experience teaches us that such is not the case. in combination with other modifying principles, hereafter to be noticed, they may be (and probably are) exceedingly important; yet, when taken singly and alone, we have no evidence to show that their consequences are of such primary significance as might be anticipated. mr. darwin, in describing the fauna (which includes many mundane forms) of the galapagos archipelago, situated immediately under the equator, remarks: "the birds, plants, and insects have a desert character, and are not more brilliantly coloured than those from patagonia; we may therefore conclude, that the usual gaudy colouring of the intertropical productions is not related either to the heat or light of those zones, but to some other cause,--perhaps to the conditions of existence being generally favourable to life[ ]." although it is true, in a broad sense, that the nearer we approach the line the grander and more gorgeous are the animate beings which tenant the surface of our earth, there are at the same time so many exceptions to this law, that it cannot he regarded as by any means universal; and whatever, therefore, be our ideas on a subject which might perchance _seem_ to be self-evident, we are compelled to infer that climatal causes, of themselves, will not suffice to account for the numerous cases of aberration which we so constantly meet with in representatives of the same species exposed, through a long series of centuries, to opposite conditions of atmosphere. we need not, however, go so far as the galapagos to convince ourselves of this. the madeiran group is placed between the nd and rd parallels of north latitude, off the coast of africa, and contains a coleopterous fauna (as hitherto ascertained) of about species. now of these, at least, occur also in europe (many of them even in our own country); hence, if a more southern climate may be presumed, of itself, to exercise any very decided modifying influence on insect development, we have an amount of material for comparison which should surely afford us some definite and tangible result. my own experience in those islands would tend to prove, that, amongst the many aberrations from their northern types which are there everywhere displayed, comparatively few of them can be referred for explanation to causes strictly climatal. i do not say that _none_ can be thus accounted for; yet i trust to make it obvious in the following pages that there are even greater agencies at work than climatal ones in regulating (albeit within prescribed limits, and by slow gradations) the outward contour of the insect tribes. when viewed geographically, there are two heads under which the insects of every individual area may be classed: namely, those which were created within its bounds, and which constitute its true aborigines (in the strictest sense); and, secondly, those which _have reached it_, either by ordinary migration over an intervening land, or by accidental introduction through human or other agencies. now it is to the members of the _latter_ of these ideal divisions, that we principally look for any positive evidence, whilst discussing the causes of variation: since, by the nature of the case, we _must_ have identical, or at any rate closely allied species to reason upon before any sound conclusions can be drawn concerning them from the circumstances and conditions to which they are severally exposed; and it is clear, that the fact of creatures being specifically coincident, and yet under influences remote, does, for the most part, actually _imply_ a transportation of them (from their primeval centres) beyond the limits of a naturally acquired range. moreover, the autochthones of the soil (if we may be excused the idiom) are in all instances adjusted to the peculiarities of the region in which they were formed; and, consequently, where they have not (as very frequently happens) diffused themselves to a sufficient distance from the birthplace of their kind to be acted upon in two opposite manners from without, the date _they_ supply, during our inquiry into specific modifications as dependent on external disturbing elements, cannot be very considerable. in spite of this severe distinction, however, which i would urge between the insect _aborigines_ of a country and _those which_ (whether by compulsion or not) _have colonized it_, and of the preference which (as just stated) must be given to the latter whilst investigating the controlling principles of aberration, i would not wish to reject _in toto_ the testimony which the former likewise may indirectly furnish,--especially under the present section, in which climatal causes on a large scale have to be taken into account. true it is that we cannot hope to descry _physical results_ amongst phænomena which are due to the _creative_ force alone; yet we may, in the contemplation of them, recognize such an amount of _design_, or a primary adaptation to conditions from without, as shall afford, through its permanence and method, fresh presumptive evidence that the "conditions" _themselves_ may have some inherent modifying power of their own on the aggressors from other districts, in which a contrary influence may perchance prevail, and for the overspreading of which they were, in the beginning, more peculiarly constituted and ordained. it has been already mentioned (and, despite the exceptional cases which are to be found, it is in a _general_ sense true), that the splendour and extravagance of the insect world attain their maximum within the tropics; and that the nearer we approach the central heat, the more and more unmistakeable is the existence of this law. it has been also hinted, that when viewed on a very extensive scale, we shall not derive much _direct_ assistance (whilst examining insect-variation, with reference to climate) from the consideration of a fact thus seemingly important,--since there are but few species whose range is so comprehensive as to embrace, at the same time, the equatorial and temperate regions of the earth; and since, as lately suggested, it is not from a comparison of the _aborigines_ of countries far removed that we can hope to derive much positive information during our present inquiry. it may be useful however to speculate, why the creative energy should have been thus lavished, as it were, in the torrid zone, whilst the fauna of the cold north is so unpretending and sombre. i believe that in the actual _number_, both of individuals and species, which they contain, the difference is not so great, between the two latitudes, as might be imagined; and that, were the minims of scandinavia to be suddenly magnified into the giants of brazil, the laplanders and swedes might stand a fair chance of being temporarily alarmed: nevertheless, as regards the multitude and eccentricity of her forms, there can be no question in which field it is that nature has ever delighted more particularly to sport. laying aside, therefore, the numerical statistics from our account, is not the exuberance of the tropics at once responsive to the conditions imposed upon them? do we ask why it is that the insect population is there moulded upon a type comparatively so colossal?--let the redundancy of the vegetation reply. have not, also, more rapid laws of putrefaction and decay been prescribed than in our cooler clime; and can we imagine that it was _not_ in obedience to this decree, that larger and more active scavengers were framed? the gaudy wings that float idly on the breeze, and the coats of mail which glitter in the light, have they nothing to tell of the local circumstances around them; or, is it too much to infer, that a more glorious and stimulating sun required creatures of superior brilliancy to bask in its rays? a moderate degree of heat, and that only during a certain portion of the year, may suffice in quiescent regions to keep up the equilibrium of the organic world, the various members of which, whether animals or plants, are ensured, in such countries, their alternate seasons of activity and rest; but within the tropics, life, in all its aspects, is ever vigorous; and, though the several species may have their appointed times of partial repose, there is no such thing as tranquillity for the mass. hence it is, that to meet the requirements of a flora[ ] such as there obtains, a less magnificent fauna would have been inadequate; and we cannot but recognize, that, in the wonderful and almost endless modifications of the insect tribes which people those zones, a special provision has been made to check the overgrowth of other created things. but how, it may be asked, does this _primary adaptation_ to external conditions affect the question of specific development? perhaps not much: nevertheless, as lately urged, it is well that such adaptations should be borne in mind, not merely that due importance may be given to influences in conformity with which the creative act was at the first expressly regulated; but also that we may be prepared, if any qualifying power be admitted to reside in those influences themselves, for the _kind_ of aberration which reason and experience would seem alike to imply that we should, in the various instances, anticipate. we have already stated, that climate, when taken alone, does not appear to produce any very decided modifying effect on insect form, seeing that there are vast numbers of species of a wide geographical range which do not display, on their northern and southern limits, differences sufficiently constant to be regarded as purely climatal ones; and it is clear that, if climatal causes of themselves were of real primary significance, we should probably seldom fail to trace out, from their long-continued operation, some steady and positive result. yet when combined with other principles, there is evidence that a considerable amount of influence must be conceded to the action of mere heat and cold, working permanently and according to fixed laws, on the members of the insect world. such being the case, it is perhaps not surprising that a slight difficulty should arise, through our employment of separate sections under which to examine the causes of variation; for, since it is ordinarily by the union of several disturbing influences that aberrations are brought about, it is for the most part impossible, to refer the results, however conspicuous they may be, to a solitary controlling element. and hence, though we may be able at times to point out perchance the _single_ reason for certain phænomena with comparative precision, it will generally happen that two or three agents must be appealed to before we can arrive at a conclusion by any means satisfactory. i would desire, therefore, that the examples hereafter to be noticed may be judged of in the mass; and may not be considered as severally assigned, of necessity, to an isolated deranging cause, through the fact of their being placed, for the sake of convenience, and because of the _predominance_ which special controlling principles have had in maturing them, under sections, both, as it were, exclusive and particular. that climate of itself possesses but a limited modifying power on insect development, is evident from the consideration (just alluded to), that numerous species of comparatively wide distribution are totally unaffected by it. thus, for instance, the _pissodes notatus_, fab., a weevil which occurs in pine forests from lapland to barbary, and which has been naturalized even in the madeira islands, passes through the alternations to which it is specifically subject, irrespective of country. in like manner, the _lixus angustatus_, fab., so abundant in central and southern europe, the north of africa, malta, madeira, and the canaries, and which has been detected in persia, would seem to be perfectly free from atmospheric control. the _coccinella -punctata_, linn., which exists in nearly every portion of the old world, is apparently unacted upon geographically. numberless beetles which follow in the track of man, or at any rate are liable to do so, almost everywhere (such as _carpophilus hemipterus_, linn., _trogosita mauritanica_, linn., _læmophloe us pusillus_, schönh., _dermestes vulpinus_, fab., _anobium striatum_, oliv., _rhizopertha pusilla_, fab., _sitophilus granarius_ and _oryzæ_, linn., and _tribolium ferrugineum_, fab.), show little or no tendency to variation. nor is this independence of climate to be observed less frequently in the aquatic forms, than in the terrestrial ones: the _agabus bipustulatus_, linn., common in the streams and pools of the whole of europe, the north of africa, and in madeira, although naturally somewhat inconstant, offers no aberration, _the result of latitude_; as is equally the case with the _hydroporus confluens_, fab., which is found from sweden to the canaries, and the _eunectes sticticus_, linn.,--an insect literally cosmopolitan. the swallow-tail butterfly (_papilio machaon_, linn.), the clouded yellow (_colias edusa_, fab.) and the painted lady (_cynthia cardui_, linn.),--the first and second of which occur throughout europe, in siberia, syria, egypt, barbary, nepaul, and cashmere; whilst the third (so general in our own country) has been recorded from india, north america, the brazils, africa, java, and new south wales,--however irregular they may be, afford no indications[ ] of undoubted geographical instability. we need not however multiply examples, since our space will scarcely admit of it, and numbers of them will be at once suggested to the entomologist: what it mainly concerns us here to corroborate, is the thesis, _that climatal operation_, although by no means invested with a universal qualifying power, _has an amount of influence on certain species, even whilst unconnected with other elements,--and therefore_, à fortiori, _when in combination with them_. the two principal conditions on which climatal causes generally may be said to rest, are latitude and altitude. as regards the former of these, however, whilst the equatorial and arctic regions of the earth will of course give us the extremes of heat and cold, we shall often perceive differences of temperature (the result perhaps of local circumstances) in areas but slightly removed from each other, sufficient to affect very materially, though by what means it is difficult to understand, the outward contour of the insect tribes. thus, to go no further than ireland, we find that the specimens of _silpha atrata_, linn., so abundant throughout england and the whole of europe, have put on (it may be from the moisture of the atmosphere, or from some other obscure influence) the appearance of a distinct race,--so distinct indeed as to have long received another name, _s. subrotundata_, from british naturalists. i think it far from improbable that the _tachyporus nitidicollis_, steph., an insect eminently characteristic of that country (and one on which i have lately offered some remarks[ ]), is but a darker climatal modification of the common _t. obtusus_: and it is well known that the examples of _pelophila borealis_, payk., from killarney and loch neagh are permanently larger, and much more metallic, than those from the orkneys. the _nebria complanata_, linn., assumes a more pallid hue in the neighbourhood of bordeaux than it does on the sandy coasts of devonshire and wales: and i have but little doubt that the _omaseus nigerrimus_, dej., of spain, the north of africa, and madeira, is a geographical state of the _o. aterrimus_ of central europe. the _sitona gressoria_, illig., so universal throughout the mediterranean districts, madeira and the canaries, may be but the subaustral form of _s. grisea_. the _bembidium obtusum_, sturm, is shorter and less parallel in our own latitude than it is in the madeiran group and along the mediterranean shores: whilst the _holoparamecus niger_, aubé, of madeira and sardinia is very much paler than the same beetle when taken in sicily. specimens of _pieris brassicæ_, linn. (the white cabbage-butterfly,--an insect of widely acquired range), from nepaul and japan, are recorded[ ] to have differed so strongly from the ordinary european type as to have been referred, by boisduval, in doubt to that species. mr. westwood has received the _vanessa atalanta_, linn., from north america, receding slightly from its british analogue; but which he, nevertheless, does not regard as specifically distinct: and such also (he adds) was the opinion of mr. kirby, who has described his american examples under that name. the common _hipparchia_ of madeira i believe to be a fixed geographical modification of the _h. semele_, linn., of our own country,--in which the paler bars of the upper surface are evanescent;--there are, however, i imagine, but few entomologists who would concur with me in this hypothesis. the madeiran specimens of _lycæna phloeas_, linn. (the small copper butterfly), are invariably darker, and more suffused, than the english ones: and mr. westwood remarks that he possesses examples from north america which "differ in the decided black spotting of the under side of the hind wings, in the bright red streak near their hind margin, and in wanting the minute spot on the costa of the fore wings; but that these characters can scarcely be held to constitute a distinct species[ ]." few observers can have failed to remark, that increased _altitude_ frequently corresponds, both in its fauna and flora, to a higher _latitude_; and that, consequently, if we ascend the mountains of a southern land, we shall be struck, at times, by the presence of a host of species which obtain at a lower level in more temperate zones. this is peculiarly traceable in the madeira islands,--which, from their subaustral position, and height (the loftiest peak of the central mass exceeding feet above the sea), afford a rich field to the student of zoological geography. yet, though the degrees of mere heat and cold are such as to allow, in the two cases, species positively identical to flourish; we should surely anticipate some slight change from the different atmospheric conditions (especially when in union with other circumstances) to which they have been, through a lapse of ages, respectively exposed: it may be well therefore to inquire, whether experience does at all tend to strengthen what our reason has an _à priori_ inclination to endorse. it must be recollected however that, in the instances to which we would draw attention, _small_ aberrations are all that can be usually looked for, since climate _of itself_ does not appear to be very potent in its action. we should remember, also, that the boundaries of insect instability are restricted; and, although we would advocate freedom of development within limits which are more or less comprehensive according to the species, to pass beyond them would be confusion, and such as could result from a _lapsus naturæ_ only, rather than from a power of legitimate variation. in exact conformity with what the above remarks will have prepared us for, we find that the _dromius obscuroguttatus_, dufts., of central europe, has undergone on the mountain summits of madeira changes precisely to that extent which we should have calculated upon; and although they would seem in reality to be referable to climate _and isolation_ combined, yet, since it is not always possible (as lately stated) to treat the elements of disturbance separately, and it is my object in this short treatise to bring forward a few prominent examples in support of the considerations proposed, rather than to accumulate a mass of material for the registry of which my space would be inadequate, i will quote _in extenso_ the reflections which, during the compilation of the 'insecta maderensia,' suggested themselves to me. "the _dromius obscuroguttatus_ is a common european insect, and the madeiran specimens recede from the ordinary ones in being slightly larger, and in having their elytra more obscurely striated, with the humeral patch less distinct: their entire surface, moreover, is of a deeper black, a difference which is especially perceptible on the legs. it occurs in the greatest profusion in madeira proper, though only from about to feet above the sea. although so common throughout europe, it is perhaps, when geographically considered, one of the most interesting of the madeiran coleoptera, as affording a striking example, not only of the modification of form in a normally northern insect when on its southern limit, but as showing likewise how a species, abundant on the low sandy shores and sheltered sea-cliffs of more temperate regions, finds its position here only on the summits of the loftiest mountains. it is true that the aberration from the typical state is not in the present instance very considerable; yet when the circumstances producing it are taken into account, i am persuaded that the difference is exactly of that nature on which too great stress cannot possibly be placed, when discussing the general question of geographical distribution as having a tendency, more or less directly, to affect both colour and form. it is well known to naturalists that a multitude of insects from the new world, receding from their european analogues merely in certain excessively minute characters, have usually been pronounced at once as new to science, first because those differences are constant, and secondly because the specimens have been received from the other side of the atlantic. and yet in instances like the present one,--in an island which, while it belongs artificially to europe, is yet naturally sufficiently distinct from it as to form at any rate a stepping-stone to the coast of africa and the mountains of barbary,--species similarly circumstanced are not necessarily received as new (and rightly so, i apprehend), though in every respect affording differences not only _analogous_ to those already mentioned, but in many instances positively identical with them. if, however, a specific line of demarcation does of necessity exist between the creatures of the old and new worlds, the problem yet remains unsolved, so long as intermediate islands present parallel modifications, where that line is to be drawn. meanwhile, how far geographical varieties of this kind, concerning the non-specific claims of which confessedly but little doubt can exist, may lead to the explanation of the transatlantic ones just referred to, i will not venture to suggest. yet certain it is, that the one case bears directly on the other; and that, if we can prove that common european insects, when isolated in the ocean, become in nearly all cases more or less modified externally in form, there is at least presumptive evidence that the law will hold good on a wider scale, and may be extended, not only to the atlantic itself, but even to countries beyond. the differences of the present _dromius_ from its more northern representatives are, as just stated, small; nevertheless, since they are _fixed_, those naturalists who do not believe in geographical influence might choose to consider them of sufficient importance to erect a new species upon. but after a careful comparison of this with other insects similarly circumstanced, i am convinced that the modifications in question are merely local ones, and such as may be reasonably accounted for by the combined agencies of latitude and isolation, and the consequently altered habits of the creature, which is thus compelled to seek alpine localities in lieu of its natural ones[ ]." in like manner the _calathus fuscus_, fab., the _anchomenus marginatus_, linn., and the _anthicus fenestratus_, schmidt, which occur almost exclusively in the _lower_ regions of northern latitudes, are found in madeira on the mountain tops; each, moreover, possessing characters which are just sufficient (although slight) to distinguish them from their european representatives. and if we inquire, on the other hand, into the _aboriginal_ species of those islands,--or, at any rate, into such of them whose naturally acquired range embraces the opposite extremes of atmosphere,--we shall detect no less surely (albeit within a narrower space) the result of climatal action on insect form. the _helops confertus_, woll., "varies according to the altitude at which it is found; being usually deeply striated and rugose on its lower, but subpicescent and much more lightly sculptured on its upper limits. i have taken specimens indeed on pico ruivo, and on the mountain-plain of the fateiras, which are so far diminished in roughness as almost to resemble, at first sight, the _h. pluto_[ ]." the _pecteropus maderensis_, woll., which ranges from about feet above the sea to the summits of the loftiest hills, although usually with pale legs, is distinguished by having its femora almost invariably dusky when on its highest elevation; and, following out the analogy with that beetle, the _trechus alticola_, woll., should perhaps be regarded as an alpine state of the _t. custos_. the _calathus complanatus_, koll., assumes along the upland heights a very different aspect to what it does in the regions below, being generally more piceous and convex, altogether broader (in proportion) and shorter, and with _both_ sexes (though, of course, especially the male) shining. nor is this principle of topographical variability (the result of climate) less apparent in other countries also. the _notiophili_, for instance, "are extremely unstable, both in their sculpture and hue, being subject to considerable local modifications, though more particularly affected, it would appear, by altitude. thus, in our own country, the _n. semipunctatus_, fab., one of the common representatives of the plains, is found likewise on the summits of the mountains; but at that elevation it becomes liable to great alternations of colour, ranging from pale brassy-brown, with the apex testaceous, into deep black. the sculpture, however, perhaps is nearly as much dependent on other circumstances for its modification as upon altitude, since it seems tolerably clear that proximity to the sea-shore, especially where the localities are saline, will frequently produce a more faintly impressed surface[ ]." it has indeed been lately suggested, that the _helobia nivalis_, payk., may be perhaps, after all, but a mountain variety of the _h. brevicollis_; the _leistus montanus_, steph., of the _l. fulvibarbis_, and the _patrobus septentrionis_, dej., of the _p. excavatus_; but of this i think further proof is needed, seeing that certain species do appear to exist which are _strictly_ alpine (that is to say, which have not been, severally, detected in the lower regions of more northern zones); and, in _most_ instances, where aberrations are to be met with from the effect of _altitude_, we have a right to inquire (provided the types from which they are supposed to have originally sprung obtain in the less-elevated portions of the same country), _where are the intermediate links_? now i am not aware that any such links have, in the examples above cited, ever been observed; whilst i can vouch that in at any rate many districts where the _quasi_ variety is found, the descendants of its assumed progenitor _do_ occur in the plains beneath. i have remarked that the _cicindelidæ_ often become inconstant in colouring as they approach their maximum of height above the sea; and i have but little doubt that the _c. fasciatopunctata_[ ], germ., from asia minor and turkey, is the _c. sylvatica_ modified by a long residence in elevated regions. and so it is with the _chrysomelæ_, many of which become, in the loftiest altitudes to which they ascend (as i have noticed at the head of the st. gotthard pass of the swiss alps), subject to unusual changes, both in lustre and hue. the above examples, although few and indiscriminately selected, will serve to illustrate the principle which we have been contending for,--that climatal influences generally, may (and in most instances do) tend to affect, more or less directly, the outward contour of the insect tribes. it will be remarked that, in the cases hitherto cited no great disturbing power has been made evident,--the aberrations to which we have appealed being, most of them, comparatively minute. this, however, is simply in harmony with the belief which we have already expressed, that climatal causes, when taken singly and alone, are not of primary importance whilst discussing the question of specific modification. it remains for us, in the following sections, to inquire, whether there are any other elements at work from which greater results are to be expected. meanwhile, let us not forget that differences _may_ be, in the strictest sense, significant, even whilst small; and that it is their _constancy_, rather than their magnitude, which more particularly concerns us in the present treatise, seeing that it is with reference to those distinctions which are less conspicuous that the greatest amount of misunderstanding (through the fact of their being _fixed_) usually prevails; whilst it is our main object to show that dissimilarities do not _necessarily_ imply the specific isolation of the creatures which display them, merely because they are, in their several localities, _permanent_. § ii. _temporary heat or cold, of an unusual degree._ it is perhaps unnecessary that the action of temporary heat and cold, of an unusual degree, should be considered under a separate head from that of climatal causes generally; nevertheless, since the latter are, in a certain sense, permanent in their operation, it may be thought desirable that i should offer a few words on the effect of sudden exceptions to the ordinary routine of things, such as, for instance, seasons of peculiar intensity. it does not however appear that any very important modifications do often occur from conditions thus abnormal, and as it were _accidentally_ brought about: on the contrary, indeed, it is a well-known fact, that the members of the insect world are singularly independent of such contingencies; and that, in the same manner as their times of maturation are neither hastened nor retarded by them, their external development is for the most part free from their control. yet, in spite of this, specific results _are_ wont to happen, ever and anon, from such circumstances, as though it were a fundamental axiom, that every agent which nature can press (regularly or irregularly) into her service should have, though it may not always exercise its privilege, some qualifying voice. i believe that almost the only deviation from the typical state, in insect form, which has been observed to originate, _par excellence_, from the occasional continuance of undue heat or cold, is curiously enough an organic one,--having reference to the enlargement of the wings. every entomologist must be aware that a vast proportion of the coleoptera (especially the _carabidæ_) are subject to great inconstancy in their metathoracic organs of flight. many species, as the common _calathus mollis_ of our own country (to which my attention has been more particularly drawn by the rev. j. f. dawson), have the hind wings at one time ample, at another rudimentary, and at a third nearly obsolete. now, although other causes, hereafter to be noticed, would seem to have far greater power than climatal ones in _permanently_ regulating the size and capacity of these appendages; i think it will be found on examination (and i may add that mr. westwood is of the same opinion[ ]), that the greater or less development of them may be frequently explained by the unusual severity of the seasons. my own researches would certainly tend to prove, that _heat_ does (in the main) favour, and _cold_ retard, their presence. exceptions (often rendered intelligible from the evident working of counter influences) will of course arise in abundance to this hypothesis; yet my impression is that, upon a broad scale, it will stand the ordeal of a rigid inquiry. speaking of certain representatives of the hymenoptera (_chalcididæ_), mr. westwood observes: "a curious peculiarity exists in one at least of these apterous species, which has been noticed by no previous author, namely, _choreius ineptus_, westw., which, although ordinarily found in an apterous state, was discovered by me in considerable numbers during the hot summer of , with wings[ ]". and, touching the irregularity of the alary organs in the homopterous _fulgoridæ_, he remarks: "other instances, in which the wings undergo a deficiency of development, occur in the genus _delphax_, the majority of which, in our english species, have the upper wings not covering more than one half of the abdomen,--the terminal membrane being deficient, _as well as the hind wings_. in certain seasons, however, especially hot ones, the wings are fully developed[ ]". mr. curtis has indeed formed the undeveloped specimens into a different genus, _criomorphus_. although the result of a more stimulating sun may be often neutralized by that of _isolation_ (which, as we shall hereafter see, is a resistless agent, amongst a host of species, in weakening, and frequently rendering abortive, the powers of flight); yet _heat_, when freed from counter influences, may be traced in its _permanent_ effect on the alary system of insects, no less than when temporarily applied. the consideration of this, however, belongs strictly to the preceding pages, and we will not therefore discuss it here. the common bed-bug (_cimex lectularius_, linn.) is almost invariably apterous, or with very short rudimental hemelytra; yet scopoli (_ent. carn._ p. ) mentions its occurrence with perfect wings. fallen, also, and latreille, state that it has been found winged; whilst westwood remarks that it has been reported as occasionally winged in the east indies; and it would seem extremely probable that, in these examples, as in numerous others which are on record, we may detect the consequences of heat; either as temporarily applied (in an unusual degree), or through the accidental transportation of the insect into a naturally warmer atmosphere. § iii. _nature of the country and of the soil._ before we proceed to inquire to what extent the outward aspect of insects is liable to be controlled by the physical state of the areas in which they severally obtain, it may not be altogether out of place to offer a few reflections on the superiority which some regions possess intrinsically over others, both for the _increase_ and _diffusion_ of the animal tribes. to suppose that all countries within the same parallels of latitude are equally favourable for the development of life (not to mention the after-dispersion of it), is contrary to experience; for although (as we have already pointed out) the organic world does certainly, when viewed in the mass, approach its maximum as we near the tropics, there are at the same time so many violations of this law, that we cannot admit its operation except in a broad and general sense. in a former section of this chapter, i drew attention to the fact, that certain islands, equatorial and subaustral, are anything but suggestive of their actual positions with respect to the line of central heat on the surface of the earth. it was with regard to _climate alone_, however, that i wished them to be understood: and it is not until now that i have ventured to urge the necessity of taking other influences into account also, if we would desire to recognize anything like design and adaptation (i will hardly call it cause and effect) between the continent and the thing contained. it is almost needless to add, that there are _many_ elements to be considered, such as local atmospheric conditions, excess or deficiency of electricity, superabundant moisture, diminished light, and the geological composition of the soil, before we can hope either to appreciate zoological phænomena as a whole, or to reconcile the apparent inconsistencies which they are accustomed to display. mr. darwin, to whom we are indebted for so much valuable information concerning the natural history of various portions of the world, in his notes on tierra del fuego, observes: "beetles occur in very small numbers; it was long before i could believe that a country as large as scotland, covered with vegetable productions and with a variety of stations, could be so unproductive. the few which i found were alpine species of _harpalidæ_ and _heteromera_, living beneath stones. the vegetable-feeding _chrysomelidæ_, so eminently characteristic of the tropics, are here almost entirely absent. i saw very few flies, butterflies, or bees, and no crickets or orthoptera. in the pools of water i found but few aquatic beetles. i have already contrasted the climate as well as the general appearance of tierra del fuego with that of patagonia; and the difference is strongly exemplified in the entomology. i do not believe they have one species in common; certainly the general character of the insects is widely dissimilar[ ]." now, it is impossible to read this account without being at once struck with two primary considerations: first, that there must exist some great peculiarity (apart from climate) in a region the fauna of which is thus singularly constituted; and, secondly, that latitude (however important it may be in a comprehensive point of view) must exercise in this case a very secondary influence, to allow of localities separated only by the straits of magellan to present differences thus extraordinary. although so dissimilar in many respects, madeira and tierra del fuego have evidently much in common as regards the conditions which they afford for the increase of organic life. mr. darwin describes the latter as "a mountainous region, partly submerged in the sea." so is madeira. he also adds, that it is "covered to the water's edge with one dense, gloomy forest;" that "to find an acre of level land in any part of the country is most rare;" and that "within the forest, the ground is concealed by a mass of slowly putrefying vegetable matter, which, from being soaked with water, yields to the foot." such _was_ madeira, in its normal state[ ]; and such it still is throughout a large district towards the northern coast. i cannot indeed refrain from quoting the following, since it portrays the characteristic features of madeira so vividly, as to be, literally, as suggestive of that island as it doubtless is of tierra del fuego. "finding it nearly hopeless," says darwin, "to push my way through the wood, i followed the course of a mountain-torrent. at first, from the waterfalls and number of dead trees, i could hardly crawl along; but the bed of the stream soon became a little more open, from the floods having swept the sides. i continued slowly to advance for an hour along the broken and rocky banks, and was amply repaid by the grandeur of the scene. the gloomy depth of the ravine well accorded with the universal signs of violence. on every side were lying irregular masses of rock and torn-up trees; other trees, though still erect, were decayed to the heart and ready to fall. the entangled mass of the thriving and the fallen reminded me of the forests within the tropics; yet there was a difference,--for in these still solitudes, death, instead of life, seemed the predominant spirit[ ]." as regards the paucity of species in tierra del fuego, there are many instances on record of other countries, and in various latitudes, in which the same anomaly (though perhaps in a less degree) prevails. i have myself observed, in madeira, large forest tracts, at a considerable elevation above the sea, and which are so densely clothed with wood as to be scarcely penetrable, almost destitute of insect life. around such altitudes however the clouds perpetually cling, and the rain is well nigh incessant; and it would seem as if the very dampness which causes the vegetation (especially the ferns) to flourish in such rank luxuriance, and the timber to rot with such rapidity that the gigantic trunks are washed, reeking with moisture, down the mountain-slopes, was too extreme for animal existence. now, it will be remembered that the madeiran group is situated at a corresponding distance from the equator as morocco, algeria, the lower limits of syria, texas, and upper florida are,--all of which literally teem with life; and that tierra del fuego lies between the same parallels of south latitude as durham and central russia do in the northern hemisphere. from which it is evident, that the equal removal of countries from the earth's greatest heat does not necessarily imply an equal _exuberance_ in their faunas,--seeing that in both the regions just appealed to, we not only perceive a vast difference in the _numbers_ of the insects which they respectively contain, from those in other districts which have a similar divergence from the tropics; but we are even able to recognize a certain _resemblance of physical conditions_ (and, therefore, of the creatures which have been either adapted to, or modified by, them) in lands so far asunder, not merely with respect to latitude, but longitude also, as madeira and tierra del fuego. other instances might be cited, in support of the immediate principle for which we are now contending,--namely, that many areas have (from local circumstances) a natural superiority over others for the increase of the animal tribes, even _apart from the direct action of heat and cold_:--but space will only permit me to glance at a very few of them. we may detect evidences of this fact, in ireland; which, in spite of the narrowness of the straits which separate it from our own country, and of its independent commerce with all parts of the civilized world, has an insect fauna curiously limited. from what cause this may arise,--whether from some obscure physical influences peculiar to the soil, or (as professor e. forbes has suggested) from the sudden impediment which the establishment of st. george's channel presented to the westward progress of the various species from the germanic plains,--it is difficult to speculate: yet the _fact_ of its poverty remains, and we must explain it as best we are able. there can be no question, that, from more frequent communication with england, its entomological fauna has of late years been considerably increased; and it is equally easy to detect, through an examination of its less inhabited provinces, that at a period geologically recent its insect population must have been singularly scanty. i know of few regions (not even excepting the uplands of madeira) which are more deficient in insect life than the mountains of kerry. although abounding, throughout extensive districts, with wood and water, and presenting every apparent requisite for its full development; the naturalist will often be disappointed by finding that a hard day's work has not ensured him the same amount of success as he would have reaped in less than half an hour in many an english meadow. do we ask, why this is so?--it is impossible to reply, except on the supposition that there are real physical agents, independently of heat and cold, which are unfavourable in ireland to the existence of these lower creatures. we may perhaps be told, by the advocates of professor forbes's theory, that it is the result of isolation,--the quondam land of passage having been broken up before the proper complement of species had reached this large portion of their western destination. but even this, although i believe it to contain much presumptive truth, will not altogether suffice to account for the phænomena which we see; for ireland is not only remarkable for the paucity of its _species_, but also for the paucity of its _individuals_,--and the latter fact cannot be explained by any stretch of the migration-hypothesis. we are compelled therefore to conclude, that ireland, like the other countries to which we have already alluded, presents conditions (altogether irrespective of _latitude_) which must be regarded as adverse to the general prosperity of the insect races. and so it is with _localities_ (no less than with larger countries),--many of which are eminently unproductive, when compared with others situated at but a short distance from them. thus, the south-western corner of england is by far the most unprofitable portion of our island, unless indeed i am much mistaken, for insect ascendency. i have made some remarks on this subject in the 'zoologist,'--from which i extract the following: "unlike the easy collecting to which we are accustomed in the more favoured east, miles of unprofitable country have often to be gone over, be it swampy moorland or iron-bound coast, where scarcely an insect is to be seen; or, at any rate, where the few which exist are so ordinary, and so sparingly dispersed, as to be scarcely worth the labour of obtaining them,--more especially since the identical species are many of them to be met with in the utmost profusion in more central, or eastern districts. whether it be the moisture of the climate, or the violence of the south-west winds, which (continually sweeping, as they do, over the high central mass of devonshire and the bleak, barren downs of cornwall) present as great an obstacle to the development of animal, as they clearly do of vegetable life, i will not venture to suggest; yet certain it is, from observation, that insects not only become fewer in number in proportion as they are exposed to these external agencies of wind and water; but likewise, in many instances, diminish so considerably in stature as to be scarcely reconcileable with their normal types[ ]." there can be no doubt that islands are, for the most part, more unproductive (even in proportion) than continents; and that, the smaller the area, the less favourable will it be for the development of insect life. mr. darwin has noticed this fact in the galapagos (which he remarks are only equalled by tierra del fuego, in barrenness), on keeling island (in the indian ocean), where he succeeded in detecting but thirteen species, in st. helena, and at ascension; and i have added fresh evidence to the same in the various portions of the madeiran group[ ]. it is however to geological causes that we must mainly look for the explanation of this phænomenon; and, therefore, since i propose to examine that branch of our subject in a future chapter of this treatise, we will not discuss it now. it will also be better perhaps to defer for the present the general question of self-_diffusion_, which, at the opening of this section, we proposed to consider, along with that of insect _productiveness_ (as dependent on other local influences, besides climatal ones),--it being scarcely possible to render the problem of dispersion in any degree intelligible without calling in geology to our aid. having then disposed of this preliminary appendage to our inquiry, by expressing our belief (which i am satisfied that observation will tend more and more to corroborate) _that certain countries and spots are by constitution more favourable than others for the increase_ (apart from the after dissemination) _of the insect tribes_,--and that too through local influences amongst which mere heat and cold are but secondary in importance; let us proceed to consider, how far the _nature of the several districts_ may assist us in accounting for some of those numerous aberrations from the typical state which various insects are accustomed to display, and on which it has too often happened that "species" (so called) have been attempted to be established. i may premise however, that, whilst (as already urged) i would regard climate _per se_ as subsidiary to many other agents, i would not wish to ignore its action altogether even under the present section, since in combination with peculiar circumstances and conditions it may have (and probably has) considerable controlling power: nevertheless i would desire it to be looked upon here as, at any rate, an inferior element, and as working in conjunction with physical influences of greater significance than itself. if therefore under the preceding heads it has been treated (so far at least as the exceptions would permit) as a great geographical principle, possessing a certain modifying quality on a large scale, let us now merely recognize it to the extent in which we are actually compelled to do, when dealing with areas of smaller magnitude,--namely as a _topo_graphical one. from amongst the many results which i have been long accustomed to associate (whether rightly so, or not, i leave it for others to decide) with certain special situations, i would draw attention to the singular inconstancy which numerous insects are liable to when existing on the coast,--and which frequently causes them to assume an aspect so permanently different from their inland types, that, without local knowledge to guide us, they might be supposed at first sight to be specifically distinct. ten years ago i offered a few comments on this fact in the pages of the 'zoologist'; which, as i have seen no reason subsequently to modify them, i will transcribe at length:-- "the extraordinary changes which many insects are subject to when occurring near the sea, is a fact worthy of notice, and one which i do not remember to have seen recorded. the strictly maritime species must be left out of the question; for although many of them are exceedingly variable both in size and colour, still we have no means of ascertaining whether that variation is referable to the locality in which they are placed,--for, never being found inland, nobody can have an opportunity of asserting that the same changes would not take place, were they to occur in positions far removed from the influence of the sea. when we find, however, the same insects in profusion both inland and on the coast, and observe also numerous and marked deviations from the typical forms peculiar to the latter situation; then, _à priori_, we have strong presumptive evidence that the changes in question are the result of local circumstances, and not referable to chance. the alteration in size i have almost always observed to be from large to small, and scarcely ever the reverse; whereas in colour the change takes place very nearly as much from light to dark as it does from dark to light: nevertheless the majority of instances i possess come under the latter department. it has been remarked that all the specimens of _mesites tardii_, which i captured in devonshire, were much smaller than the original series taken by mr. tardy at powerscourt waterfall, in the county of wicklow; and so decided was the difference, that many of my friends, at first sight, concluded the two to be distinct species. this, however, i consider entirely owing to their locality, for my specimens were found only on the coast, and mr. tardy's at a considerable distance inland. and, inasmuch as neither of these instances rested on mere individual examples, but on long and conspicuous series, the certainty of the change from large to small was the more apparent. mr. holme of oxford mentions having taken _olisthopus rotundatus_ in the scilly islands, in great profusion, none of the specimens of which exceeded two lines and a half in length. at whitsand bay in cornwall i have captured _gymnaëtron campanulæ_, none of which exceeded three-quarters of a line,--the usual length being from a line to a line and three-quarters. _anthonomus ater_, the average length of which is two lines, i have taken a series of in lundy island, none of which exceeded one. in the same locality, also, the common _ceutorhynchus contractus_ scarcely ever reaches its natural size; and is, moreover, so variable in colour, that i was long before i could persuade myself that the species was not distinct. instead of the bluish-black elytra which i had always considered invariable, they all possess a yellowish or brassy tinge; and the legs, instead of being black, are in most instances entirely of a light yellow,--and in all, more or less inclined to that colour. i have received from mr. hardy, of gateshead, specimens of _haltica rufipes_[ ], captured by him on the coast, in which the entire insect is of a uniform brownish-red hue. of the rare _mantura chrysanthemi_ i have taken beautiful varieties at mount edgcumbe and in lundy island,--many of which inclined to a rich metallic-yellow, instead of the brassy-brown of the ordinary specimens: also, in the latter locality, particularly dark specimens of _telephorus testaceus_. in like manner, i might enumerate other species equally remarkable; but i trust that those already mentioned are sufficient to verify my observations, of the extreme liability to change which, more or less, most insects possess when placed within the immediate influence of the sea. how to account for it, i know not. i mention it as a mere fact, and leave it for others to assign a reason for its existence[ ]." apparently dependent, in a large measure, on the same circumstance (namely proximity to the coast), the _bembidium saxatile_, gyll., so common at the edges of the mountain streams in the north of england, in scotland, and throughout a portion of ireland, presents itself along our southern shores in the form of a permanent variety; being, as the rev. j. f. dawson remarks, "more depressed, never narrower in front (the sides therefore more parallel), whilst the colour is always much paler and the spots larger,--that before the apex being round and very conspicuous, and the anterior one occasionally expanding over the surface very considerably[ ]." i have taken it in profusion on the coasts of the isle of wight, dorsetshire, and devon. and so with the _cistela sulphurea_, linn., which in certain maritime localities (as i have particularly noticed on the sand-hills at deal) is liable to become so dark in colouring, that, without the intermediate shades to judge from (which however may usually be obtained _in situ_), it might stand a fair chance, occasionally, of being mistaken for a separate species. a _psylliodes_ in lundy island, allied to (if not identical with) the _chrysocephala_, linn., found in abundance on a _brassica_ along the ascent from the eastern landing-place, varies "in every consecutive shade between the limits of light yellow and dark metallic-green[ ]," the former of which states (the normal one on that rock) might have been fairly set down as specifically distinct from the latter, did not observation on the spot decide the question for us without doubt. another curious example of the effect of local influences (amongst which proximity to the shore plays, in all probability, an important part) on the external aspect of insects exists in the _aphodius plagiatus_, linn.,--which in this country is generally deep black. "it is a circumstance worth noticing," i remarked in the 'zoologist,' in , "that the form which is looked upon by the continental naturalists _as the variety_, is in england evidently the typical one,--for out of about sixty specimens which i captured [at tenby in south wales], only _two_ possess the conspicuous red dashes on the elytra which are considered abroad as the almost invariable accompaniment." i have observed the same peculiarity in the flat and damp spots between the sand-hills at deal, where i have never detected a single individual which is not perfectly dark; and i believe that the greater number of the specimens which were originally taken at wisbeach, in cambridgeshire, offered the same geographical characteristics; whilst those which were found near the more inland towns of peterborough and norwich present a larger proportion of the ordinary european state. the _blood-red dashes_, however, with which the elytra of numerous insects are adorned, i have constantly remarked possess a singular tendency to become evanescent. it is indeed almost diagnostic of the genus _gymnaëtron_, either that its representatives should be thus ornamented typically, or else that those which are normally black should, _when they vary_, keep in view, as it were, _this principle_ for their wanderers to subscribe to. thus, i have no doubt that the _g. veronicæ_, germ., is but a variety of the _g. niger_,--an opinion which i expressed in the 'zoologist' nine years ago. whilst commenting on the coleoptera of dorsetshire, i then stated, that "for my own part i must confess i should have doubted the _g. veronicæ_ being really distinct from the _g. niger_, for red dashes on the elytra seem naturally peculiar, more or less, to the whole genus; and i should therefore have suspected that, had occasional aberrations from a black type existed (which is not unlikely), those aberrations would probably assume a form which is so common in the other species of the generic group[ ]." the _bembidium bistriatum_, dufts., is usually much paler when found in saline districts (under which circumstances it was described as a distinct species by mr. stephens) than when occurring in more inland positions. the _blemus areolatus_, creutz., i have frequently remarked is similarly affected in brackish places: and i think it far from improbable that the _stenolophus skrimshiranus_, steph., is but a local modification (though not altogether, perhaps, through marine influences) of the _s. teutonus_, schr. the _dromius fasciatus_, gyll., not being _exclusively_ littoral, may be quoted as another case in point,--the specimens which are collected near the coast being for the most part singularly pale. in speaking of the _anthicus bimaculatus_, illig., m. de la ferté observes: "il y a sculement lieu de remarquer que les individus du bord de l'océan sont généralement plus pâles que ceux des contrées orientales de l'europe, et que ceux des côtes de france et de belgique sent entièrement dépourvus de tache discoïdale[ ]." and bearing, in much the same manner, on the subject of variations, the _anthicus humilis_, germ., "est une des espèces le plus généralement répandues en europe; mais il lui faut le voisinage de l'eau salée. aussi on le rencontre non-seulement sur les rivages de toutes les mers, même de la baltique, mais encore aux bords des lacs salés, tels que celui de mannsfeld, en saxe. _ceux de cette dernière localité sont généralement noirs_; ceux que j'ai pris à perpignan sont d'un rouge très-clair, ce qui me porte à croire que cette espèce est dans le même cas que quelques autres _anthicus_, dont les variétés les plus foncées appartiennent au nord de l'europe, et les plus pâles au midi[ ]." whilst touching on this immediate question of variability _as dependent to a great extent_, in numerous cases, _on proximity to the sea_, we may just notice the marked tendency which even the insects _peculiar to_ saline spots would seem in a large measure to possess, of converging, more or less obviously, to a lurid-testaceous, or pale brassy hue, in their colouring. true it is that we cannot (as above suggested) deduce any evidence of direct physical modifications from amongst species which are _strictly maritime_,--seeing that we have no means of judging in such instances whether similar phænomena would or would not be produced in central districts also: nevertheless we may perhaps detect in this general law some slight indication of the effects which an atmosphere and soil constantly impregnated with salt would be likely to bring about in the external aspect of those members of the insect tribes whose range is sufficiently extensive to expose them to its operation. the bare mention of such names as _nebria complanata_ and _livida_, _calathus mollis_, _pogonus luridipennis_, _trechus lapidosus_, _aëpus marinus_ and _robinii_, _cillenum laterale_, _bembidium scutellare_, _ephippium_ and _pallidipenne_, _ochthebius marinus_, _psylliodes marcida_, _phaleria cadaverina_, _helops testaceus_, and _anthicus instabilis_, so eminently characteristic as they are of briny situations, will at once appeal to our native entomologists; whilst the acknowledgement of the same principle is no less conspicuous in a host of other species which are not included in the british fauna. hence, when we see the tendencies of coloration (not to mention other particulars, often readily apparent) essentially the same, both in insects which are peculiar to, and in those which have overspread (from without) certain regions or localities, it is impossible not to associate some inherent controlling power with the regions themselves; and we are driven to the conclusion, that _either_ well-defined _races_ have been gradually shaped out, by means of the physical influences to which they have been exposed, or else that the _species themselves_ (as witnessed by the intermediate geographical links, which, although sometimes rare, are in all instances to be found) do assuredly merge into each other. in addition to those which we have been just discussing, there are other influences (equally independent of mere heat and cold) by which insect modifications may be brought about,--modifications moreover of that precise character which must be referred, in general terms, to the nature of the country and of the soil in which they severally obtain: a very few examples, however, in illustration of their action, must suffice for our present purpose. the _tarus lineatus_, schönh., is slightly shorter in madeira, as also somewhat darker on its head and prothoracic disk (and with its elytral striæ less deeply impressed), than it is in algeria and spain. the madeiran specimens of the _aphodius nitidulus_, fabr., are usually a little paler, and more distinctly punctulated, than their northern analogues; as are also, in the latter respect, those of the _clypeaster pusillus_, gyll. the _scydmænus helferi_, schaum, is permanently smaller in the madeiran group than it is in sicily; and i believe that the _achenium hartungii_, heer, of those islands, is but a local state of the _a. depressum_, grav., of central europe. the _bembidium tabellatum_ and _schmidtii_, woll., may be in reality but geographical modifications of the _b. tibiale_ and _callosum_ of higher latitudes; and the _malthodes kiesenwetteri_, woll., of the common european _m. brevicollis_. calcareous deposits would appear, ever and anon, to have considerable efficacy in regulating the outward aspect of such species as are able to adapt themselves to different geological districts; and when in juxtaposition with the shore, their effects are often very conspicuous. the _dromius arenicola_, woll., is the portosantan representative of the _d. obscuroguttatus_, dufts.; and distinct as it is in colouring from that insect (as evinced both in madeira proper and throughout europe), i believe it to be in reality but a local condition of it, occasioned by a residence through a long series of ages on a calcareous soil. for the same reason perhaps (though assisted, in all probability, by the qualifying power of isolation), the _hadrus illotus_, woll., may be specifically identical with the madeiran _h. cinerascens_. in like manner, the _bembidium atlanticum_, woll., which in madeira proper is frequently so dark that its elytral patches are sub-obsolete, and which is but seldom brightly arrayed in that island, assumes in porto santo (which is not only more calcareous than the central mass; but is strongly impregnated, as its streams and rills everywhere testify, with muriate of soda) a permanently paler hue,--being at times almost testaceous. some districts seem to be more prolific in varieties, generally, than others. the neighbourhood of ipswich, in our own country, has been cited by mr. curtis[ ] as possessing this peculiarity; and i have remarked a similar tendency in certain parts of ireland. the common _haliplus obliquus_, indeed, of the blackwater river, in the county of cork, is usually so dark and suffused in colouring, that it might be almost taken for a distinct species,--its fasciæ, especially the hinder ones, being occasionally evanescent. one more example must satisfy us under this section,--namely, the _harpalus vividus_, dej., of the madeiran group. so curiously is that insect affected by the nature of the areas through which it successively ascends, and that too irrespectively of heat and cold (as may be gathered from the fact that its phases on the shore and upland heights are well nigh coincident), that it may be appropriately singled out as a concluding instance of the effects of those obscure local influences to which we have been drawing attention. "ranging from the beach to the extreme summits of the loftiest mountains, accommodating itself at one time to a low barren rock of yards circumference, at another to the deep-wooded ravines of intermediate altitudes, around which the clouds perpetually cling, and where vegetation and decay are ever rampant, or harbouring beneath the rough basaltic blocks of the weather-beaten peaks ( feet above the sea); we should naturally expect, _à priori_, to discover some slight modifications of outward structure, according as the respective localities differed in condition. and such we find to be everywhere the case. i am satisfied, moreover, that it is only by a careful observation on the spot that an insect like the present one can be properly understood; for, to anybody acquainted with it practically in all its phases, it is but too evident how many 'species' (so called) might be established on undoubted varieties, where there exists a desire for creating them, and where our sole knowledge is gathered from a few stray specimens collected by another person, and unaccompanied by local information to render the aberrations intelligible. for it must be tracked from the shore to an elevation of more than feet before we are enabled to discern the causes by which its development is controlled, or even to connect by slow and easy gradations its opposite extremes of form. and it is an interesting fact, that the distance between its variations does not increase in proportion to the distance between its altitudes. on the contrary, it would seem to pass through its minimum of size and maximum of sculpture at about the elevation of from to feet; both above and below which,--that is to say, as it recedes from the upper and lower limits of the sylvan districts,--it becomes gradually modified, and almost in a similar manner. thus, to a person who had visited madeira and had picked up specimens on the coast, and to another who had perchance penetrated into the interior, as passing visitors from the vessels are accustomed to do, and had brought away examples from the wooded mountain-slopes, the two insects would appear altogether distinct. for, commencing on the level of the beach, the usual type is broad, flat, more or less opake, with the prothorax almost impunctate, and the elytra soldered together. as we ascend higher, the breadth invariably diminishes, the brightness, and depth of sculpture, seem (up to a certain altitude) to increase, and the elytra are seldom, or but very imperfectly united; until, on entering the lower limits of the forest region, at an elevation perhaps, _ore rotundo_, of feet, we find that it has gradually put on a very different aspect,--being small, narrow, bright, convex, comparatively ovate and deeply striated; the legs and antennæ have become exceedingly pale; the prothorax has altered considerably in shape, being much narrowed behind and punctured; and the elytra are nearly always free. in this state it continues for about feet; when again emerging into the broad daylight of the open hills, it recommences to mould itself as it did below; until, having reached the summits of the loftiest peaks, more than feet above the sea, it has almost (though not entirely) assumed the features which characterized it on the shores beneath[ ]." § iv. _isolation; and exposure to a stormy atmosphere._ having in the preceding pages touched upon the subject of insect variability, as the occasional result, to a greater or less extent, of climatal and other influences; let us now proceed to consider the importance of a certain physical condition, which will be found, i believe, on inquiry, to be accompanied by a more decided modifying power than any which we have yet discussed. every one who has examined the natural history of islands, both in theory and practice, must be aware of the many difficulties which have constantly to be encountered, before the several phænomena can be satisfactorily explained. laying aside those forms which are manifestly endemic (the numerical proportion of which usually accords with the _distance_ from the nearest mainland), again and again are we baffled by the near resemblance of the various creatures to continental types,--whilst the minute _differences_ which they display, from them, are at the same time so permanently fixed, that we are almost precluded, under the ordinary acceptation of a "species," from regarding the two as undoubted descendants of a common stock: and thus it is that insular faunas have frequently been magnified, in the novelties which they are supposed to contain, far beyond what is right. a person however who looks to the causes of things, and is prepared to recognize _effects_ where there are fair grounds for anticipating them, will not be slow to perceive, that, in the small deviations which we are so often accustomed under such circumstances to behold, _the results of isolation itself_ (as an active controlling principle) may be traced out; whilst geology, ever ready to lend a helping hand when appealed to, will seldom fail to supply those intermediate links of probability which the believer in specific centres of creation must needs subscribe to, before he can draw any deductions on a broad scale, or be competent to analyse even the general bearings of a question thus necessarily comprehensive. having thought it desirable to defer to a subsequent chapter of this treatise the few geological reflections which our subject may give rise to, it will not be my aim to allude to them in the present section more than is absolutely requisite. i propose rather to consider some of the ordinary effects of isolation, as mere matters of experience; and to allow geology to tell its own tale when we come to examine the problem of _self-dispersion, as occasionally interrupted by subsidence_. if we except a few of the _heteromera_ and apterous _curculionidæ_, which appear to be influenced in a different manner, the power of isolation over insect form is perhaps more especially to be detected in a deterioration of stature. whether this principally emanates from the constant irritation of a stormy atmosphere, such as small islands are of course exposed to, and which would seem to have stunted the development (during a long series of ages) of the animal and vegetable worlds, or from a diminution of area consequent on the breaking up of a continuous land, it is difficult to pronounce: nevertheless, it is most consistent with both reason and analogy to suppose that each of those causes has operated to induce a similar result; and that we must therefore view them as working in concert, if we would appreciate their action aright. it is a law to which a large proportion of the organic creation would appear to be subject, that the exuberance of life (not so much, however, as regards the number of individuals which the various species may present, as in the grandeur of their size) has reference to the magnitude of the spot over which it is permitted to range. the unnatural breeding-in of a single race, which must of necessity happen unless the intercourse with other varieties of its kind be possible, has always been attended with effects more or less pernicious; and in the annulose tribes i believe that the reduction of space which geological convulsions have at various epochs brought about, has been commonly succeeded (_inter alia_) by a reduction of stature in those species which have been cut off from their fellows. i do not assert that there are no exceptions to this rule; for counter-influences may at times prevail (as we shall shortly see), to neutralize the above tendency. i hold it, however, as an absolute truism, in physics, that a law without an exception is an anomaly. if, therefore, we were once to admit the latter to negative the former, no such thing as a law could exist. hence it follows, as a corollary (unless, indeed, we are prepared to endorse that conclusion), _that_ _where there is a law there must be an exception to it_; and that, consequently, exceptional cases, if not exceedingly numerous, should never pervert our belief from an otherwise presumptive truth. this dwindling-down of size has seldom failed to attract my attention, more or less, in almost every island which i have hitherto had an opportunity of exploring: space, however, will not permit me to dwell upon many instances. i have already adverted to the diminished stature of _anthonomus ater_, mshm, and _ceutorhynchus contractus_, mshm, in lundy island,--the first of which scarcely ever reaches, on that rock, more than half its natural bulk. the late mr. holme, of corpus christi college, oxford, in like manner, captured the common _calathus melanocephalus_, linn., and _olisthopus rotundatus_, payk., in scilly,--the former of which seldom exceeded two lines, and the latter two and a half, in length: and he also recorded, that the _bolitochara assimilis_, kby, is invariably smaller in those islands than it is in the neighbourhood of penzance[ ]. the _vanessa callirhoë_, fabr. (a geographical analogue of the red admiral butterfly[ ], so common in our own country), is permanently smaller in porto santo than it is on the larger, more luxuriant and varied, and therefore more protected, island of madeira proper. and, as regards the _ptini_ of that group, so completely are some of them "affected by isolation, and by exposure to a perpetually stormy atmosphere, that they do not attain half the bulk on many of the adjacent rocks that they do in the more sheltered districts of the central mass; and so marvellously is this verified in a particular instance, that i have but little doubt that five or six _species_ (so called) might have been recorded out of one, had only a few stray specimens been brought home for identification, without any regard having been paid to the respective circumstances under which they were found[ ]." that "one," protean, representative is the _ptinus albopictus_, woll.; and it is so eminently a case in point, that it may be admissible to quote, _in extenso_, a few of the observations which i have already published concerning it:-- "the _p. albopictus_ is the commonest of the madeiran _ptini_, and by far the most variable, having a separate radiating-form for almost every island of the group,--whilst, at the same time, the whole are so intimately connected together (and merge into each other) by innumerable intermediate links, that it is impossible to regard them, in spite of the opposite contour of the _extremes_, in any other light than as different aspects of a single species, according as circumstances may favour, retard, or otherwise regulate its development. instability in fact (in its broadest sense) may be considered to be one of its most prominent characteristics, since it appears to be more sensitive to isolation and altitude than any of the other members of the genus with which we have here to do,--as may be proved to a demonstration by a careful study of its habits on the spot, where the influences of position and exposure are, in nearly all instances, more than sufficient to account for the successive phases assumed. thus, commencing with _var._ alpha, which reaches its maximum in the sheltered ravines of the central mass, the bulk is usually large, and the tints comparatively intense. _var._ beta. is likewise brightly variegated, but it is smaller. now, if our premises be correct, that locality and the action of the external elements have much to do with the changes in question, we might have expected, _à priori_, that this state, from its peculiarity to the dezerta grande, would not only have reduced in dimensions (which it is), but in colour also (which it is not). here, therefore, observation, _in situ_, becomes extremely important; since such does at once convince us that its almost exclusive attachment to the interior of the stalks of the _silybum marianum_, grtn. (the _holy thistle_ of the ancients), with which the more protected portions of that island everywhere abound, affords it ample conditions, even on so bleak a rock, for its completion. nevertheless, its _stature_ (as already stated) is slightly diminished in spite of this: and when we come to examine the individuals which infest the lichen of more open situations (aberrant however on the dezerta grande, and answering to the _var._ gamma. of the diagnosis), we immediately perceive that _both_ of our required results are indicated,--the reduction not being limited to size, but extended also to hue. in porto santo this modification is the normal one,--where the insect likewise displays the same lichenophagous tendency, and where the districts in which it exists are equally barren. but, if its maximum be attained in madeira proper, and a certain number of minor deviations range throughout porto santo and the dezerta grande, it still remains for us to show where its _minimum_ is to be obtained:--which, true to the _modus operandi_ by which we have conjectured its divers degrees of abortion to have been brought about, would seem to be centred on the northern dezerta, or ilheo chão. when we bear in mind the minute dimensions of that flattened rock, which does not include so much as a single valley, or depression, within its bounds, and is consequently seldom free from the violence of the winds (which sweep across it incessantly, from whatever quarter they may arise); it could hardly be supposed that an insect which is so obviously subservient to atmospheric control should not have become materially affected, in its outward guise, through long seclusion on such a spot:--and accordingly we are not astonished to find the race which has been thus cut off for ages on this extraordinary little island, itself _as_ extraordinary. it is indeed very remarkable to trace out how clearly the agencies we are discussing have here operated on the species under consideration,--for both sexes (though especially the male) descend on the ilheo chão to somewhat less than half a line in length, being literally of scarcely greater magnitude than some of the larger representatives of the _ptiliadæ_!"[ ] i stated above, that, although this diminution of stature is a very general accompaniment of isolation, amongst insects which have been _long_ cut off from the rest of their kind, there is no rule without an exception to it; and that, therefore, we must not always anticipate the result which has been described. we should remember that _immense_ periods of time are apparently necessary before any perceptible change can come over creatures from the stoppage of their migratory progress, and the unnatural in-breeding of their several tribes; so that in islands geologically recent (which often implies, however, their existence through epochs which would sound vast indeed to ears unscientific) we must not invariably expect to discover evidences of this law. on the contrary, we must first of all take into account the age of their formation, before we can judge _à priori_ as to the probability of its operation through a sufficient interval of time to have become conspicuous in its effects. i say "through a sufficient interval of time," because the process of deterioration may be silently going on, even now, in many an island, _which has not yet shown any matured traces of its action_, except perhaps in the case of a few species which appear to be more particularly susceptible to contingencies from without. we should then call to mind, that an enormous proportion of nearly every insular fauna is composed of accidental colonists during the last few centuries, in which civilization and commerce have been unintentionally at work in the cause of animal diffusion; and that, therefore, if modifications in outward contour have not necessarily resulted during a positive _geological_ interval, it would be absurd to look for them in the mere settlers (as it were) of yesterday. thus, it will be perceived, how necessary it is to take every element and contingency into account before we venture to pronounce dogmatically on either the existence or non-existence of any physical law; and how cautious we should be of denying the legitimate operation of external influences in one region, because they would seem, _primâ facie_, to be contradicted in another. it is surely more philosophical to endeavour to reconcile the two, by tracing out (as may frequently be done) some opposing principle in the latter, which shall enable us to understand the discrepancy, and to believe that the same action may be going on in both cases, but that in one of them it is either overruled by a greater controlling power than itself, or else has not had sufficient time to bring its fruits to maturity. if a proposition be true, we should recollect that it is _always_ so (under all the circumstances and conditions to which it is applicable); for, otherwise, it would be both true and false,--which is absurd: hence, _if_ my premises be true, that the general tendency of isolation is to diminish the stature of those insects which have become isolated; it follows that that tendency must remain, so long as there are no other special disturbing influences to absorb or neutralize it. "when any observation," says a writer of the last century, "hath hitherto constantly held true, or hath _most commonly_ proved to be so, it has by this acquired an established credit: the cause may be presumed to retain its former force; and the effect may be taken as probable, _if in the example before us there doth not appear something particular,--some reason for exception_[ ]." hence it is, that, even amongst the _opposite_ phænomena which one island may occasionally present from those of another, i have often been able to recognize the working of a selfsame law; and clearly to detect, that it is not from _its failure_, in either instance, that contending results are brought about, but simply that some counteracting agent has been exerting its energy in the one case, so as to nullify what would have otherwise come to pass. the main object however of the present section being to show that a considerable amount of power is due to isolation itself, in regulating (after a long series of ages) the outward aspect of the insect tribes, it is not strictly necessary that we should so rigidly insist on deterioration of size as one of its primary consequences,--since (whether it be so or not) we are merely concerned here to demonstrate, that its influence, _in some shape or other_, is absolute and real. after the above remarks, we shall not be surprised that the phænomena displayed in certain islands, as regards size, are sometimes (though i believe it to be an exception to the ordinary rule) the exact opposite of what we have been describing. let us not however be alarmed at this fact, on the bare statement of it,--as though the proposition which we have been lately advancing were at once disproved; since we shall find, on inquiry, that the case is not so desperate as might be imagined; and that in many islands where even this principle is to be detected, we may recognize traces of the other also. but how, it will be asked, can this be? for, since the influences are the same, creatures similarly exposed to them must be similarly affected. now, although, on a broad scale, such a notion contains much presumptive truth; on a narrower one it does not always apply; for species are differently constituted _ab ovo_, and will sometimes give a different result from the operation of causes which are identical. moreover, there is a curious tendency which i have remarked in most islands, that the wings (especially the metathoracic ones) of their insect inhabitants are liable to be retarded in their development,--often indeed to such an extent as to become actually evanescent: and i believe it to be a law of nature, that when any particular organ is either stunted or taken away, the creature receives a compensation for its loss either by the undue enlargement of some other one[ ], or else in a general increase of its bulk. if such be the case, the presence of two apparently conflicting effects in a single island is rendered somewhat more intelligible; nevertheless, on the above hypothesis, the specimens which increase in dimensions should undoubtedly have their organs of flight more or less enfeebled, whilst those which diminish should be regularly winged. and hence we arrive at the question, is this so? my own experience would certainly tend to prove that it is; and i suspect that future observations will confirm the fact. meanwhile, i must content myself with simply advancing the subject for consideration, and with recording such few examples, in support of the theory, as space will permit, and which occur to me almost spontaneously. the madeiras would seem to inherit, as it were, a more than usual control over the alary system of their insect population; for, out of about species of coleoptera which i have hitherto met with in that group, nearly are either altogether apterous, or else have their organs of flight so imperfectly developed, that they may be practically regarded as such; so that, if our preceding conclusions (from the compensation-hypothesis) be correct, we should _à priori_ anticipate an increase of bulk in those islands, rather than a decrease of it. unfortunately the greater number of these representatives are now, through the submergence of the once surrounding continent, _endemic_, so that we have no means of judging whether the obsoleteness of their wings is to be referred to the long action of madeiran influences[ ], or whether they were thus created severally in the beginning; and, for the same reason (that is to say, having no others of their kind to compare them with), we cannot pronounce, even if we might assume this partial organic decay to be the consequence of their isolation on these rocks, whether their general stature has been subsequently augmented or not. still, there are some few, out of the just alluded to, which are of common european distribution; and, as these would appear to have obeyed the principles to which we have been calling attention, it is not unreasonable to suppose, that many of the others (could we but behold them as they formerly were,--emigrants over a vast continuous land) would be found to have done so also. i alluded, in a previous section, to the _dromius obscuroguttatus_, dufts., as presenting permanent characteristics in madeira,--the combined result of latitude and isolation; and i also stated that it was not always possible, whilst dealing with physical agents which are necessarily obscure, to refer the respective phænomena (whatsoever they may be), which would seem to have departed from their types, to a single disturbing cause. hence, whilst i there acknowledged latitude as in part answerable for the changes which that insect has undergone, i may here suggest that it is, in all probability, to _isolation_ that we must mainly look, if we would understand those changes aright. but what _are_ the distinctive features, it may be asked, which the _d. obscuroguttatus_ has adopted, since its first arrival from more northern latitudes over an unbroken[ ] continent? it has not altered much, after all: it is, however, the _nature_ of the alterations, and their constancy, which give them their real importance. in a few words then, the insect is rather larger and more robust than its european analogue, and (to omit other minor differences) _its wings are evanescent_. but this, on our above hypothesis, is precisely what we should have expected: for, since it is self-evident that the species cannot have been naturalized accidentally on these mountains, and since geology informs us that a _vast_ interval has elapsed since the madeiran islands were portions of a continuous whole, we have at once a sufficient _time_ assured us for the modifications to be completed, and to appear at length permanently adjusted in accordance with the conditions and influences which locally prevail. there are other examples which might be quoted in support of my theory,--that isolation, when involving a sufficient period of time, has a direct tendency either to diminish the stature of the insect tribes, or else to neutralize their power of flight; but that, in the latter case, the creatures, when thus despoiled of a function, do, on the contrary (instead of deteriorating in size), often receive a compensation for their loss by an actual _increase_ in their bulk. the common _bradycellus fulvus_, mshm, is another instance in point. from its occurrence in the almost inaccessible districts of the madeiran group, far removed from cultivation, i am inclined to refer its entry into this southern region to that remote period when a connective land offered a natural passage to wanderers from the north. hence our first stipulation, that of _sufficient time_, is satisfied; and what is the result? the insect is a trifle more robust than its ordinary european representatives, and it is _invariably apterous_. the _calathus fuscus_, fabr., is also, as is clear from its special attachment to the mountain tops, strictly indigenous in madeira (that is to say, it must have arrived there during the migratory epoch); and the consequence is, that, although usually winged in our own country, it is permanently subapterous in that island. i think it far from unlikely that the _dromius negrita_, woll., may be the ultimate phasis (from isolation) of the common _d. glabratus_, dufts.,--from which it may be distinguished by its somewhat larger bulk, more robust head and prothorax, and by the obsoleteness of its wings. true it is, that the latter species flourishes alongside it in madeira; but, like the _vanessa atalanta_ (when considered with respect to the _v. callirhoë_), may it not be of more recent importation from the european continent, and as yet in a transition state?--an idea which the _smallness_ of its wing, as compared with those of its british analogues, would seem rather to corroborate. but, if this slight increase of stature would appear generally to accompany that gradual extinction of the powers of flight which isolation is apt to induce, it follows, on the other hand (as indeed i have lately intimated), that where wings are so essential to the continuance of a species that they cannot, without its positive destruction, be taken from it, the _primary_ effect of isolation,--namely a diminution of bulk,--will for the most part happen instead. as this fact, however, has been already commented upon, we will not discuss it afresh. why it is, in the insecta, that _islands_[ ] should predispose to an apterous state more than continents, it is not easy to speculate. mr. darwin has indeed suggested, and with much apparent reason, that, were wings fully developed, the indiscriminate use of them might lead to unhappy results, by tempting the creatures to venture too far from their native rocks; and that, therefore, this partial decay is, under such circumstances, a wise provision in their favour: whilst it has been urged, on the other hand, that since insular species are at all times liable during heavy gales to be blown out to sea, they should in reality be gifted with _stronger_ powers of flight (rather than weaker ones), to fortify them against such disasters; and that, consequently, the above phænomena are not explicable on mr. darwin's hypothesis. for my own part, i am inclined to accept that theory, in all its fullness; and, furthermore, i do not believe that the latter consideration (though it unquestionably contains much presumptive truth) does at all interfere with the admission of it,--seeing that either requirement may be fulfilled, according to the nature of the several species which are destined to be acted upon. thus, if _flight_ is absolutely indispensable, as in the greater number of the lepidoptera, and beetles of a flower-infesting tendency, we shall find that the wings remain unaltered (if indeed they be not actually increased in capacity, of which i am by no means certain), and that the effect of isolation is more particularly evident in a diminution of stature. but if, on the contrary, the creatures are less dependent on aërial progression for their sustenance, as in the predacious tribes generally, especially those of nocturnal habits, the reduced area in which they are confined, in conjunction, it may be, with the danger to which they would constantly expose themselves by the promiscuous employment of organs which their modes of life do not positively need, would seem to render the presence of wings unnecessary; and they are accordingly, by degrees, removed:--in which case, however, a compensation for the loss is not unfrequently granted by an increase (more or less perceptible) in bulk. in the madeiras, this diminution and enlargement of stature, accompanied for the most part respectively by the retention and annihilation of the powers of flight, is singularly traceable on the selfsame rocks, particularly the smaller ones of the group. thus, on the flat deserta, or ilheo chão, the _scarites abbreviatus_, koll., _laparocerus morio_, schön., and the _helops vulcanus_, woll., attain a gigantic size; yet it is on that very island that the _ptinus albopictus_, woll., finds its minimum of development,--scarcely exceeding in dimensions some of the larger members of the _trichopterygia_. the deserta grande has some special modifying capability of its own,--the _eurygnathus latreillei_, lap., _notiophilus geminatus_, dej., _zargus pellucidus_, woll., _calathus complanatus_, koll., _olisthopus maderensis_, woll., _caulotrupis conicollis_, woll., _laparocerus morio_, schön., _omias waterhousei_, woll., _helops vulcanus_, woll., and the _ellipsodes glabratus_, fab., being also larger on that rock than is typical: all of them, however, with the exception of _notiophilus geminatus_, are there, as elsewhere, apterous. other qualifying results, from isolation, are equally apparent. take _colour_, for instance; and we shall perceive that in the _dromius sigma_, rossi, it is sensibly affected. the normal state of that insect "does not occur at all in madeira proper, but only in porto santo. true it is that the modifications in the several islands present but slight differences _inter se_; nevertheless, being constant, i would lay particular stress upon them, since they go very materially to prove that the effects of isolation on external insect form are even more important, if possible, than those of latitude. that this is the case in the present instance, appears clear from facts so minute as these. for, out of the many specimens which have come under my observation from various countries of europe, if there is one point more constant than another in this otherwise variable species, it is, i believe, to all circumstances, its immaculate prothorax. now, whilst this (we may almost say essential) character obtains in porto santo, in madeira it does not hold good: the prothorax there is invariably infuscate in the centre; and on a small adjacent rock (the ilheo de fora) it is entirely dark. nor let anyone suppose that details apparently so trivial are beneath our notice, or the mere result of chance, since it is by the observation of such-like points, and by marking their development according to the circumstances of the several localities in which they obtain, that we are alone able to appreciate their importance, and so to form, in a wider and geographical sense, a correct estimate of their value[ ]." the _olisthopus maderensis_, woll., is much paler, larger, and more opake, on the dezerta grande than it is in madeira proper. so great indeed is the change which it has undergone through a long isolation on that rock, "that, had the case been a solitary one, i should not have hesitated in regarding the specimens obtained from thence as specifically distinct; nevertheless, with the knowledge both of the modifying effects of isolation, and also of the _kind_ of modification essentially peculiar to that island, i am perfectly satisfied that it is a mere local state, although a very remarkable one, and has no claim whatsoever to be otherwise considered[ ]." the _pecteropus maderensis_, woll., is of a greenish-brassy tinge in porto santo, and much acuminated in front; whereas on the dezerta grande it is almost invariably _coppery_, and less narrowed anteriorly. the _caulotrupis lucifugus_, woll., although ranging through no very opposite phases, either of outline or sculpture, "appears to possess a slight modification for every island of the madeiran group: and hence small shades of difference, which might otherwise be regarded as trifling, become directly important, and cannot be ignored in a local fauna,--even though a general collector may deem it unnecessary to recognize them. in real fact, however, such distinctions, when viewed geographically, are of the greatest interest, as serving to illustrate what we have so often had occasion to comment upon, namely the influence of isolation and other circumstances on external insect form[ ]." the _psylliodes vehemens_, woll., is permanently paler in porto santo than it is in madeira proper, being almost entirely testaceous. "that the species is identical, however, with the madeiran one i have not the slightest doubt,--the sculpture and colour, as i conceive, having merely undergone a change since the remote period of its isolation on a comparatively calcareous soil[ ]." the _scarites abbreviatus_, koll., occupies the loftiest peaks of nearly all the madeiran islands, and was probably once abundant over the entire ancient continent, whatsoever its limits may have been, of which the present group forms but an isolated part. "there are traces of it in the canaries, from whence occasional specimens have been brought, and which, from the want of local data and of sufficient numbers to reason upon, have in their turn been severally regarded as distinct. the fact however is, that the species in question is an extremely variable one, assuming differences of size according to the altitude at which it lives, and differences of sculpture according to the circumstances of the spot on which it is isolated. that such is actually the case, a careful observation of the many minute changes which the insect has undergone in the various islands and altitudes of the madeiran group will, i think, prove to a demonstration. for it is impossible to suppose that every rock contains its own _species_, that is to say, has had a separate creation expressly for itself,--a conclusion at which we must assuredly arrive, if small and even constant differences are _of necessity_ specific. rejecting therefore this hypothesis as utterly untenable, and as contrary to all experience, we are driven to acknowledge that isolation _does_, in nearly every instance, in the course of time, affect, more or less sensibly, external insect form;--which being admitted, we have at once an intelligible principle whereby to account for modifications innumerable, each of which, when viewed simply as a difference, independently of the circumstances producing it, might have been regarded as sufficient to erect a 'species' upon, had the desire for multiplying them overbalanced the love of truth[ ]." * * * * * such are a few of the circumstances, influences, and conditions, by which the outward aspect of the insect tribes is liable, within definite limits, to be more or less regulated: and it is impossible to view them with an unbiassed mind and not arrive at the conclusion, that physical agents generally have a very decided control over the external contour of these lower creatures. in selecting the examples which we have lately discussed, i have avoided as much as possible those startling instances of variation which distant quarters of the globe will readily supply, because there are vast numbers of our naturalists who will not acknowledge the validity of any evidence which would tend to amalgamate, in a broad sense, the species of the old and new worlds. i have therefore contented myself with such data as must fall within our common experience, feeling satisfied that if the principle be allowed in the one case, it cannot long be objected to in the other. there are few entomologists who would not recognize, in the abstract, a legitimate capacity for adaptation in every insect with which they have to do; yet i believe there are not many, who, if modifications were to be shown them as the fixed result of disturbances from without, would be prepared at once practically to accept them as such. the collectors of the present day are so prone to regard every _permanent_ difference as a specific one, that a large proportion of them do not sufficiently realize, that well-marked races, or states, are no longer matters of hypothesis, but of fact; and that, therefore, a sensible amount of aberration should not only be _conceded_ to the action of certain physical combinations and elements, but even anticipated and looked for. such however ought not to be; and earnestly therefore would i advocate a greater latitude for geographical influences than has been hitherto admitted by many of us. especially would i urge the necessity for a more careful study of _insular_ phænomena, for i am convinced that a due allowance is seldom, if ever, made for the qualifying power of isolation, _per se_,--the most significant perhaps of all the conditions which we have attempted in the preceding pages to examine. "felix qui potuit rerum cognoscere causas" is a motto which the student of nature should keep constantly in view; for it is undoubtedly a more honourable task to discover the _reasons_ for what we see, than the mere appearances themselves. he who has dived deeply into the everyday circumstances around him will be reluctant to ascribe so much as a single item of all that comes within his ken, to chance; for to him the whole system of created things is, from first to last, replete with design. _natura nil agit sine causâ_ is as true now as it ever was, and it will be so to the end. let us not therefore be discouraged at the apparent smallness of the data from which many of our conclusions have to be drawn, for nothing is in reality trivial which is the effect of a wisely appointed law; and, even were such the case, it would not be thereby proved that the investigation of the law itself (however liable it may be to exceptions) is unimportant. nor ought we, on the other hand, to be discouraged if we cannot always reconcile conflicting phænomena, and detect in each a primary controlling cause. we should rather bear in mind, that the elements with which we have to deal are obscure, and subject to permutations from which various results must of necessity arise; and that it is only, therefore, on a broad scale that we can look for uniformity of action, even from conditions which may appear to be identical. "nature is not irregular, or without method, because there are some _seeming_ deviations from the common rule. these are generally the effects of that influence which free agents, and various circumstances, have upon natural productions[ ]." footnotes: [ ] religion of nature delineated, p. . [ ] journal of researches (london, ), p. . [ ] the great preponderance of the phytophagous over the predacious tribes, in the hotter regions of the earth, is a remarkable fact, and strongly suggestive of the relation which the insect and vegetable worlds (both of which attain their maximum in those zones) bear to each other. "the carnivorous beetles, or _carabidæ_," says mr. darwin, "appear in extremely few numbers within the tropics. the carrion-feeders and _brachelytra_ are very uncommon; on the other hand, the _rhynchophora_ and _chrysomelidæ_, all of which depend on the vegetable world for subsistence, are present in astonishing numbers. the orders _orthoptera_ and _hemiptera_ are peculiarly numerous; as is, likewise, the stinging division of the _hymenoptera_, the bees, perhaps, being excepted."--journal of researches, p. . [ ] mr. westwood states that he possesses an individual of the _papilio machaon_ from the himalayan mountains, captured by professor royle, "which scarcely exhibits the slightest differences when compared with english specimens."--_the butterflies of great britain_, p. . [ ] zoologist, xiii. p. . [ ] the butterflies of great britain (london, ), p. . [ ] _id._ p. . [ ] insecta maderensia (london, ), pp. , , . [ ] insecta maderensia, p. . [ ] insecta maderensia, p. . [ ] i possess specimens of this insect captured on the summit of mount olympus by my friend e. armitage, esq., who is also of opinion that it may be but a mountain state of the _c. sylvatica_, linn. [ ] introduction to the modern classification of insects (london, ), ii. p. . [ ] id. ii. p. . [ ] introduction to the modern classification of insects, ii. p. . [ ] journal of researches, p. . [ ] that i may not be misunderstood by those of my readers who conceive madeira to be a kind of "arva beata," with the sky for ever blue, and (as a consequence) an unclouded sun; i would repeat, that i am not speaking of the vicinity of funchal only (from which the invalids, who resort thither for their health, almost exclusively draw their deductions), but of _madeira_,--and, more-over, of madeira _as it was_, and not of madeira as it is. more or less of cultivation during a period exceeding four centuries, in conjunction with the overwhelming fire which completely devastated the entire south of the island, immediately after its first settlers had taken possession of it, and which is stated (in the accounts which are transmitted to us) to have smouldered on for nearly seven years, have so altered the features of the country, that it is only in the untouched regions of the north (on which the woodman's axe is nevertheless encroaching, season after season, with lamentable rapidity) that we can catch even a glimpse of its pristine condition. the dense forests which then everywhere abounded must have caused an amount of moisture and exhalation of which even the northern districts as they now are (though saturated, even yet, with dampness; and at a certain elevation almost constantly enveloped with clouds) can give us but a faint idea. so tremendous indeed must have been the aqueous accumulations which then hung around the island, that even the splendour of a southern sun cannot have penetrated the atmosphere as it does at present; and, hence, the historical fact that madeira proper (although separated by a channel of only thirty miles in breadth, and _now_ usually visible in bold relief against the sky, during a portion, at least, of every day, from a far greater distance) was not discovered for _an entire year_ after the colonization of porto santo, on account of the thickness of the canopy which shrouded it from view, is at once rendered intelligible. it is narrated, that, in the year , prince henry of portugal organized an expedition to attempt the doubling of cape bojador; but the commanders, having lost their reckoning, were driven ashore on an island,--which they named porto _santo_, in commemoration of their escape from the perils of the sea. "on their return," says mr. harcourt, "prince henry sent out zargo, vaz, and pestrello, to plant a new colony in the island. it was not long before a dark spot was observed on the western horizon of porto santo. this was regarded by some with superstitious awe; but zargo concluded it to be clouds attracted by high land; and shaping his course in that direction, in spite of the endeavours of his crew (by menaces and supplications) to prevent him, he discovered, in the year , the island to which, from the trees that covered it, he gave the name of _madeira_."--_a sketch of madeira_, london, , p. . [ ] journal of researches, pp. , . [ ] zoologist, x. . [ ] considering that i have already detected more than one thousand species in those islands, it may perhaps be questioned whether the same truth _is_ to be gathered from the result of my madeiran researches. i would wish it therefore to be understood, first, that my statement refers to that group _as contrasted with countries in a similar latitude_; and, secondly, that its _real_ fauna is alone taken into account,--the host of introductions from more northern regions, a large proportion of which have probably taken place within a very recent period (as may be fairly presumed from the knowledge that fresh arrivals, an almost necessary consequence of the importation of plants, _are_ occurring nearly every season), having been dismissed from our present inquiry. [ ] i perceive, on reference to the original examples, still in my collection, that this was wrongly quoted as the _haltica rufipes_. it is the _h. exoleta_, fabr., and it is thus entered in messrs. hardy and bold's 'catalogue of the insects of northumberland and durham;' where they make the observation, "variable in colour; specimens from the sea-coast are frequently of a dark mahogany tint." i have myself indeed, since i communicated the above remarks to the 'zoologist,' taken its precise counterpart, in abundance, along the yorkshire coast,--from bridlington to the extremity of flamborough head; so that it may perhaps be regarded as a topographical state which is more especially peculiar to the eastern shores of england, north of the humber. [ ] zoologist, iv. pp. , . [ ] geodephaga britannica (london, ), p. . [ ] zoologist, iii. p. . [ ] zoologist, v. p. . [ ] monographie des _anthicus_ (paris, ), p. . [ ] _id._ pp. , . [ ] proceedings of the entomological society of london (part . new series), p. . [ ] insecta maderensia, pp. , . [ ] trans. of the ent. soc. of london, ii. pp. , . [ ] considering that the true _vanessa atalanta_, of more northern latitudes, _does_ occasionally occur around funchal, it may be reasonably contended that the fact of its coexistence (on the same spot) with the _v. callirhoë_ is strong presumptive proof that the latter is a true species, and no climatal or insular modification of the former. and so, judging from a distance, and without local evidence to explain this phænomenon, i should have concluded myself: nevertheless, recollecting how easy of transport the larvæ and pupæ of lepidoptera necessarily are (of which we have the plainest assurance in the almost certain introduction of the _pontia brassicæ_, _sphinx convolvuli_, _acherontia atropos_, &c. into those islands), especially in a region which for more than a century has been receiving a constant supply of vegetables and ornamental plants from western europe; i am induced to believe that the appearance of the _atalanta_ is a comparatively recent one, whilst that of the _callirhoë_ (which, unlike the typical _red admiral_, has naturalized itself in nearly all portions of the group) must be referred to the remote period when migrations over a long-lost continuous land were in regular operation. the _slowness_ of the change, in external aspect, which the isolation of insects from geological causes would seem to bring about (and which follows, as a corollary, if the above conclusion be true), i propose to discuss in a subsequent chapter of this work. [ ] insecta maderensia, p. . [ ] insecta maderensia, pp. , . [ ] religion of nature delineated, p. . [ ] although the result of a primary (or creative) adjustment to special circumstances, rather than of a secondary adaptation, brought about by a self-modifying capability; we may just call attention to the fact, that most of the blind insects, whether associates within the nests of ants, or natives of subterranean caverns, have either their palpi _or_ antennæ anomalously developed,--as though, partially (although how, and in what degree, we cannot possibly ascertain), to make amends for the inconvenience which a total want of sight must, necessarily entail. [ ] this is certainly rendered _probable_, however, from the fact that a large proportion of these apterous species are members of _genera_ which are usually winged,--such as _tarus_, _loricera_, _calathus_, _olisthopus_, _argutor_, _trechus_, _hydrobius_, _ephistemus_, _syncalypta_, _phloe ophagus_, _tychius_, _longitarsus_, _chrysomela_, _scymnus_, _corylophus_, _helops_, and _othius_,--whilst the knowledge that, out of twenty-nine genera which i believe to be endemic in those islands, six only are winged (the remaining twenty-three being apterous), will not tend to diminish the probability that there is something peculiar in the action of madeiran influences generally on the alary system of the insect tribes. [ ] i do not think it necessary to apologize for the apparent disposal of this _quæstio vexata_; because, from the wildness of the upland ridges to which the _d. obscuroguttatus_ is in madeira exclusively confined, i deem it an absolute impossibility that it could ever have been _introduced_, through any chance agencies whatsoever. and hence, unless we reject the doctrine of specific centres _in toto_, i contend that it must have migrated, together with other insects similarly circumstanced, by ordinary means, and without natural impediments, from its own area of diffusion. [ ] i am informed by dr. hooker, that the only two insects (belonging respectively to the orders coleoptera and lepidoptera) which he detected in kerguelen's land were wingless. [ ] insecta maderensia, p. . [ ] insecta maderensia, p. . [ ] _id._ p. . [ ] insecta maderensia, p. . [ ] insecta maderensia, p. . [ ] religion of nature delineated, p. . chapter iv. organs and characters of variation. having in the preceding chapter briefly alluded to some of the principal causes by which the outward aspect of the insect tribes would seem to be in a large measure (though within definite specific limits) regulated, it may perhaps be desirable to gather into a small compass, from those remarks, what the chief organs and characters are which appear to be more peculiarly beneath the control of the various influences which we have been just discussing. to imagine that when an insect has become much altered in its general contour, all the parts of which it is composed are equally affected, is contrary to experience; since observation warns us that there are but few actual _members_ which are capable of change,--whilst even the external features, or secondary diagnostics, are only interfered with according to a fixed law, the workings of which are necessarily modified, in proportion as the constitutions of the several animals are differently organized and acted upon. as regards positive structure, indeed, we can have but few observations to communicate,--seeing that the limbs and appendages themselves are usually of so constant a nature, that disturbing agencies have little or no power to divert them from their typical states. still, there are occasional facts on record, which would tend to prove that even these are not altogether exempt from the deranging force of certain contingencies from without: the number of the antennal joints, for instance, in the tribes where those organs are multiarticulate, is said to vary; but how far this may be dependent on physical influences, i am not in a position to decide. the connateness of the elytra, again, is a character which we may at any rate define as _sub_-structural; and this i have myself noticed, at times, to fluctuate, according to the circumstances and conditions of the respective localities in which the particular species obtain. such is eminently the case with the universal _harpalus_ (the _h. vividus_, dej.) of the madeiran group. speaking of this peculiarity, in my volume on the coleoptera of those islands, i made the following remarks: "but perhaps its most singular character, and in which it differs from every other _harpalus_ with which i am acquainted, consists in the tendency of its elytra to become united or soldered together. i say 'the tendency,' because it is not always the case that they are joined (which, since the law exists at all, is perhaps the more remarkable), although in most instances, especially in localities much exposed and but slightly elevated above the sea-shore, they are. i have examples, however, from the upper as well as the lower regions, in which both states are represented; and others again in which the elytra are only partially connected, being free at the apex though firmly attached towards the scutellum. in every instance, however, even where they are united throughout their entire length, a little force will succeed in separating them, showing their structure, as i have indicated in the diagnosis, to be _sub_-connate rather than connate. but that it does require force to effect the disjunction, when they are really in the condition described, is proved to a demonstration to any one who has seen the _remains_ of the insect beneath the slabs of stone on many of the small adjacent islands where it most abounds, or drifting about over the surface of the rocks,--under which circumstances i have observed them in immense numbers, apparently the accumulation of two or three generations, which the violence of the elements had not been able to sever. it is rare in the sylvan districts to find them joined; nevertheless such is sometimes the case,--thus proving that the peculiarity is not actually essential, but merely one which it is the tendency of the species to assume, and which is more developed in some specimens, and under certain conditions, than in others.[ ]" but by far the greatest amount of variability to which insect structure is liable, is presented by the _wings_,--especially the metathoracic ones. the wings, indeed, unless i am much mistaken, are essentially (as compared with other primary details) organs of variation, capable of being more or less developed, according as the several countries in which the creatures are placed may necessitate their action. i will not recapitulate the evidence which i have already adduced, proving that islands have an especial capability of their own, either for increasing or neutralizing, as it may happen, the powers of flight (in which _latter_ case, however, a compensation is usually made for the loss); but i will point to the data which are there brought together, in support of the hypothesis for which i am now pleading,--believing that they will be found sufficient, on inquiry, to establish the doctrine of alary mutability, so far at least as it is connected with isolation as an element of control. if, however (irrespectively of its cause), the thing itself be recognized, the _principle_ is at once established; and we may reason upon it as a matter of fact. so that, if we can ensure this concession or acknowledgment, the occasional _proneness_ to variation of these thoracic appendages is, as a law, admitted. the only questions which would then appear immediately to suggest themselves, are: under what circumstances do they principally fluctuate? and why should it happen that organs which are apparently so necessary as a medium of subsistence, should be subject to inconstancy? both of these have, in reality, been already replied to in the preceding chapter. nevertheless, we may briefly repeat, that, so far as the first is concerned, it is in islands that we detect the maximum of instability to which the wings of the insecta are liable, and that it is in seasons of extraordinary heat that their development is everywhere inclined (if at all) to be especially stimulated: whilst, as regards the second, it will be sufficient to state, that in _continents_, when any decided alteration takes place in the organs of flight, it for the most part comes to pass that an _increased_ (rather than diminished) action is the result; whereas in _islands_, provided that the species are not absolutely dependent on aërial progression for their food (in which case, in order to prepare for the contingency of being blown out to sea, the capacity of the wings is commonly augmented), the _reverse_ is nearer the truth. so that the _second_ problem,--the _reason why_ appendages thus apparently essential should be subject to inconstancy,--is at once rendered intelligible from the consideration, that it is only under circumstances in which the indiscriminate employment of those organs would be apt to bring the creatures into trouble that (when not an actual _sine quâ non_ to their existence) they are liable to be taken away; whilst, even in that case, it generally happens that some partial equivalent for the privation incurred is granted, as a recompense. mr. westwood, in his admirable _introduction to the modern classification of insects_, has recorded many instances of alary variation; which, however, as he does not appear to have noticed the peculiarity of island faunas, are principally in corroboration of what i have just insisted upon as the usual tendency in continents,--namely, an _enlargement_ of the erratic powers. speaking of the _aphelocheirus æstivalis_ (a member of the hemiptera), he observes: "my british specimens have but short, rudimental, oval hemelytra, like those of the bed-bug; but i possess one of bosc's original examples, described by fabricius, not quite so large as the others, in which _the wings are fully developed_. i do not, however, on that account, regard the former either as pupæ or distinct species, but as undeveloped specimens in the imago state[ ]." and whilst discussing the _hydrometridæ_, he expresses himself thus: "it appears to me, that, from causes of which we are ignorant, numerous individuals of many of the species of these tribes are subjected to an inferior kind of development in the imago state, which does not allow the acquirement of wings,--which, however, in certain cases, _acquire their full size_. hence, i consider that the apterous specimens of _hydrometra stagnorum_, those with very short elytra, and those with the full-sized wings and wing-covers, are all in the imago state, although some are more perfect than others[ ]." and, again, in his reflections on the hemiptera, mr. westwood says (and most entomologists are aware of the fact): "the species of _gerris_, _hydrometra_, and _velia_ are mostly found perfectly apterous, though _occasionally with full-sized wings_. _chorosoma miriforme_, _prostemma guttula_, _pachymerus brevipennis_, &c., are generally found with very short wing-covers, but sometimes with full-sized wings[ ]." in like manner, the _cimex apterus_, linn. (one of the _lygæidæ_) "exhibits, in an eminent degree, the ordinary occurrence of an imperfect perfect-state; whilst individuals are occasionally found _with fully developed organs of flight_[ ]". _lyæus brevipennis_, lat., also ordinarily occurs with abbreviated hemelytra; but it has been found with them perfect by westwood, as well as with metathoracic wings. none of the above examples however would appear to do more than refer to the alary instability of the insecta, as a matter of fact; but this is all for which we are now contending,--the preceding chapter having been in part devoted to some of the presumptive _causes_ of it. whether the specimens of _oncocephalus griseus_, to which spinola called attention, were insular ones, i cannot say; but he seems to have noted an example in which an _opposite_ phænomenon to those which mr. westwood has cited, was displayed, and moreover to have speculated on the conditions producing it, when he suggests: "l'influence du climat septentrional parait avoir arrêté le développement des organes du vol[ ]." and, again, when commenting upon the other tendency in a representative of the _reduviadæ_, he says ('essai,' p. ): "je pense que la présence des ailes et leur développement dépendent du climat." whilst treating of two british species of the same family, mr. westwood observes: "the _prostemma guttula_, fab., and _coranus subapterus_, curt., are interesting on account of their being generally found in an undeveloped imago state,--the latter being either entirely apterous or with the fore-wings rudimental, although occasionally to be met with having the fore-wings completely developed[ ]." the common _phosphuga atrata_ of our own country has the organs of flight very rudimentary, and much too small for use: yet the late mr. holme of oxford has mentioned[ ], that he has several times taken it on the wing, during the hot sunshine. and, concerning the _olisthopus rotundatus_, he states[ ] that every specimen which he captured in the scilly islands was subapterous. but facts like those are, after all, nothing more than such as we may trace the counterpart of in higher animals than the insecta. mr. gould informs me, that the swallows of malta, which have but a comparatively narrow space to cross over, to the african continent, constitute (although specifically identical with them) a distinct race from those of england,--all of which, he believes, winter in morocco. but, what are the differences displayed? from amongst many minor ones, of a climatal or geographical nature, the most conspicuous is _the length of the wings_,--those which have annually a longer journey to perform having, through a course of ages, acquired, as a race, a superior capacity for flight. and, in answer to a late query on this subject, he adds that _all_ the sylvan birds in malta, such as the black-caps, willow-wrens, &c., though unquestionably of the same species as those of great britain, exhibit small local characteristics by which they may be immediately distinguished,--such as the length of the wings, size of the bills, and tints of the plumage. so that the migratory birds generally, which pass to and fro between europe and africa in that particular latitude, would appear to form separate races from those which traverse the ocean to our own country; and to be, most of them, remarkable, _inter alia_, for a slight shortening of their organs of transit. if, however, the members of the insect tribes are capable of but small variation in actual _structure_, with the exception, in certain instances, of the greater or less development of the wings; we shall find that their external characters are much more prone to instability. there is not an item indeed of all their secondary diagnostics which does not admit of a positive change; and, though it be only within fixed limits that the several modifications can occur, those boundaries are frequently far apart, and include at times numerous phases within their embrace which have been too often looked upon as specific. thus, whether we regard their bulk, outline, colour, or sculpture, anything like absolute constancy, under all circumstances and conditions, does not so much as exist; and we are driven to admit, that the physical influences to which these various creatures are exposed have a very decided power over their general configuration and aspect. it would be needless, however, to attempt to discuss the above details of aberration separately; because, where any one of them is especially interfered with, it usually happens that the others are more or less involved with them: but we may offer a few desultory remarks, which will tend to show that disturbing agents are apt to mar them both individually and as a whole,--and not only so, but to affect them in a permanent manner (as indeed has been already intimated), according as similar combinations of them are, from local causes (as it were), _selected_, to be acted upon. i have stated in the last section of the preceding chapter that insect stature is eminently beneath the control of contingences from without; adducing, amongst other examples, in support of this, the madeiran _ptinus albopictus_,--a species which, whilst it averages more than a line in length on the central island of the group, is reduced to _less than half_ that bulk on a small and weather-beaten rock (the ilheo chão) at a distance from it. judging indeed from many hundred specimens of the _ptini_ which i have submitted to a close comparison, "the most constant of their characters would seem to be outline and sculpture, whilst size and colour are apparently the least to be depended upon:--so that trifling differences may be of specific indication in the former case, where in the latter much larger ones are worthless[ ]." i have in fact generally noticed, that size and colour are more peculiarly liable to be affected _together_. this, however, is nothing more than what we should anticipate, since the same causes which have stunted the dimensions, during a long series of ages, of any particular creature, will for the most part be found to have also impaired the brilliancy of its tints. luxuriance of vegetation and sheltered districts are alike conducive, in the annulosa, to the development both of the body and its adornment; or, in other words, where the vegetable creation attains its maximum (which it certainly does not do in situations which are exposed to the irritating consequences of a perpetually stormy atmosphere), there the animal world will be usually observed to thrive. there are many insects which appear to have _two distinct states_, both in magnitude and hue, which we are seldom (in some instances, i believe, never) able to unite by intermediate links, or grades; and yet which are universally admitted, although found in actually the self-same spots (a fact which prevents their being looked upon as separate, local modifications of a common type), to be mere varieties of each other. they are, however, exceptions to the general rule; and, although infringing on the strict definition of a "variety," as given at a preceding page[ ], we nevertheless feel an _à priori_ conviction that they are by no means specifically dissimilar _inter se_. such phases, as regards _stature_, are presented by the _brachinus crepitans_ and _lamprias chlorocephalus_ of our own country; whilst, as regards _colour_, the _philhydrus melanocephalus_, _aphodius plagiatus_, and the _psylliodes erythrocephala_ (constituting in its paler garb the _p. nigricollis_, mshm) may be quoted, as cases in point. thus, also, in madeira, the _mycetoporus pronus_, erich., has a large and small form, living in communion,--which i have never been able to connect, and yet which are unquestionably identical (differing in no respect except in size): and so have the _stenus heeri_, woll., and the _saprinus nitidulus_, fab.[ ] as regards the instability displayed by _colour_, in the insect tribes, when subjected to the action of certain conditions and influences from without, so much has been said in the fourth section of the preceding chapter, that it is unnecessary to repeat it here. true it is that it was then my sole province to discuss the _causes_ which would appear to regulate, in a large measure, the external aspect of the annulosa; yet the _existence_ of inconstancy, in the several organs and characters involved (with which alone we are now concerned), was, by the nature of the case, implied: so that if the _disturbing element_ was demonstrated, the mere fact that the thing (whatsoever it may have been) _was interfered with_, was surely proved _à fortiori_. i there pointed out the great proneness to a change in hue which divers circumstances are apt to induce; and i particularly instanced proximity to the sea-shore, and other saline spots, as well as an attachment to calcareous districts, as amongst the most powerful of the deranging contingences. in case, however, that any further evidence should be looked for, on this immediate subject, i will quote the following,--relating to the _bembidium atlanticum_ of the madeira islands, which was but just touched upon in that chapter,--as a concluding example of the general effect of physical agents on the colour of these lower creatures. "throughout all the madeiran coleoptera there is perhaps no insect which displays such an extraordinary range of colouring as the present one does; and although it is true that the section of _bembidium_ to which it belongs is essentially a variable one, yet i am not acquainted with any _peryphus_ in which the paler patches of the elytra are so remarkably unstable, or which appear to be so completely under the control of external circumstances, as are those of the _b. atlanticum_: and indeed unless viewed in the mass, we should scarcely be inclined to recognize the same species in the many aspects which it puts on between its extremes. the examination, however, of a very large number of examples, and a careful consideration of the several localities and altitudes in which they were taken, has convinced me that there is unquestionably but a single type of form amongst my entire series, since the whole are so intimately connected, by successive gradations both of outline and colour, that it is perfectly impossible to isolate even a single specimen, or to draw a line of specific demarcation between any two consecutive members of the chain. it will be perceived, by a reference to the diagnosis, that the insect in question passes imperceptibly from nearly a pure green, through a well-defined spotted state, into one which has the elytra almost testaceous,--the paler portions being at last so largely developed as to become confluent, and almost to cover the entire surface. in madeira proper the darker varieties would seem to be typical; whereas in porto santo the brightly coloured ones preponderate, and in fact are all but universal. both extremes do nevertheless occur in both islands, the tendency being merely, in either case, to assume the particular modification characteristic of the spot[ ]". and so it is with the outline and sculpture (no less than with bulk and hue): they also are equally liable to disturbance from physical causes, as indeed has been already insisted upon. like most of the minutiæ of variation, however, to which we have called attention, it is more particularly on islands that this is to be observed,--isolation, during an interval sufficiently long, appearing to possess some especial control over the external contour and surface of the insect races. thus, in the madeiras, for instance, the _caulotropis lucifugus_ has its prothorax more distinctly punctured, and its elytra more perceptibly striated, in the principal island, than on any of the smaller members of the group; in porto santo, indeed, it is almost free from sculpture of any kind; whilst its ally, the _c. conicollis_, apart from being somewhat larger, is, on the contrary, both more punctured and striated on the dezerta grande than it is in madeira proper. the _omias waterhousei_, again (in addition to its slightly increased bulk and less shining envelope, in that locality), is more lightly impressed on the dezerta than it is in madeira and, not to mention other differences, the _ellipsodes glabratus_ is densely beset with most minute granules on that same rock--whereas on the mountain slopes of the central mass, it is highly polished and glabrous. the _helops confertus_, we have intimated at a previous page, is less coarsely sculptured in the lofty regions of madeira, than in the lower ones: and the _h. futilis_ has its elytral tubercles apparent in madeira proper, but evanescent on the dezerta grande. the _eurygnathus latreillei_ assumes a permanent variety on the dezerta, the insect having become modified through a long isolation on those weather-beaten heights,--here it not only attains a more gigantic stature than in porto santo, but is invariably also more parallel and opake, has the sides of its prothorax more recurved, with the punctures towards the lateral angles almost obsolete, and the striæ of its elytra somewhat more evidently punctate[ ]. such examples, however, might be multiplied _ad infinitum_; and i will not therefore devote further space to the bringing together of facts which it is hardly possible will be disputed,--especially as it has been my wish, in the present chapter, merely to _enumerate_ what the organs and characters principally are which are more peculiarly sensitive to change, throughout the annulose tribes. this i may venture to hope, though briefly, i have in part done; and i will consequently pass on to other considerations, which, even if somewhat alien to the immediate question of insect instability, should scarcely be altogether omitted in a treatise like this. footnotes: [ ] insecta maderensia, pp. , . [ ] introduction to the modern classification of insects, ii. p. . [ ] _id._ ii. p. . [ ] _id._ ii. p. . [ ] introduction to the modern classification of insects, ii. p. . [ ] essai, p. . [ ] introduction to the modern classification of insects, ii. p. . [ ] trans. of the ent. soc. of london, ii. p. . [ ] _id._ ii. p. . [ ] insecta maderensia, pp. , . [ ] vide _supra_, p. . [ ] although, in our ignorance of their real nature, we cannot cite them as actually analogous to these separate phases in certain members of the insecta, yet we are forcibly reminded by the latter of the distinct states which many of the terrestrial mollusca present (frequently in equal proportions) in the same localities. thus, most of the _pupæ_ have at least two abruptly-marked forms,--a larger and smaller one. many of the _helices_ also exhibit this tendency in an eminent degree: i have indeed been shown specimens by sir charles lyell of the _helix hirsuta_, say, from north america, one state of which is considerably more than double the dimensions of the other; and i believe it is a well-known fact that intermediate links _have_ not yet been observed to connect the extremes. may not therefore the gigantic _h. lowei_ and _bowdichiana_, which are now extinct in the madeira islands, have been but forms of the _h. portosanctana_ and _punctulata_, respectively,--co-existent with them, though more sensitive to the great diminutions of altitude and area which were consequent on the breaking-up of a once continuous land? if such be the case, however, it is certain that they were far commoner at an early period than their smaller colleagues (which, now, in their proper districts, absolutely teem),--seeing that the _latter_ are extremely rare in the fossil deposits, whilst they themselves literally abound. [ ] insecta maderensia, p. . [ ] insecta maderensia, pp. , . chapter v. geological reflections. we frequently hear it asserted, that, since the members of the insecta are so numerous and minute, when compared with those of other departments of the organic world, the entomologist, whose province it is to collect and classify them, can have but little time, if he attempt the real advancement of his particular science, for generalizations on a broad scale. now, whilst there is necessarily some reason in this remark (for the investigation of species is a work of such labour and drudgery that it is apt to monopolize all the leisure hours which the greater number of us are able to command), we should recollect, on the other hand, that the soundest theorists have ever been the most patient and accurate observers; and have, many of them, spent whole years of their lives as humble students in nature's domain. we need not be afraid that an occupation amongst what is microscopically small is liable to cramp the mind, and render it unfit for wider processes of induction, since the very opposite of this would seem to come nearer to the truth. the understanding which has been well tutored by a system of close and steady observation, which has been trained to seize upon differences amongst the objects of our common experience, to balance the importance of generic and specific characters, as tested in the acquisitions of our daily walks; and which has been gradually brightened and matured by the habitual exercise of its judgment on the most trifling phænomena around us, has usually gained strength enough to form conclusions from such data, which will not only stand the test of analysis, but will be free from those eccentricities of genius which too often mar the speculations of less practical naturalists. the mind, moreover, having been chained and fettered for a season to the mere detail of facts, breaks forth, under such circumstances, with all the vigour with which the contemplation of truth has gifted it, and takes its flight as it were to a clearer sky; and, though a reaction may at times set in, hurrying it away into regions beyond its sphere, it will assuredly return at length, fraught with the soberness which its vocation has inspired, and commence to build up its hypotheses, step by step, in harmony with the material which it has amassed. yet though entomologists may be in reality as well qualified as any other natural historians for drawing general conclusions from the result of their researches, it is impossible to conceal the fact, that, as a body, they have not ordinarily done so. whether this has happened through an accidental disinclination on their part to occupy themselves in such matters, or (which is more probable) from their whole time having been engrossed by the dry routine of their science, i do not pretend to determine: be the solution, however, what it may, the inference is practically the same,--that the annulosa have not hitherto been sufficiently regarded, in the great questions of zoological geography. but especially have they been ignored during that most significant of considerations which has been so ably brought forward of late years by some of our keenest observers,--namely, the distribution of animals, as affected by geological changes, on the earth's surface. it would be well if the collector of insects would devote at least a tithe of his energies to the speculative branch of his subject. certain it is that much would probably be advanced, at first, on slender premises; and would, as a consequence, fall to the ground, leaving no record behind it. yet such must inevitably be the case, at the outset, in every region of inquiry; and we are prepared to expect it. it does not however follow that _good_ would not be developed also; whilst we are confident of the fact, that unless the trial be made, it cannot possibly arise. no question has ever yet been mooted without beneficial results: it has either been shown to be absurd, and has received its death-blow on the spot, or else truth has been elicited (indirectly perhaps), which has at once shed a new ray of light on some of its obscurest bearings. and so, assuredly, it would be in the present instance. we cannot doubt that there is much to be discovered in the past history of insect dissemination, which would tend, when rightly interpreted, to explain many of the occult phænomena of the present day; and we may be equally satisfied that this cannot by any possibility be attempted without the assistance of geology. let us therefore glance hastily at a few of those more undeniable convulsions which we are aware have, at various epochs, taken place; and endeavour to catch a glimpse of how, in the common course of things, that portion of the insect world would be affected which was exposed to their influence. first and foremost, perhaps, in importance, of all the changes which it is self-evident have happened, may be mentioned _subsidence_. including, as it does, both the general lowering of some countries, and the actual isolation of others, there are, i believe, no physical crises to which we could point, through the instrumentality of which the very _existence_ of the insect races (not to allude to their diffusion) has been, by the nature of the case, more seriously interfered with. we know that there are certain species of an alpine and boreal character, which cannot live except in a climate of low temperature,--guaranteed to them either by _elevation_ in one land, or by a higher latitude in another: and let us picture the consequences of the gradual sinking of a mountain chain, even to a small extent, the _summits_ of which only just afforded the conditions of atmosphere necessary for the continuance of creatures like these. now this is an example by no means far-fetched, and such as _must_ have occurred in instances innumerable. but, what would be the many results of a diminution in the level of our imaginary range? it needs no argument to prove, that _one_ at least would be manifest in the total extinction of those forms which could not adapt themselves to the increased heat. others, which were able with difficulty to endure the alteration, would in all probability, even though they had now emigrated to the loftiest peaks, flourish less vigorously than before; and it is not unlikely, moreover, that they would become _somewhat modified from their normal states_,--states which, be it recollected (for this is an instructive lesson), would still exist in more northern zones. during my researches in mountain tracts, i have usually remarked, that the highest points of land either teem with life, or else are perfectly barren. my own experience would certainly tend to prove, that, in a general sense, one or the other of these extremes does almost constantly obtain. and, although i would not wish to dogmatize on phænomena which may in reality be explicable on other hypotheses, it would perhaps be worth while to inquire whether the geological movements of subsidence and elevation will not afford some clew to the right interpretation of them. be this, however, as it may, i can answer, that in many countries, where there are strong indications of the former, the alpine summits harbour an insect population to a singular extent; whilst in others, where the latter is as distinctly traceable, the upland ridges are comparatively untenanted. now we have already shown, that where the gradual lowering of a region has taken place, there will be, of necessity, an undue accumulation of life on its loftiest pinnacles,--for, even allowing a certain number of species (which _even formerly_ were only just able to find a sufficient altitude for their development) to have perished, we shall have concentrated at that single elevation the residue of all those which have survived _from the ancient elevations above it_. but, if, on the other hand, an area, already peopled, be in parts greatly upheaved, there will be _either_ a universal dying-out, from the cold, of a large proportion of its inhabitants, or else an instinctive striving amongst them to desert the higher grounds on which they have been lifted up, and to descend to their normal altitudes: in both cases, however, the present summits will display the same feature,--namely, utter desolation. such are a few of the effects which elevation and subsidence, even on a small scale, would seem (when tested by theory and practice) to produce. it yet remains for us to suggest, that the latter, when carried to its maximum, so as to cause the actual separation by the sea of one district from another, is a contingency of immense significance in regulating the distribution of the annulose tribes. their outward contour and aspect we have shown in a previous chapter to be very largely beneath the control of isolation, provided a sufficient _time_ can be granted for the change: but their ultimate absence from any particular place, through the impediment which it offers to their migratory progress, we have not yet touched upon. let us conceive, therefore, an extensive continent; and, since the insects which at present inhabit our earth must, if the doctrine of specific centres be true, have been originally created in certain definite spots, let us suppose a limited proportion of them to have been first produced upon this tract. self-dissemination, we will assume, has been going on for centuries: those species which were gifted with quick diffusive powers have become pretty evenly dispersed over its surface; whilst those of naturally slow or sedentary habits have peopled, comparatively, but small areas around the respective localities of their birth. such may have been the case, at some fixed period, amongst the aboriginal beings of any country which we choose to select as an illustration. but there is another element to be considered. if this region be not insular, it will have received colonists from foci of radiation situated beyond its bounds; and these, therefore, according to their several capabilities for progression, will have, likewise, in parts, overspread, or tenanted, it. now it is impossible to cite a more simple example than this. but let us endeavour to realize what would be the necessary consequence of the breaking up of such a district as that which we have imagined. if a _general_ sinking should take place, causing its higher points to be alone visible above the ocean, or merely a _partial_ one, so as to admit of the sea encompassing portions of it which would remain unaffected in their altitude; the result practically would be the same,--namely, the constitution of a group of islands out of a once continuous land. then, as regards the animal population of this tract, the main phænomena are almost self-evident. should any of its isolated fragments chance to contain a portion of one of _those limited areas_ which a species of slow progressive powers had succeeded in colonizing, it would of course harbour (provided that the other portion has disappeared) what would now be defined as _endemic_. numbers of these small areas, or, in other words, of the species which had overspread them, would in all probability be lost for ever; whilst the occurrence of any of the surviving ones in more than a single island would manifestly depend on the proximity of the islands _inter se_. those forms which had diffused themselves over the whole original continent would now be found in all the detachments of the cluster; whilst others, which had wandered over the greater portion of it only, might be traceable perhaps in every island _except a few_. such are the primary facts which suggest themselves, whilst discussing the question of isolation as regulating the _distribution_ of the annulose tribes. its _after effects_, on their external configuration and development, we have examined in a preceding chapter of this treatise; and we have also lately intimated what might be a few of the presumptive consequences of a subsidence (in a general sense), _apart from_ the still more important principle of isolation. before, however, we dismiss these brief and elementary reflexions on the upward and downward movements which geology testifies to have occurred, at various epochs, on the earth's surface, i shall perhaps be pardoned if i digress so far from my immediate subject as to trace out some of the actual results of isolation in the diffusion of the insecta (especially recognizable in the stoppage of a former migratory progress) in a few of the northern atlantic groups. i should premise, however, that it is from the coleoptera alone that i shall attempt to draw my inferences; nevertheless, since that order is more extensive than any of the others, and has moreover been closely investigated in most of those islands, it may possibly afford us data of sufficient comprehensiveness and accuracy for practical purposes. to commence, then, with the madeiras and canaries; the first facts which isolation discloses to us, concerning the statistics of a region which was once continuous throughout that portion of the atlantic, are the _slowness_ and the _direction_ of the ancient migratory movements. the former of these is rendered evident from the vast number of endemic species which are at present contained, not merely in the two groups combined, but in the several islands of which each of them is composed. true it is, that these peculiar forms are, most of them, apterous, and of naturally sluggish self-disseminating powers; yet, still the circumstance remains, that these various creatures had not overrun areas of any extent before the land of passage was destroyed,--for otherwise they must have occurred, now, on islands and rocks but slightly removed from each other, _which they do not_. the latter of the above conclusions, namely, the _direction_ of the migratory current, will become apparent in the sequel. we may premise however, that, so far as the aborigines of this province are concerned, their course will be found, upon the whole, to have been a _northerly_ one. as regards the slowness, and the direction, of the _quondam_ migration (questions which can scarcely be treated apart from each other), some light may be thrown on the subject from considerations like the following. the canaries are the head-quarters of the genus _hegeter_; teneriffe may indeed be called the land of hegeters. no less than thirteen or fourteen species have been recorded as indigenous to those islands; and there can be no reasonable doubt whatsoever that that ancient region (when continuous and entire) was the primæval centre, or range, of that heteromerous group. the hegeters are an apterous race, and of a sedentary temperament; hence, when the area (whether by general or partial subsidence, it signifies not) was broken up, it is not surprising that those local fragments of it should have become the nucleus of reception, as it were, for the members of that genus. nevertheless, a few of these many representatives (of more discursive capabilities perhaps than the rest) had found their way, before the period of dissolution, to a considerable distance from their original haunts. thus, one of them (the _h. latebricola_, woll.) had arrived at what now constitutes the rocks of the salvages; another (the _h. elongatus_, oliv.), at least, if not two, had colonized the madeiras, and is said (though i believe incorrectly) to have even reached the present coast of portugal. this latter species is clearly of a more adaptive nature than its allies, inasmuch as it has, also, naturalized itself (though this may be a more recent, and accidental, circumstance) on the opposite shores of africa. one thing, however, is at any rate manifest,--that the hegeters attain their maximum in the canaries, and that a few members only have been sent off, in a northerly, or north-easterly, direction, from thence. in like manner, the genus _tarphius_ is distinctively madeiran. i have detected nearly twenty well-defined species of it in that group; yet, out of so large a number, two only have occurred beyond the central island. now the _tarphii_ are, also, wingless; and creatures of very sluggish propensities,--scarcely ever stirring from the masses of loose rotting timber which they so assimilate in hue, and to the under sides of which they affix themselves, day and night. although difficult to investigate in their precise economy, it is extremely probable (may i not say, certain?) that some important and peculiar office is assigned to them in the remote upland districts to which they exclusively belong: and there cannot be any question, to a person who has studied them carefully on the spot, but that the region which they now inhabit is the actual area of their primæval appearance on this earth. many kindred species may of course have been lost, during those gigantic subsidences which caused the madeiras to be shaped out, and to tell their tale above the waves as ruins of an ancient land; yet our existing cluster of forms could not have wandered far at that early period, from the serras and ridges of their birth,--perhaps not _so_ far indeed (considering the limited bounds within which they are now confined, and that time should in reality have increased their range rather than diminished it) as they have succeeded in doing at the present day. hence we may reasonably conclude, that madeira proper is an example of what we have alluded to in a preceding page,--namely, of the accidental retention, during a vast downward movement, of a nucleus of small specific areas of colonization, the colonizers of which _had not extended elsewhere_. but i stated, that two of the above-mentioned _tarphii_ have occurred beyond the central mass. it is in porto santo that they make their appearance; nevertheless, since one of them is apparently peculiar to that island, it is only the _t. lowei_, woll. (an insect of a different, and more active, nature than the rest) which has violated that _local exclusiveness_ which would seem to be almost a generic character, as it were, of its allies. that species, however, both in its manners and aspect, recedes materially from the remainder. although, like them, nocturnal in its habits, it is able to run with considerable velocity; and, instead of attaching itself to the blocks of putrefying wood, which both fall and decay _in situ_ on those elevated tracts, it hides within the bunches of _evernia scopulorum_ and _prunastri_ which clothe the trunks of living trees, and fill up the crevices of the weather-beaten peaks. hence, when contrasted with its comrades, we can easily understand how the varied processes of accidental transportation would operate to increase the range of a creature which differs so essentially, in many respects, from them. it is indeed, not unfrequently, brought down, at the present day, by _human_ agencies from the mountain-slopes; for, since the cutting of faggots is one of the few sources of livelihood to a large proportion of the poor of funchal, numerous insects of subcortical and lichen-infesting tendencies are subject to be naturalized (provided they can adapt themselves to the change) in altitudes lower than their normal ones: so that there are many chances, even _à priori_, in favour of the _t. lowei_ having overspread, whether by natural or artificial means, a wider area than its congeners. i believe that there is no such thing as a _tarphius_ in the canarian group: nevertheless, singularly enough, a representative, which is more akin to the _t. lowei_ than to any other hitherto discovered (and which was imagined until lately to have been the sole exponent of the genus), namely, the _t. gibbulus_, germ., occurs in sicily. from which data we arrive at this significant fact: that, whilst madeira proper is, without doubt, the original centre of the _tarphii_, two species (one of which is, likewise, madeiran) are found in porto santo, to the north-east of it; whilst a third makes its appearance in an island of the mediterranean. the genus _acalles_ presents a nucleus of species in the canaries, moulded on a very large pattern. a closely allied member, the _a. neptunus_, woll. (which may perhaps be in reality but an insular modification of the _a. argillosus_, schön., from teneriffe), has been detected on the rocks of the salvages, to the north of them; whilst on the dezerta grande, one of the most southern stations of the madeiran group, we have a third, which displays far more in common with the canarian type than it does with that which obtains in madeira proper;--which last is gradually, in its turn, merged into the ordinary european form. the genus _pecteropus_, woll., is another instance in point. i possess three or four species from the grand canary, fuertaventura, and teneriffe; and i believe it will be found, on inquiry, to attain its maximum in that cluster. unlike the others, however, which we have just cited, it is powerfully winged; and we should consequently expect to trace the evidences of its northward progression with comparative perspicuity. can we therefore do so? yes: in madeira proper it has two representatives, and in porto santo (to the north of it) one. and so with _xenostrongylus_, woll. (which is likewise winged), we have two species, at least, in the canaries; one in the madeiras; and a third, unless i am mistaken, in sicily. the genus _ditylus_ is shadowed forth in the canary islands by two or three singular representatives of a pallid, testaceous hue; and, although the group is entirely absent in madeira, a species (the _d. fulvus_, woll.) is found on the 'great piton' of the salvages, so nearly resembling, except in its smaller size, one of those from the canaries that i think it far from improbable that it is a fixed insular state of that insect. _deucalion_, also, may be quoted in support of this twofold hypothesis, of the direction, and the slowness, of the former migratory movements. it is an apterous genus, and of eminently sluggish habits; and what is the consequence?--we have a very remarkable species (the _d. oceanicum_, woll.) on one of the rocks of the salvages, whilst another (the _d. desertarum_, woll.) has been isolated on the two southernmost islands of the madeiran group; and of so sedentary a nature is this last, that, although physically unimpeded, it has not, even to this day, overrun the diminutive areas on which, when the surrounding region was submerged, it was originally saved from destruction. so strongly indeed was this fact impressed upon me, when i first detected it, that i shall perhaps be excused for recapitulating _in extenso_ the few reflexions which then suggested themselves to my mind. "there is no genus, perhaps, throughout all the madeiran coleoptera, more truly indigenous than _deucalion_. confined apparently, so far as these islands are concerned, to the remote and almost inaccessible ridges of the two southern dezertas, it would seem to bid defiance to the most enthusiastic adventurer who would scale those dangerous heights. its excessive rarity, moreover, even when the localities are attained, must ever impart to it a peculiar value in the eyes of a naturalist; whilst its anomalous structure and sedentary[ ] mode of life give it an additional interest in connexion with that ancient continent, of which these ocean ruins, on which for so many ages it has been cut off, are the undoubted witnesses. approximating in affinity to _parmena_ and _dorcadion_, yet presenting a modification essentially its own, it becomes doubly important in a geographical point of view; and it was therefore with the greater pleasure that i lately received a second representative, from the distant rocks of the salvages,--midway between madeira and the canaries. differing widely in specific minutiæ, yet agreeing to an identity in everything generic, they offer conjointly the strongest presumptive evidence to the _quondam_ existence of many subsidiary links (long since lost, and radiating in all probability from some intermediate type) during the period when the whole of these islands were portions, and perhaps very elevated ones, of a vast continuous land. * * * * * the _deucalion desertarum_ is of the utmost rarity, the only two[ ] specimens which i have seen having been captured (the first by myself, in ; and the second by the rev. r. t. lowe, in ) on the respective summits of the middle and southern dezertas. so local indeed does it seem to be, that it, apparently, has not extended itself even over the dezerta grande (where there are no external obstacles to bar its progress); but retains the very position which in all probability constituted its original centre of dissemination at the remote period of time when this ancient continent received its allotted forms. judging from the slowness with which creatures of such habits must necessarily, under any circumstances, be diffused, it is at least unlikely that the present one could have circulated far, when the now submerged portions of that region began to give way; and hence it is not impossible that the southern dezerta, with the adjacent part (then united to it) of the central one, may have embraced the _whole area_ of its actual primæval range,--the remains of which (though they be now separated by a channel) it still continues to occupy, and from which, even when physically unimpeded, it has never roamed[ ]." although it is not my province in this volume to draw inferences from data which are not strictly entomological, i shall perhaps be pardoned for adding a few words on the testimony which the land mollusca of the madeiras would seem to afford, in support of the general slowness of the animal migrations over that primæval continent. the researches of the rev. r. t. lowe, and of myself, on every rock and island of the group, have, it appears, so nearly exhausted the whole number of species which lately remained to be found, that the conchological statistics are perhaps, at the present time, more accurate than those of any other department of the fauna: and, independently of the modifications which have been manifestly brought about, in some few instances, by isolation, since the periods of subsidence, it is truly singular to remark how every detached portion of the entire cluster harbours real species, which are now peculiarly its own. thus (to select an illustration from amongst the most anomalous of the endemic forms), we have in madeira proper, porto santo, and on the southern dezerta, respectively, true representatives, in the _helix tiarella_, _coronata_, and _coronula_,--which in all probability still occupy the positions (or nearly so) of their original _début_ upon this earth. considering the sluggish, or sedentary, nature of the terrestrial mollusks, it is extremely likely (nay, almost certain) that many intermediate links, radiating from the same type, were lost for ever, when the gigantic movements which rent this ancient region were in course of operation: so that, if such were in reality the case, we need not be surprised that one at least of this small geographical nucleus should have been preserved on three of the existing islands of the group. that these are actual species (saved alive from their fellows, after the wholesale destructions in this atlantic province had been completed), and no results of insular development, is demonstrated by the fact that two of them (for the third has apparently become extinct) have not altered one iota since the _fossil period_, which, in the opinion of sir charles lyell, is anterior to the dissolution of the intermediate land;--whereas, had they been mere modifications of each other, induced by the local conditions and influences to which they have been, through a long series of ages, severally exposed, the difference between their recent contour and that of their fossil homologues would have been doubtless at once conspicuous. i gather, therefore, that like the _tarphii_, to which we have lately drawn attention, they are veritable surviving members of an esoteric assemblage which found its birth-place on this post-miocene (?) tract. in a similar manner, the _h. undata_ in madeira proper, the _h. vulcania_ on the dezertas, and the _h. porto-sanctana_ in porto santo, are representative species,--each occupying the same position, and being equally abundant, on their respective islands: and, although it may be a problem whether the second of these is not an insular modification of the first (or _vice versâ_); yet, with the analogy of the three already mentioned before us, i am inclined _à priori_ to view it as distinct. these, also, occur in a subfossil state; and no alteration appears to have been brought about, by either circumstances or time. and so it is with numerous others (as the _h. latens_ in madeira, and the _h. obtecta_ in porto santo; the _h. squalida_ in madeira, and the _h. depauperata_ in porto santo; the _h. delphinula_ in madeira, and the _h. tectiformis_ in porto santo), which are no less representative _inter se_. from which we are driven to conclude;--first, that this _quondam_ continent was densely stocked at the beginning with foci of radiation created expressly for itself[ ]; and, secondly, that the areas which these various creatures had overspread, before the land of passage was broken up, was extremely limited,--or, which amounts to the same thing, that _their migratory progress was unusually slow_. touching the two-fold question, of the _local engagement_ of this atlantic district with specific centres of diffusion, and the _extreme slowness of their diffusive progress_, much instruction may be derived from a contemplation of the conchological statistics. porto santo, for instance, is a very small island (not more than seven miles in length), yet the number of endemic species which it includes is so perfectly astounding that it may be appropriately termed a _generic area of radiation_. nor does this primæval excess of its aboriginal beings strike us more forcibly than does the utter quiescence (if i may so express it) which has been going on amongst them since the remote era of their birth. although a few have apparently died out[ ] since that epoch, consequent perhaps on the change of level and diminished range which took place during the process of subsidence; we are amazed to find that certain species which are now limited to particular spots (even whilst unopposed by physical barriers) have been absolutely peculiar to them from the first,--or, in other words, that, whilst the fossil deposits extend throughout the lower regions of the island, far and wide, it is only in those respective portions of the beds which join on to the present "habitats" that the fossil homologues of several of the species are to be met with. the _h. wollastoni_ is eminently a case in point. that most interesting of the madeiran mollusks was first detected by myself on the southern ascent of the pico de conseilho, of porto santo, april , ; and the subsequent explorations of the rev. r. t. lowe, in conjunction with my own, have, i think, satisfactorily proved that it occurs nowhere else except upon that single slope. throughout the large expanse of calcareous incrustations which are spread over the island elsewhere, and on the adjoining ilheo de baixo, all of which teem with shells, i think i may assert, without fear of contradiction, that the _h. wollastoni_ does not so much as exist. yet at the zimbral d'areia, which the pico de conseilho directly overhangs,--a rich tract for these fossil remains,--as well as in the muddy composition of a cliff near at hand, it literally abounds. in like manner, we might recall many others which are peculiar, _recent and fossil_, to the self-same precincts. such, for example, are the _h. calculus_ and _commixta_, which swarm on the summit of the ilheo de baixo, in both states. the _h. attrita_, again, is the pico d'anna ferreira modification of the _h. polymorpha_; and it is only in the beds towards the base of that mountain that its fossil homologue is found. but what do these facts indicate? surely they tell us plainly of what we have already so often insisted upon,--namely, the redundancy of this once continuous land with specific foci of its own, and the sluggish or sedentary nature of those primæval radiating forms. we must not however omit to notice, that some few of these endemic _helices_ appear to have been gifted (as we should _à priori_ anticipate) with more rapid capabilities for diffusion than the rest. thus, the _h. erubescens_ and _paupercula_ seem not only to have colonized the entire province of which the madeiras are detached fragments, but to have even found their way to that distant portion of it which now constitutes the azores. the _h. polymorpha_ has also penetrated the madeiran region throughout; and being, like the _h. erubescens_, peculiarly sensitive to the action of external influences, we perceive, in consequence, that almost every island and rock has now its own especial phasis of it. so greatly indeed is that species beneath the control of local circumstances, that the very districts of an island as insignificant as porto santo have each their separate races to boast of. on the pico d'anna ferreira it assumes a form to which the name of _h. attrita_ has been applied; when on the ilheo de baixo, it is the _h. papilio_; at the zimbra d'areia, on the pico de conseilho, and in the ribeira da coxinha, it is the _h. pulvinata_; and, in many other situations widely removed _inter se_, it puts on the shape (variable, both in size and hue) to which the title of _h. discina_ has been given. but, if we leave porto santo, and follow this protean _helix_ into the other divisions of the group; we meet with it on the dezertas as the _h. senilis_ (those moreover from the central island having a much more open umbilicus than is the case in the northern and southern ones), whilst in madeira proper it constitutes the _h. lincta_ (with an additional pale variety for the calcareous district of caniçal),--and the _h. saccharata_, from the são lourenço promontory. in the same way we might pursue the _h. erubescens_, and show that in the sylvan regions, and on the low barren ponta são lourenço of madeira, on the pico de facho of porto santo, on the ilheo chão, on the central dezerta, and on the bugio (where it attains a gigantic size), it has its distinct and permanent phases,--the evident results of isolation, and other topographical influences, since the subsidence of the intervening tracts. and in like manner, the _clausilia deltostoma_ is universal throughout the madeiran archipelago,--displaying, however, in porto santo a fixed and strongly ribbed state, peculiar to that island. thus, if the examples which we previously cited tend to establish the extreme slowness of the migratory movements of the terrestrial mollusca across this former continent, the present ones (which refer to a few exceptional species of quicker self-diffusive powers) will show, no less than the _insects_ to which i have lately called attention, that where sufficient areas had been overspread (before the periods of subsidence) for the creatures to have reached what now constitute the various islands of the cluster, we at once detect traces of this fact, through their more or less altered aspects,--the result of isolation, and diminished range, during the enormous interval which has elapsed since the successive convulsions which caused the partial destruction of this atlantic province were brought to a close. to return, however, to the insects, after this long conchological digression,--i need not multiply evidence, in corroboration of my theory. enough has been said to render intelligible the idea which i wished to convey, concerning the _general direction_ of the migratory current over that ancient tract, and the _extreme slowness of its progress_,--the former of which i consider probable from the north-easterly course in which creatures _generically identical_ were, if we may so express it, "given-off;" whilst the circumstance of their being for the most part _specifically dissimilar_ (or, in other words, of the islands harbouring, many of them, species which are endemic) would seem as it were to establish the latter. we must not however forget, that it is only to the _aborigines_ of this _quondam_ land that the above speculations apply. assuming the region not to have been insular, that is to say, to have been connected, on its outer limits, with a european, or mediterranean, continent; it would necessarily follow, that a certain number of colonists must have found their way over its area, and moreover _in an opposite direction_ to the living stream (if we may so call it) which had been long flowing in a north-easterly course across its surface. whatever be the length of the periods, however, during which these counter migrations were going on, i think it sufficient to state that i would refer them to epochs altogether different,--so that, accompanied as they may have been by special geological phænomena, which, if known, would in all probability become at once explanatory, we should be the less inclined to regard as absurd what might appear at first sight difficult to understand. in the case of the british isles indeed, no less than five of these distinct migratory eras have been assumed, and specified[ ], by professor edward forbes; therefore (whatever value be attached to his able and interesting theory) i do not consider it necessary to apologize for requiring _at least two_ in behalf of this ancient atlantic province. not to insist upon those of his faunas and floras which are of a less evident, or more questionable, character, he has at any rate proved, i think, almost to a demonstration, the _westward progress_ of the great mass of our british animals and plants, over a then unbroken land (the upheaved bed of the glacial sea), from the central germanic plains; whilst the accurate calculations of the late mr. thompson of belfast, concerning the reptile statistics of ireland, england, and belgium, respectively, have succeeded in showing, with much presumptive reason, how the formation of st. george's channel, _before_ that of the german ocean, interrupted the march of these wanderers to the far west, and debarred an immense proportion of them from an entry into ireland,--which would otherwise have colonized that country equally with our own. as regards professor forbes's views of the creation of a vast continent (reaching far into the atlantic[ ]) at the close of the miocene epoch, through the upheaved bed of a shallow miocene sea,--a region moreover of such an extent as to have connected the various island groups between the fucus bank and the shores of the old world, not only with each other, but with a mediterranean province, asturias, and even the south-west of ireland,--i must be content to pass them by, hazarding only a few crude and desultory remarks. so large a question, indeed, cannot be safely handled without a corresponding amount of data, in all departments of natural science, to reason from,--which i do not possess: still, if a speculation from entomological premises, _per se_, be not altogether worthless, i would point to the conclusions (lately adverted to) which my madeiran researches have forced upon me, concerning the _direction_ of the former insect migrations,--inferences which are, from first to last, of necessity erroneous, if the requisite medium for transit (into south-european latitudes, at all events) be a mere conjecture or romance. such a notion, however, i would not for a moment entertain,--for there is too much direct evidence in support of distinct epochs of diffusion, to allow of any hypothesis, when endeavouring to account for the phænomena which we now behold, to supersede the assumption of a once continuous tract. no matter if we be compelled to suppose, whilst attempting to interpret what we see, that the disseminating current has flowed in exactly opposite courses, at different and remote periods, over the surface of that ancient land,--seeing that the _fact_ (if such in reality it be) remains untouched, that _the land itself is_ at any rate _there_. i am not, however, prepared to assert that the opinion at which i had independently arrived, from the insect statistics, does positively require a northerly prolongation of that area beyond the line of the central mediterranean districts; yet, after making every possible allowance for accidental introductions since the subsidences have taken place, there is still left a large residuum which i am convinced can never be explained (unless the doctrine of specific centres be a myth) except through the means of ordinary and regular migration over an unbroken continent. nevertheless, though i would not presume, from insufficient material, to insist upon an extension of this atlantic region into higher latitudes than those which i have just referred to, i must express my individual belief that, the more the subject is examined, with reference to the distribution of the annulosa, the less will professor forbes's idea suffer from the inquiry. in the 'insecta maderensia,' i have already thrown out a few scattered hints which bear on this immediate consideration; and, since no subsequent reason has induced me either to withdraw or modify them (but rather the reverse), i will select the following,--extracted from my preface to that work. "taking a cursory view of the coleoptera here described, the fauna may perhaps be pronounced as having a greater affinity with that of sicily than of any other country which has been hitherto properly investigated. apart from the large number of our genera (and even species) which are diffused over more or less of the entire mediterranean basin, this is especially evinced in some of the most characteristic forms,--such as _apotomus_, _xenostrongylus_, _tarphius_, _cholovocera_, _holoparamecus_, _berginus_, _litargus_, _thorictus_, and _boromorphus_. there is, moreover, strange though it may appear to be, some slight (though decided) collective assimilation with what we observe in the south-western extremity of our own country and of ireland,--nearly all the species which are common to madeira and the british isles being found in those particular regions; whilst one point of coincidence at any rate, and of a very remarkable nature, has been fully discussed under _mesites_. whether or not this partial parallelism may be employed to further professor e. forbes's theory of the _quondam_ approximation, by means of a continuous land, of the kerry and gallician hills, and of a huge miocene continent extending beyond the azores, and including all these atlantic clusters within its embrace, i will not venture to suggest: nevertheless, it is impossible to deny that, so far as the madeiras betoken, everything would go to favour this grand and comprehensive idea. partaking in the main of a mediterranean fauna, the _northern tendency_ of which is in the evident direction of the south-western portions of england and ireland, and with a profusion of endemic modifications of its own (bearing witness to the engorgement of this ancient tract with centres of radiation created expressly for itself), whilst geology proclaims the fact that _subsidences_ on a stupendous scale have taken place, by which means the ocean's groups were constituted; we seem to trace out on every side records of the past, and to catch the glimpses, as it were, of a _veritable_ atlantis from beneath the waves of time[ ]." the _mesites maderensis_, woll., to which i alluded in the above quotation, is undoubtedly a strong case in point. although specifically dissimilar from the _m. tardii_, its irish counterpart, it nevertheless approaches it so closely, that it might be literally mistaken, _primâ facie_, for that insect; and we know that it is one of the plans on which nature commonly proceeds, that species which are not merely representative of (or analogous to) each other, but which are actual homologues, or allies, should usually emanate at first from foci not far removed _inter se_; or, at all events, if distant, connected by an intervening land:--in other words, that _generic areas_, no less than specific centres, of radiation, form a substantial item of the comprehensive scheme on which the system of created things was originally planned. we detect traces of this primary law in each division, or class, of the organic world; nor is its reality _as a law_ interfered with, through the occasional exceptions which are liable, as in every other instance, to present themselves. such deviations are often easily to be accounted for, whether by natural or artificial means; and do not affect the subject, as a whole. sometimes indeed they become at once intelligible from the historical records connected with them, proving that human agencies have been at work acting as transporting media, within a period comparatively recent; whilst at others, the fact of the creature having been endowed with self-diffusive powers to an extravagant degree may succeed equally in rendering the phænomena explicable. but, even where neither of these solutions would seem to suffice, we should still recollect that it is only in the mass that such questions can be pronounced upon; and that, consequently, where we are able to discover a rule which is _for the most part_ adhered to, it is more philosophical to conclude that the departures from it are the result of special disturbing causes (whatsoever they may have been), than to permit them to undermine our faith in what would be otherwise universally true. thus, the botanist tells us of ixias, stapelias, mesembrianthemums, pelargoniums, and euphorbias, as concentrated in southern africa; of magnolias in central america; of calceolarias on the andes; of myrtles, banksias, mimosas, and _eucalypti_, in australia; and of the bread-fruit trees in the south sea islands: the ornithologist points, _inter alia_, to the toucans and humming-birds from south america and the west indies; whilst the student of the higher animals informs us of the kangaroos (indeed of the whole of the subclass _marsupialia_, except the genus _didelphys_) as peculiar to australia and a few islands to the north of it; of _lemur_ _proper_ to madagascar; of the sloths, armadillos, tree porcupines, and of alligators, and of the _platyrrhini_ (amongst the monkeys), to south america; and of the ourangs to the islands of the indian archipelago. and so it is with the insecta; many of the larger groups of which (as _amycterus_ and _paropsis_, in australia; _pachyrhynchus_ and _apocyrtus_, in the philippine islands; _hipporhinus_, _monochelus_, _dichelus_, and _moluris_, in southern africa; _macronota_, in java; and _naupactus_, _hypsonotus_, _centrinus_, _platyomus_, and _cyrtonota_, in south america) are confined to countries of proportionate magnitude, whilst the smaller ones are more commonly (as it were) shaped out for special provinces or regions, according as local circumstances may require primary adaptations to harmonize with them. thus, whilst we frequently find an extensive genus diffused over the greater portion of the known world, we perceive that even its _structural_ characteristics are not uniform throughout, but afford fixed geographical modifications (_not_, in this case, however, the effect of development),--which have often, in their turn, obtained the name of 'genera,' and have been described as such. whether genera, however, or not, they are undeniably small topographical assemblages, satellites around their central types; and they may therefore be safely regarded as genera, if we choose to view them in that light. of such a nature i have already pointed out[ ] is _saprinus_, as compared with _hister_; _atlantis_ with _laparocerus_; and _oxyomus_ with _aphodius_; and, i might also add, _mesites_ with _cossonus_. i believe indeed that _mesites_ will be found to attain its maximum on the pyrenees (i already possess two or three species, in abundance, from that region); and, if such should be the case, we shall be able to appreciate the significance of two representatives so closely allied as the _m. tardii_ and _maderensis_,--one of which has been given off in the direction of ireland, and the other of the madeiran archipelago. but i will not digress further on the subject of this atlantic province; since, however much i may individually regard it as a reality of the past (which the coleopterous statistics have compelled me to do), it must of necessity remain, as heretofore, a matter of much controversy and doubt. i should indeed apologize for having trespassed on the reader's attention, in wandering this far from the immediate results of _subsidences_,--which i proposed, at the outset of this chapter, to examine, with reference to the impeded diffusion of the annulose races. nevertheless, concluding that a practical illustration of the effects of one of those great downward movements to which geology so repeatedly bears witness would not be irrelevant to the _assumed consequences_ which i had previously ventured to define, i have acted on that judgment; and, having finished my task, will now proceed to notice, briefly, a few other considerations which should not be omitted, when inquiring into insect distribution as influenced by geological phænomena. next in importance, perhaps, to the elevations and sinkings (traces of one or the other of which are more or less manifest in almost every region of the world), _natural barriers_ may be cited,--as presenting, not unfrequently, insurmountable obstacles to the self-dissemination of the insect tribes. by natural barriers, however, i would be understood to imply natural _primary_ barriers,--or, in other words, such as have continued as barriers ever since the present animals and plants came into existence upon the earth. for, the _ocean_ (by way of illustration) is a natural barrier; and yet it is not necessarily a primary one, as may be readily gathered from the above remarks, in which the results of _subsidences_ are discussed,--subsidences which have had the effect of letting it in over portions of an _already tenanted_, and unbroken, continent. mountain-chains, also, are barriers; but it may happen that they have not been so from the beginning,--as in instances, for example, where they have been gradually upraised during periods geologically recent. but both sea and alpine ranges are barriers, when (as usually happens) they have remained as such since the creation of the several species which now inhabit our globe. mr. darwin has acknowledged this distinction, whilst commenting upon the marked divergence of the faunas on the eastern and western slopes of the cordillera. "this fact," says he, "is in perfect accordance with the geological history of the andes; for these mountains have existed as a great barrier since the present races of animals have appeared; and therefore, unless we suppose the same species to have been created in two different places, we ought not to expect any closer similarity between the organic beings on the opposite sides of the andes, than on the opposite shores of the ocean. in both cases, we must leave out of the question those kinds which have been able to cross the barrier,--whether of solid rock or salt-water[ ]." conceding, therefore, this distinction between barriers of a primæval and more recent character, it is not difficult to understand why the opposite sides of an alpine chain, as well as countries separated by the sea, should display different phænomena from each other. on the contrary indeed, if we could feel satisfied that no means of accidental transportation had operated to take them there, and that the animals themselves were incapable of enduring great diversities of temperature, and other contingencies; we should be startled to discover creatures specifically identical in such regions,--so long at least as the doctrine of unique centres of radiation formed part of our zoological creed. we must not, however, be too hasty in questioning (if i may be pardoned for the completion of a metaphor of which i thoroughly disapprove) this article of our faith, through the occurrence of similar beings in areas between which there exist barriers, both primary and well-defined; for the methods of diffusion are so complicated and numerous, that, even where human agency (that most important of elements) is not concerned, what at first sight may frequently appear to be impossible becomes clear enough when more critically inquired into. some species, we know, are gifted with greater powers for horizontal and vertical progression than their comrades, and can (though they are doubtless exceptions to the general rule) pass through extremes of atmosphere sufficient to render even lofty mountain summits no obstacles to _them_. others, as the _calosoma syncophanta_ of europe, have been stated to traverse the ocean unhurt[ ]; and i believe that many do at times accidentally arrive, in a half-drowned state, especially after boisterous weather, across channels of considerable breadth. mr. kirby, on examining the marine _rejectamenta_, during one of these apparent occurrences, along the suffolk coast, writes as follows: "whether the insects i observed upon the beach, wetted by the waves, had flown from our own shores, and, falling into the water, had been brought back by the tide; or whether they had succeeded in the attempt to pass from the continent to us, by flying as far as they could, and then falling had been brought by the waves, cannot certainly be ascertained; but kalm's observation inclines me to the latter opinion[ ]." and sir charles lyell remarks:--"exotic beetles are sometimes thrown on our shore, which revive after being drenched in salt water[ ]." nor should we forget that chance agencies of every description, which we are too apt to overlook, are daily at work (and have been so since, at any rate, the last creative epoch) to transport these variously organized beings beyond their original spheres. sometimes they are carried on, or within, the bodies of larger animals, which is especially the case with the parasitic tribes; at others on floating trunks of trees, and casual substances of divers kinds, which are able to resist for a definite period the destructive action of an element saturated with salt. unwilling victims, again, are ever and anon hurried to comparatively distant lands by the very winds that blow; and not only to distant lands, but over altitudes in which the severity of the cold would quickly annihilate them, were they (as perhaps usually happens) to be deposited there on their headlong and compulsory course. "as almost all insects are winged[ ]," says sir charles lyell, "they can readily spread themselves wherever their progress is not opposed by uncongenial climates, or by seas, mountains, and other physical impediments; and _these_ barriers they can sometimes surmount by abandoning themselves to violent gales, which may in a few hours carry them to very considerable distances. on the andes some sphinxes and flies have been observed by humboldt, at the height of , feet above the sea, and which appeared to him to have been involuntarily carried into those regions by ascending currents of air[ ]." with respect to the accidental conveyance of numerous species across the sea, it is not to the winds alone that we must look for an explanation. large and rapid rivers are liable to inundate their banks and bring down insects in prodigious masses,--which are disgorged into the ocean, and carried to a distance from the coast, in proportion to the violence of the ejecting stream. when the body of water is considerable, the sea becomes diluted to an unusual extent; and creatures which must have otherwise perished, from the action of the salt, are able to survive for a time, and may be deposited, by means of rapid currents into which they are borne, on neighbouring islands and continents. even the _hydradephaga_ are thus occasionally transported; for darwin mentions having captured a _colymbetes_ off cape s^{ta} maria (to the north of the rio de la plata), when forty-five miles from the shore. and, in his 'journal of researches,' he records the following remarkable facts, which bear upon this immediate question. "on another occasion, when seventeen miles off cape corrientes, i had a net overboard to catch pelagic animals. upon drawing it up, to my surprise i found a considerable number of beetles in it, and, although in the open sea, they did not appear much injured by the salt water. i lost some of the specimens; but those which i preserved belonged to the genera _colymbetes_, _hydroporus_, _hydrobius_, _notaphus_, _cynucus_, _adimonia_, and _scarabæus_. at first i thought that these insects had been blown from the shore; but upon reflecting that, out of the eight species, four were aquatic (and two partly so) in their habits, it appeared to me most probable that they were floated into the sea by a small stream which drains a lake near cape corrientes. on any supposition, it is an interesting circumstance to find live insects swimming in the open ocean seventeen miles from the nearest point of land[ ]." accidental means of dissemination, such as those to which i have just alluded, and others to which we might appeal, will generally account, and with much presumptive truth, for the many exceptional cases which present themselves, during our investigation into the effects of natural barriers, as visible in the distribution of the annulose races, on the earth's surface. i say "exceptional cases," because any one who has laboured practically in mountain tracts cannot have failed to recognize the marked difference which is often displayed by the insect population on opposite sides of some alpine chain; whilst he whose lot has been cast amidst island groups, will have become even more conscious than the former of the permanency of those impediments which have been placed (in this instance by the broad arms of the mighty ocean) as checks upon a too rapid system of diffusion. but if the sea and mountain ranges, when of a sufficient age _in situ_, are amongst the most effectual of nature's barriers against the self-dispersion of the animate tribes; it follows that, if the two could be (as it were) _united_, we should have found the greatest obstacle which physical conditions can ordinarily present against the wandering capabilities of the latter. the question therefore arises,--is it possible for them to _be_ so joined? undoubtedly it is: and hence we arrive at the conclusion, that a _mountain island_ should afford us the _minimum of size, as regards the areas its species have overspread_, which any country is able to furnish. madeira is a mountain island,--its highest peaks rising, although resting on so small a base, to an altitude of more than feet. yet it is only partially a case in point; for, although it was a mountain mass, and perhaps a very elevated one, when its endemic beings made their first appearance upon its surface, we have already intimated that it has become isolated _since_ that epoch: so that, whilst _one_ of the natural barriers against dispersion which it involves (namely, mountain ridges) may be considered as primary; the _other_ (to wit, the sea, as it now obtains) has played, as an agent of obstruction, but a secondary part. still, there is good reason to believe that the ancient tract of which it is a portion was broken up at a comparatively early date after the creation of those peculiar organic forms which found their birthplace within its bounds; and that, consequently, the latter could not have wandered far (if we except those species on which unusual powers of diffusion were bestowed) when the land of passage began to give way. hence, even the sea, in this particular instance, partakes almost of the character (no less than the mountain heights) of an original impediment; and madeira therefore may be safely quoted as an example in which two barriers, of a primary nature, are united; and where, consequently, we may anticipate those ultra phænomena of _areal limitation_ upon which we have been just commenting. but let us now inquire, whether the hypothesis at which we have arrived will stand the test of experience; for unless it will do so, we might have been spared the labour of propounding it. madeira is a country composed of narrow mountain ridges, which radiate from central crests, and form the lateral boundaries of deep and precipitous ravines. modifications of this structural type are of course traceable everywhere; the upland tracts are often undulating and broad, and the buttresses which slope towards the sea are sometimes expansive and irregular: yet upon the whole the above description is correct, and we may accept it in a generic sense. now we may premise that, even to this day, it is an island of floods; therefore, how much more must it have been so when its primæval forests, in all their splendour, caused an amount of exhalation and moisture of which at present we can have but a remote conception! hence, it is hardly to be imagined, that (however limited may have been the naturally acquired areas of those of its inmates which are most sluggish and sedentary) a fusion would not have taken place, in the course of ages, so as to render its modern fauna, in a large measure, homogeneous throughout. yet, in spite of this esoteric tendency, it is surprising how little amalgamation has been effected amongst the tenants of its several districts. scarcely a gorge or woodland serra exists within its bounds which does not harbour some species essentially its own; and in many instances the ranges of these creatures are so local or confined, that they might be easily overlooked even in their respective neighbourhoods. it is certain, however, that the floods (which happen periodically) have done considerable work in naturalizing many of the subalpine forms, which could adapt themselves to the climatal change, in altitudes below their normal ones: and, in the north of the island, where the temperature is cooler than on the opposite side, and where the lofty defiles terminate, even at their lowest outlets, in abrupt precipices along the coast, so that the _rejectamenta_ during the annual rains are brought into direct contact with the shore, this gradual process of deportation is particularly evident,--a circumstance to which i have already alluded elsewhere[ ]. but, after making due allowance for these powerful means of dissemination (which, in the common order of things, must necessarily obtain in _mountain islands_, as it were, _par excellence_), the fact still remains, that in the madeiran group the acquired areas, even up to the present date, of a vast proportion of the insect inhabitants, are wonderfully circumscribed. the real state of the case, however, would appear to be simply this: that the floods, although they may have tended to diffuse the members of a comparatively uniform alpine fauna in the various clefts or gorges beneath, can have had no power to combine the aborigines of the several gorges themselves; and, since a large proportion of the endemic species of those islands are (as i have previously stated) apterous, the perpendicular edges of the ravines, which in many instances rise to an elevation of feet, have acted (and ever _will_ act) as impassable barriers to vast numbers of the insect tribes. with this single example (by way of illustration), which the madeiras have supplied, i will take my leave of the question of _natural barriers, as tending to regulate the topographical diffusion of the annulosa_,--feeling that i have already devoted too much time and space to this portion of the subject (if such indeed it be) which i had proposed in the present treatise to discuss. other barriers might have been adverted to,--such as large rivers, extensive deserts, and thickly set forests (especially of pine-trees, which frequently offer a very decided impediment to insect progress),--but they are of secondary importance, when compared with marine and alpine ones; and their consequences may be, to a certain extent, deduced from the considerations which i have just entered into. my main object has been to draw attention to the fact, that the great obstacles which nature has placed against the too rapid dispersion of animal life should be more strictly taken into account (as a matter of positive reality) than it is, during our investigations into entomological geography. to be aware that these barriers exist, and yet to feel surprised, especially in a country where the species are principally wingless, that we do not discover indications of a general uniformity in its fauna, involves an absurdity,--unless the doctrine of specific centres of creation be a mere coinage of the brain. but, if we believe in that theory (which, until it can be shown to be impossible, i hold that we are _à priori_ bound to do), we must at least act consistently with ourselves, and not anticipate phænomena where we have neither reason nor right to look for them. we are too apt to draw a line of imaginary demarcation between the sciences, as though each had its own propositions to establish, and nothing more: indeed, some of us would appear to assume (though perhaps tacitly), that what is proved to be true in one department may be, at least, rendered inconsistent (if not actually negatived) in another. but surely this requires no argument to refute,--since a _principle_ which is _true_, is true under every circumstance and condition; for otherwise, it could be both true and false. we need not therefore be afraid of comparing truth with truth, under whatever shape it may arrive, as though it were possible that either of its phases could ever suffer from the ordeal of a close contact; since, if they be really true, and free from deception, they must needs go hand in hand, and _may_ become (however opposite they be in their subjects) directly explanatory of each other. the astronomer who is not intimately acquainted with pure mathematical analysis, in its various aspects and bearings, is in fact no astronomer at all. the geologist who would interpret the grand phænomena of the earth's crust apart from statical and dynamical knowledge, and without the help which the chemist, mineralogist, anatomist, zoologist, and botanist can afford him, stands a fair chance of leaving his problems unsolved; whilst the students of zoology and botany who would endeavour to understand, and account for, what they see in the animal and vegetable worlds around them, without calling in geology to their aid, must assuredly be prepared to fail signally in their attempts. all indeed must work in concert, if the whole is to be advanced,--and not only in concert, but as mutually assisting each other. "by the help of truths already known, more may be discovered; for those inferences which arise from the application of general truths to the particular things and cases contained under them, must be just.[ ]" footnotes: [ ] "when we consider indeed the apterous nature of _deucalion_, its subconnate elytra, and its attachment (at any rate in the larva state) to the interior of the stems of particular, local plants, or its retiring propensities within the crevices of rocks; we are at once struck with the conviction, that, during the enormous interval of time which has elapsed since the mighty convulsions which rent asunder these regions terminated, it has probably never removed many yards from the weather-beaten ledges which it now inhabits." [ ] since the above was published, i have succeeded in detecting one more example,--namely (in june ) on the summit of the ilheo bugio, or southern dezerta, within a few yards of the self-same spot where it was found by the rev. r. t. lowe in may . although i searched diligently on the dezerta grande, during my late campaign in the madeira islands, i was not able (so great is its rarity) to discover farther traces of it on that rock. [ ] insecta maderensia, p. . [ ] it would seem, when viewed on a broad scale, as if particular districts throughout the world had been made as it were the special fields for the exercise of the creative force,--or that, _generic areas of radiation_ were part of the elementary design. thus, professor e. forbes records his belief that most, if not indeed _all_, of the terrestrial animals and plants now inhabiting britain are members of specific centres beyond bounds,--they having migrated to it over a continuous land, before, during, or after the glacial epoch. hence, since the greater number of them are supposed to have come from the central germanic plains, we may assume that those plains were one of the primary areas of diffusion for a large mass of created beings. there is good cause for suspecting that the pyrenean region may have been another; and certainly all evidence would tend to prove that this vast atlantic province was, also, well stocked with aboriginal forms. [ ] assuming the _helix lowei_ and _bowdichiana_ to be gigantic phases of the _h. portosanctana_ and _punctulata_, respectively; four only, namely _h. fluctuosa_ and _lapicida_, _achatina eulina_, and _cyclostoma lucidum_ (the first three of which are extinct throughout the entire group), seem to have altogether disappeared. nevertheless, the gradual dying-out, as it were, of species, both here and in madeira proper, is singularly evident. thus, in the latter, the caniçal beds show the _h. tiarella_ to have been once most abundant (it literally teems in those calcareous formations). yet so rare is it in a recent state, that, until the summer of , when it was detected by myself and the rev. r. t. lowe in two remote spots along the perpendicular cliffs of the northern coast, it was supposed to have been lost for ages. and the same may be said of its counterpart, the _h. coronata_, in porto santo,--which, likewise, swarms in every fossil-bed of that island; but which was, also, until i met with it, on the th of december , adhering to slabs of stone at a considerable depth beneath the ground, on the extreme eastern peak (opposite to the ilheo de cima), imagined to have long passed away. and so, reasoning from analogy, i think it far from improbable that the third representative of this little geographical assemblage,--the _h. coronula_ of the bugio (which has hitherto only occurred in the mud deposits on the summit of that rock),--may be still alive, though perhaps in very small numbers, on some of the inaccessible ridges of those dangerous heights. [ ] origin of the fauna and flora of the british isles (in mem. of the geol. survey of great britain, vol. i. p. , a.d. ). [ ] "my own belief," says professor forbes, "is, that the great belt of gulf-weed, ranging between the th and th degrees of north latitude, and constant in its place, marks the position of the coast-line of that ancient land." [ ] although, for want of a better name, it may be admissible, when speaking either figuratively or poetically, to allude to this former region (as i have done in the above quotation) under the title of "atlantis;" yet it seems incredible that certain writers (assuming its _quondam_ existence) should have recently referred to it seriously as the possible "atlantis _of the ancients_!" considering that there is good reason to believe that all these islands _were islands in a miocene sea_, and that, if (through a general elevation) they were subsequently connected, the land of passage was broken up long anterior to the appearance of man upon the earth, "the ancients" must have assuredly merited their appellation, if they could have thrown any light on a problem which belongs to an epoch thus remote. whether the "atlantis" had any being at all except in the imagination of the latin poets, or whether (as lord bacon has suggested) it was the new world, will probably never now be known; yet the fact that the _insulæ fortunatæ_ of juba are almost universally identified with the present canarian group (as indeed the accurate description of pliny well nigh demonstrates), and the _purpurariæ_ with the madeiras, ought at once, apart from geological evidence, to point out the absurdity of the hypothesis, that an atlantic continent, _in the very position which those islands occupy_, could have been acknowledged to have any existence by the literature of either rome or greece. [ ] insecta maderensia, p. . [ ] journal of researches, pp. , . [ ] many of the _calosomata_ would appear to possess this power of crossing, either by flight or by abandoning themselves to the waves (though more probably by the assistance of both), even marine barriers with impunity. numerous instances are on record to this effect; and i am informed by mr. darwin that a _calosoma_ flew on board the 'beagle,' off the bay of san blas, in south america, whilst they were ten miles from shore. it seems likely, therefore, that the occasional occurrence of the _c. syncophanta_ in our own country, along the southern and eastern coasts, is due to this generic capability,--and consequently (as indeed it is usually acknowledged to be), the result of accident. [ ] introduction to entomology, ii. p. . [ ] principles of geology, th ed. p. . [ ] although this is true on a broad scale, a reference to my observations in a preceding chapter will show, that in some countries, especially islands, the reverse will frequently be found to obtain. [ ] principles of geology, p. . [ ] journal of researches, p. . [ ] insecta maderensia, p. . [ ] religion of nature delineated, pp. , . chapter vi. the generic theory. how glorious to the observant eye is the great system of the organic world, how perfect in each separate part, how complete and harmonious the whole! the unity of the comprehensive plan, amidst the infinite modifications which it includes, has ever been a theme of admiration and delight; for the mind, which has once caught a glimpse, even in physics, of what it is not possible to disprove, instinctively clings to it, as to a grand material truth. the discovery, at all times, of what we feel to be actually _certain_ is in itself so fascinating, that the very data which it gives us are scarcely more prized than the mere knowledge that we have gained a single additional light to guide us on our forward way: for, since in the inductive sciences we can but climb from step to step, at a slow and even pace, we hail with inward satisfaction whatsoever may tend to lighten our task, and to lead us more quickly onwards (gradually though we must of necessity advance) towards its final accomplishment. but how, it may be asked, is this general harmony of the organic creation to be insisted upon, when beings so extravagant and dissimilar are everywhere to be met with? is it possible to recognize anything like a unity of type amongst creatures so differently constructed, and so widely removed from each other in their habits, aspects, functions, and attributes? such questions as these, however, though they may occasionally perplex the tyro, or amateur, are not likely to be raised by anyone who has mastered the merest alphabet of zoology,--and who is aware that the integrity of nature is something real and positive, as experience indeed is ever tending more and more to corroborate, and by no means the day-dream of an enthusiastic, or fertile, imagination. to trace out the progressive development of animal life, from its humblest phases; and to mark, as they become visible in the intermediate grades, the first rudiments of organs and instincts which are destined to attain their maximum in the higher ones, embody but a small portion of what it is the naturalist's mission to investigate. to him belongs the special privilege of inquiring dogmatically into this structural advancement; and of suggesting methods of classification which shall accord, in their several component divisions, so far at least as is practicable, with the constitutional change. we should recollect, however, that this system, being based upon truth, must, if it would be consonant throughout, adapt itself to all the various phænomena (in their respective positions, in the scale), from the consideration of which it should be exclusively deduced, or built. to draw broad conclusions of any kind, or to attempt the establishment of propositions and principles, from simple dialectics, without a previous training in the practical bearings of the subject, would be absurd, and almost certain to beget error. "it cannot be that axioms established by means of _reasoning_ [alone] should be of any value for the discovery of new results; because the subtilty of nature far exceeds the subtilty of reasoning. but axioms duly and orderly abstracted from _particulars_, in their turn easily point out and mark off new particulars; and so render the sciences active[ ]." such were the words of the greatest philosopher which this country has ever produced; and it would be well, whilst examining the causes of what we see, and endeavouring to obtain some faint and distant notion of the vast scheme of nature as originally designed, to keep them constantly in view,--lest, by trusting to theory only, apart from observation and facts; or by venturing to pervert the latter (instead of being led by them), so as to tally with our preconceived ideas of what ought to be, we miss our road, and become lost in the mazy labyrinth of our own fanciful inventions. with this preliminary stricture on the express duty which devolves upon the naturalist (with whom the phænomena of the organic world principally rest, for interpretation) to make facts, rather than reason and argument, the basis of his various doctrines,--at any rate of those in which the critical subject of _arrangement_ is concerned; i shall perhaps be pardoned, after having been drawn, in the preceding chapters (however involuntarily), into the question of 'species,' as rigidly defined, if i now offer a few passing remarks on the theory of _genera_. there can be no doubt that amongst a large class of ordinary observers a clear perception of the generic system, in an abstract sense, does not by any means prevail. what the nature of a genus really is, would appear to have been very commonly overlooked, or perhaps misunderstood, by people of this stamp; and the consequence has been, that the wildest notions have frequently arisen, even from men of sound _specific_ attainments, as to the claims (for annihilation or retention, as 'genera') of certain subsidiary zoological assemblages. the terms 'genus' and 'species' have been conjointly so long associated in our minds with the selfsame things (whatsoever they may be), that they have become almost part and parcel of the objects themselves; so that the student who does not sufficiently reflect on their true signification, is apt to regard them as of equal importance,--or, rather, more often perhaps than otherwise, to make the latter subservient (or inferior) to the former! this however is, in reality, the very reverse of what should be the case, as a moment's consideration will indeed at once convince us: for what are genera, after all, but _dilatations_ (as it were) along a chain _which is itself composed of separate_, though differently shaped, _links_? the links (or the actual, independent bodies which constitute the chain) are the species; but the knobs, or swellings, which their several forms may tend, _by degrees_, to establish along its course (through the slight disparity which each of them presents from that which is next in succession to it; and therefore through the gradual manner in which the bulbs, or nodules, may be said, _on the whole_, to be produced), are the groups into which those species naturally fall. it matters not a straw whether these assemblages be primary, secondary, tertiary, &c.,--in other words, whether they be departments, families, or genera, as usually understood,--the _principle_ is in every instance the same; the difference being merely relative, and not absolute. or, if we choose to vary the simile, we may compare the whole system to a cord, upon which beads, of innumerable sizes, patterns, and colours, have been densely strung. now, if there were no such things as natural divisions in the organic world, these beads (which represent the separate species) might have been disposed of anyhow,--their positions, with respect to each other, would under those circumstances have been of no importance. but such is not the case: there is an order and method throughout nature, which shows that every individual portion of it has been adjusted by the master's hand, and that nothing has been left to chance. those beads (to follow up the metaphor) of countless magnitudes and hues, have had their proper places allotted to them,--and moreover with such care and regularity, that a complete plan, or scheme, of distribution is at once conspicuous. although there are not even two, amongst that enormous multitude, which are _precisely_ alike (for every species, however it may resemble its next ally, has _some_ distinctive feature of its own), we immediately perceive that those beads which have most in common, are, as it were, attracted to each other,--so as, by their close approximation, or contact, to create excrescences and stripes, of divers kinds, along the entire length of the cord. if we assume now that the red beads have been collected together, to the length (for instance) of a yard, and that within that space a dozen protuberances, of discordant aspects and dimensions, have (by the union of those beads which more nearly simulate each other) been brought about; we shall have a very fair idea of the ordinary grouping of the animate tribes. the red beads, taken in the mass, may be likened to a perfect "family;" the differing gibbosities to twelve well-marked "genera," which that family includes; whilst the "species" (the real _dramatis personæ_, of independent existence, which are nevertheless compelled to occupy the situations we have described,--thus _causing_ the divisions to be mapped out) are here typified, as everywhere, by the several beads themselves. i have not thought it necessary to pursue this reasoning into higher divisions than "families;" but of course it may be extended to any amount,--so as to shadow forth, equally, the compartments of _primary_ significance. nor would i wish to imply, by the above similes, that i regard a _lineal_ method of arrangement as the correct one. every zoologist is aware, that in nature such does not exist: but the mode of illustration which i have selected is applicable to all systems alike, so far as the _principle_ is concerned. it will consequently be seen, from what has been said, that the terms "genus" and "species" not only differ very considerably in _importance_, but in signification also. whilst the former is merely suggestive of a particular _position_ which a creature occupies in a systematic scale (a position, however, which depends upon the various structural peculiarities which it possesses _in common with other beings_,--which thus more or less resemble it); the latter expresses the actual creature itself: so that while one applies to _several_ animals (of distinct natures and origins, though bound together by a certain bond of imitation), the other belongs to _a single race alone_, which it therefore exclusively indicates. but if such be the case, it will perhaps be asked,--why then insist upon a generic name at all, if the specific one be sufficient to denote all that is required, namely, the _animal itself_? to which, however, we may reply, that the binomial nomenclature is demanded for two elementary reasons,--first, because it is founded upon a natural truth, which (to say the least) it would be unwise to violate; and, secondly, because it is _convenient_, both for simplification and analysis. we should assuredly be surprised were a man to object to his surname, as unnecessary, because he has a christian (or specific[ ]) one which is the exponent of him _alone_. true it is that his family (or generic) title applies to the rest of his kin also; but, since there are other people (of other families) who may have the same _individual_ appellation as himself, it is clearly desirable, even as a matter of expediency alone, that patronymic and christian name should be alike retained. we need not, however, plead expediency, in favour of this acceptance of what has been so long tested, and shown to be correct; we appeal to a higher tribunal,--that of experience,--in proof that it draws its origin from nature itself, and is implied by the very existence, or reality, of _natural groups_. the 'méthode mononomique' has indeed been attempted[ ]; and it has failed,--or at any rate it has shown itself to be inferior, both ideally and in practice, to the plan commonly in use: and if i might be pardoned a passing conjecture on its ultimate success, i should be inclined, since it is contrary to the canon of the organic world, to regard its case as utterly hopeless. let us not be unfair, however, towards those who have sought to establish a nomenclature which they conceived would be less open to objections than that which we have been hitherto accustomed to endorse. the notion did, at any rate, arise out of an apparent defect in the binomial process,--for the inconveniences which they complained of are real ones; and, having felt them practically, they aspired to sweep them away by remodelling the whole system afresh. but, had it not been for an evident misconception of the generic theory, in the abstract, the trial would in all probability have never been made; and we should have been spared the downfall of a contrivance which has had but little to recommend it beyond the ingenuity of its machinery and detail. if we analyse the motives for this experiment, we shall find that it originated from a belief, that genera are _either_ purely imaginary, or else that they must (like species) have a definite and isolated existence. now both of these conclusions appear to be equally gratuitous and untenable; and such as a lack of observation could alone beget. genera are _not_ mere phantoms of the brain (as most naturalists will readily admit); but they are, likewise, by no means abrupt, or well-marked, on their outer limits (except indeed by accident,--of which hereafter), but merge into each other by gradations, more or less slow and perceptible. such being the case, we can easily understand why it is that the followers of the 'méthode mononomique' (who, paralysed by the fact that genera are seldom _clearly defined at their extremes_, would seem to repudiate them _in toto_) have rashly regarded the binomial system as intolerable. finding that it was possible for numerous species, whose structural characteristics were less conspicuously pronounced than those of their allies, to be enumerated, and with equal plausibility, under two consecutive groups; they immediately inferred that the groups themselves could not be upheld on account of these connective links: and so it was resolved (through a new and artificial scheme) to ignore them; and to fall back upon the creed, that species alone (and not genera) are to be recognized in the organic world. this was but the device, however, at the outset, of a single mind; and the perverts to it have been but few. it is in direct opposition to the first principles of nomenclature, and sets at defiance a great natural truth. but what, it may be inquired, is this great primary truth which the monomial system tends to violate? i repeat what i have already stated, that it is the _existence of natural assemblages_ which that scheme would, if it were practicable, discountenance. order and symmetry, however (which involve classification, or arrangement), are the law of nature, and it is not possible to set them aside. it matters not if harsh lines of demarcation are undiscernible between the several consecutive groups,--the _groups themselves_ must still remain (however equivocal it may be where they exactly commence or terminate), and cannot be wiped out. to suppose _à priori_ that the allied divisions of the animate creation are perfectly disconnected _inter se_, is in fact to break the chain on which the unity of the organic world depends; whilst to assume that groups cease to be groups when they can be discovered to merge into each other, would no less destroy the harmony of that admirable method, or array, which the naturalist, above all others, delights to contemplate. if things are no longer to be regarded as dissimilar because they unite on their outer limits, differences may be given up, as having no special meaning, and as therefore unworthy of investigation. it requires but a slight insight into the physical universe to be convinced, that nearly everything which we see (and, moreover, _without injuring its individual reality_) is blended into that to which it is the most akin. night is distinct from day; yet, so long as the twilight intervenes, no man can pronounce where the one ends, and the other begins. heat is opposed to cold; yet, if by degrees they be respectively diminished, they will at last amalgamate, in a central temperature. and thus it is with things material. the sea and the land are essentially unlike; yet the precise boundary between the two is never clearly defined,--the ebb and flow are constantly going on, and the line of separation is variable. the mountain-range is moulded on a different type to the level country beneath it; yet the turning-point of them both is, in all instances, on neutral ground. we need not however adduce further evidence in support of this fact,--that, throughout the whole of nature, the _general principle_ of fusion (either absolute or apparent) is most obvious. from first to last, traces of it are everywhere to be detected; not only between _clusters_, or material combinations, of objects (in which case it is absolute), but even between the objects themselves,--under which circumstances, however, it is merely apparent; for, since they are specifically dissimilar, it can only arise from their _near resemblance_ to each other, and not from their positive coalescence. but, admitting that this universal blending, throughout the animate world, does not interfere with the gradual conformation of its several groups, which _therefore_ should be recognized; we may perhaps be told by the believers in the 'méthode mononomique,' that they do not intend to ignore the _arrangement_ which nature has so broadly laid down, but that, on the contrary, they tacitly endorse it,--their device having reference to the _names_ only. to this however it will be sufficient to reply, that, if they deem it necessary (of which i am by no means convinced) to accept the natural genera of the organic creation at all, why not _acknowledge_ them? and how can they be so well acknowledged, either in principle or practice, as through the medium of a binomial nomenclature? such a system is the only consistent one, on the hypothesis that they _do_ consider them of primary importance; it is more in unison with our notions of what ought to be; more suggestive of what actually _is_; more honest and generous to those who have laboured (as describers), with such care and diligence, before us. it will be perceived, from the above remarks, that, although professedly criticizing the 'méthode mononomique,' into the analysis of which my subject has unintentionally drawn me, it is the absurdity of objecting to genera _because they are not rigidly defined throughout_, that i have been mainly striving to condemn. it is indeed well nigh incredible that any such strictures could ever have been advanced; for it must surely have occurred to the most superficial inquirer, that genera, after all, _cannot_ be homogeneous,--seeing that they are necessarily composed of detached species, no two of which are _precisely_ similar, even in the few structural details which may have been accidentally chosen for generic diagnostics. how is it possible, therefore, that mere _groups_, even though they be in accordance with nature, should be so far isolated and uniform in their character as to occupy an analogous position to that of the absolutely independent species (of distinct origins) which they severally contain? taking the preceding considerations into account, the question will perhaps arise,--how then is a genus to be defined? to which i may reply that, were i asked whether genera had any real existence in the animate world, my answer would be that they undoubtedly have,--though not in the sense (which is so commonly supposed) of abrupt and disconnected groups. i conceive them to be gradually formed nuclei, through the gathering together of creatures which more or less resemble each other, around a central type: they are the _dilatations_ (to use our late simile) along a chain which is itself composed of separate, though differently shaped links,--the links being the actual species themselves, and the swellings, or nodes, the slowly developed genera into which they naturally fall. when i say "slowly developed," my meaning may possibly require some slight comment. it is simply therefore to guard against the fallacy, which i have so often disclaimed, that genera are abruptly (or suddenly) terminated on their outer limits, that the expression has been employed. though i believe that a series of _species_, each partially imitating the next in contact with it, is nature's truest system; yet we must be all of us aware that those species do certainly tend, in the main, to map out assemblages of divers phases and magnitudes, distinguished by peculiar characteristics which the several members of each squadron have more or less in common. so that it is only in the middle points that these various groups, respectively, attain their maximum,--every one of which (by way of illustration) may be described as a _concentric bulb_, which becomes denser, as it were, in its successive component layers, and more typical, as it approaches its core. if, then, the theory of genera be such as i have endeavoured to expound, it results from what has been said, _that every generic type is to be looked for in, or about, the centre of its peculiar group_,--or at any rate in that region of it which would seem to be the most characteristically, or evenly, pronounced. i lay particular stress upon this conclusion, because (if correct) it will somewhat modify the notions which are occasionally entertained upon the subject. a stricture, however, may here be required upon what i have advanced, lest, through using the metaphors _which i selected for the elucidation of a principle_, it be supposed that i would wish them to apply to the smaller details, likewise, of the problem. if a genus has been portrayed under the similitude of a bulb, or of a nodule (formed by the approximation of beads which more or less resemble each other in their primary aspect), it does not follow that either bulb or nodule are to diminish in a similar ratio towards their respective circumferences,--or, which is the same thing, that they are to be symmetrical; whether spherical, ovoid, or otherwise. the general method of the organic creation is a progressive one; and its successive types, therefore, will not always be found to radiate _equally_ from their normal foci: so that it is in the direction of the _higher_ (rather than the lower) extremities of the assemblages that those foci are usually to be discerned;--and where the groups are large, it is not often difficult to pronounce which of their ends are, as a whole, the more perfectly developed. it will, moreover, be further acknowledged (if my premises are allowed), that, since it is a somewhat central position which the typical member of a genus usually occupies, _the diagnostic characters_, although (in combination) carried out to the full, _are more evenly balanced in a generic type than in any of its associates_; or, in other words, that a species in which any single organ is monstrously enlarged, at the expense of the rest, is seldom typical of the assemblage with which it is placed; but may be _à priori_ regarded as in all probability a transition form, leading us onwards into some neighbouring group[ ]. i will not, however, venture too closely into this question in its minor bearings;--suffice it to have demonstrated that, whatever be the rate, law, or direction, of the advancement of the various groups towards a more perfect model; or in whatsoever position the several types are to be discerned, with respect to their immediate associates, genera _cannot_ be isolated and distinct, but must of necessity merge (each into two or more others) on their outer limits. hence, if such be the case, as i contend that it usually is (the exceptions to the rule being, as i shall hope shortly to prove, the result of accident, and by no means a part of the original design), it may perhaps be a problem, how far we are justified in rejecting many large and natural assemblages, through the fact that they blend, both at their commencement and termination, imperceptibly, with others,--their precise boundaries being dimly defined. that the recognition of genera is necessary, even as a matter of mere convenience, is self-evident; for in many extensive departments they combine with each other so completely at their extremities (although sufficiently well-marked in the mass), that, unless we are prepared to accept them as they are, we must needs repudiate them altogether: under which circumstances, our difficulties, both in determination and nomenclature, would be increased tenfold. we should also recollect, that clusters which seem abruptly chalked out whilst our knowledge is imperfect, are very frequently united with others when fresh discoveries are made, and the intermediate grades brought to light: so that their apparent isolation may oftentimes arise from our ignorance of the absent links, rather than from the fact itself. it would surely be more desirable, therefore, when viewed even in the light of expediency alone, to submit to the possibility of a few neutral species being conceded, _with equal reason_, to different groups, than to amalgamate the whole, and so lose sight of the general method or arrangement, into which the various creatures do unquestionably (in a broad sense) dispose themselves. if, however, there be any truth in the generic doctrine as above enunciated, the question of _convenience_ may be omitted from our speculations _in toto_,--seeing that _all_ genera (except those whose present abruptness is the effect of accident) fuse into others with which they are in immediate contact: so that in reality, unless we ignore these natural assemblages from first to last, we have no choice left us as regards the equivocal forms; but must consent to recognize them as of doubtful location, and as possessing an equal right to be placed in one or the other of two consecutive groups,--according to the judgment of the particular naturalist who has to deal with them. but let us glance at the subject through the medium of an example, and endeavour to realize what would be the consequence of that wholesale combination at which we must sooner or latter arrive, if genera are not to be upheld because they slowly merge into each other as we recede from their respective types. the immense department _carabidæ_, of the coleoptera, is eminently a case in point. in the details of their oral organs the _whole_ of that family display (as i have elsewhere[ ] remarked) so great a similarity _inter se_, or rather shade off into each other by such imperceptible gradations, that the _tendency_ which various clusters of them possess to assume modifications of form which attain their maximum only in successive centres of radiation, must oftentimes be regarded as _generic_, if we would not shut our eyes altogether to the natural collective masses into which the numerous species (however gradually) are, in the main, so manifestly distributed. it is possible indeed that, as our knowledge advances and new discoveries take place, we shall so far unite many of the consecutive nuclei which are now considered pretty clearly defined, that we shall be driven at last _either_ to accept the linnæan genera only, or else the entire host of subsidiary ones (albeit perhaps in a secondary sense) which are, one by one, being expunged. and, since under the former contingency the _determination of species_ would become practically well nigh hopeless, it is far from unlikely that we shall eventually hail the latter as, after all (at any rate to a certain extent), the more convenient of the two. look, for instance, at the great genus _pterostichus_, which has nearly representatives in europe alone: true it is that its several sections (_poe cilus_, _argutor_, _omaseus_, _corax_, _steropus_, _platysma_, _cophosus_, _pterostichus_ proper, _abax_, _percus_, and _molops_), although easily recognized in the mass, do unquestionably blend into each other; yet i believe that it has arisen from a too rigid promulgation of the generic theory that they have not been retained as separate. and this opinion may be rendered somewhat more plausible, from the knowledge that certain of the _pterostichi_ (the argutors, for instance) approach so closely, in their trophi, to _calathus_, as to be hardly discernible from it; which latter genus is scarcely distinguishable (structurally) from _pristonychus_,--a form which, in its turn, leads us on towards another type. who would have imagined, again, some fifty years ago, that the widely distributed groups, _calosoma_ and _carabus_, were not thoroughly detached _inter se_? yet what naturalist _now_ can draw an exact line of demarcation between them? and so it is with numerous others, which it is needless to recall. the practical inference, however, from the whole, is this: _that if genera must be rejected because they are not homogeneous and isolated throughout, the only ones that will remain are those which have become abrupt from causes which are merely accidental_. having now, however, examined the question in its broadest phasis, that is to say, on the supposition that nature is _complete_ in her several links and parts; i shall perhaps be expected to offer a few passing words on what i have already hinted at,--namely, the possibility of genera being absolutely well-defined, even on their outer limits, _from accident_. briefly, then, it is through the extinction of species that groups may, in some instances, be abruptly expressed: but, as such contingences are at all times liable (whether from natural or artificial causes) to happen; it would be unfair to build up our generic _definition_ from examples which are the exception, and not the rule,--and, _more_ than mere "exceptions" (as commonly understood by that term), the result of positive disturbances from without. yet, that genera thus distinctly bounded, at either end, do actually occur, must be self-evident to any one who has attempted to study the distribution of organic beings with reference to the geological changes which have taken place on the earth's surface; for it is clear that a vast proportion of the creatures which inhabit our globe came into existence at periods _anterior_ to many of those great convulsions which altered finally the positions of sea and land, apportioning to each the areas which they now embrace: so that, if _generic provinces_ of radiation (no less than specific centres) be more than a fancy or romance, it is certain that numerous members of many geographical assemblages must have perished for ever during the gigantic sinkings which have at various epochs been brought about. from which it follows, _that those groups, or clusters, of which but few representatives (comparatively) are extant, will be more or less abruptly terminated, according as the original type to which they severally belong was peculiar, and in proportion as the number of its exponents has been reduced_. although there are many means through which species may become annihilated, yet, since the subsidence of a tract into the sea involves the maximum of loss which a space of that magnitude can sustain, the above conclusion gives rise to a corollary: _that it is in islands that we should mainly look for genera which are to be rigidly pronounced_. the question therefore naturally suggests itself,--is this in harmony with what we see; or, in other words, is it consistent with experience, or not? i believe that it is; for i think it will be found, on inquiry, _that the greater proportion of those groups which are more especially isolated in their character_ (i do not say, necessarily, the most anomalous; though this in some measure follows from the fact of their detachment) _are peculiar to countries which are insular_. but, however important an element, in the eradication of species, submergence may be; we must not entirely omit to notice other methods also, through the medium of which genera may become well-defined. we should recollect that the removal of a _very few_ links from an endemic cluster is sufficient to cause its disjunction from the type to which it is next akin, and that where the creatures which unite in composing it are of slow diffusive powers, or sedentary habits, the elimination of such links is (through the smallness of the areas which have been overspread) a comparatively easy operation. the accidental introduction of organic beings amongst others to the interests of which they are hostile, may be a powerful means, as mr. darwin has suggested, of keeping the latter in check, and of finally destroying them[ ]. the gradual upheaval of a tract which has been well-stored with specific centres of radiation, created expressly for itself, may (through the climatal changes which have been brought about) succeed in extirpating races innumerable,--those only surviving which are able to adapt themselves to the altered conditions; and which would _now_ be consequently looked upon as abrupt topographical assemblages. the over-whelming effect of a volcanic eruption, in a region where the aborigines of the soil have not wandered far from their primæval haunts, may, as sir charles lyell has well remarked, put an end to others, and so effect the separation of their allies from the central stock. and, lastly, the intervention of man, with all the various concomitants which civilization, art, and agriculture bring in his train, is the most irresistible of every agency in the extensive (though often accidental) demolition of a greater or less proportion of the animate tribes. the whole of these ultimate assortments, however, are dependent, as it were, for their outline, upon contingency or chance; and we must not deduce our ideas of genera from the examples which _they_ supply. we should rather reflect, that it is no matter of mere speculation, that many organic links, now absent, have, through the crises and occurrences to which we have just drawn attention, become lost. on the contrary, indeed, we know that, in the common course of things, it _must_ have been so; and therefore we are induced to regard those cases as exceptional, and as in no way expository of nature's universal scheme. the more we look into the question, whether by the light of analogy or the evidence of facts, the more are we convinced that lines of rigid demarcation (either between genera or species, though especially the former) do not anywhere, except through accident, exist. and hence it is that we ascend, by degrees, to a comprehension of that _unity_ at which i have already glanced; and are led to believe that, could the entire living panorama, in all its magnificence and breadth, be spread out before our eyes, with its long-lost links (of the past and present epochs) replaced, it would be found, from first to last, to be complete and continuous throughout,--a very marvel of perfection, the work of a master's hand. footnotes: [ ] "nullo modo fieri potest, ut axiomata per argumentationem constituta ad inventionem novorum operum valeant; quia subtilitas naturæ subtilitatem argumentandi multis partibus superat. sed axiomata a particularibus rite et ordine abstracta, nova particularia rursus facile indicant et designant; itaque scientias reddunt activas."--_novum organum_, aphoris. xxiv. [ ] in selecting this simple method to illustrate the _principle_ of a binomial system of nomenclature, it is scarcely necessary to remind the reader that i do not intend to imply that every man is _specifically distinct_ from his neighbour! [ ] considérations sur un nouveau système de nomenclature, par c. j. b. amyot (_rev. zool._, p. , a.d. ). [ ] i may add, that this suggestion, as to the evenly balanced state of generic types, is in accordance with the views of mr. waterhouse,--whose extensive knowledge in the higher departments of zoological science gives a value to his opinion, especially on questions such as these, which i am glad to have an opportunity of acknowledging. [ ] annals of nat. hist. ( nd series), xiv., p. . [ ] a familiar example of this disappearance of a creature before the aggressive powers of another, which is either hostile to or stronger than itself, is presented by the black rat (_mus rattus_) of our own country,--which is said to have been extremely abundant formerly, but which is now replaced by the common brown (or "hanoverian") one of northern europe. the british species, however, although it has become extremely scarce, is not yet _quite_ exterminated: it has been recorded (_vide_ 'zoologist,' ) in essex, and in devonshire ('zoologist,' ); and it still swarms on a small rock off lundy island, in the bristol channel. it is reported, moreover, to have been lately re-introduced at liverpool. chapter vii. conclusion depositâ sarcinâ, levior volabo ad coe lum.--_s. jerome._ having now completed the short task which i had undertaken to perform, i will, in conclusion, offer a few brief comments on the results at which we have arrived, and endeavour to realize to what extent the consideration of them is likely to be found useful, during our inquiries into the general subject of entomological geography. commencing with the thesis, that specific variation, whether as a matter of experience or as probable from analogy, does _ipso facto_ exist; i have endeavoured to maintain that position, by evidence of divers kinds; and i have sought to strengthen the inferences deduced, by an appeal to some of those external agents and circumstances which may be reasonably presumed (if not indeed actually demonstrated) to have had a considerable share in bringing it about. i have also suggested what the principal organs and characters are, in the insecta, which would appear to be more peculiarly sensitive to the action of local influences; and i have then diverged to the question of topographical distribution, in connection with the geological changes on the earth's surface; and, lastly, to some practical hints arising out of a proper interpretation of the generic theory. how far i have succeeded in elucidating the several points which i proposed to examine, is a problem which must be solved by others; meanwhile, if i have failed at times to interpret what seems scarcely to admit of positive proof, i shall at least have had the advantage of propounding the enigmas for discussion, and of so paving the way for future research. we must remember, however, that, where certainty is not to be had, probability must be accepted in its stead; or, as an old writer has well expressed it: "that we ought to follow probability when certainty leaves us, is plain,--because it then becomes the only light and guide that we have. for, unless it is better to wander and fluctuate in _absolute_ uncertainty than to follow such a guide; unless it be reasonable to put out our candle because we have not the light of the sun, it _must_ be reasonable to direct our steps by probability, when we have nothing clearer to walk by".[ ] what my chief aim in the present treatise has been, will be easily perceived,--namely, to substantiate, as such, those _elements of disturbance_ (on the outward contour of the annulose tribes) with which the physical world does everywhere abound: and, thereupon, to provoke the inquiry, whether entomologists, as a mass, have usually taken them into sufficient account, when describing as "species," from distant quarters of the globe, insects which recede in only minute particulars from their ordinary states. my own impression is, that they have not done so; and, moreover, that, if they had, our catalogues would have worn a very different appearance to what they now do: for, when once the subject is fairly looked into and analysed, it is impossible not to be convinced, that the _primâ-facie_ aspect of these creatures is eminently beneath the control of the several conditions to which they have been long exposed. but let me not be misunderstood in the conclusion which i have been thus compelled to endorse, or be supposed to ignore the fact that truly _representative species_ may frequently occur in countries far removed from each other; which cannot therefore be regarded as modifications of a common type. i believe, however, that this doctrine of _representation_, whatever truth it may contain, has been too much relied upon; and that we have been over-ready to take advantage of it (unproved as it is) for the multiplication of our, so called, "specific novelties." i suspect, indeed, that _actual_ representative species (if they may be thus expressed) are more often to be recognized on the isolated portions of a formerly continuous tract, than in regions which have been widely separated since the last creative epoch; and that, in the instances where beings of a _nearly_ identical aspect are detected in opposite divisions of the earth, it is more often the case that members of them have been transported at a remote period (either by natural or artificial means) from their primæval haunts, and have become gradually altered by the circumstances amongst which they have been placed, than that the respective phases were produced _in situ_ on patterns almost coincident. i have before announced my conviction, that _generic areas_ have a real existence in nature's scheme; and that, consequently, where species which are so intimately allied that they can with difficulty be distinguished, prevail, there is presumptive reason to suspect (until at least the contrary is rendered probable) that the areas which they now colonize were once connected by an intervening land,--or, in other words, that the migrations of the latter were brought about, through ordinary diffusive powers, from specific centres within a moderate distance of each other. i say "_presumptive_ reason," because there are undoubted exceptions to this law (as to every other), and it can therefore be only judged of on a broad scale. still, i contend that in a wide sense it holds good; and that, consequently, if closely related "species" are traceable in countries which geology demonstrates to have been far asunder during the _entire_ interval since the first appearance of the present animals and plants upon our earth, there is at any rate an _à priori_ probability that they are no _species_ at all,--but permanent geographical states, which have been slowly matured since their casual introduction beyond their legitimate bounds. if we except those forms which are in reality but modifications, from climatal and other causes (and which have, therefore, been wrongly quoted as distinct); i believe that a vast proportion of the species which have been usually considered to be "representative" ones, were members, in the first instance, of the self-same assemblages,--which had wandered to a distance from their primæval haunts, and were afterwards, through the submergence of the intervening land, cut off from their allies. i have adduced, in a preceding chapter, some remarkable examples in illustration of this hypothesis,--an hypothesis which i believe to be the true clue to a very large item of the "specific representation" theory. a considerable number of the madeiran _helices_ may be cited (which i have already done[ ]) as, in the strictest sense, representative of each other,--and as therefore specifically distinct: and i may add, that it is to island groups that we must mainly look for this system in its full development. but, apart from the fact that i would not wish to resign _in toto_ the doctrine of "specific representation," even as frequently understood (that is to say, as recognizable in countries which have been altogether disconnected since the last creative epoch), and therefore, _à fortiori_, in what i conceive to be its truer meaning; there is yet another point on which i would desire to be interpreted aright, whilst endeavouring to substantiate the action of local influences on the members of the insect world. it has been my aim, in the preceding pages, to call attention to the importance of external circumstances and conditions in regulating, within definite limits, the outward aspect of the articulate tribes. i do not, however, assert that _every_ species is liable to be interfered with _ab extra_; that is a question which the greater or less susceptibility of the several races, as originally constituted, can alone decide; still less would i willingly lend a helping hand to that most mischievous of dogmas, that they are _all_-important in their operation,--or, in other words, that they possess within themselves the inherent power (though it may not invariably be exercised) of shaping out (provided a sufficient time be granted them, and in conjunction with the advancing requirements of the creatures themselves) those permanent organic states to which the name of species (in a true sense) is now applied. such a doctrine is in reality nothing more than the transmutation theory, in all its unvarnished fulness; and i do not see how it can be for a moment maintained, so long as facts (and not reasoning only) are to be the basis of our speculations. i repeat, that it is merely _within fixed specific bounds_ that i would advocate a freedom of development, in obedience to influences from without: only i would widen those limits to a much greater extent than has been ordinarily done,--so as to let in the controlling principle of physical agents, as a significant adjunct for our contemplation. it does indeed appear strange that naturalists, who have combined great synthetic qualities with a profound knowledge of minutiæ and detail, should ever have upheld so monstrous a doctrine as that of the transmission of one species into another,--a doctrine, however, which arises almost spontaneously,--if we are to assume that there exists in every race the tendency to _an unlimited progressive improvement_. there are certainly no observations on record which would, in the smallest degree, countenance such an hypothesis. many animals and plants, it is true, are capable of considerable modifications and changes, for the better,--very much more than is the case with others. but what does this prove, except that their capacity for advancement has a slightly wider compass than that of their allies? it touches not the fact, that the boundaries of their respective ranges are absolutely and critically defined. it is moreover a singular phænomenon, and one in which the strongest proofs of design (or a primary adjustment of limits with a view to the future) may be discerned, that the members of the organic creation which display the greatest adaptive powers, are those which were apparently destined to become peculiarly attendant upon man. "the best-authenticated examples," says sir charles lyell, "of the extent to which species can be made to vary may be looked for in the history of domesticated animals and cultivated plants. it usually happens that those species which have the greatest pliability of organization, those which are most capable of accommodating themselves to a great variety of new circumstances, are most serviceable to man. these only can be carried by him into different climates, and can have their properties or instincts variously diversified by differences of nourishment and habits. if the resources of a species be so limited, and its habits and faculties be of such a confined and local character, that it can only flourish in a few particular spots, it can rarely be of great utility. we may consider, therefore, that in the domestication of animals and the cultivation of plants, mankind have first selected those species which have the most flexible frames and constitutions, and have then been engaged for ages in conducting a series of experiments, with much patience and at great cost, to ascertain what may be the greatest possible deviation from a common type which can be elicited in these extreme cases[ ]." the fact, however, that all areas of aberration (however large they may be) are positively circumscribed, need scarcely be appealed to, in exposing the absurdity of the transmutation hypothesis. the whole theory is full of inconsistencies from beginning to end; and from whatever point we view it, it is equally unsound. how, for instance, can any amount of local influences, or the progressive requirements of the creatures themselves, give rise to the appearance of several well-marked representatives of a genus on the self-same spot,--where the physical conditions for each of them are absolutely the same? look, for example, at the _tarphii_ (to which i have already alluded[ ]) of madeira: i have detected about eighteen abundantly defined species; and, as stated in a previous chapter, i have but little doubt, from their sedentary habits, and the evident manner in which they are adjusted to the peculiarities of the region in which they obtain, that they are strictly an esoteric assemblage, inhabiting the actual sites (or nearly so) of their original _début_ upon this earth. here, then, we have a sufficient length of time for developments to have taken place; they are all exposed to the self-same agencies from without (for they live principally in communion); yet, though i have examined carefully more than a thousand specimens (a large proportion of them beneath the microscope), i have never discovered a single intermediate link which could be regarded as in a transition state between any of the remainder. but how is this?--is it possible to account for differences so decided, yet each of such amazing constancy, amongst the several creatures of a central type which have been exposed to identical conditions through, at any rate, generations innumerable? they clearly cannot be explained on the doctrine of transmutation: yet they are no exceptions to the ordinary rule,--occupying an analogous position to the members of every other endemic group. but i will not occupy more space on the transmutation theory: suffice it to have shown that, in thus conceding a legitimate power of self-adaptation, in accordance with external circumstances, to the members of the insect world; and in suggesting the inquiry, whether the action of physical influences has been adequately allowed for by entomologists generally (or, in other words, whether the small shades of difference which have often, because permanent, been at once regarded as specific, may not be _sometimes_ rendered intelligible by a knowledge of the localities in which the creatures have been matured), i do not necessarily open the door to the disciples of lamarck, or infringe upon the strict orthodoxy of our zoological creed. on the contrary, indeed, i believe that the actual reverse is nearer the truth; and, moreover, that those very hyper-accurate definers who recognize a "species" wheresoever the minutest decrepancy is shadowed forth, will be found eventually (however unaware of it themselves) to have been the most determined abettors of that dogma,--seeing that their species, if such they be, do most assuredly pass into each other. we must not, however, omit to notice, briefly, how this perversion of nature's economy took its rise. it was from the desire, which is almost inherent within us, to account for everything by physical laws; and to dispense with that constant intervention of the direct creative act which the successive races of animals and plants, such as are proved by geology to have made their appearance at distinct epochs upon this earth, would seem to require. or, which amounts to the same thing, it resulted through an endeavour to explain by material processes what is placed beyond their reach. but, if this be the case, it may be reasonably asked,--are material laws then not to be inquired into, and should the various influences which operate in the organic world around us be debarred from analysis? unquestionably not. truth is truth, under whatever aspect it may come; and cannot possibly contradict another truth. to exercise our intellectual faculties, by tracing out, through slow, inductive methods, the _modus operandi_ of even a single natural law, is an honourable task; nor should the apparent smallness of the media which we are at times compelled to employ, render it less so (else would this present treatise, like many others of a kindred stamp, have been best unwritten): but it is from the conceit that our own imperfect interpretations have left nothing more to be found out, that the great danger is to be anticipated. an effect may be literally dependent upon a certain proximate cause; and if we be so fortunate as to ascertain that cause, we have done something; but it does not necessarily follow that we have done _much_. on the contrary, it often happens that, in so doing, we have achieved wonderfully little,--seeing that the problem may be self-evident. behind that "cause," we should recollect, others lie concealed, of a far deeper nature, each depending upon the next in succession to it; until, in the order of causation, we are at length led back, step by step, to the final one,--with which alone the mind can be thoroughly content. "we make discovery after discovery," says dr. whewell, "in the various regions of science; each, it may be, satisfactory, and in itself complete, but none final. something always remains undone. the last question answered, the answer suggests still another question. the strain of music from the lyre of science flows on, rich and sweet, full and harmonious; but never reaches a close: no cadence is heard with which the intellectual ear can feel satisfied[ ]." as regards that most obscure of questions, _what the limits of species really are_, observation alone can decide the point. it frequently happens indeed that even observation itself is insufficient to render the lines of demarcation intelligible,--therefore, how much more mere dialectics! to attempt to argue such a subject on abstract principles, would be simply absurd; for, as lord bacon has remarked, "the subtilty of nature far exceeds the subtilty of reasoning:" but if, by a careful collation of _facts_, and the sifting of minute particulars gathered from without, the problem be fairly and deliberately surveyed, the various disturbing elements which the creatures have been severally exposed to having been duly taken into account, the boundaries will not often be difficult to define. albeit, we must except those races of animals and plants which, through a long course of centuries, have become modified by man,--the starting-points of which will perhaps continue to the last shrouded in mystery and doubt. it would be scarcely consistent indeed to weigh tribes which have been thus unnaturally tampered with by the same standard of evidence as we require for those which have remained for ever untouched and free,--especially so, since (as we have already observed) it does absolutely appear, that those species, the external aspects of which have been thus artificially controlled, are by constitution more tractile (and possess, therefore, more decided powers for aberration) than the rest. whether traces of design may be recognized in this circumstance, or whether those forms were originally selected by man _on account_ of their pliability, it is not for me to conjecture; nevertheless, the first of these inferences is the one which i should, myself, be _à priori_ inclined to subscribe to. in examining, however, this enigma, _of the limits within which variation is_ (as such) _to be recognized_; it should never be forgotten, that it is possible for those boundaries to be absolutely and critically marked out even where we are not able to discern them: so that the difficulty which a few domesticated creatures of a singularly flexible organization present, should not unnecessarily predispose us to dispute the question in its larger and more general bearings. nor should we be unmindful that (as sir charles lyell has aptly suggested) "some mere varieties present greater differences, _inter se_, than do many individuals of distinct species;" for it is a truth of considerable importance, and one which may help us out of many an apparent dilemma. but, whatever be the several ranges within which the members of the organic creation are free to vary; we are positively certain that, _unless the definition of a species, as involving relationship, be more than a delusion or romance_, their circumferences are of necessity real, and must be indicated _somewhere_,--as strictly, moreover, and rigidly, as it is possible for anything in nature to be chalked out. the whole problem, in that case, does in effect resolve itself to this,--where, and how, are the lines of demarcation to be drawn? no amount of inconstancy, provided its limits be fixed, is irreconcilable with the doctrine of specific similitudes. like the ever-shifting curves which the white foam of the untiring tide describes upon the shore, races may ebb and flow; but they have their boundaries, in either direction, beyond which they can never pass. and thus in every species we may detect, to a greater or less extent, the emblem of instability and permanence combined: although perceived, when inquired into, to be fickle and fluctuating in their component parts, in their general outline they remain steadfast and unaltered, as of old,-- "still changing, yet unchanged; still doom'd to feel _endless mutation, in perpetual rest_." footnotes: [ ] religion of nature delineated, p. . [ ] vide _supra_, p. . [ ] principles of geology, th edition, pp. , . [ ] vide _supra_, p. . [ ] indications of the creator (london, ), p. . index. aberration, perhaps indicated universally, , , . aborigines, insect, unimportant for climatal modifications, , , . _acalles_, the canarian type of, apparent on the salvages and dezertas, . _---- neptunus_, woll., perhaps a state of _a. argillosus_, . _achatina eulima_, lowe, its extinction in porto santo, . _achenium hartungii_, heer, a form of _a. depressum_, . _acherontia atropos_, linn., its introduction into madeira perhaps recent, . _adimonia_, the capture of, out at sea, . _aëpus marinus_, ström., pallid hue of, . _---- robinii_, lab., pallid hue of, . _agabus bipustulatus_, linn., unaffected by climate, . alligators, their peculiarity to s. america, . alpine species, some peculiarly so, . altitude and latitude, sometimes reciprocal, , . _amyeterus_, its concentration in australia, . amyot, m., his 'méthode mononomique,' . analogies, lord bacon on the importance of, ; why necessary to be studied, . analogy, argument from, , , . _anchomenus marginatus_, linn., slightly modified in madeira, . andes, dissimilarity of the fauna on the opposite sides of the, . _anobium striatum_, oliv., unaffected by climate, . antennæ, joints of, said occasionally to vary, . _anthicus bimaculatus_, illig., variability of, near the sea, . _---- fenestratus_, schmidt, slightly modified in madeira, . _---- humilis_, germ., variability of in salt places, . _---- instabilis_, hoffm., pallid hue of, . _anthonomus ater_, mshm, very small in lundy island, , . _aphelocheirus æstivalis_, fabr., the hemelytra of, sometimes fully developed, . _aphodius nitidulus_, fabr., paler in madeira than in europe generally, . _aphodius plagiatus_, linn., usually black in england, ; two distinct states of, indicated, . _apocyrtus_, its concentration in the philippine islands, . _apotomus_, common to madeira and sicily, . _argutor_, always apterous in madeira, ; trophi of, almost identical with those of _calathus_, . armadillos, their peculiarity to s. america, . armitage, mr., on _cicindela fasciatopunctata_ from mount olympus, . arrangement, a lineal one is not indicated in nature, . atlantic continent, prof. e. forbes on the former existence of, . atlantis of the ancients, the impossibility of its being identified with a former atlantic region, ; perhaps the new world, . _atlantis_, the genus, a modification of _laparocerus_, . azores, the colonization of, by two madeiran _helices_, . bacon, lord, on the importance of analogies, ; on the atlantis of the ancients, ; on the necessity of observation for forming science, . banksias, their concentration in australia, . barriers, natural, the difference between primary and recent, ; their hindrance to insect diffusion, . _bembidium atlanticum_, woll., paler in porto santo than in madeira, ; the variations to which it is subject, , . _---- bistriatum_, dufts., paler in saline districts, . _---- ephippium_, mshm, pallid hue of, . _---- obtusum_, sturm, varies in southern latitudes, . _---- pallidipenne_, illig., pallid hue of, . _---- saxatile_, gyll., variety of, on the south coast of england, . _---- schmidtii_, woll., perhaps a state of _b. callosum_, . _---- scutellare_, germ., pallid hue of, . _---- tabellatum_, woll., perhaps a state of _b. tibiale_, . _berginus_, common to madeira and sicily, . black rat, nearly exterminated in england, . _blemus areolatus_, creutz., paler in brackish places, . _bolitochara assimilis_, kby, smallness of, in the scilly islands, . _boromorphus_, common to madeira and sicily, . _brachinus crepitans_, linn., two distinct sizes of, frequently indicated, . _bradycellus fulvus_, mshm, apterous in madeira, . bread-fruit trees, their peculiarity to the south sea islands, . _calathus_, apterous in madeira, ; its trophi almost identical with those of _pristonychus_, . _---- complanatus_, koll., varies from altitude, ; variety of, on one of the madeira islands, . _---- fuscus_, fabr., slightly modified in madeira, , . _calathus melanocephalus_, linn., smallness of, in the scilly islands, . _---- mollis_, mshm, variable in its wings, ; lurid colour of, . calcareous soils, effect of, on the aspect of insects, . calceolarias, their concentration on the andes, . _calosoma_, a species of, ten miles from shore, ; the genus, mergescgradually into _carabus_, . _---- syncophanta_, linn., its power of crossing the sea, . canary islands, migratory direction of their insect population, . _carabidæ_, inconstant in their organs of flight, ; family of, nearly similar throughout in its oral organs, . _carpophilus hemipterus_, linn., unaffected by climate, . _caulotrupis conicollis_, woll., large size of, on one of the madeira islands, , . _---- lucifugus_, woll., varies from isolation, , . causes, never final ones which we investigate, . _centrinus_, its concentration in s. america, . _ceutorhynchus contractus_, mshm, smallness of, in lundy island, , . _cholovocera_, common to madeira and sicily, . _choreius ineptus_, westw., on a winged state of, . _chorosoma miriforme_, the development of the wings of, . _chrysomela_, apterous in madeira, . _chrysomelæ_, vary from altitude, . _chrysomelidæ_, almost absent in tierra del fuego, . _cicindela fasciatopunctata_, germ., a state of _c. sylvatica_ . _cicindelidæ_, often variable, . _cillenum laterale_, sam., lurid hue of, . _cimex apterus_, linn., the development of the wings of, . _---- lectularius_, linn., on the development of the wings of, . _cistela sulphurea_, linn., its variability near the sea, . _clausilia deltostoma_, lowe, a porto-santan form of, . climatal modifications significant, although small, . climate, not important as a disturbing cause, , , , , . clouded-yellow butterfly, unaffected by climate, . _clypeaster pusillus_, gyll., differs slightly in madeira, . coast, inconstancy of insects in the vicinity of the, . _coccinella -punctata_, linn., unaffected by climate, . _colias edusa_, fabr., unaffected by climate, . colour, its inconstancy in insects found near the sea, , . ---- of insects, affected by isolation, . _colymbetes_, a species of, captured forty-five miles from shore, , . compensation, generally apparent when an insect is deprived of an organ or sense, . _coranus subapterus_, curt., the development of the wings of, . cordillera, mr. darwin on the fauna of the, . _corylophus_, apterous in madeira, . _criomorphus_, curtis, referable to the genus _delphax_, . _cyclostoma lucidum_, lowe, its extinction in porto santo, . _cynthia cardui_, linn., unaffected by climate, . _cynucus_, a species of, seventeen miles from shore, . _cyrtonota_, its concentration in s. america, . darwin, mr., on the fauna of the galapagos, ; relative proportions of the insect tribes in the tropics, , ; on the insects of tierra del fuego, ; on the natural features of tierra del fuego, ; on the insects of keeling island, ; on the insects of st. helena, ; on the insects of ascension, ; on the apterous condition of insular species, ; on the fauna of the cordillera, ; on a _calosoma_ captured at sea, ; on insects captured in the sea, , ; on the disappearance of animals before more powerful ones than themselves, . dawson, rev. j. f., on a variety of _bembidium saxatile_, . definition of the term 'species,' ; of the term 'variety,' . _delphax_, on the development of the wings of, . _dermestes vulpinus_, fabr., unaffected by climate, . _deucalion_, its occurrence on the salvages and dezertas, . _---- desertarum_, woll., its sedentary nature, , , . _dichelus_, its concentration in s. africa, . differences, when to be regarded as specific, ; too exclusively studied, . diffusion, various means of, which operate on the insect tribes, . disturbing agents, prof. henfrey on, . _ditylus_, the same type of, indicated in the canaries and salvages, . domesticated animals, pliable nature of, , . _dromius arenicola_, woll., representative of _d. obscuroguttatus_, . _---- fasciatus_, gyll., its paleness near the sea, . _---- negrita_, woll., perhaps an ultimate state of _d. glabratus_, . _---- obscuroguttatus_, dufts., its changes in madeira, , , ; apterous in madeira, . _---- sigma_, rossi, its colour affected by isolation, , . elevation, sometimes corresponds with latitude, , . _ellipsodes glabratus_, fabr., singular variety of, on one of the madeira islands, , . elytra, connateness of, a variable character, . 'endemic,' to what species the term is applicable, . entomology, the study of, does not necessarily cramp the mind, . _ephistemus_, apterous in madeira, . _eucalypti_, their concentration in australia, . _eunectes sticticus_, linn., unaffected by climate, . euphorbias, their concentration in southern africa, . _eurygnathus latreillei_, lap., variety of, on one of the madeira islands, , . exceptions, not be allowed to negative a law, , . extinction of species, as indicated in the madeiran _helices_, ; the only cause by which genera may be abruptly defined, . forbes, prof. e., on the origin of the british animals and plants, ; his epochs of migration of the british animals and plants, ; on the existence of a former atlantic continent, . forests, the hindrance which they offer to insect-diffusion, . "fortunate islands" of the ancients, probably the canarian group, . galapagos, fauna of, . genera, the nature of, often misunderstood, ; a familiar explanation of, , , ; cannot be abrupt except from accident, ; how to be defined, ; the types of, usually situated towards the centres of the several groups, ; the types of, usually evenly balanced in their structural characters, , ; may be abruptly defined from accidental causes, , . generic areas, an important feature throughout nature, , , . geology, a necessary item in the study of insect-diffusion, . germanic plains, the, probably a primary area of diffusion, . _gerris_, on the development of the wings of, . gould, mr., on the swallows of malta, . _gymnaëtron_, blood-red dashes characteristic of, . _---- campanulæ_, linn., its smallness on the cornish coast, . _---- veronicæ_, germ., a variety of _g. niger_, . _hadrus illotus_, woll., perhaps a form of _h. cinerascens_, . _haliplus obliquus_, gyll., dark state of, in ireland, . _haltica exoleta_, fabr., its variability on the coast, . harcourt, mr., on the discovery of madeira, , . _harpalus vividus_, dej., changes to which it is subject, , , ; variable in the connateness of its elytra, , . _hegeter_, its maximum attained in the canaries, . _---- elongatus_, oliv., its migration from the canaries, ; of a more adaptive nature than its allies, . _---- latebricola_, woll., its occurrence in the salvages, . _helices_, have often two distinct states, ; many of them representative in the madeira islands, , ; those in the madeiras chiefly of slow migratory powers, , . _helix attrita_, lowe, its local character, . _---- bowdichiana_, fér., perhaps a gigantic state of _h. punctulata_, . _---- calculus_, lowe, sedentary nature of, . _helix commixta_, lowe, sedentary nature of, . _---- coronata_, desh., its peculiarity to porto santo, ; its occurrence beneath the surface of the ground, . _---- coronula_, lowe, its peculiarity to the southern dezerta, . _---- delphinula_, lowe, the madeiran representative of _h. tectiformis_ in porto santo, . _---- discina_, lowe, a form of _h. polymorpha_, . _---- erubescens_, lowe, its powers of diffusion greater than those of its allies, ; sensitive to external influences, . _---- fluctuosa_, lowe, its extinction in porto santo, . _---- hirsuta_, say, two distinct states of, . _---- lapicida_, linn., its extinction in porto santo, . _---- latens_, lowe, the madeiran representative of _h. obtecta_ in porto santo, . _---- lincta_, lowe, the common madeiran form of _h. polymorpha_, . _---- lowei_, pfr., perhaps a gigantic state of _h. portosanctana_, . _---- papilio_, lowe, a form of _h. polymorpha_, . _---- paupercula_, lowe, its powers of diffusion greater than those of its allies, . _---- polymorpha_, lowe, sensitive to external influences, and of great diffusive powers, . _---- portosanctana_, sow., its peculiarity to porto santo, . _---- pulvinata_, lowe, a form of _h. polymorpha_, . _---- saccharata_, lowe, a local state of _h. polymorpha_, . _---- senilis_, lowe, the dezertan form of _h. polymorpha_, . _---- squalida_, lowe, the madeiran representative of _h. depauperata_ in porto santo, . _---- tiarella_, webb, its sedentary nature, . _---- undata_, lowe, its peculiarity to madeira proper, . _---- vulcania_, lowe, its peculiarity to the dezertas, . _---- wollastoni_, lowe, sedentary nature of, . _helobia nivalis_, payk., perhaps a state of _h. brevicollis_, . _helops_, always apterous in madeira, . _---- confertus_, woll., varies from altitude, . _---- futilis_, woll., varies from isolation, . _---- testaceus_, küst., pallid hue of, . _---- vulcanus_, woll., large size of, on one of the madeira islands, . henfrey, prof., on disturbing agents, . herschel, sir john, on the requisites for an observer, . _hipparchia semele_, linn., has a distinct aspect in madeira, . _hipporhinus_, its concentration in s. africa, . holme, mr., on _olisthopus rotundatus_ in the scilly islands, , ; on a winged state of _phosphuga atrata_, . _holoparamecus_, common to madeira and sicily, . _---- niger_, aubé, different in madeira and sicily, . hooker, dr., on the insects of kerguelen's land, . humboldt, his notice of sphinxes and flies high up on the andes, . humming-birds, their peculiarity to s. america and the w. indies, . _hydrobius_, apterous in madeira, ; the capture of, out at sea, . _hydrometridæ_, on the development of the wings of, . _hydroporus_, the capture of, out at sea, . _---- confluens_, fabr., unaffected by climate, . _hypsonotus_, its concentration in s. america, . influence of climate not important, . insect-aberration, perhaps a universal fact, , , . _insulæ fortunatæ_ of juba, probably the canarian group, . ireland, poverty of the fauna of, , ; the south-west of, has something in common with madeira, . islands, faunas of, often too greatly magnified, ; the species of, generally more isolated in their structure than those of continents, . isolation, effects of, on insect-stature, . ixias, their concentration in southern africa, . kangaroos, their concentration in australia, . kerguelen's land, insects of, . kirby, rev. w., on insects washed up on the suffolk coast, . _læmophloe us pusillus_, schönh., unaffected by climate, . _lamprias chlorocephalus_, ent. h., two distinct sizes of, frequently indicated, . _laparocerus morio_, schönh., large size of, on one of the madeira islands, . latitude and altitude, sometimes reciprocal, . _leistus montanus_, steph., has been supposed to be equal to _l. fulvibarbis_, . _lemur_, its peculiarity to madagascar, . _litargus_, common to madeira and sicily, . _lixus angustatus_, fabr., unaffected by climate, . localities, some naturally more productive than others, , . _longitarsus_, the native species of, apterous in madeira, . _loricera_, apterous in madeira, . lowe, rev. r. t., his capture of the _deucalion desertarum_, . lundy island, smallness of the insects in, , ; occurrence of the black rat in, . _lycæna phloe as_, linn., darker in madeira than in england, . lyell, sir charles, on _helix hirsuta_, ; on the fossil period of the madeiran _helices_, ; on insects washed up on the shore, ; on the effect of gales in the transportation of insects, ; on the effects of a volcanic eruption in destroying species, ; on the flexible nature of certain animals and plants, ; on the greater differences which varieties often present than do species, . _lygæus brevipennis_, latr., on the development of the wings of, . _macronota_, its peculiarity to java, . madeira, has some features in common with tierra del fuego, , , , ; former state of, , ; great fire on the southern side of, ; origin of the name of, ; the insects of, ; the tendency of its insects to become apterous, ; the migratory direction of its insect population, ; the local nature of its various species, , . magnolias, their concentration in central america, . malta, mr. gould on the birds of, . _malthodes kiesenwetteri_, woll., perhaps a state of _m. brevicollis_, . man, agency of, in the destruction of species, . _mantura chrysanthemi_, ent. h., variability of, in lundy island, . _marsupialia_, their concentration in australia, . mesembryanthemums, their concentration in southern africa, . _mesites_, a modification of _cossonus_, . _---- maderensis_, woll., its near relationship to the _m. tardii_, . _---- tardii_, curtis, its variability near the coast, . 'méthode mononomique,' the unsoundness of, - . migratory powers, slowness of, in the madeiran _helices_, - . ---- progress, direction of, in the madeiran animals, , . mimosas, their concentration in australia, . mollusca, terrestrial, often present two distinct states, . _moluris_, its concentration in s. africa, . _monochelus_, its concentration in s. africa, . mountain-chains, their hindrance to insect-diffusion, . mountain-tops, either very prolific in insect life, or else barren, . _mus rattus_, almost exterminated in england, . _mycetoporus pronus_, erichs., two distinct states of, indicated, . myrtles, their concentration in australia, . naturalist, the, what his province to investigate, . nature, not irregular because presenting occasional anomalies, . _naupactus_, its concentration in s. america, . _nebria complanata_, linn., unusually pale near bordeaux, ; pallid hue of, . new world, some of its insects perhaps but states of those of the old, . nomenclature, a binomial system the only true one, , . _notaphus_, the capture of, out at sea, . _notiophili_, extremely variable, . _notiophilus geminatus_, dej., large size of, on one of the madeira islands, . observation, indispensable in natural science, , , . ocean, the, its hindrance to insect-diffusion, . _ochthebius marinus_, payk., lurid hue of, . _olisthopus_, apterous in madeira, . _---- maderensis_, woll., large state of, on one of the madeira islands, , . _---- rotundatus_, payk., very small in the scilly islands, , ; subapterous in the scilly islands, . _omaseus nigerrimus_, dej., a form of _o. aterrimus_, . _omias waterhousei_, woll., large state of, on one of the madeira islands, , . _oncocephalus griseus_, development of the wings of, . _othius_, apterous in madeira, . ourangs, their peculiarity to the indian islands, . _oxyomus_, a modification of _aphodius_, . _pachymerus brevipennis_, the development of the wings of, . _pachyrhynchus_, its concentration in the philippine islands, . painted-lady butterfly, unaffected by climate, . _papilio machaon_, linn., unaffected by climate, . _paropsis_, its concentration in australia, . patagonia, insects of, distinct from those of tierra del fuego, , . _patrobus septentrionis_, dej., has been supposed to be a state of _p. excavatus_, . _pecteropus_, its maximum attained in the canaries, . _---- maderensis_, woll., varies from altitude, . _---- rostratus_, woll., varies from isolation, . pelargoniums, their concentration in southern africa, . _pelophila borealis_, payk., larger in ireland than in the orkneys, . _phaleria cadaverina_, fabr., pallid hue of, . _philhydrus melanocephalus_, oliv., two states of, frequently indicated, . _phlæophagus_, apterous in madeira, . _phosphuga atrata_, linn., taken with the wings developed, . _---- subrotundata_, leach, the irish form of the _p. atrata_, . _phytophaga_, preponderance of, in the tropics, , . _pieris brassicæ_, linn., varies in nepaul and japan, . _pissodes notatus_, fabr., unaffected by climate, . _platyomus_, its concentration in s. america, . _platyrrhini_, their peculiarity to s. america, . _pogonus luridipennis_, germ., lurid hue of, . _pontia brassicæ_, linn., its introduction into madeira probably recent, . porto santo, origin of the name of, ; a generic area of radiation for certain _helices_, . predacious insects, less numerous in the tropics, , . _prostemma guttula_, fabr., the development of the wings of, , . _psylliodes_, a variable species of, in lundy island, . _---- erythrocephala_, linn., two distinct states of, frequently indicated, . _---- marcida_, illig., pallid hue of, . _---- nigricollis_, mshm, a pale state of the _p. erythrocephala_, . _---- vehemens_, woll., varies from isolation, . _pterostichus_, its various divisions are natural ones, . _ptini_, their stature affected by isolation, ; which characters of, are the most constant, . _ptinus albopictus_, woll., its changes on the islands of the madeiran group, - . _pupa_, often two distinct states of, . _purpurariæ_ of the ancients, probably the madeiran group, . pyrenean region, the, perhaps a primary area of diffusion, . reasoning, not sufficient of itself for the formation of science, . red-admiral butterfly, its introduction into madeira perhaps recent, . _reduviadæ_, on the development of the wings of a representative of the, . representative species, exemplified by the madeiran _helices_, , , ; where frequently to be recognized, . _rhyzopertha pusilla_, fabr., unaffected by climate, . rivers, their power of transporting insects along their course, . saline spots, variation of insects in, . salvages, occurrence of a canarian form on the, , . _saprinus_, a modification of _hister_ proper, . _---- nitidulus_, fabr., two distinct states of, indicated, . _scarabæus_, the capture of, out at sea, . _scarites abbreviatus_, koll., large size of, on one of the madeira islands, ; varies both from isolation and altitude, . sciences, the, should assist rather than oppose each other, , . _scydmænus helferi_, schaum, smaller in madeira than in sicily, . _scymnus_, an apterous species of, in porto santo, . sea, inconstancy of insects in the vicinity of the, . sicily, the fauna of, has much in common with that of madeira, . _silpha atrata_, linn., presents a distinct state in ireland, . _silybum marianum_, grtn., its stalks the food of a _ptinus_, . similitudes, lord bacon on the importance of, . _sitonia gressoria_, illig., perhaps a form of the _s. grisea_, . _sitophilus granarius_, linn., unaffected by climate, . _sitophilus oryzæ_, linn., unaffected by climate, . sloths, their peculiarity to s. america, . species, definition of the term, ; familiar explanation concerning the nature of, , ; limitation of, how to be attempted, ; limits of, real, though often difficult to trace out, ; in a certain sense both unstable and permanent, . specific centres of creation, . _sphinx convolvuli_, linn., its introduction into madeira probably recent, . spinola, on one of the _reduviadæ_, ; on _oncocephalus griseus_, . stapelias, their concentration in southern africa, . states, large and small ones indicated in some insects, . stature of insects, smaller in islands than on continents, . _stenolophus skrimshiranus_, steph., perhaps a state of _s. teutonus_, . _stenus heeri_, woll., two distinct states of, indicated, . structural characters, seldom variable in the insecta, . subsidences, the effect of, on insect life, . swallow-tail butterfly, unaffected by climate, . _syncalypta_, apterous in madeira, . _tachyporus nitidicollis_, steph., perhaps a state of _t. obtusus_, . _tarphii_, their economy in the madeira group, . _tarphius_, its maximum attained in madeira proper, ; common to madeira and sicily, . _---- gibbulus_, germ., the sicilian exponent of the genus, . _---- lowei_, woll., of a more adaptive nature than its allies, . _tarus_, always apterous in madeira, . _---- lineatus_, schönh., assumes a distinct state in madeira, . _telephorus testaceus_, linn., its variability in lundy island, . thompson, mr., on the reptiles of ireland, england, and belgium, . _thorictus_, common to madeira and sicily, . tierra del fuego, insects of, ; has many characters in common with madeira, - . time, an important item in the question of modifications, . toucans, their peculiarity to s. america and the w. indies, . transmutation-theory, unsoundness of the, - ; how it took its rise, . _trechus_, always apterous in madeira, . _---- alticola_, woll., perhaps a state of _t. custos_, . _---- lapidosus_, daws., pallid hue of, . tree-porcupines, their peculiarity to s. america, . _tribolium ferrugineum_, fabr., unaffected by climate, . _trogosita mauritanica_, linn., unaffected by climate, . tropics, exuberance of the, , ; relative proportions of the insect tribes within the, , . _tychius_, always apterous in madeira, . unity, indicated in the organic creation, , . _vanessa atalanta_, linn., has a different aspect in n. america, ; perhaps a recent introduction into madeira, . _---- callirhoë_, fabr., smaller in porto santo than in madeira, . variation in the insecta, a matter of experience, , , ; probable from analogy, ; perhaps indicated in every individual, , , ; restricted, . variety, definition of the term, . _velia_, on the development of the wings of, . waterhouse, mr., his opinion concerning generic types, . westwood, mr., on _papilio machaon_ from the himalayas, ; on american specimens of _lycæna phloe as_, ; on the effect of heat in developing the wings of insects, ; on a winged state of _choreius ineptus_, ; on the development of the wings in _delphax_, ; on a winged state of _cimex lectularius_, ; on _aphelocheirus æstivalis_, ; on the development of the wings of the _hydrometridæ_, ; on _cimex apterus_, ; on _prostemma guttala_ and _coranus subapteras_, ; on the development of the wings of _lygæus brevipennis_, . whewell, dr., on the natural causes which science has to investigate, . white-cabbage butterfly, varies in nepaul and japan, . winds, the effects of, in the diffusion of insects, . wings of insects, subject to undue development in hot seasons, ; liable to become gradually obsolete in islands, ; more variable than other organs, . _xenostrongylus_, its geographical distribution, ; common to madeira and sicily, . _zargus pellucidus_, woll., variety of, on one of the madeira islands, . finis. printed by taylor and francis, red lion court, fleet street. lately published, by the same author, in large to (with thirteen coloured plates), price £ _s._, insecta maderensia; being an account of the insects of the islands of the madeiran group. london: john van voorst, , paternoster row. transcriber's notes: inconsistent/archaic spelling and punctuation left as in original. none biological publications. the primary factors of organic evolution. by _prof. e. d. cope_. cuts, . pp., xvi, . cl., $ . 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( s. d.). the open court pub. co., chicago. * * * * * on germinal selection as a source of definite variation by august weismann translated from the german by thomas j. mccormack * * * * * second edition * * * * * chicago the open court publishing company. london agents: kegan paul, trench, trÜbner & co., ltd. . * * * * * copyright by the open court publishing co. * * * * * { } preface. the present paper was read in the first general meeting of the international congress of zoölogists at leyden on september , . several points, which for reasons of brevity were omitted when the paper was read, have been re-embodied in the text, and an appendix has been added where a number of topics receive fuller treatment than could well be accorded to them in a lecture. the address was first printed in _the monist_ for january, , and afterwards in a german pamphlet. the basal idea of the essay--the existence of germinal selection--was propounded by me some time since,[ ] but it is here for the first time fully set forth and tentatively shown to be the necessary complement of the process of selection. knowing this factor, we remove, it seems to me, the patent contradiction of the assumption that the general fitness of organisms, or the adaptations _necessary_ to their existence, are produced by _accidental_ variations--a contradiction which formed a serious stumbling-block to the theory of selection. though still assuming that the _primary_ variations are "accidental," i yet hope to have demonstrated that an interior mechanism exists which compels them to go on increasing in a definite direction, the moment selection intervenes. _definitely directed { } variation exists_, but not predestined variation, running on independently of the life-conditions of the organism, as naegeli, to mention the most extreme advocate of this doctrine, has assumed; on the contrary, the variation is such as is elicited and controlled by those conditions themselves, though indirectly. in basing my proof of the doctrine of germinal selection on the fundamental conceptions of my theory of heredity, a few words of justification are necessary, owing to the fact that the last-mentioned theory has been widely and severely assailed since its first emergence into light and even repudiated as absolutely futile and erroneous. in the first place, many critics have characterised it as a "pure creation of the imagination." and to a certain extent it is such, as every theory is. but is it on that account necessarily wrong? can not its fundamental ideas still be quite correct, and it itself therefore perfectly justified as a means of further progress? surely my critics cannot be ignorant of the prominent part which imagination has recently played in the exactest of all natural sciences--physics? are they unaware that the english physicist maxwell "constructed from liquid vortices and friction-pulleys enclosed in cells with elastic walls, a wonderful mechanism, which served as a mechanical model for electromagnetism"?[ ] he hoped "that further research in the domain of theoretical electricity would be promoted rather than hindered by such mechanical { } fictions." and so it actually happened, for maxwell found by means of them "the very equations, whose singular and almost incomprehensible power hertz has so beautifully portrayed in his lecture on the relations between light and electricity." "maxwell's formulæ were the direct outcome of his mechanical models." "these ideal mechanisms"--so relates boltzmann in the same interesting essay--"were at first widely ridiculed, but gradually the new ideas worked their way into all fields. they were themselves more convenient than the old hypotheses. for the latter could be maintained only in the event of everything's proceeding smoothly; whereas now little inconsistencies were fraught with no peril, for no one can take amiss a slight hitch in a mere analogy.--ultimately maxwell's ideas were philosophically generalised as the theory that all knowledge consists in the disclosure of analogies." but not only does it seem that there is little appreciation among biologists for the scientific import of imagination, they also appear to have little sense for the significance of theory. it is a favorite attitude nowadays to look upon theory as a sort of superfluous ballast, as a worthless survival from the epoch of decrepit "nature-philosophies." people pronounce with pride the miscomprehended utterance of newton, _hypotheses non fingo_, and place the value of the slightest new fact infinitely higher than that of "the most beautiful theory."[ ] and yet theory originally { } fashions science out of facts and is the indispensable precondition of every important scientific advance. heinrich hertz,[ ] the discoverer of electric undulations, had the same thought in mind when he said: "we form inward representations or constructs of outward objects, so constituted that the results that follow logically and necessarily from the constructs are in turn always constructs of the results flowing naturally and necessarily from the objects." "these constructs or mental images copied after familiar objects possessed of familiar properties, so constituted that from their manipulation effects result similar to those which we observe in the objects to be explained. experience teaches us that the requirements here made can be fulfilled and that consequently such 'correspondences' between reality and the supposed images [or, as hertz says, between nature and mind] actually exist. having succeeded in extracting from the accumulated experience of the past, representative images or constructs fulfilling all these necessary requirements, we can then reproduce by them in a short space of time, as we might by models, results that in the outward world require a long space of time for their actualisation or can be produced only through our personal intervention," etc. { } such representative models, or constructs, now, in my theory of heredity, are the _determinants_, which may be conceived as indefinitely fashioned packages of units (biophores) which are set into activity by definite impressions and put a distinctive stamp upon some small part of the organism, on some cell or group of cells, evoking definite phenomena somewhat as a piece of fireworks when lighted produces a brilliant sun, a shower of sparks, or the glowing characters of a name. the _ids_, also, are such representative models, and may be compared to a definitely ordered but variously compounded aggregate of fireworks, in which the single pieces are so connected as to go off in fixed succession and to produce a definite resultant phenomenon like a complete inscription surrounded by a hail of fire and glowing spheres. owing to the greater complexity of the phenomena in biology we can never hope to reach the same distinctness in our constructs and models as in physics, and the attempt to derive from them mathematical formulæ by the independent development of which research could be continued, would at present be utterly fruitless. in the meantime it seems preferable to have some sort of adequate model to which the imagination can always resort and with which it can easily operate, rather than to have to revert, in considering every special problem of heredity, to the mutual actions of the molecules of living substance and outward agents--processes which we know only in their roughest outlines. or is any one presumptuous enough to believe we can infer from our slight knowledge of the chemical and physical constitution of the germs of a trout and a salmon the real cause { } of the one's becoming a trout and of the other's becoming a salmon? the fact is, we can make no show of accounting for the complex phenomena of heredity with mere _material_ units; we can never reach these phenomena from below, but must begin farther up and make the assumption of _vital_ units and _hereditary_ units, if there is to be any advance in this field. it is undoubtedly a splendid aim which the newly founded science of developmental mechanics has set itself of laying bare the entire causal line leading from the egg to the finished organism; yet, however much we may wish to see the success of this plan realised, we cannot disguise the fact that little or nothing is to be accomplished by it in the settlement of the problems of heredity. it is impossible to suspend the study of heredity until this mechanics is completed, and even if we could it would help us little, for the riddles of heredity are not concealed in the ontogenesis of types, or, to give an example, in the developmental history of man _as a race_, but in the ontogenesis of _individuals_, in that of a _definite and particular_ man. this last ontogenesis exhibits the phenomena of variation, of reversion, of the predominance of the one or the other parent, etc., and no one is likely to believe that inductive evolutional inquiry alone will ever afford us knowledge of these minute and delicate processes, which, in their bearing on the total resultant development, phylogenesis, are after all the most important of all. there is, accordingly, no choice left. if we are really bent on scientifically investigating the question of heredity, we are obliged perforce to form from the observed facts of heredity a highly detailed and { } elaborate theory, on the basis of which we can propound new questions, which will give rise in turn to new facts, and thus will exercise a retroactive influence on the theory, improving and transforming it. this is precisely what i have sought to accomplish by my theory of germ-plasm, as i stated in the preface to the book bearing that name. it was never intended as a theory of life, nor, indeed, primarily, as a theory of evolution, but first and above all as a theory of heredity. i cannot understand, therefore, the animadversion, that my theory in no way furthers our insight into the mechanics of development. that is not its purpose; in fact, it takes the ultimate physical and chemical processes which make up the vital processes for granted; and inevitably it is constrained to do so. its aim is to put into our hands a serviceable formula by means of which we can go on working in the field of heredity at any rate, and, if i am not mistaken, also in that of evolution. to me, at least, the newest results of developmental mechanics do not seem so widely at variance with the theory of determinants as might appear at first sight; so far as i can see, they can be quite readily made to harmonise with the theory, provided only the initial stage of the disintegration of the germ-plasm in the determinant groups be not invariably placed at the beginning of the process of segmentation, but be transferred according to circumstances to a subsequent period. the exact state of things cannot as yet be determined, so long as the mass of facts is still in constant flux. in any event i still hold fast to the hope which i expressed in the preface to my _germ-plasm_, that despite the unavoidable uncertainties in its foundation my theory would yet prove more than a mere work { } of imagination, and that the future would find in it some durable points which would outlive the mutations of opinion. it is possible that one of these durable gains is my much impugned idea of determinants, and in fact not only will the present essay be made to rest on this idea, but it will also defend it on new grounds, although primarily only as a representation of something which we do not as yet exactly know, but which still exists and on which we can reckon, leaving it to the future to decide the greater or less resemblance of our hypothetical construct to nature. the real aim of the present essay is to rehabilitate the principle of selection. if i should succeed in reinstating this principle in its emperilled rights, it would be a source of extreme satisfaction to me; for i am so thoroughly convinced of its indispensability as to believe that its demolition would be synonymous with the renunciation of all inquiry concerning the causal relation of vital phenomena. if we could understand the adaptations of nature, whose number is infinite, only upon the assumption of a teleological principle, then, i think, there would be little inducement to trouble ourselves about the causal connexion of the stages of ontogenesis, for no good reason would exist for excluding teleological principles from this field. their introduction, however, means the ruin of science. august weismann. freiburg, nov. , . * * * * * { } germinal selection. * * * * * numerous and varied are the objections that have been advanced against the theory of selection since it was first enunciated by darwin and wallace--from the unreasoning strictures of richard owen and the acute and thoughtful criticisms of albert wigand and nägeli to the opposition of our own day, which contends that selection cannot create but only reject, and which fails to see that precisely through this rejection its creative efficacy is asserted. the champions of this view are for discovering the motive forces of evolution in the _laws_ that govern organisms--as if the norm according to which an event happens were the event itself, as if the rails which determine the direction of a train could supplant the locomotive. of course, from every form of life there proceeds only a definite, though extremely large, number of tracks, _the possible variations_, whilst between them lie stretches without tracks, _the impossible variations_, on which locomotion is impossible. but the actual travelling of a track is not performed by the track, but by the locomotive, and on the other hand, the choice of a track, the decision whether the destination of the train shall be berlin or paris, is not made by the locomotive, the cause of the variation, but by the driver of the locomotive, who directs the engine on the right track. in the theory of selection the engine-driver is represented by utility, for with utility rests the decision { } as to what particular variational track shall be travelled. the cogency, the irresistible cogency, as i take it, of the principle of selection is precisely its capacity of explaining why fit structures always arise, and that certainly is the great problem of life. not the fact of change, but the _manner_ of the change, whereby all things are maintained capable of life and existence, is the pressing question. it is, therefore, a very remarkable fact, and one deserving of consideration, that to-day ( ), after science has been in possession of this principle for something over thirty years and during this time has steadily and zealously busied itself with its critical elaboration and with the exact determination of its scope, that now the estimation in which it is held should apparently be on the decrease. it would be easy to enumerate a long list of living writers who assign to it a subordinate part only in evolution, or none at all. one of our youngest biologists speaks without ado of the "pretensions of the refuted darwinian theory, so called,"[ ] and one of the oldest and most talented inquirers of our time, a pioneer in the theory of evolution, who, unfortunately, is now gone to his rest, thomas huxley, implicitly yet distinctly intimated a doubt regarding the principle of selection when he said: "even if the darwinian hypothesis were swept away, evolution would still stand where it is." therefore, he, too, regarded it as not impossible that this hypothesis should disappear from among { } the great explanatory principles by which we seek to approach nearer to the secrets of nature. i am not of that opinion. i see in the growth of doubts regarding the principle of selection and in the pronounced and frequently bitter opposition which it encounters, a transient depression only of the wave of opinion, in which every scientific theory must descend after having been exalted, here perhaps with undue swiftness, to the highest pitch of recognition. it is the natural reaction from its overestimation, which is now followed by an equally exaggerated underestimation. the principle of selection was not overrated in the sense of ascribing to it too much explanatory efficacy, or of extending too far its sphere of operation, but in the sense that naturalists imagined that they perfectly understood its ways of working and had a distinct comprehension of its factors, which was not so. on the contrary, the deeper they penetrated into its workings the clearer it appeared that something was lacking, that the action of the principle, though upon the whole clear and representable, yet when carefully looked into encountered numerous difficulties, which were formidable, for the reason that we were unsuccessful in tracing out the actual details of the individual process, and, therefore, in _fixing_ the phenomenon as it actually occurred. we can state in no single case how great a variation must be to have selective value, nor how frequently it must occur to acquire stability. we do not know when and whether a desired useful variation really occurs, nor on what its appearance depends; and we have no means of ascertaining the space of time required for the fulfilment of the selective processes of nature, and hence cannot calculate the exact number of such { } processes that do and can take place at the same time in the same species. yet all this is necessary if we wish to follow out the precise details of a given case. but perhaps the most discouraging circumstance of all is, that in scarcely a single actual instance in nature can we assert whether an observed variation is useful or not--a drawback that i distinctly pointed out some time ago.[ ] nor is there much hope of betterment in this respect, for think how impossible it would be for us to observe all the individuals of a species in all their acts of life, be their habitat ever so limited--and to observe all this with a precision enabling us to say that this or that variation possessed selective value, that is, was a decisive factor in determining the existence of the species. in many cases we can reach at least a probable inference, and say, for example, that the great fecundity of the frog is a property having selective value, basing our inference on the observation that in spite of this fertility the frogs of a given district do not increase. but even such inferences offer only a modicum of certainty. for who can say precisely how large this number is? or whether it is on the increase or on the decrease? and besides, the exact degree of the fecundity of these animals is far from being known. rigorously viewed, we can only say that great fecundity must be advantageous to a much-persecuted animal. and thus it is everywhere. even in the most indubitable cases of adaptation, as, for instance, in that of the striking protective coloring of many butterflies, { } the sole ground of inference that the species upon the whole is adequately adapted to its conditions of life, is the simple fact that the species is, to all appearances, preserved undiminished, and the inference is not at all permissible that just this protective coloring has selective value for the species, that is, that if it were lacking, the species would necessarily have perished. it is not inconceivable that in many species today these colorings are actually unnecessary for the preservation of the species, that they formerly were, but that now the enemies which preyed on the resting butterflies have grown scarce or have died out entirely, and that the protective coloring will continue to exist by the law of inertia[ ] only for a short while till panmixia or new adaptations shall modify it. discouraging, therefore, as it may be, that the control of nature in her minutest details is here gainsaid us, yet it were equivalent to sacrificing the gold to the dross, if simply from our inability to follow out the details of the individual case we should renounce altogether the principle of selection, or should proclaim it as only subsidiary, on the ground that we believe the protective coloring of the butterfly is not a protective coloring, but a combination of colors inevitably resulting from internal causes. the protective coloring remains a protective coloring whether at the time in question it is or is not necessary for the species; and it arose as protective coloring--arose not because it was a constitutional necessity of the animal's organism that here a red and there a white, black, or yellow spot should be produced, but because it was { } advantageous, because it was necessary for the animal. there is only one explanation possible for such patent adaptations and that is selection. what is more, no other natural way of their originating is conceivable, for we have no right to assume teleological forces in the domain of natural phenomena. i have selected the example of the butterfly's wing, not solely because it is so widely known, but because it is so exceedingly instructive, because we are still able to learn so much from it. it has been frequently asserted that the color-patterns of the butterfly's wings have originated from internal causes, independently of selection and conformably to inward laws of evolution. eimer has attempted to prove this assertion by establishing in a division of the genus papilio the fact that the species there admit of arrangement in series according to affinity of design. but is a proof that the markings are modified in definite directions during the course of the species's development equivalent to a definite statement as to the _causes_ that have produced these gradual transformations? or, is our present inability to determine with exactness the biological significance of these markings and their modifications, a proof that the same have no significance whatever? on the contrary, i believe it can be clearly proved that the wing of the butterfly is a tablet on which nature has inscribed everything she has deemed advantageous to the preservation and welfare of her creatures, and nothing else; or, to abandon the simile, that these color-patterns have not proceeded from inward evolutional forces, but are the result of selection. at least in all places where we do understand their biological significance these patterns are constituted and distributed over the wing exactly as utility would require. { } i do not pledge myself, of course, to give an explanation of every spot and every line on a wing. the inscription is often a very complicated one, dating from remote and widely separated ages; for every single existing species has inherited the patterns of its ancestral species and that again the patterns of a still older species. even at its origin, therefore, the wing was far from being a _tabula rasa_, but was a closely written and fully covered sheet, on which there was no room for new writing until a portion of the old had been effaced. but other parts were preserved, or only slightly modified, and thus in many cases gradually arose designs of almost undecipherable complexity. i should be far from maintaining that the markings arose unconformably to law. here, as elsewhere, the dominance of law is certain. but i take it, that the laws involved here, that is, the physiological conditions of the variation, are without exception subservient to the ends of a higher power--utility; and that it is utility primarily that determines the kind of colors, spots, streaks and bands that shall originate, as also their place and mode of disposition. the laws come into consideration only to the extent of conditioning the quality of the constructive materials--the variations, out of which selection fashions the designs in question. and this also is subject to important restrictions, as will appear in the sequel. the meaning of formative laws here is that definite spots on the surfaces of the wings are linked together in such a manner by inner, invisible bonds, as to represent the same spots or streaks, so that we can predict from the appearance of a point at one spot the appearance of another similar point at another, and { } so on. it is an undoubted fact that such relations exist, that the markings frequently exhibit a certain symmetry, that--to use the words of the most recent observer on this subject, bateson[ ]--a meristic representation of equivalent design-elements occurs. but i believe we should be very cautious in deducing laws from these facts, because all the rules traceable in the markings apply only to small groups of forms and are never comprehensive nor decisive for the entire class or even for the single sub-class of diurnal butterflies, in fact, often not so for a whole genus. all this points to special causes operative only within this group. if internal laws controlled the marking on butterflies' wings, we should expect that some general rule could be established, requiring that the upper and under surfaces of the wings should be alike, or that they should be different, or that the fore wings should be colored the same as or differently from the hind wings, etc. but in reality all possible kinds of combinations occur simultaneously, and no rule holds throughout. or, it might be supposed that bright colors should occur only on the upper surface or only on the under surface, or on the fore wings or only on the hind wings. but the fact is, they occur indiscriminately, now here, now there, and no one method of appearance is uniform throughout all the species. but the fitness of the various distributions of colors is apparent, and the moment we apply the principle of utility we know why in the diurnal butterflies the upper surface alone is usually variegated and the under surface protectively colored, or why in the nocturnal { } butterflies the fore wings have the appearance of bark, of old wood, or of a leaf, whilst the hind wings, which are covered while resting, alone are brilliantly colored. on this theory we also understand the exceptions to these rules. we comprehend why danaids, heliconids, euploids, and acracids, in fact all diurnal butterflies, offensive to the taste and smell, are mostly brightly marked and equally so on both surfaces, whilst all species not thus exempt from persecution have the protective coloring on the under surface and are frequently quite differently colored there from what they are on the upper. in any event, the supposed formative laws are not obligatory. dispensations from them can be issued and are issued _whenever utility requires it_. indeed, so far may these transgressions of the law extend, that in the very midst of the diurnal butterflies is found a genus, the south american ageronia, which, like the nocturnal butterfly, shows on the entire _upper_ surface of both wings a pronounced bark-coloration, and concerning which we also know (and in this respect it is an isolated genus and differs from almost all other diurnal butterflies), that it spreads out its wings when at rest like the nocturnal butterfly, and does not close them above it as its relatives do. therefore, entirely apart from cases of mimicry, which after all constitute the strongest proof, the facts here cited are alone sufficient to remove all doubt that not inner necessities or so-called formative laws have painted the surface of the butterflies' wings, but that the conditions of life have wielded the brush. this becomes more apparent on considering the details. i have remarked that the usually striking colorations of exempt butterflies, as of the heliconids, { } are the same on both the upper and the lower surfaces of the wings. possibly the expression of a law might be seen in this fact, and it might be said, the coloration of the heliconids _runs through_ from the upper to the under surface. but among numerous imitators of the heliconids is the genus protogonius, which has the coloration of the heliconids on its upper surface, but on its lower exhibits a magnificent leaf-design. during flight it appears to be a heliconid and at rest a leaf. how is it possible that two such totally different types of coloration should be combined in a single species, if any sort of _inner_ rigorous necessity existed, regulating the coloration of the two wing-surfaces? now, although we are unable to prove that the protogonius species would have perished unless they possessed this duplex coloration, yet it would be nothing less than intellectual blindness to deny that the butterflies in question are effectively protected, both at rest and during flight, _that their colorations are adaptive_. we do not know their primitive history, but we shall hardly go astray if we assume that the ancestors of the protogonius species were forest-butterflies and already possessed an under surface resembling a leaf. by this device they were protected when at rest. afterwards, when this protection was no longer sufficient, they acquired on their upper surface the coloration of the exempt species with which they most harmonised in abode, habits of life, and outward appearance. at the same time it is explained why these butterflies did not acquire the coloration of the heliconids on the under surface. the reason is, that in the attitude of repose they were already protected, and that in an admirable manner. { } that _exempt_ diurnal butterflies should be colored on the upper and under surfaces alike, and should never resemble in the attitude of repose their ordinary surroundings, is intelligible when we reflect that it is a much greater protection to be despised when discovered than to be well, or very well, but never absolutely, protected from discovery. it has been so often reiterated that diurnal butterflies, as a rule, are protectively colored on the under surfaces, that one has some misgivings in stating the fact again. and yet the least of those who hold this to be a trivial commonplace know how strongly its implications militate against the inner motive and formative forces of the organism, which are ever and anon appealed to. no less than sixty-two genera are counted today in the family of diurnal butterflies known as the nymphalidæ. of these by far the largest majority are sympathetically colored underneath, that is, they show in the posture of rest the colorings of their usual environment. in a large number of the species belonging to this group the entire surface of the hind wings possesses such a sympathetic coloration, as does also the distant apex of the fore wings. why? the reason is obvious. this part only of the fore wing is visible in the attitude of repose. here, then,--as a zealous opponent of the theory of selection once exclaimed,--there is undoubted "correlation" between the coloring of the surface of the hind wing and of the apex of the fore wing. correlation is unquestionably a fine word, but in the present instance it contributes nothing to the understanding of the problem, for there are near relatives and often species of the same genera in which this correlation is not restricted to the apex of the { } fore wings, but extends to a third or even more of their wings, and these species are also in the habit of drawing back their wings less completely in the state of rest, thus rendering a larger portion of them visible. there are species, too, like the forest-butterflies of south america just mentioned, the protogonius, anæa, kallima species, etc., which have nearly the _whole_ of the under surfaces of their fore wings marked according to the same pattern with their hind wings, and these butterflies when at rest hold their fore wings free and uncovered by their hind wings. where are the formative laws in such cases? or, perhaps some one will say: "the covering by the hind wings hinders the formation of scales on the wing, or impedes the formation of the colors in the scales." such a person should examine one of these species. he will find that the scales are just as dense on the covered as on the uncovered surface of the wing, and in many species, for example, in katagramma, the scales of the covered surface are colored most brilliantly of all. but the facts are still more irresistible, when we consider _special adaptations_; for example, the imitation of leaves, which is so often cited. it is to be noted, first, that this sort of imitation is by no means restricted to a few genera, still less to a few species. all the numerous species of the genus anæa, which are distributed over the forests of tropical south america, exhibit this imitation in pronounced and varied forms, as do likewise the american genera hypna and siderone, the asiatic symphaedra, the african salamis, eurypheme, etc. i have observed fifty-three genera in which it is present in one, several, or in many species, but there are many others. { } these genera, now, are by no means all so nearly allied that they could have inherited the leaf-markings from a common ancestral form. they belong to different continents and have probably for the most part acquired their protective colorings themselves. but one resemblance they have in common--they are all _forest-butterflies_. now what is it that has put so many genera of forest-butterflies and no others into positions where they could acquire this resemblance to leaves? was it directive formative laws? if we closely examine the markings by which the similarity of the leaf is determined, we shall find, for example, in kallima inachis, and parallecta, the indian leaf-butterflies, that the leaf-markings are executed _in absolute independence of the other uniformities governing the wing_. from the tail of the wing to the apex of the fore wings runs with a beautiful curvature a thick, doubly-contoured dark line accompanied by a brighter one, representing the midrib of the leaf. this line cuts the "veins" and the "cells" of the wing in the most disregardful fashion, here in acute and here in obtuse angles, and in absolute independence of the regular system of divisions of the wing, which should assuredly be the expression of the "formative law of the wing," if that were the product of an internal directive principle. but leaving this last question aside, this much is certain with regard to the markings, that they are dependent, not on an _internal_, but on an _external_ directive power. should any one be still unconvinced by the evidence we have adduced, let him give the leaf-markings a closer inspection. he will find that the midrib is composed of two pieces of which the one belongs to the { } hind wing and the other to the fore wing, and that the two fit each other exactly when the butterfly is in the attitude of repose, but not otherwise. now these two pieces of the leaf-rib do not begin on corresponding spots of the two wings, but on absolutely non-identical spots. and the same is also true of the lines which represent the lateral ribs of the leaf. these lines proceed in acute angles from the rib; to the right and to the left in the same angle, those of the same side parallel with each other. here, too, no relation is noticeable between the parts of the wings over which the lines pass. the venation of the wing is utterly ignored by the leaf-markings, and its surface is treated as a _tabula rasa_ upon which anything conceivable can be drawn. in other words, we are presented here with a _bilaterally symmetrical_ figure engraved on a surface which is essentially _radially symmetrical_ in its divisions. i lay unusual stress upon this point because it shows that we are dealing here with one of those cases which cannot be explained by mechanical, that is, by natural means, unless natural selection actually exists and is actually competent to create new properties; for the lamarckian principle is excluded here _ab initio_, seeing that we are dealing with a formation which is only passive in its effects; the leaf-markings are effectual simply by their existence and not by any function which they perform; they are present in flight as well as at rest, during the absence of danger, as well as during the approach of an enemy. nor are we helped here by the assumption of _purely internal motive forces_, which nägeli, askenasy, and others have put forward as supplying a _mechanical_ force of evolution. it is impossible to regard the { } coincidence of an indian butterfly with the leaf of a tree now growing in an indian forest as fortuitous, as a _lusus naturæ_. assuming this seemingly mechanical force, therefore, we should be led back inevitably to a teleological principle which produces adaptive characters and which must have deposited the directive principle in the very first germ of terrestrial organisms, so that after untold ages at a definite time and place the illusive leaf-markings should be developed. the assumption of pre-established harmony between the evolution of the ancestral line of the tree with its pre-figurative leaf, and that of the butterfly with its imitating wing, is absolutely necessary here--a fact which i pointed out many years ago,[ ] but which is constantly forgotten by the promulgators of the theory of internal evolutionary forces. for the present i leave out of consideration altogether the question as to the conceivable extent of the sphere of operation of natural selection; i am primarily concerned only with elucidating the process of selection itself, wholly irrespective of the comprehensiveness or limitedness of its sphere of action. for this purpose it is sufficient to show, as i have just done, _that cases exist wherein all natural explanations except that of selection fail us_. but let us now see how far the principle of selection will carry us in the explanation of such cases--natural selection, i mean, as it was formulated by darwin and wallace. there can be no doubt but the leaf-markings readily admit of production in this manner, slowly and with a gradual but constant increase of fidelity, provided a single condition is fulfilled: _the occurrence of the { } right variations at the right place_. but just here, it would seem, is the insurmountable barrier to the explanatory power of our principle, for who, or what, is to be our guarantee that dark scales shall appear at the exact spots on the wing where the midrib of the leaf must grow? and that later dark scales shall appear at the exact spots to which the midrib must be prolonged? and that still later such dark spots shall appear at the places whence the lateral ribs start, and that here also a definite acute angle shall be accurately preserved, and the mutual distances of the lateral ribs shall be alike and their courses parallel? and that the prolongation of the median rib from the hind wing to the fore wing shall be extended exactly to that spot where the fore wing is not covered by the hind wing in the attitude of repose? and so on. if i could go more minutely into this matter, i should attempt to prove that the markings, as i have just assumed, have not arisen suddenly, but were perfected very, very gradually; that in one species they began on the fore wing and in another on the hind wing; and that in many they never until recently proceeded beyond one wing, in other species they went only a little way, and in only a few did they spread over the entire surface of both wings. that these markings advanced slowly and gradually, but with marvelous accuracy, is no mere conjecture. but it follows that the right variations at the right places must never have been wanting, or, as i expressed it before: _the useful variations were always present_. but how is that possible in such long extensive lines of dissimilar variations as have gradually come to constitute markings of the complexity here presented? suppose that the useful colors had not { } appeared at all, or had not appeared at the right places? it is a fact that in constant species, that is, in such as are not in process of transformation, the variations of the markings are by no means frequent or abundant. or, suppose that they had really appeared, but occurred only in individuals, or in a small percentage of individuals? such are the objections raised against the theory of selection by its opponents, and put forward as insurmountable obstacles to the process. nor are such objections relevant only in the case of protective colorings; they are applicable in all cases where the process of selection is concerned. take the case of instincts that are called into action only once in life, as, for example, the pupal performances of insects, the artificial fabrication of cocoons, etc. how is it that the useful variations were always present here? and yet they must have been present, if such complicated spinning instincts could have taken their rise as are observable in the silk-worm, or in the emperor-moth. and they have been developed, and that in whole families, in forms varying in all species, and in every case adapted to the special wants of the species. particularly striking is the proof afforded of this constant presence of the useful variations by cases where we meet with the development of highly special adaptations that are uncommon even for the group of organisms concerned. such a case, for example, is the apparatus designed for the capture of small animals and their digestion, found in widely different plants and widely separated families. on the other hand, very common adaptations, such as the eyes of animals, show distinctly that in all cases where it was necessary, the useful variations for the formation of { } an eye were presented, and were presented further exactly at spots at which organs of vision could perform their best work: thus, in turbellaria and many other worms that live in the light, at the anterior extremity of the body and on the dorsal surface; in certain mussels, on the edge of the mantle; in terrestrial snails, on the antennæ; in certain tropical marine snails inhabiting shallow waters, on the back; and in the chitons even on the dorsal surface of the shell! but even taking the very simplest cases of selection, it is impossible to do without this assumption, that the useful variations are always present, or that _they always exist in a sufficiently large number of individuals for the selective process_. you know the thickness and power of resistance of the egg-shells of round-worms. the eggs of the round-worms of horses have been known to continue their course of development undisturbed even after they had been thrown into strong alcohol and all other kinds of injurious liquids--much to the vexation of the embryologists, who wished to preserve a definite stage of development and sought to kill the embryo at that stage. indeed, think of the result, if in the course of their phylogenesis stout and resistant variations of egg-shells had not been presented in these worms, or had not always been presented, or had not been presented in every generation and not in sufficient quantities. the cogency of the facts is absolutely overpowering when we consider that practically no modification occurs _alone_, that every primary modification brings in its train secondary ones, and that these induce forced modifications in many parts of the body, frequently of the most diversified, or even self-contradictory, forms. recently herbert spencer has drawn { } fresh attention to these secondary modifications, which must always occur in harmony with the primary one, and has, as he thinks, advanced in this set of facts, a convincing disproof of the contention that such coadaptive modifications of numerous cofunctioning parts can rest on natural selection. now, although i deem his conclusion precipitate, yet the very fact of a simultaneous, functionally concordant, yet essentially diversified modification of numerous parts, points conclusively to the circumstance that _something is still wanting to the selection of darwin and wallace, which it is obligatory on us to discover, if we possibly can_, and without which selection as yet offers no complete explanation of the phyletic processes of transformation. there is a hidden secret to be unriddled here before we can obtain a satisfactory insight into the phenomena in question. _we must seek to discover why it happens that the useful variations are always present._ herbert spencer appealed to lamarck's principle for the explanation of coadaptation, and it is certain that functional adaptation is operative during the individual life, and that it compensates in a certain measure the inequalities of the inherited constitutions. i shall not repeat what i have said before on this subject, nor maintain, in refutation of spencer's contention, that functional adaptation is itself nothing more than the efflux of _intra-biontic_ selective processes, as spencer himself once suggested in a prophetic moment, but which it was left for wilhelm roux to introduce into science as "the struggle of the parts" of organisms.[ ] i shall only remark that if functional adaptations were themselves inheritable, this would still be insufficient { } for the explanation of coadaptation, for the reason that precisely similar coadaptive modifications occur in _purely passively_ functioning parts, in which, consequently, modification _by_ function is excluded. this is the case with the skeletal parts of articulata; e. g., it is true of their articular surfaces with their complex adaptations to the most varied forms of locomotion. in all these cases the ready-made, hard, unalterable, chitinous part is _first_ set into activity; consequently its adaptation to the function must have been _previously_ effected, independently of that function. these joints, and divers other parts, accordingly, have been developed in the precisest manner for the function, and the latter could have had no direct share in their formation. when we consider, now, that it is impossible that every one of the numerous surfaces, ridges, furrows, and corners found in a single such articulation, let alone in all the articulations of the body, should hold in its hands the power of life and death over individuals for untold successions of generations, the fact is again unmistakably impressed upon our attention that the conception of the selective processes which has hitherto obtained is insufficient, that the root of the process in fact lies deeper, that it is to be found in the place where it is determined what variations of the parts of the organism shall appear--namely _in the germ_. the phenomena observed in the _stunting_, or _degeneration_, _of parts rendered useless_, point to the same conclusion. they show distinctly that ordinary selection which operates by the removal of entire persons, _personal selection_, as i prefer to call it, cannot be the only cause of degeneration; for in most cases of degeneration it cannot be assumed that slight individual { } vacillations in the size of the organ in question have possessed selective value. on the contrary, we see such retrogressions affected apparently _in the shape of a continuous evolutionary process determined by internal causes_, in the case of which there can be no question whatever of selection of persons or of a survival of the fittest, that is, of individuals with the smallest rudiments. it is this consideration principally that has won so many adherents for the lamarckian principle in recent times, particularly among the paleontologists. they see the outer toes of hoofed animals constantly and steadily degenerating through long successions of generations and species, concurrently with the re-enforcement of one or two middle toes, which are preferred or are afterwards used exclusively for stepping, and they believe correctly enough that these results should not be ascribed to the effects of personal selection alone. they demand a principle which shall effect the degeneration by internal forces, and believe that they have found it in functional adaptation.[ ] { } on this last point, now, i believe, they are mistaken, be they ever so strongly convinced of the correctness of their view and ever so aggressive and embittered in their defence of it. recently, an inquirer of great caution and calmness of judgment, prof. c. lloyd morgan, has expressed the opinion that the lamarckian principle must at least be admitted as a working hypothesis. but with this i cannot agree, at least as things stand at present. a working hypothesis may be false, and yet lead to further progress; that is, it may constitute an advance to the extent of being useful in formulating the problem and in illuminating paths that are likely to lead to results. but it seems to me that a hypothesis of this kind has performed its services and must be discarded the moment it is found to be at hopeless variance with the facts. if it can be proved that precisely the same degenerative processes also take place in such superfluous parts as have only _passive_ and not active functions, as is the case with the _chitinous parts of the skeleton of arthropoda_, then it is a demonstrated fact, that the cessation of functional action is not the efficient cause of the process of degeneration. at once your legitimate working hypothesis is transformed into an illegitimate dogma--illegitimate because it no longer serves as a guide on the path to knowledge but { } blocks that path. for the person who is convinced he has found the right explanation is not going to seek for it. i can understand perfectly well the hesitation that has prevailed on this point in many minds, from their having seen _one_ aspect of the facts more distinctly than the other. from this sceptical point of view osborn has drawn the following perfectly correct conclusion: "if acquired variations are transmitted, there must be some unknown principle in heredity; if they are not transmitted, there must be some unknown factor in evolution."[ ] such in fact is the case and i shall attempt to point out to you what this factor is. my inference is a very simple one: if we are forced by the facts on all hands to the assumption that the useful variations which render selection possible are always present, then _some profound connection must exist between the utility of a variation and its actual appearance_, or, in other words, _the direction of the variation of a part must be determined by utility_, and we shall have to see whether facts exist that confirm our conjecture. the facts do indeed exist and lie before our very eyes, despite their not having been recognised as such before. all _artificial selection_ practised by man rests on the fact that by means of the selection of individuals having a given character slightly more pronounced than usual, there is gradually produced a general augmentation of this character, which subsequently reaches a point never before attained by any individual { } of this species. i shall choose an example which seems to me especially clear and simple because only one character has been substantially modified here. the long-tailed variety of domestic cock, now bred in japan and corea, owes its existence to skilful selection and not at all to the circumstance that at some period of the race's history a cock with tail-feathers six feet in length suddenly and spasmodically appeared. at the present day even, as professor ishikawa of tokio writes me, the breeders still make extraordinary efforts to increase the length of the tail, and every inch gained adds considerably to the value of the bird. now nothing has been done here whatever except always to select for purposes of breeding the cocks with the longest feathers; and in this way alone were these feathers, after the lapse of many generations, prolonged to a length far exceeding every previous variation. i once asked a famous dove-fancier, mr. w. b. tegetmeier of london, whether it was his opinion that by artificial selection alone a character could be augmented. he thought a long time and finally said: "it is without our power to do anything if the variation which we seek is not presented, but once that variation is given, then i think the augmentation can be effected." and that in fact is the case. if cocks had never existed whose tail-feathers were a little longer than usual the japanese breed could never have originated; but as the facts are, always the cocks with the longest feathers were chosen from each generation, and these only were bred, and thus a hereditary augmentation of the character in question was effected, which would hardly have been deemed possible. now what does this mean? simply that the { } hereditary diathesis, the constitutional predisposition (_anlage_) of the breed was changed in the respect in question, and our conclusion from this and numerous similar facts of artificial selection runs as follows: _by the selection alone of the plus or minus variations of a character is the constant modification of that character in the plus or minus direction determined._ obviously the hereditary _diminution_ of a part is also effected by the simple selection of the individuals in each generation possessing the smallest parts, as is proved, for example, by the tiny bills and feet of numerous breeds of doves. we may assert, therefore, in general terms: a definitely directed progressive variation of a given part is produced by continued selection in that definite direction. this is no hypothesis, but a direct inference from the facts and may also be expressed as follows: _by a selection of the kind referred to the germ is progressively modified in a manner corresponding with the production of a definitely directed progressive variation of the part._ in this general form the proposition is not likely to encounter opposition, as certainly no one is prepared to uphold the view that the germ remains unchanged whilst the products proceeding from it, its descendants, are modified. on the contrary, all will agree when i say that the germ in this case must have undergone modifications, and that their character must correspond with the modifications undergone by its products. thus far, then, we find ourselves, not on the ground of the hypothesis that has been lately so much maligned, but on the ground of facts and of direct inferences from facts. but if we attempt to pierce deeper into the problem, we are in need of the hypothesis. { } the first and most natural explanation will be this--that through selection the zero-point, about which, figuratively speaking, the organ may be said to oscillate in its plus and minus variations, is displaced upwards or downwards. darwin himself assumed that the variations oscillated about a mean point, and the statistical researches of galton, weldon, and others have furnished a proof of the assumption. if selection, now, always picks out the plus variations for imitation, perforce, then, the mean or zero-point will be displaced in the upward direction, and the variations of the following generation will oscillate about a higher mean than before. this elevation of the zero-point of a variation would be continued in this manner until the total equilibrium of the organism was in danger of being disturbed. there is involved here, however, an assumption which is by no means self-evident, that every advancement gained by the variation in question constitutes a new centre for the variations occurring in the following generation. _that this is a fact_, is proved by such actual results of selection as are obtained in the case of the japanese cock. but the question remains, why is this the fact? now here, i think, my theory of determinants gives a satisfactory answer. according to that theory every independently and hereditarily variable part is represented in the germ by a _determinant_, that is by a determinative group of vital units, whose size and power of assimilation correspond to the size and vigor of the part. these determinants multiply, as do all vital units, by growth and division, and necessarily they increase rapidly in every individual, and the more rapidly the greater the quantity of the germinal cells { } the individual produces. and since there is no more reason for excluding irregularities of passive nutrition, and of the supply of nutriment in these minute, microscopically invisible parts, than there is in the larger visible parts of the cells, tissues, and organs, consequently the descendants of a determinant can never all be exactly alike in size and capacity of assimilation, but they will oscillate in this respect to and fro about the maternal determinant as about their zero-point, and will be partly greater, partly smaller, and partly of the same size as that. in these oscillations, now, the material for further selection is presented, and in the inevitable fluctuations of the nutrient supply i see the reason why every stage attained becomes immediately the zero-point of new fluctuations, and consequently why the size of a part can be augmented or diminished by selection without limit, solely by the displacement of the zero-point of variation as the result of selection. we should err, however, if we believed that we had penetrated to the root of the phenomenon by this insight. there is certainly some other and mightier factor involved here than the simple selection of persons and the consequent displacement of the zero-point of variation. it would seem, indeed, as if in one case, _videlicet_, in that of the japanese cock, the augmentation of the character in question were completely explained by this factor _alone_. in fact, in this and similar cases we cannot penetrate deeper into the processes of variation, and therefore cannot say _a priori_ whether other factors have or have not been involved in the augmentation of the character in question--other characters, that is, than the simple displacement of the zero-point. there is, however, another class of phyletic modifications, which point { } unmistakably to the conclusion that the displacement of the zero-point of variation by personal selection is not and cannot be the only factor in the determination and accomplishment of the direction of variation. i refer to _retrogressive development_, the gradual degeneration of parts or characters that have grown useless, the gradual disappearance of the eye in cave-animals, of the legs in snakes and whales, of the wings in certain female butterflies, in short, to that entire enormous mass of facts comprehended under the designation of "rudimentary organs." i have endeavored on a previous occasion to point out the significance of the part played in the great process of animate evolution by these retrogressive growths, and i made at the time the statement that "the phenomena of retrogressive growth enabled us in a greater measure almost than those of progressive growth to penetrate to the causes which produce the transformations of animate nature." although at that time[ ] i had no inkling of certain processes which today i shall seek to prove the existence of, yet my statement receives a fresh confirmation from these facts. for, in most retrogressive processes _active_ selection in darwin's sense plays no part, and advocates of the lamarckian principle, as above remarked, have rightly denied that active selection, that is, the selection of individuals possessing the useless organ in its most reduced state, is sufficient to explain the process of degeneration. i, for my part, have never assumed this, { } and i enunciated precisely on this account the _principle of panmixia_. now, although this, as i still have no reason for doubting, is a perfectly correct principle, which really does have an essential and indispensable share in the process of retrogression, still it is not _alone_ sufficient for a full explanation of the phenomena. my opponents, in advancing this objection, were right, to the extent indicated and as i expressly acknowledge, although they were unable to substitute anything positive in its stead or to render my explanation complete. the very fact of the cessation of control over the organ is sufficient to explain its _degeneration_, that is, its deterioration, the disharmony of its parts, but not the fact which actually and always occurs where an organ has become useless--viz., _its gradual and unceasing diminution continuing for thousands and thousands of years culminating in its final and absolute effacement._ if, now, neither the selection of persons nor the cessation of personal selection can explain this phenomenon, assuredly some other principle must be the efficient cause here, and this cause i believe i have indicated in an essay written at the close of last year and only recently published.[ ] i call it _germinal selection_. the principle in question reposes on the application, made some fifteen years ago by wilhelm roux, of the principle of selection to the _parts_ of organisms--on the _struggle of the parts_, as he called it. if such a struggle obtains among organs, tissues, and cells, it must also obtain between the smallest and for us invisible vital particles, not only between those of the body-cells, strictly so called, but also between those of the { } germinal cells. roux himself spoke of the struggle of the molecules, by which he presumably understood the smallest ultimate units of vital phenomena--elements which de vries designated pangenes, wiesner plasomes, and i _biophores_, after brücke's ingenious conception[ ] of these invisible entities had been almost totally forgotten, or at least had lain unnoticed for thirty years. no struggle, as that is understood in the theory of selection, could take place between real { } molecules, for molecules are neither nourished, subject to growth, nor propagated. the gradual degeneration of organs grown useless may be explained, now, by the theory of determinants very simply and without any co-operation on the part of active personal selection, as follows. nutrition, it is known, is not merely a passive process. a part is not only _nourished_ but also actively _nourishes_ itself, and the more vigorously, the more powerful and capable of assimilation it is. hence powerful determinants in the germ will absorb nutriment more rapidly than weaker determinants. the latter, accordingly, will grow more slowly and will produce weaker descendants than the former. let us assume, now, that a part of the body, say the hinder extremities of the quadruped ancestors of { } our common whales, are rendered useless. panmixia steps in, _i. e._, selection ceases to influence these organs. individuals with large and individuals with small hind legs are equally favored in the struggle for existence. from this fact alone would result a degradation of the organ, but of course it would not be very marked in extent, seeing that the minus variations which occur are no longer removed. according to our assumption, however, such minus variations repose on the weaker determinants of the germ, that is, on such as absorb nutriment less powerfully than the rest. and since every determinant battles stoutly with its neighbors for food, that is, takes to itself as much of it as it can, consonantly with its power of assimilation and proportionately to the nutrient supply, therefore the unimpoverished neighbors of this minus determinant will deprive it of its nutriment more rapidly than was the case with its more robust ancestors; hence, it will be unable to obtain the full quantum of food corresponding even to its weakened capacity of assimilation, and the result will be that its ancestors will be weakened still more. inasmuch, now, as no weeding out of the weaker determinants of the hind leg by personal selection takes place on our hypothesis, inevitably the average strength of this determinant must slowly but constantly diminish, that is, the leg must grow smaller and smaller until finally it disappears altogether. the determinants[ ] of the useless organ are constantly at { } a disadvantage as compared with the determinants of their environment in the germinal tenement, because no assistance is offered to them by personal selection after they have once been weakened by a decrease of the passive nutrient influx. nor is the degeneration stopped by the uninterrupted crossing of individuals in sexual propagation, but only slightly retarded. the number of individuals with weaker determinants must, despite this fact, go on increasing from generation to generation, so that soon every determinant that still happens to be endowed with exceptional vigor will be confronted by a decided overplus of weaker determinants, and by continued crossing therefore will become more and more impoverished. panmixia is the indispensable precondition of the whole process; for owing to the fact that persons with weak determinants are just as capable of life as those with strong, owing to the fact that they cannot now, as formerly, when the organ was still useful, be removed by personal selection, solely by this means is a further weakening effected in the following generations--in short, only by this means are the determinants of the useless organ brought upon the inclined plane, down which they are destined slowly but incessantly to slide towards their completed extinction. the foregoing explanation will be probably accepted as satisfactory _in a purely formal regard_, but it will be objected that, even granting this, it has not yet been proved to be the correct one. in answer i can of course adduce nothing except that it is at present the only one that can be given. it may be that the actual state of things in nature is different, but if it can be shown that a self-direction of variation merely from the need of it is at all conceivable by mechanical means, { } that in itself, it seems to me, is a decided gain. it must also not be forgotten that some process or other _must_ take place in the germ-plasm when an organ becomes rudimentary, and that as the result of it this organ, and only this organ, must disappear. now in what shall this process consist, if not in a modification of the constitution of the germ? and how could the effect of such a modification be limited only to _one_ organ which was becoming rudimentary if the modification itself were not a local one? these are questions which it is incumbent on those to answer who conceive the germinal substance to be composed of like units. applying, now, the explanation derived from the disappearance of organs to the opposed transformation, namely, to the _enlargement_ of a part, the presumption lies close at hand that the production of the long tail-feathers of the japanese cock does not repose solely on the displacement directly effected by personal selection, of the zero-point of variation upwards, but that _it is also fostered and strengthened by germinal selection_. were that not so, the phenomena of the transmutation of species, in so far as fresh growth and the enlargement and complication of organs already present are concerned, _would not be a whit more intelligible than they were before_. we should know probably how it comes to pass that the constitutional predisposition (group of determinants) of a _single_ organ is intensified by selection, but the flood of objections against the theory of selection touching its inability to modify _many_ parts at once would not be repressed by such knowledge. the initial impulse conditioning the independent maintenance of the useful direction of variation in the germ-plasm must rather be sought { } in the utility of the modification itself, and this also seems to me intelligible from the side of the theory. for as soon as personal selection favors the more powerful variations of a determinant, the moment that these come to predominate in the germ-plasm of the species, at once the tendency must arise for them to vary _still more strongly_ in the plus direction, not solely because the zero-point has been pushed farther upwards, but because they themselves now oppose a relatively more powerful front to their neighbors, that is, actively absorb more nutriment, and upon the whole increase in vigor and produce more robust descendants. from the relative vigor or dynamic status of the particles of the germ-plasm, thus, will issue spontaneously an ascending line of variation, precisely as the facts of evolution require. for, as i have already said, it is not sufficient that the augmentation of a character should be brought about by uninterrupted personal selection, even supposing that the displacement of the zero-point were possible without germinal selection. thus, i think, may be explained how personal selection imparts the initial impulse to processes in the germ-plasm, which, when they are once set agoing, persist of themselves in the same direction, and are, therefore, in no need of the continued supplementary help of personal selection, _as directed exclusively to a definite part_. if but from time to time, that is, if upon the average the poorest individuals, the bearers of the weakest determinants, are eliminated, the variational direction of the part in question, now reposing on germinal selection, must persist, and it will very slowly but very surely increase until further development is impeded by its inutility and personal selection { } arrests the process, that is, ceases to eliminate the weaker individuals. in this manner it becomes intelligible how a large number of modifications varying in kind and far more so in degree can be guided _simultaneously_ by personal selection; how in strict conformity with its adaptive wants every part is modified, or preserved unmodified; how a given articulation can undergo modifications, causing it to disappear on one side, to grow in volume on another, and to continue unaltered on a third. for every part that is perfectly adapted, although it can fluctuate slightly, yet can never undergo a permanent alteration in the ascending or descending direction because every plus and every minus variation which has attained selective value would be eliminated by personal selection in the course of time. therefore, a definite direction of variation cannot arise in such cases and we have also reached, as it seems to me, a satisfactory explanation of the _constancy_ of well-adapted species and characters. hitherto i have spoken only of plus and minus variation. but there exist, as we know, not only variations of size but also variations of _kind_; and the coloration of the wings of butterflies, which we chose above as our example, would fall, according to the ordinary usage of speech, under just this head of variations of quality. the question arises, therefore, have the principles just developed any claim to validity in the explanation of _qualitative_ modifications? in considering this question it should be carefully borne in mind that by far the largest part of the qualitative modifications falling under this head rest on _quantitative_ changes. of course, chemical transformations, which usually also involve quantitative { } alterations, cannot be reduced to the processes of augmentation described, inasmuch as these, by their very nature, can be effected only in living elements capable of increase by propagation; but the interference of selection does not begin originally with the constitutional predisposition (_anlagen_) of the germ, i. e. with the determinants, but with the ultimate units of life, the _biophores_. a determinant must be composed of heterogeneous biophores, and on their numerical proportion reposes, according to our hypothesis, their specific nature. if that proportion is altered, so also is the character of the determinant. but disturbances of this numerical proportion must result at once on proof of their usefulness, or as soon as the modifications determined thereby in the inward character of the determinant turn out to be of utility. for fluctuations of nutriment and the struggle for nutriment, with its sequent preference of the strongest, must take place between the various species of the biophores as well as between the species of the determinants. but changes in the quantitative ratios of the biophores appear to us qualitative changes in the corresponding determinants, somewhat as a simple augmentation of a determinant, for example, that of a hair, may on its development appear to us as a qualitative change, a spot on the skin where previously only isolated hairs stood being now densely crowded with them, and assuming thus the character of a downy piece of fur. the single hair need not have changed in this process, and yet the spot has virtually undergone a qualitative modification. the majority of the changes that appear to us qualitative rest on invisible _quantitative_ changes, and such can be produced at all times and _at all stages_ { } _of the vital units_ by germinal selection. in a similar manner are induced the most varied qualitative changes of the corresponding determinants and of the characters conditioned thereby, just as changes in the numerical proportions of atoms produce essential changes in the properties of a chemical molecule. in this way we acquire an approximate conception of the possible mechanical _modus operandi_ of actual events--namely, of the manner in which the useful variations required by the conditions of life _can_ always, that is, very frequently, make their appearance. this possibility is the sole condition of our being able to understand how different parts of the body, absolutely undefined in extent, can appear as variational units and vary in the same or in different directions, according to the special needs of the case, or as the conditions of life prescribe. thus, for example, in the case of the butterfly's wings it rests entirely with utility to decide the size and the shape of the spots that shall vary simultaneously in the same direction. at one time the whole under surface of the wing appears as the variational unit and has the same color; at another the inside half, which is dark, is contrasted with the outside half which is bright; or the same contrast will exist between the anterior and posterior halves; or, finally, narrow stripes or line-shaped streaks will behave as variational units and form contrasts with manifold kinds of spots or with the broader intervals between them, with the result that the picture of a leaf or of another protected species is produced. i must refrain from entering into the details of such cases and shall illustrate my views regarding the color-transformations of butterflies' wings by the simplest { } conceivable example--viz. that of the uniform change of color on the entire under surface of the wing. suppose, for example, that the ancestral species of a certain forest-butterfly habitually reposed on branches which hung near the ground and were covered with dry or rotten leaves; such a species would assume on its under surface a protective coloring which by its dark, brown, yellow, or red tints would tend toward similarity with such leaves. if, however, the descendants of this species should be subsequently compelled, no matter from what cause, to adopt the habit of resting on the green-leafed branches higher up, then from that period on the brown coloring would act less protectively than the shades verging towards green. and a process of selection will have set in which consisted first in giving preference only to such persons whose brown and yellow tints showed a tendency to green. only on the assumption that such shades were possible by a displacement in the quantitative proportions of the different kinds of biophores composing the determinants of the scales affected, was a further development in the direction of green possible. such being the case, however, that development _had to_ result; because fluctuations in the numerical proportions of the biophores are always taking place, and consequently the material for germinal selection is always at hand. at present it is impossible to determine exactly the magnitude of the initial stages of the deviations thus brought about and promoted by the sexual blending of characters; but it may perhaps be ascertained in the future, with exceptionally favorable material. pending such special observations, however, it can only be said _a priori_ that slight changes in the composition of a determinant do not necessarily { } condition similar slight deviations of the corresponding character,--in this case the color,--just as slight changes in the atomic composition of a molecule may result in bestowing upon the latter widely different properties. as soon, however, as the beginning has been made and a definite direction has been imparted to the variation, as the result of this or that primary variation's being preferred, the selective process must continue until the highest degree of faithfulness required by the species in the imitation of fresh leaves has been attained. that the foregoing process has actually taken place is evidenced not only by the presence of the beginnings of such transformations, as found for example in some greenish-tinted specimens of kallima, but mainly by certain species of the south american genus catonephele, all of which are forest-butterflies, and which, with many species having dark-brown under surfaces, present some also with bright green under surfaces--a green that is not like the fresh green of our beech and oak trees, but resembles the bright under surface of the cherry-laurel leaf, and is the color of the under surfaces of the thick, leathery leaves, colored dark-green above, borne by many trees in the tropics. the difference between this and the old conception of the selection-process consists not only in the fact that a large number of individuals with the initial stages of the desired variation is present from the beginning, for always innumerable plus and minus variations exist, but principally in the circumstance that the constant uninterrupted progress of the process after it is once begun is assured, that there can never be a lack of progressively advantageous variations in a large number of individuals. selection, { } therefore, is now not compelled to wait for accidental variations but produces such itself, whenever the required elements for the purpose are present. now, where it is a question simply of the enlargement or diminution of a part, or of a part of a part, these variations are always present, and in modifications of quality they are at least present in many cases. this is the only way in which i can see a possibility of explaining phenomena of _mimicry_--the imitation of one species by another. the useful variations must be produced in the germ itself by internal selection-processes if this class of facts is to be rendered intelligible. i refer to the mimicry of an exempt species by two or three other species, or, the aping of _different_ exempt patterns by _one_ species in need of protection. it must be conceded to darwin and wallace that some degree of similarity between the copy and the imitation was present from the start, at least in very many cases;[ ] but in no case would this have been sufficient had not slight shades of coloring afforded some hold for personal selection, and in this way furnished a basis for independent germinal selection acting only in the direction indicated. it would have been impossible for such a minute similarity in the design, and particularly in the shades of the coloration, ever to have arisen, if the process of adaptation rested entirely { } on personal selection. were this so, a complete scale of the most varied shades of color must have been continually presented as variations in every species, which certainly is not the case. for example, when the exempt species _acræa egina_, whose coloration is a brick-red, a color common only in the genus acræa, is mimicked by two other butterflies, a papilio and a pseudacræa, so deceptively that not only the cut of the wings and the pattern of their markings, but also that precise shade of brick-red, which is scarcely ever met with in diurnal butterflies, are produced, assuredly such a result cannot rest on accidental, but must be the outcome of a _definitely directed_, variation, produced by utility. we cannot assume that such a coloration has appeared as an _accidental_ variation in just and in only these two species, which fly together with the _acræa_ in the same localities of the same country and same part of the world--the gold coast of africa. it is conceivable, indeed, that non-directed variation should have accidentally produced this brick-red _in a single case_, but that it should have done so three times and in three species, which live together but are otherwise not related, is a far more violent and improbable assumption than that of a causal connexion of this coincidence. now hundreds of cases of such mimicry exist in which the color-tints of the copy are met with again in more or less precise and sometimes in exceedingly exact imitations, and there are thousands of cases in which the color-tint of a bark, of a definite leaf, of a definite blossom, is repeated _exactly_ in the protectively colored insect. in such cases there can be no question of accident, but _the variations presented to personal selection must themselves have been produced by the principle of the survival of the_ { } _fit!_ and this is effected, as i am inclined to believe, through such profound processes of selection in the interior of the germ-plasm as i have endeavored to sketch to you to-day under the title of germinal selection. i am perfectly well aware how schematic my presentation of this process is, and must be at present, owing mainly to our inability to gain exact knowledge concerning the fundamental germinal constituents here assumed. but i regard its existence as assured, although i by no means underrate the fact that eminent thinkers, like herbert spencer, contest its validity and believe they are warranted in assuming a germ which is composed of _similar units_. i strongly doubt whether even so much as a _formal_ explanation of the phenomena can be arrived at in this manner. so far as direct observation is concerned, the two theories stand on an equal footing, for neither my dissimilar, nor spencer's similar, units of germinal substance can be _seen_ directly. the attempt has been recently made to discredit my _anlagen_, or constitutional germ-elements, on the ground that they are simply a subtilised reproduction of bonnet's old theory of preformation.[ ] this { } impression is very likely based upon ignorance of the real character of bonnet's theory. i will not go into further details here, particularly as whitman, in several excellently written and finely conceived essays, has recently afforded opportunity for every one to inform himself on the subject. my determinants and groups of determinants have nothing to do with the preformations of bonnet; in a sense they are the exact opposites of them; they are simply _those living parts of the germ whose presence determines the appearance of a definite organ of a definite character in { } the course of normal evolution_. in this form they appear to me to be an absolutely necessary and unavoidable inference from the facts. there _must_ be contained in the germ parts that correspond to definite parts of the complete organism, that is, parts that constitute the reason why such other parts are formed. it is conceded even by my opponents that the reason why one egg produces a chicken and another a duck is not to be sought in external conditions, but lies in a difference of the germinal substance. nor can they deny that a difference of germinal substance must also constitute the reason why a slight _hereditary_ difference should exist between two filial organisms. should there now, in a possible instance, be present between them a second, a third, a fourth, or a hundredth difference of hereditary character, each of which could vary from the germ, then, certainly, some second, third, fourth, or hundredth part of the germ must have been different; for whence, otherwise, should the heredity of the differences be derived, seeing that external influences affecting the organism in the course of evolution induce only non-transmissible and transient deviations? but the fact that every complex organism is actually composed of a very large number of parts independently alterable from the germ, follows not only from the comparison of allied species, but also and principally from the experiments long conducted by man in artificial selection, and by the consequent and not infrequent change of only a single part which happens to claim his interest; for example, the tail-feathers of the cock, the fruit of the gooseberry, the color of a single feather or group of feathers, and so on. but a still more cogent proof is furnished by the degeneration of parts grown { } useless, for this process can be carried on to almost any extent without the rest of the body necessarily becoming involved in sympathetic alteration. whole members may become rudimentary, like the hind limbs of the whale, or it may be only single toes or parts of toes; the whole wing may degenerate in the females of a butterfly species, or only a small circular group of wing-scales, in the place of which a so-called "window" arises. a single vein of the wing also may degenerate and disappear, or the process may affect only a part of it, and this may happen in one sex only of a species. in such cases the rest of the body may remain absolutely unaltered; only a stone is taken out of the mosaic. the assumption, thus, appears to me irresistible, that every such hereditary and likewise independent and very slight change of the body rests on some alteration of a _single_ definite particle of the germinal substance, and not as spencer and his followers would have it, on a change of _all_ the units of the germ. if the germinal substance consisted wholly of like units, then in every change, were it only of a single character, _each_ of these units would have to undergo exactly the same modification. now i do not see how this is possible. but it may be that spencer's assumption is the _simpler_ one? quite the contrary, its simplicity is merely apparent. whilst my theory needs for each modification only a modification of _one_ constitutional element of the germ, that is, of _one_ particle of the germinal substance, according to spencer _every_ particle of that substance must change, for they are all supposed to be and to remain alike. but seeing that all hereditary differences, be they of individuals, races, { } or species, must be contained in the germ, the obligation rests on these similar units, or rather the capacity is required of them, to produce in themselves a truly enormous number of differences. but this is possible only provided their composition is an exceedingly complex one, or only on the condition that in every one of them are contained as many alterable particles as according to my view there are contained determinants in the whole germ. _the differences that i put into the whole germ, spencer and his followers are obliged to put into every single unit of the germinal substance._ my position on this point appears to me incontrovertible so long as it is certain that the single characters can vary hereditarily; for, if a thing can vary independently, that is, _of its own accord_, and _from the germ_, then that thing must be represented in the germ by some particle of the substance, _and be represented there in such wise that a change of the representative particle produces no other change in the organism developing from the germ than such as are connected with the part which depends on it_. i conceive that even on the assumption of my constitutional elements (_anlagen_) the germ-plasm is complex enough, and that there is no need of increasing its complexity to a fabulous extent. be that as it may, the person who fancies he can produce a complex organism from a _really_ simple germinal substance is mistaken: he has not yet thoroughly pondered the problem. the so-called "epigenetic" theory with its _similar_ germinal units is therefore naught else than an evolution-theory where the primary constitutional elements are reduced to the molecules and atoms--a view which in my judgment is inadmissible. a _real_ { } epigenesis from absolutely _homogeneous_ and not merely _like_ units is not thinkable. all value has been denied my doctrine of determinants[ ] on the ground that it only shifts the riddles of evolution to an invisible terrain where it is impossible for research to gain a foothold. now i have indeed to admit that no information can be gained concerning my determinants, either with the aided or with the unaided eye. but fortunately there exists in man another organ which may be of use in fathoming the riddles of nature and this organ which is called the brain has in times past often borne him out in the assumption of invisible entities--entities that have not always proved unfruitful for science by reason of that defect, in proof whereof we may instance the familiar assumptions of atoms and molecules. probably the biophores also will be included under that head if the determinants should be adjudged utterly unproductive. but so far i have always held that assumptions of this kind _are_ really productive, if they are only capable of being used, so to speak, as a _formula_, whereby to perform our computations, unconcerned for the time being as to what shall be its subsequent fate. now, as i take it, the determinants have had fruitful results, as their application to various biological problems shows. is it no advance that we are able to reduce the scission of a form of life into two or several forms subject to separately continued but recurrent changes,--i refer to dimorphism and polymorphism,--that we are able to reduce such phenomena to the formula of male, female, and worker determinants? it has been, i think, { } rendered conceivable how these diverse and extremely minute adaptations could have developed side by side in the same germ-plasm, under the guidance of selection; how sterile forms could be _hereditarily_ established and transformed in just that manner which best suits with their special duties; and how they themselves under the right circumstances could subsequently split up into two or even into three new forms. surely at least the unclear conception of an _adaptively_ transformative influence of food must be discarded. it is true, we cannot penetrate by this hypothesis to the last root of the phenomena. the hotspurs of biology, who clamor to know forthwith how the molecules behave, will scarcely repress their dissatisfaction[ ] with such provisional knowledge--forgetful that _all our knowledge is and remains throughout provisional_. but i shall not enter more minutely into the question whether epigenesis or evolution is the right foundation of the theory of development, but shall content myself with having shown, first, that it is illusory to imagine that epigenesis admits of a simpler structure of the germ, (the precise opposite is true,) and secondly, that there are phenomena that can be understood only by an evolution-theory. such a phenomenon is { } the _guidance of variation by utility_, which we have considered to-day. for without primary constituents of the germ, whether they are called as i call them, determinants, or something else, _germinal selection_, or guidance of variation by personal selection, is impossible; for where all units are alike there can be no struggle, no preference of the best. and yet such a guidance of variation exists and demands its explanation, and the early assumptions of a "definitely directed variation" such as nägeli and askenasy made are insufficient, for the reason that they posit only _internal_ forces as the foundations thereof, and because, as i have attempted to show, the harmony of the direction of variation with the requirements of the conditions of life subsists and represents the riddle to be solved. _the degree of adaptiveness which a part possesses itself evokes the direction of variation of that part._ this proposition seems to me to round off the whole theory of selection and to give to it that degree of inner perfection and completeness which is necessary to protect it against the many doubts which have gathered around it on all sides like so many lowering thunder-clouds. the moment variation is determined substantially though not exclusively by the adaptiveness itself, all these doubts fall to the ground, with _one_ exception, that of the utility of the initial steps. but just this objection is the least weighty. without doubt the theory requires that the initial steps of a variation should also have selective value; otherwise personal selection and hence germinal selection could not set in. since, however, as i have before pointed out, _in no case can we pretend to a judgment regarding the selective value of a modification, or have any_ { } _experience thereof_, therefore the assumption that in a given case where a character is transformed the original initial steps of the variation did have selective value, is not only as probable as the opposed assumption that they had none, but is _infinitely more probable_, for with this we can give an intelligible explanation of the mysterious fact of adaptation, while with that we cannot. consequently, unless we are resolved to give up all attempts whatsoever at explanation, we are forced to the assumption that the initial steps of all actually affected adaptations possessed selective value. the principal and fundamental objection that selection is unable to create the variations with which it works, is removed by the apprehension that a germinal selection exists. natural selection is not compelled to wait until "chance" presents the favorable variations, but supposing merely that the groundwork for favorable variations is present in the transforming species, that is, supposing merely that in the constitutional basis of the part to be changed are contained components which render favorable variations possible by a change of their numerical ratio, then those variations _must_ occur, for the reason that quantitative fluctuations are always happening, and they must also be augmented as soon as personal selection intervenes and permanently holds over them her protecting hand. not only is the marvelous _certainty and exactitude_ with which adaptation has operated in so many individual cases, rendered intelligible in this manner, but what is more difficult, we are able to understand the _simultaneity_ of numerous and totally different modifications of the most diverse parts co-operant towards some collective end, such as we see so frequently occur, { } for example, in the simultaneous rise of instincts and protective similarities, or in the harmonious and simultaneous augmentation of two co-operant but independent organs, as of the eye and of the centre of vision, or of the nerve and its muscle, etc. the "secret law," of which wolff prophetically speaks in his criticism of selection, is in all likelihood naught else than germinal selection. this it is that brings it about that the necessary variations are always present, that symmetrical parts, for example, the two eyes, usually vary alike, but under circumstances may vary differently, for example, the two visual halves of soles; that homodynamic parts, (for instance, the member-pairs of arthropoda,) have frequently varied alike, and not infrequently and in conformity with the needs of the animal, have varied differently. it brings it about also that conversely species of quite different fundamental constitutions occasionally vary alike, as instances of mimicry and numerous other cases of convergence show us. as soon as utility itself is supposed to exercise a determinative influence on the direction of variation, we get an insight into the entire process and into much else besides that has hitherto been regarded as a stumbling-block to the theory of selection, and which did indeed present difficulties that for the moment were insuperable--as, for example, the like-directed variation of a large number of already existing similar parts, seen in the origin of feathers from the scales of reptiles. the utility in the last-mentioned instance consisted, not in the transformation of one or two, but of _all_ the scales; consequently the line of variation of _all_ the scales must have been started simultaneously in the same direction. a large part of the objections to the theory of selection { } that have been recently brought forward by the acutest critics, as for example by wigand, but particularly by wolff,[ ] find, as i believe, their refutation in this doctrine of germinal selection. the principle extends precisely as far as utility extends, inasmuch as it creates, not only the direction of variation for every increase or diminution demanded by the circumstances, but also every qualitative direction of variation attainable by changes of quantity, so far as that is at all possible for the organism in question. considering also the contrary process, the degeneration of useless parts by the cessation of selection in regard to the normal size of that part, a clear light is shed on that whole complex system of ascending and descending modifications which makes up most of the transformations of a living form, and we are led to understand how the fore extremity of a mammal can change into a fin at the same time that the _hinder_ extremity is growing rudimentary, or how one or two toes of a hoofed animal can continue to develop more and more powerfully, whilst the others in the same degree grow weaker and weaker until finally they have disappeared entirely from the germ of most of the individuals of the species. possibly some of that large body of inquirers, mostly paleontologists, who till now have considered the lamarckian principle indispensable for the explanation of these phenomena--perhaps some, i say, will not utterly close their eyes to the insight that germinal selection performs the same services for the understanding of observed transformations, particularly of { } the degeneration of superfluous parts, that a heredity of acquired characters would perform, without rendering necessary so violent an assumption. i have always conceded that many transformations actually do run parallel to the use and disuse of the parts,[ ] that therefore it does really look as if functional acquisitions of the individual life were hereditary. but if it be found that _passively functioning parts_, that is, parts which are not alterable during the individual life by function, obey the same laws and also degenerate when they become useless, then we shall scarcely be able to refuse our assent to a view which explains both cases. it certainly cannot be the physiological function which provokes modifications in the individual, which are then subsequently transmitted to the germ and in this way made hereditary, if _functionless parts also change_ when they become useless. it is precisely this _uselessness_, then, from which the initial impulse emanates, and the primary modification is not in the soma but in the germ. the lamarckians were right when they maintained that the factor for which hitherto the name of natural selection had been exclusively reserved, viz., _personal_ selection, was insufficient for the explanation of the phenomena. they were also right when they declared that panmixia in the form in which until recently i held the theory was also insufficient to explain the degeneration of parts that had grown useless, but they { } erred when they ascribed hereditary effects to the selection-processes which are enacted among the parts of the body (wilhelm roux) and which are rightly regarded as the results of functioning. and they did this, moreover, as they themselves admit, not because the facts of heredity directly and unmistakably required it, but because they saw no other possibility of explaining many phenomena of transformation. i am fain to relinquish myself to the hope that now after another explanation has been found, a reconciliation and unification of the hostile views is not so very distant, and that then, we can continue our work together on the newly laid foundations. that the application of the malthusian principle was thoroughly justified is now clear. _the entire process of the development of living forms is guided by this principle._ the struggle for existence, _videlicet_, for food and propagation, takes place at all the stages of life between all orders of living units from the biophores recently disclosed upwards to the elements that are accessible to direct observation, to the cells, and still higher up, to individuals and colonies. consequently, in all the divers orders of biological units lying between the two extremes of biophores and colonies, the modifications must be controlled by selective processes; therefore, these govern every change of living forms no matter what its significance, and bring it about that the latter fit their conditions of life as wax does the mould; and the various stages of these processes, as enacted between the divers orders of biological units, in all organisms not absolutely simple, are involved in incessant and mutual interaction. the three principal stages of selection, that of { } _personal_ selection[ ] as it was enunciated by darwin and wallace, that of _histonal_ selection as it was established by wilhelm roux in the form of a "struggle of the parts," and finally that of _germinal selection_ whose existence and efficacy i have endeavored to substantiate in this article--these are the factors that have co-operated to maintain the forms of life in a constant state of viability and to adapt them to their conditions of life, now modifying them _pari passu_ with their environment, and now maintaining them on the stage attained, when that environment is not altered. everything is adapted in animate nature[ ] and has been from the first beginnings of life; for adaptiveness of organisation is here equivalent to the power to exist, and they alone have had the power to exist who have permanently existed. _we know of only one natural principle of explanation for this fact--that of selection { } of the picking out of those having the power to exist from those having the power to originate._ if there is any solution possible to the riddle of adaptiveness to ends,--a riddle held by former generations to be insoluble,--it can be obtained only through the assistance of this principle of the self-regulation of the originating organisms, and we should not turn our faces and flee at the sight of the first difficulties that meet its application, but should look to it whether the apparent effects of this single principle of explanation are not founded in the imperfect application that is made of it. if i am not mistaken the situation is as follows: we had remained standing half way. we had applied the principle, but only to a portion of the natural units engaged in struggle. if we apply the principle throughout we reach a satisfactory explanation. selection of _persons_ alone is _not sufficient_ to explain the phenomena; _germinal_ selection must be added. germinal selection is the last consequence of the application of the principle of malthus to living nature. it is true it leads us into a terrain which cannot be submitted directly to observation by means of our organs of touch and by our eyes, but it shares this disadvantage in common with all other ultimate inferences in natural science, even in the domain of inorganic { } nature: in the end all of them lead us into hypothetical regions. if we are not disposed to follow here, nothing remains but to abandon utterly the hope of explaining the adaptive character of life--a renunciation which is not likely to gain our approval when we reflect that by the other method is actually offered at least in principle, not only a broad insight into the adaptation of the single forms of life to their conditions, but also into the mode of formation of the living world as a whole. the variety of the organised world, its transformation by adaptation to new, and by reversed adaptation to old conditions, the inequality of the systematic groups, the attainment of the same ends by different means, that is, by different organisations, and a thousand and one other things assume on this hypothesis in a certain measure an intelligible form, whilst without it they remain lifeless facts. and so in this case, i may say, that again doubt is the parent of all progress. for the idea of germinal selection has its roots in the necessity of putting something else in the place of the lamarckian principle, after that had been recognised as inadequate. that principle did, indeed, seem to offer an easy explanation of many phenomena, but others stood in open contradiction to it, and consequently that was the point at which the lever had to be applied if we were to penetrate deeper into the phenomena in question. for it is at the places where previous views are at variance with facts that the divining rod of the well-seekers must thrice nod. there lie the hidden waters of knowledge, and they will leap forth as from an artesian well if he who bores will only drive undaunted his drill into their depths. * * * * * { } appendix. * * * * * i. the rejection of selection. many years ago semper[ ] denied the power of selection to create an organ, declaring that the organ must have previously existed before selection could have increased and developed it. more recently wolff[ ] has distinguished himself by the vigor with which he has attacked the "task" of "setting aside the dogma of selection." henry b. orr[ ] is also of opinion that selection is not the real cause of improved organic states; he regards it as a factor checking growth in certain directions, but not as a cause producing growth. likewise yves delâge,[ ] in his recent voluminous but in many respects excellent work, regards natural selection solely as a subordinate principle which is devoid of all power to create species (p. ), although he grants to it certain functions, and even characterises it { } as "an admirable and perfectly legitimate principle" (p. ). a more pronounced opponent of selection, of any kind, as a principle creating species, is the rev. mr. henslow,[ ] whose views we shall discuss later, in division vii. of this appendix. finally, must be mentioned the name of th. eimer, as that of a pronounced and bitter enemy of the theory of selection. i shall leave it to others to decide whether he can properly be called an "opponent" of the principle, in the scientific acceptance of the word. i can see in the blind railings of the tübingen professor nothing but a reiteration of the same unproved assertions, mingled with loud praises of his own doughty performances and captious onslaughts on every one who does not value them as highly as their originator.[ ] the lack of confidence latterly placed in the theory of selection even by professed adherents of the doctrine, is well shown by such remarks as the following { } from emery,[ ] who says: "some pupils of darwin have gone beyond their master and discovered in natural selection the sole and universal factor controlling variations. thus there has arisen in the natural course of things a reaction, especially on the part of those who, while they accept evolution, will have naught to do with natural selection or darwinism as they call it." emery then professes himself a darwinian, although not in the sense of wallace and "other co-workers and pupils of darwin." for him "natural selection is a very important factor in evolution, and in determining the direction of variation plays the highest part; but it is far from being the only factor and is probably also not the most efficient factor." not the most efficient factor but plays the highest part! * * * * * ii. chemical selection. if we refer adaptation to selection, we have also to trace back to this source the origin of the organic combinations which make up the various tissues of the body and which go by the collective name of muscular, nervous, glandular substance, etc. lloyd morgan has prettily likened the vital processes to the periodic formation and discharge of explosive substances.[ ] unstable combinations are upon the application of a { } stimulus suddenly disintegrated into simpler and more stable compounds; through this disintegration they evoke what is called the function of the disintegrating part--for example, certain changes of form (muscular contractions) or the excretion of the disintegrated products, etc. now how is it possible that such unstable chemical combinations, answering exactly to the needs of life, could have arisen in such marvellous perfection if the _useful_ variations had not always been presented to the ceaselessly working processes of selection? or, if the constantly increasing adaptation to the constantly augmenting delicacy of operation of physiological substances had depended in its last resort on _accidental_ variations? hence, not only with regard to the "form" of organs, but also with regard to the chemical and physiological composition of their materials, we are referred to the constant presence of appropriate variations. * * * * * iii. variation and mutation. i have still to add a few remarks on the subject touched on in the footnote at page . the view there referred to was discussed by professor scott before in an article published in the _american journal of science_, vol. xlviii., for november, , entitled "on variations and mutations." following the precedent of waagen and neumayr, scott sharply discriminates between the inconstant vacillating variations which it is supposed [?] produce simultaneously occurring "varieties," and "mutations," or the successively evolved _time_-variations of a phylum, which constitute the stages of phyletic development. the facts on which this view is based are those already { } adduced in the text--the _zielstrebigkeit_ (to use k. e. von bär's phraseology) displayed in the visible paleontological development, the directness of advance of the modifications to a final "goal." "the direct, unswerving way in which development proceeds, however slowly, is not suggestive of many trials and failures in all directions save one." and again, "the march of transformation is the resultant of forces both internal and external which operate in a _definite manner_ upon a changeable organism and similarly affect _large numbers of individuals_." the two points which i have here italicised are actually the facts which separate phylogenetic from common individual variation: the definite _manner_ of the change, repeated again and again without modification, and its occurrence in a _large number of individuals_. still the two are not solely a result of observation, deduced from paleontological data; they are also _a consequence of the theory of selection_, as was shown in the text. if the theory in its previous form was unable to fulfil this requirement, it is certainly now able to do so after germinal selection has been added, and it is not in any sense necessary to assume a difference of _character_ between phylogenetic and ontogenetic variations. bateson and scott are wrong in imagining that i ask them "to abrogate reason" in pronouncing the "omnipotence of natural selection." on the contrary, the theory seems to me to accord so perfectly with the facts that we might, by reversing the process, actually construct the facts from the theory. what other than the actual conditions could be expected, if it is a fact that selection favors only the useful variations and singles them out from the rest by producing them in { } increasing distinctness and volume with every generation, and also in an increasing number of individuals? the mere displacement of the zero-point of useful variations alone must produce this effect, especially when it is supported by germinal selection. it is impossible, indeed, to see how considerable, that is perceptible, deviations could arise at all on the path of phyletic development if in each generation a large number of individuals always possessed the useful, that is, the phyletic variations? in fact, by the assumption itself, the difference between useful and less useful variations is merely one of degree, and that a slight one. hence, as i before remarked at page , i see no reason for assuming two kinds of hereditary variations, _distinct as to their origin_, such as scott and the other palæontologists mentioned have been led to adopt, although with the utmost caution. i believe there is only one kind of variation proceeding from the germ, and that these germinal variations play quite different rôles according as they lie or do not lie on the path of adaptive transformation of the species, and consequently are or are not favored by germinal selection. to repeat what i have said in the footnote to page only a relatively small portion of the numberless individual variations lie on the path of phyletic advancement and so mark out under the _guidance_ of germinal selection the way of further development; and hence it would be quite possible to distinguish continuous, _definitely directed_ variations from such as fluctuate hither and thither with no uniformity in the course of generations. the origin of the two is the same; they bear in them nothing that distinguishes the one from the other, and their success alone, that { } is, the actual resultant phyletic modification, permits their being known as phyletic or as vacillating variations. uncertain fluctuations along the path of evolution are what the geologists would be naturally led to expect from the theory of selection, but which they were unable to discover in the facts; it is evident, however, that these fluctuations are not a logical consequence of the theory of selection as that is perfected by germinal selection, and there seems to me to be no reason now for attributing "variations" to the union of changing hereditary tendencies, while "mutations" are ascribed to the effect "of dynamical agencies acting long in a uniform way, and the results controlled by natural selection." the idea which the grecian philosophers evolved of the thousands of non-adaptive formations that nature brings forth by the side of adaptive ones, and which must subsequently all perish as being unfit to live, is certainly correct in its ultimate foundations. but it is in need of far more radical refinement than it underwent in the hands of empedocles, or than it seems likely to undergo at the hands of many contemporary inquirers. we know now that nature did not produce isolated eyes, ears, arms, legs, and trunks, and afterwards permit them to be joined together just as the play of the fundamental forces of love and hatred directed, leaving the monsters to perish and granting permanent existence only to harmonious products. yet there is a weak echo of this conception, although infinitely far removed from its prototype, in the question as to where all the non-adaptive individuals are preserved that have perished in the struggle for existence and been eliminated from development by selection? where, for example, are the fossil remains { } of the rejected individuals in the line of the horses? certainly they should be forthcoming in far larger numbers than the individuals lying directly in the path of development, for by our very assumption the latter were greatly in the minority in every generation. doubtless the question would be a proper one if our eyes were sufficiently keen-sighted to assign the life-value of the various minute differences that distinguish the "better" from the "worse" individuals of every generation. but this is a task which we can accomplish at best only with selective processes which are artificially directed by ourselves, as in the case of doves and chickens, and even there only with the utmost difficulty and only with reference to a single characteristic and not with any species which to-day exists in the state of nature. picture, then, the difficulties attending such a task as applied to the meagre fossilic bones of prehistoric species, touching which the richest discoveries never so much as remotely approach to the actual number of individuals that have lived together for a _single_ generation in the same habitat. if the differences between good and bad in a single generation were striking enough to be immediately remarked _as such_ in fossil bones, the development of species would take place so rapidly that we could directly witness it in living species. * * * * * iv. remarks on the history of definitely directed variations. as to the attempt here made to apply the selective process to the elements of the germinal substance (the idioplasm) and thus to acquire a foothold for definitely directed variation not blind in its tendency but { } proceeding in the direction of adaptive growth, it is remarkable that the same was not made long ago by some one or other of the many who have thought and written on selection and evolution. allusions to a connexion between the direction of variation and the selective processes are to be found, but they remained unnoticed or undeveloped. i have been able to find at least two such observations, but would not wish to assert that there are not more of them hidden somewhere in the literature of the subject. one of them is old and comes from fritz müller. it was appended by his brother hermann as a "supplementary remark" to his book _die befruchtung der blumen durch insecten_ ( ) and is dated november , . we read there: "my brother fritz müller communicates to me in a letter which reached my hands only after the bulk of the present work had passed through the press, the following law discovered by him, which materially facilitates the explanation by natural selection of the pronounced characters of sharply distinguished species: 'the moment a choice in a definite direction is made in a variable species, progressive modification from generation to generation in the same direction will set in as the result of this choice, wholly apart from the influence of external conditions. transformation into new forms is thus greatly facilitated and accelerated.'" the facts on which f. müller based the enunciation of his law, are the results of several experiments with plants, the numbers of whose grains (maize), or styles, or flowering leaves, were, by the exercise of choice in the cultivation, made to change in definite directions. accurately viewed their significance is the same as that of numerous other cases of artificial selection, for { } example, that of the long-tailed japanese cock which was laid at the foundation of the theory in the text, although the numerical form of the observation gives more precision and distinctness to the reasoning based on them, than is to be observed in cases where we speak of characters as being simply "longer" or "shorter." f. müller's opinion regarding the increase of characters by selection is expressed as follows: "the simplest explanation of these facts appears to be that every species possesses the faculty of varying within certain limits; the crossing of different individuals, so long as no choice is effected in a definite direction, maintains the mean round which the oscillations take place at the same points, and consequently the extremes also remain unaltered. if, however, one side is preferred by natural or artificial selection, the mean is shifted in the direction of this side and accordingly the extreme forms are also displaced towards that side, going now beyond the original limit. however, this explanation does not satisfy me in all cases." it is not known to me that f. müller ever returned to this conception subsequently to the year or gave further developments of the same, nor have i been able to discover that it has been mentioned by other writers or incorporated in previous notions regarding selection. the second naturalist who has approached the fundamental idea of my doctrine of germinal selection, is a more recent writer. i refer to the english botanist thiselton-dyer, a scientist whose occasional utterances on the general questions of biology have more than once evoked my sympathetic approval. in an article, "variation and specific stability," which appeared in { } _nature_ for march , , this author enunciates twenty theses touching this subject, many of which appear to me apposite and correct, particularly the following: in every species there is a mean specific form round which the variations are symmetrically grouped like shots around the bull's eye of a target. as soon as natural selection comes into play and favors one of these variations it must shift the centre of density. variations arise by a change in the outward conditions of life and can be useful or indifferent; only in the first case will natural selection obtain control of them and "the new variation will get the upper hand and the centre of density will be shifted." this is not germinal selection, but it is the same as what i have referred to in this and in the preceding essay as displacement of the zero-point of variation. thiselton-dyer did not draw the conclusion that a definitely directed variation answering to utility resulted from this process, which variation alone must cause the disappearance of useless parts, for the reason that he never attempted to penetrate to the causes of the shifting of the zero-point of variation. neither fritz müller, whose utterances thiselton-dyer was obviously ignorant of, nor thiselton-dyer himself pushed his inquiries beyond the thought that the shifting in question resulted entirely in consequence of personal selection. there is no gainsaying that the degeneration of useless organs cannot be explained by personal selection alone, seeing that though the minus variations may possibly have a selective value at the beginning of a degenerative process, they certainly cannot have such in the subsequent course of the same, when the organ has dwindled down to a really minimal mass of substance as compared with the whole { } body. of what advantage would it be to the whale if his hinder leg, now concealed in a mass of flesh and no longer protruding beyond the skin, should still be reduced one or several centimetres in size? (spencer.) if the minus variations have no selective value, how can the upper limit of the variational field be constantly displaced downwards, as actually happens? it is unquestionable but something different from personal selection must come here co-determinatively into play. * * * * * v. historical remarks concerning the ultimate vital units. (for this appendix which is marked "appendix v." in the german edition of _germinal selection_ see the footnote at page .) * * * * * vi. the initial stages of useful modifications. in characterising as "least" weighty the old objection that the variations are too small at the start to be useful and to be selected, i find myself diametrically opposed to many writers of the present day, who have taken up with renewed vigor this old stumbling block to the principle of selection. bateson[ ] regards the deficient proof of the utility of initial stages as the most serious objection that can be made to natural selection. new organs must in the necessity of the case have first been imperfect; how, then, could they have been selected since imperfect organs cannot be useful? answers from various quarters have already been { } made to this and to similar objections, and darwin himself has referred to the fact that even the smallest variations may have selective value; dohrn, too, has urged his principle of change of functions, which with regard to this question of the utility of initial stages has certainly a wide significance. still, every transformation and new structure in the narrow sense of the word does not rest on change of function, and neither darwin nor wallace, nor any other more recent champion of the principle of selection, can ever succeed in demonstrating in _every_ case the selective value of an initial stage. one reason why this cannot be done is because _in no case of morphological variation do we really know what these initial stages are_. to say that "new organs were at first necessarily imperfect" appears obvious enough, but it is at bottom a meaningless assertion, for it is not only possible but certain, that "imperfect" organs may still have selective value, and in by far the most cases have had selective value. the fact that we see to-day a long graduated line of forest-butterflies which possess resemblance to leaves and by this means are able in a measure to conceal themselves from prying eyes, yet that this resemblance in many species is very imperfect, in others more perfect, and in a very small number very perfect, simply proves that even "imperfect" formations may be of utility. the word "imperfect" in this connexion is itself very imperfect, for it is utterly anthropomorphic and estimates the biological value of a structure by our own peculiar artistic notions of its faithfulness to a leaf-copy, whilst we are really concerned here only with its protective value for the species in question, which is by no means dependent merely on the faithfulness of the copying, on the { } faithfulness of the imitation, but on numerous other factors, such as the frequency and sharp-sightedness of the enemies of the species, the fertility of the species, their frequency and persecution in earlier developmental stages, and so forth, in brief, on their need of protection on the one hand and on their other means of protection on the other. now all this cannot be exactly calculated in any given case, and it will be better, instead of haggling about individual cases concerning which we can never judge with certainty, to take the position adopted in the text and say: since the utility of the initial stages _must_ be assumed unless we are to renounce forever the explanation of adaptation, let us then take it for granted. no contradiction of facts is involved in this assumption; in fact, even individual variations exist whose eventual utility can be demonstrated, for example, the invisible differences enabling europeans of certain constitutions to resist the attacks of tropical malarial fevers,--or the differences of structure, likewise not directly visible, which enable palms from the summits of the cordilleras to withstand our winter climate better than palms of the same species from along the base-line of the mountains; and so on. * * * * * vii. the assumption of internal evolutionary forces definite variation was not only postulated in the last decade by nägeli and askenasy, but has also been repeatedly set up in recent years by various other authors. the rev. george henslow, in his book _the origin of species without the aid of natural selection_, , regards the variations occurring in the state { } of nature as always definite and not with darwin as indefinite, and meets the objection that modification but not adaptation to outward conditions of life can be inferred from this fact, by the bold assumption that it is precisely the outward conditions of life or the environment which "induces the best fitted to arise." he further concludes that natural selection has nothing to do with the origin of species. at the basis of his conviction lies the naturally correct view that the summation of _accidental_ variations is insufficient for transforming the species, but that definitely directed variation is necessary to this end. but concerning the way in which external conditions are always able to produce the fit variations, he can give us no information--if i am not mistaken, for the simple reason that such is not the fact, that the outward conditions only apparently determine the direction of variations whilst in truth it is the adaptive requirement itself that produces the useful direction of variation by means of selectional processes within the germ. c. lloyd morgan also has recently expressed himself in favor of the necessity of definite variation, though likewise without assigning a basis for its action, and without being able to show how its efficacy is compatible with the plain fact of adaptation to the conditions of life. he seeks to find the origin of variation in "mechanical stresses and chemical or physical influences," but this conception is too general to be of much help. he has, in fact, not been able to abandon completely the heredity of acquired characters. emery[ ] likewise sees only the alternative of a { } "definitely directed variation" from internal causes and of a summation of "accidental" variations. he says: "a summation of entirely accidental variations in a given direction is extremely difficult," because "natural selection thus always awaits its fortune at the hands of accident whereby it is possible that the little good thereby produced will be swept away by other accidents (disadvantages of position) or obliterated in the following generations by unfortunate crossings." we can, therefore, continues emery, well conceive "how many scientists look upon the whole theory of selection as a fable, or else throw themselves into the arms of lamarckism." unquestionably emery has here singled out the insufficient points in the assumption of a selection of "accidental" variations; he has recognised the necessity of operating, not with single variations, but with "directions of variation." he has not, however, attempted the derivation of directed tendencies of variation from known factors; he apparently thinks of them as of something which has sprung from unknown constitutional factors and consequently ascribes to them the capacity of shooting beyond their mark, so to speak, that is, of acting beyond and ahead of utility, and so of producing modifications which may lead to the destruction of the species. * * * * * { } index. accidental variations, , . acquired variations, . acracids, . acræa, . active selection, . adaptations, , , , , . adaptiveness, footnote, , et seq. ageronia, . anæa, . _anlagen_, , , . arthropoda, , . articulata, . artificial selection, . askenasy, , , . atoms, , . bär, k. e. von, . bateson, , , . "better" individuals, . biology, character of research in, . biophores, , , . boltzmann, , . bonnet, . bourne, footnote, . brücke, . butterflies, et seq., et seq., . catonephele, . chance, . chemical selection, . chitons, . coadaptation, . colorings, protective, et seq. constancy of species, . constructs, . cormi, footnote. correlation, . danaids, . darwin, , , , , , , , . definite variation, , , , - , . degeneration, et seq., et seq. , , , . delâge, yves, , . determinants, et seq., , et seq. , , . developmental mechanics, , . de vries, . dimorphism, . directions of variations, . directive forces, , . dixey, footnote. dohrn, . driesch, hans, . dyer, thiselton, - . eimer, , . emery, , - . empedocles, . epigenesis, footnote, , . euploids, . europeans, exempt from malarial fevers, . eurypheme, . evolution, footnote, . fireworks, determinants and ids compared to, . "fits," footnote. fluctuations of development, - . formative laws, et seq., . frog, . functional adaptation, . functionless parts, . galton, . germs, et seq., et seq. { } germinal selection, , , , - , , , - . germinal substance, et seq. germ-plasm, , , . haase, eric, . heliconids, , , footnote. henslow, g., , . heredity, et seq. hertwig, o., footnote, , . hertz, , . histonal selection, . huxley, thomas, . hypna, . hypotheses, nature of, et seq. ids, their theoretical character, . imagination, its function in science, . "imperfect" formations, . individual variations, et seq. inertia, law of organic, . internal forces of evolution, , , , , , - . intrabiontic selection, . ishikawa, professor, . japanese cocks, long-tailed, , , . kallima, , , . katagramma, . knowledge, its character, . lamarckian principles, , et seq., et seq., , - , , . leaves, imitated by butterflies, et seq. locomotive, simile of, . malthusian principle, , . markings, butterflies', et seq. maxwell, , . mean of variation, - . meristic, . mimicry, , et seq. minot, s., footnote. models, mental, et seq. molecules, . morgan, prof. c. lloyd, , , . müller, fritz, - . müller, hermann, . mussels, . mutations, footnote, - . nägeli, , , , , . neumayr, . newton, . nutrition of determinants, , , , . nymphalidæ, . ontogenesis, . orr, henry b., . osborn, prof. h. f., . owen, richard, . paleontology, , , , . palms from cordilleras, . pangenes, . panmixia, , , , , . papilio, , . parallecta, . parts, struggling of the, , , - . passively functioning parts, et seq., . personal selection, , , , , , - , . phyletic variation, - footnote. phylogenesis, . phylogenetic variations, - , . plasomes, . plus and minus variations, , , , , - . polymorphism, . poulton, footnote. predestined variation, . pre-established harmony, . preformation, . protective colorings, et seq. protogonius, . pseudocræa, . qualitative modifications, . quantitative changes, - . retrogressive development, . round-worms, eggs of, . roux, wilhelm, , , , . salamis, . scott, prof. w. b., footnote, - . segmentation, . { } selection, natural, , et seq., , , , - , , . selective value of variations, . semper, . siderone, . snails, . spencer, , , , , , , . struggle for existence, . survival of the fit, . symphædra, . _tabula rasa_, , . tegetmeier, w. b., . teleological principles, , , . theories, nature of, et seq. turbellaria, . units, vital, biological, physiological, etc., , , , , , , . useful modifications, value of initial stages of, - . utility, , , , , , , , . variations, necessary, their constant presence, et seq., et seq., ; generally, , - , , et seq. waagen, . wallace, , , , , , . weldon, . whale, hind leg of, , , . whitman, c. o., . wiesner, . wigand, albert, , . wings of butterflies, et seq., - , . wolff, k. f., , , , . "worse" individuals, . zero-point of variation, et seq., , , . * * * * * notes [ ] _neue gedanken zur vererbungsfrage, eine antwort an herbert spencer._ jena. . [ ] see boltzmann, _methoden der theor. physik_, munich, . (in the catalogue of the mathematical exhibit.) [ ] of late this saying of newton's is frequently quoted as if newton were a downright contemner of scientific hypotheses. but if we read the passage in question in its original context, we shall discover that his renunciation of hypotheses referred solely to a definite case, viz., to that of universal gravitation, of whose character newton could form no conception and hence was unwilling to construct hypotheses concerning it. indeed, such a wholesale repudiation of hypotheses is antecedently incredible on the part of the inventor of the emission-theory of light, in which, to speak of only one daring conjecture, "fits" were ascribed to the luminous particles. compare newton, _philosophiae naturalis principia mathematica_, second edition, , page . [ ] h. hertz, _die principien der mechanik_. [ ] hans driesch, _die biologie als selbstständige grundwissenschaft_, leipsic, , p. , footnote. the sentence reads: "an examination of the pretensions of the refuted darwinian theory, so called, would be an affront to our readers." [ ] _die allmacht der naturzüchtung._ a reply to herbert spencer. jena, , p. et seq. [also in the _contemporary review_ for september, .] [ ] that is, by the law of exceedingly slow retrogression of superfluous characters, which may be designated the law of organic inertia. [ ] _materials for the study of variation with especial regard to discontinuity in the origin of species._ london, . [ ] _studien zur descendenztheorie_, leipsic, . vol. ii. pp. and . [ ] compare my essay, _neue gedanken zur vererbungsfrage_, jena, , p. , second footnote. [ ] on the same day on which the present address was delivered at the international congress of zoölogists in leyden, and on the same occasion, dr. w. b. scott, professor of geology in princeton college, new jersey, read a very interesting paper on the tertiary mammalian fauna of north america, in which, without a knowledge of my paper, he took his stand precisely on this argument and arrived at the opinion that it could not possibly be the ordinary individual variations which accomplished phyletic evolution, but that it was necessary to assume in addition phyletic variations. i believe our views are not as widely remote as might be supposed. of course, i see no reason for assuming two kinds of hereditary variations, different _in origin_. still it is likely that only a relatively small portion of the numberless individual variations lie on the path of phyletic advancement and so under the _guidance_ of germinal selection mark out the way of further development; and hence it would be quite possible in this sense to distinguish continuous, _definitely directed_ individual variations from such as fluctuate hither and thither with no uniformity in the course of generations. the root of the two is of course the same, and they admit of being distinguished from each other only by their success, phyletic modification, or by their failure. [ ] h. f. osborn, "the hereditary mechanism and the search for the unknown factors of evolution," in _biological lectures delivered at the marine biolog. lab. at wood's holl in the summer session of _. boston, . [ ] in . see my paper on "retrogression in nature," published in english in nos. , , , and of _the open court_, and also in my essays on _heredity_, jena, . [ ] _neue gedanken zur vererbungsfrage_, jena, . [ ] delâge, in _la structure du protoplasma et les théories sur l'hérédité_, etc., paris, , is mistaken in attributing to herbert spencer the merit of having first pointed out the necessity of the assumption of biological units ranking between the molecule and the cell. brücke set forth this idea three years previously to spencer and established it exhaustively in a paper which in germany at least is famous ("elementarorganismen," _wiener sitzungsberichte_, october , , vol. xliv., ii., p. ). spencer's _principles of biology_ appeared between and ; consequently there can be no dispute touching the priority of the idea. strangely enough delâge cites brücke's essay in the bibliographical index at the end of his book correctly, although brücke's name and views are nowhere mentioned in the book itself. it is to be observed, however, that the elementary organisms of brücke are not merely the precursors of spencer's "physiological units," but repose on much firmer foundations than the latter, which, as delâge himself remarks, are at bottom nothing more than magnified molecules and not combinations of different molecules of such character as to produce necessarily phenomena of life. he aptly remarks on this point: "the physiological units of spencer are only chemical molecules of greater complexity than the rest, and as he defines them they would be regarded as such by every chemist. he attributes to them no property _essentially_ different from those of chemical molecules." assimilation, growth, propagation, in short the attributes of life, are not attributed by spencer to his units, while brücke by his very designation "elementary organisms" expresses the idea of "ultimate living units," to use wiesner's phrase. of course this particular aspect of the vital units was not emphasised by brücke with the same distinctness and sharpness as by recent inquirers, who took up brücke's ideas thirty years after. i refer to the conception that the union of a definite combination of heterogeneous molecules into an invisibly small unit, forms the cradle or focus of the vital phenomena. this was first done and apparently on independent considerations by de vries, and soon after by wiesner, and subsequently by myself (de vries, _intracelluläre pangenesis_, jena, ; wiesner, _die elementarstructur and das wachsthum der lebenden substanz_, vienna, ; weismann, _das keimplasma_, jena, ). let me say at the close of this note that it is not my intention in thus defending the rights of a great physiologist, to censure in the least the distinguished author of _l'hérédité_ who has set himself a remarkably high standard of exactitude in such matters. certainly, when we consider the enormous extent of the literature that had to be mastered to produce his book, embracing as it did all the various theories of recent times, such an oversight is quite excusable. [ ] i speak here of determinants, not of groups of determinants, which is the more correct expression, merely for the sake of brevity. it is a matter of course that a whole extremity, such as we have here chosen, cannot be represented in the germ by a single determinant only, but requires a large group of determinants. [ ] that this is not so in all cases has recently been shown by dixey from observations on certain white butterflies of south america which mimic the heliconids and in which a small, yellowish red streak on the under surface of the hind wing has served as the point of departure and groundwork of the development of a protective resemblance to quite differently colored heliconids. "on the relation of mimetic characters to the original form," in the _report of the british association for _. [ ] oscar hertwig, _zeit-und streitfragen der biologie_, jena, . it is customary now to look upon the preformation-theory of bonnet as a discarded monstrosity, and on the epigenesis of k. f. wolff as the only legitimate view, and to draw a parallel between these two and what might be called to-day "evolution" [i. e. unfoldment] and epigenesis. the evolution, or unfoldment, of bonnet and harvey, however, was something totally different from modern doctrines of evolution, and whitman is quite right when he says that even my theory of determinants would have appeared to the inquirers of the last century as "extravagant epigenesis." biologists in that day were concerned with quite different questions from what they are at present, and although now we probably all share the conviction of wolff that new characters do arise in the course of evolution, yet the acceptance of this view is far from settling the question _as to how these new characters are established in the germ-substance_--for in this substance they certainly must have their foundation. when, therefore, o. hertwig laments over my regarding evolution and not epigenesis as the correct foundation of the theory of development, his sorrow is almost as naïve as is the statement of bourne that epigenesis is a fact and not a theory "a statement of morphological fact," _science progress_, april, , page ), or, as is the latter's unconsciousness that facts originally receive their scientific significance from thought, i. e. from their interpretation and combination, and that thought is theory. and when s. minot, as the leader of the embryologists, carries his zeal to the pitch of issuing a general pronunciamento against me as a corruptor of youth, in which he declares it to be a "scientific duty to protest in the most positive manner against weismann's theory," i wonder greatly that he does not suggest the casting of a general ballot in the matter. (see the _biologisches centralblatt_ of august , .) we see how with these gentlemen the wisdom of the recitation-room regarding the infallibility of epigenesis has grown into a dogma, and whoever ventures to disturb its foundations must be burnt as a heretic. [ ] oscar hertwig, _zeit- und streitfragen der biologie_, jena, . [ ] nor will those, who demand a demonstration of "how the biophores and determinants are constituted in every case, and must be arranged in the architecture of the germ-plasm." (o. hertwig, _loc. cit._, p. ). as if any living being could have the temerity even so much as to guess at the actual ultimate phenomena in evolution and heredity! the whole question is a matter of symbols only, just as it is in the matter of "forces," "atoms," "ether undulations," etc., the only difference being that in biology we stumble much earlier upon the unknown than in physics. [ ] "beiträge zur kritik der darwin'schen lehre," _biologisches centralblatt_, vol. x., p. . . [ ] poulton has adverted to the fact that this is nevertheless not always the case; for example, it is not so with the teeth, whose shape it had also been sought to reduce to the mechanical effects of pressure and friction. see "the theory of selection" in _the proceedings of the boston society of natural history_, vol. xx., page . . [ ] as the highest stage of selective processes must be regarded that between the highest biological units, the colonies or cormi--a stage, however, which is not essentially different from personal selection. in this stage the persons enact the part that the organs play in personal selection. like their prototypes they also battle with one another for food and in this way maintain harmony in the colony. but the result of the struggle endures only during the life of the individual colony and can be transmitted through the germ-cells to the following generation as little as can histological changes provoked by use in the individual person. only that which issues from the germ has duration. [ ] this statement has often been declared extravagant, and it is so if it is taken in its strict literalness. on the other hand, it would also seem, by a more liberal interpretation, as if there existed non-adaptive characters, for example, rudimentary organs. adaptiveness, however, is never absolute but always conditioned, that is, is never greater than outward and inward circumstances permit. moreover, an organ can only disappear gradually and slowly when it has become superfluous; yet this does not prevent our recognising every stage of its degeneration as adapted when compared with its precursor. further, it does not militate against the correctness of the above proposition that there are also characters whose fitness consists in their being the necessary accompaniments of other directly adapted features, as, for instance, the red color of the blood. [ ] semper, _die natürlichen existenzbedingungen der thiere_, leipsic, , pp. - . [ ] wolff, "beiträge zur kritik der darwin'schen lehre," _biolog. centralblatt_, vol. x., sept. , , and "bemerkungen zum darwinismus mit einem experimentellen beitrag zur physiologie der entwicklung," _biolog. centralblatt_, vol. xiv., sept. , . [ ] henry b. orr, _a theory of development and heredity_, new york, . [ ] yves delâge, _la structure du protoplasma et les théories sur l'hérédité et les grands problèmes de la biologie générale_, paris, . [ ] henslow, _the origin of species without the aid of natural selection, a reply to wallace_. . [ ] if any one should deem these words too severe, let him read the sarcastic passages in which eimer has dispatched the late unfortunate eric haase who had been presumptuous enough to oppose the tübingen professor's deliverances on certain points. haase, as we all know, fell a victim to the climate of the tropics, shortly after resigning the post of director of the natural science collections in bangkok, in order to return to germany and to work out the fruits of his tropical sojourn. the unfortunate end of this accomplished man who had rendered important services to science had no effect in mollifying the resentment of herr eimer at the opposition which his views had encountered; and in twenty printed pages he takes him to task in the most personal and rancorous manner for this affront, remarking at the close: "in the meantime herr haase has died. nevertheless i owe it to myself, in spite of this occurrence, to make public the foregoing facts, in order," etc. any one who is interested in knowing the motives of herr eimer's excuse may find them in his book _artbildung and verwandtschaft bei den schmetterlingen_, part ii., p. . [ ] "gedanken zur descendenz- und vererbungstheorie." _biolog. centralblatt_, july , . [ ] c. lloyd morgan, _animal life and intelligence_, london, - , p. - . [ ] _materials for the study of variation with especial regard to discontinuity in the origin of species_, london, , p. . [ ] "gedanken zur descendenz- and vererbungstheorie," _biolog. centralblatt_, , vol. xiii., p. . _nature series_ are the effects of use and disuse inherited? _an examination of the view held by spencer and darwin_ by william platt ball london macmillan and co. and new york _the right of translation and reproduction is reserved_ richard clay and sons, limited, london and bungay. preface. my warmest thanks are due to mr. francis darwin, to mr. e. b. poulton (whose interest in the subject here discussed is shown by his share in the translation of weismann's _essays on heredity_), and to professor romanes, for the help afforded by their kindly suggestions and criticisms, and for the advice and recommendation under which this essay is now published. encouragement from mr. francis darwin is to me the more precious, and the more worthy of grateful recognition, from the fact that my general conclusion that acquired characters are _not_ inherited is at variance with the opinion of his revered father, who aided his great theory by the retention of some remains of lamarck's doctrine of the inherited effect of habit. i feel as if the son, as representative of his great progenitor, were carrying out the idea of an appreciative editor who writes to me: "we must say that if darwin were still alive, he would find your arguments of great weight, and undoubtedly would give to them the serious consideration which they deserve." i hope, then, that i may be acquitted of undue presumption in opposing a view sanctioned by the author of the _origin of species_, but already stoutly questioned and firmly rejected by such followers of his as weismann, wallace, poulton, ray lankester, and others, to say nothing of its practical rejection by so great an authority on heredity as francis galton. the sociological importance of the subject has already been insisted on in emphatic terms by mr. herbert spencer, and this importance may be even greater than he imagined. civilization largely sets aside the harsh but ultimately salutary action of the great law of natural selection without providing an efficient substitute for preventing degeneracy. the substitute on which moralists and legislators rely--if they think on the matter at all--is the cumulative inheritance of the beneficial effects of education, training, habits, institutions, and so forth--the inheritance, in short, of acquired characters, or of the effects of use and disuse. if this substitute is but a broken reed, then the deeper thinkers who gradually teach the teachers of the people, and ultimately even influence the legislators and moralists, must found their systems of morality and their criticisms of social and political laws and institutions and customs and ideas on the basis of the darwinian law rather than on that of lamarck. looking forward to the hope that the human race may become consciously and increasingly master of itself and of its destiny, and recognizing the darwinian principle of the selection of the fittest as the _only_ means of preventing the moral and physical degeneracy which, like an internal dry rot, has hitherto been the besetting danger of all civilizations, i desire that the thinkers who mould the opinions of mankind shall not be led astray from the true path of enduring progress and happiness by reliance on fallacious beliefs which will not bear examination. such, at least, is the feeling or motive which has prompted me to devote much time and thought to a difficult but important inquiry in a debatable region of inference and conjecture, where (i am afraid) evidence on either side can never be absolutely conclusive, and where, especially, the absolute demonstration of a universal negative cannot reasonably be expected. contents. page preface v importance and bearing of the inquiry spencer's examples and arguments - diminution of the jaws diminished biting muscles of lap-dogs crowded teeth blind cave-crabs no concomitant variation from concomitant disuse the giraffe, and necessity for concomitant variation alleged ruinous effects of natural selection adverse case of neuter insects Æsthetic faculties lack of evidence inherited epilepsy in guinea-pigs inherited insanity and nervous disorders individual and transmissible type not modified alike darwin's examples - reduced wings of birds of oceanic islands drooping ears and deteriorated instincts wings and legs of ducks and fowls pigeons' wings shortened breast-bone in pigeons shortened feet in pigeons shortened legs of rabbits blind cave-animals inherited habits tameness of rabbits modifications obviously attributable to selection similar effects of natural selection and use-inheritance inferiority of senses in europeans short-sight in watchmakers and engravers larger hands of labourers' infants thickened sole in infants a source of mental confusion weakness of use-inheritance inherited injuries - inherited mutilations the motmot's tail other inherited injuries mentioned by darwin quasi-inheritance miscellaneous considerations - true relation of parents and offspring inverse inheritance early origin of the ova marked effects of use and disuse on the individual would natural selection favour use-inheritance? use-inheritance an evil varied effects of use and disuse use-inheritance implies pangenesis pangenesis improbable spencer's explanation of use-inheritance conclusions - use-inheritance discredited as unnecessary, unproven, and improbable modern reliance on use-inheritance misplaced are the effects of use and disuse inherited? importance and bearing of the inquiry. the question whether the effects of use and disuse are inherited, or, in other words, whether acquired characters are hereditary, is of considerable interest to the general student of evolution; but it is, or should be, a matter of far deeper interest to the thoughtful philanthropist who desires to ensure the permanent welfare and happiness of the human race. so profoundly important, in fact, are the moral, social, and political conclusions that depend on the answer to this inquiry, that, as mr. herbert spencer rightly says, it "demands, beyond all other questions whatsoever, the attention of scientific men." it is obvious that we can produce important changes in the individual. we can, for example, improve his muscles by athletics, and his brain by education. the use of organs enlarges and strengthens them; the disuse of parts or faculties weakens them. and so great is the power of habit that it is proverbially spoken of as "second nature." it is thus certain that we can modify the individual. we can strengthen (or weaken) his body; we can improve (or deteriorate) his intellect, his habits, his morals. but there remains the still more important question which we are about to consider. will such modifications be inherited by the offspring of the modified individual? does individual improvement transmit itself to descendants independently of personal teaching and example? have artificially produced changes of structure or habit any inherent tendency to become congenitally transmissible and to be converted in time into fixed traits of constitution or character? can the philanthropist rely on such a tendency as a hopeful factor in the evolution of mankind?--the only sound and stable basis of a higher and happier state of things being, as he knows or ought to know, the innate and constitutionally-fixed improvement of the race as a whole. if acquired modifications are impressed on the offspring and on the race, the systematic moral training of individuals will in time produce a constitutionally moral race, and we may hope to improve mankind even in defiance of the unnatural selection by which a spurious but highly popular philanthropy would systematically favour the survival of the unfittest and the rapid multiplication of the worst. but if acquired modifications do not tend to be transmitted, if the use or disuse of organs or faculties does not similarly affect posterity by inheritance, then it is evident that no innate improvement in the race can take place without the aid of natural or artificial selection. herbert spencer maintains that the effects of use and disuse _are_ inherited in kind, and in his _factors of organic evolution_[ ] he has supported his contention with a selection of facts and reasonings which i shall have the temerity to examine and criticize. darwin also held the same view, though not so strongly. and here, to prevent misunderstanding, i may say that the admiration and reverence and gratitude due to darwin ought not to be allowed to interfere in the slightest degree with the freest criticism of his conclusions. to perfect his work by the correction of really extraneous errors is as much a sacred duty as to study and apply the great truths he has taught. footnotes: [ ] which originally appeared in the _nineteenth century_ for april and may, . spencer's examples and arguments. diminution of the jaws in civilized races. mr. spencer verified this by comparing english jaws with australian and negro jaws at the college of surgeons.[ ] he maintains that the diminution of the jaw in civilized races can _only_ have been brought about by inheritance of the effects of lessened use. but if english jaws are lighter and thinner than those of australians and negroes, so too is the rest of the skull. as the diminution in the weight and thickness of the walls of the cranium cannot well be ascribed to disuse, it must be attributed to some other cause; and this cause may have affected the jaw also. cessation of the process by which natural selection[ ] favoured strong thick bones during ages of brutal violence might bring about a change in this direction. lightness of structure, facilitating agility and being economical of material, would also be favoured by natural selection so far as strength was not too seriously diminished. sexual selection powerfully affects the human face, and so must affect the jaws--as is shown by the differences between male and female jaws, and by the relative lightness and smallness of the latter, especially in the higher races. human preference, both sexual and social, would tend to eliminate huge jaws and ferocious teeth when these were no longer needed as weapons of war or organs of prehension, &c. we can hardly assume that the lower half of the face is specially exempt from the influence of natural and sexual selection; and the effects of these undoubted factors of evolution must be fully considered before we are entitled to call in the aid of a factor whose existence is questioned. after allowing for lost teeth and the consequent alveolar absorption, and for a reduction proportional to that shown in the rest of the skull, the difference in average weight in fifty european and fourteen australian male jaws at the college of surgeons turned out to be less than a fifth of an ounce, or about per cent. this slight reduction may be much more than accounted for by such causes as disuse in the individual, human preference setting back the teeth, and partial transference of the much more marked diminution seen in female jaws. there is apparently no room for accumulated _inherited_ effects of ancestral disuse. the number of jaws is small, indeed; but weighing them is at least more decisive than mr. spencer's mere inspection. the differences between anglo-saxon male jaws and australian and tasmanian jaws are most easily explained as effects of human preference and natural selection. we can hardly suppose that disuse would maintain or develop the projecting chin, increase its perpendicular height till the jaw is deepest and strongest at its extremity, evolve a side flange, and enlarge the upper jaw-bone to form part of a more prominent nose, while drawing back the savagely obtrusive teeth and lips to a more pleasing and subdued position of retirement and of humanized beauty. if human preference and natural selection caused some of these differences, why are they incompetent to effect changes in the direction of a diminution of the jaw or teeth? and if use and disuse are the sole modifying agents in the case of the human jaw, why should men have any more chin than a gorilla or a dog? the excessive weight of the west african jaws at the college of surgeons is partly _against_ mr. spencer's contention, unless he assumes that guinea negroes use their jaws far more than the australians, a supposition which seems extremely improbable. the heavier skull and narrower molar teeth point however to other factors than increased use. the striking variability of the human jaw is strongly opposed to the idea of its being under the direct and dominant control of so uniform a cause as ancestral use and disuse. mr. spencer regards a variation of oz. as a large one, but i found that the english jaws in the college of surgeons varied from · oz. to · oz. (or oz. if lost teeth were allowed for); australian jaws varied from oz. to · oz. (with _no_ lost teeth to allow for); while in negro jaws the maximum rose to over - / oz.[ ] in spite of disuse some european jaws were twice as heavy as the lightest australian jaw, either absolutely or (in some cases) relatively to the cranium. the uniformity of change relied upon by mr. spencer is scarcely borne out by the facts so far as male jaws are concerned. the great reduction in the weight of _female_ jaws _and skulls_ evidently points to sexual selection and to panmixia under male protection. i think, on the whole, we must conclude that the human jaws do not afford satisfactory proof of the inheritance of the effects of use and disuse, inasmuch as the differences in their weight and shape and size can be more reasonably and consistently accounted for as the result of less disputable causes. diminished biting muscles of lap-dogs. the next example, the reduced biting muscles, &c., of lap-dogs is also unsatisfactory as a proof of the inheritance of the effects of disuse; for the change can readily be accounted for without the introduction of such a factor. the previous natural selection of strong jaws and teeth and muscles is reversed. the conscious or unconscious selection of lap-dogs with the least tendency to bite would easily bring about a general enfeeblement of the whole biting apparatus--weakness of the parts concerned favouring harmlessness. mr. spencer maintains that the dwindling of the parts concerned in clenching the jaw is certainly not due to artificial selection because the modifications offer no appreciable external signs. surely hard biting is sufficiently appreciable by the person bitten without any visual admeasurement of the masseter muscles or the zygomatic arches. disuse during lifetime would also cause some amount of degeneracy; and i am not sure that mr. spencer is right in _entirely_ excluding economy of nutrition from the problem. breeders would not over-feed these dogs; and the puppies that grew most rapidly would usually be favoured. crowded teeth. the too closely-packed teeth in the "decreasing" jaws of modern men (p. )[ ] are also suggestive of other causes than use and disuse. why is there not simultaneous variation in teeth and jaws, if disuse is the governing factor? are we to suppose that the size of the human teeth is maintained by use at the same time that the jaws are being diminished by disuse? mr. spencer acknowledges that the crowding of bull-dogs' and lap-dogs' teeth is caused by the artificial selection of shortened jaws. if a similar change is really occurring in man, could it not be similarly explained by some factor, such as sexual selection, which might affect the outward appearance at the cost of less obvious defects or inconveniences? mr. spencer points to the decay of modern teeth as a sign or result of their being overcrowded through the diminution of the jaw by disuse.[ ] but the teeth which are the most frequently overcrowded are the lower incisors. the upper incisors are less overcrowded, being commonly pressed outwards by the lower arc of teeth fitting inside them in biting. the lower incisors are correspondingly pressed inwards and closer together. yet the upper incisors decay--or at least are extracted--about twenty times as frequently as the closely packed lower incisors.[ ] surely this must indicate that the cause of decay is not overcrowding. the lateness and irregularity of the wisdom teeth are sometimes supposed to indicate their gradual disappearance through want of room in a diminishing jaw. but a note on tasmanian skulls in the _catalogue of the college of surgeons_ (p. ) shows that this lateness and irregularity have been common among tasmanians as well as among civilized races, so that the change can hardly be attributed to the effects of disuse under civilization. blind cave-crabs. the cave-crabs which have lost their disused eyes but _not the disused eye-stalks_ appear to illustrate the effects of natural selection rather than of disuse. the loss of the exposed, sensitive, and worse-than-useless eye, would be a decided gain, while the disused eye-stalk, being no particular detriment to the crab, would be but slightly affected by natural selection, though open to the cumulative effects of disuse. the disused but better protected eyes of the blind cave-rat are still "of large size" (_origin of species_, p. ). no concomitant variation from concomitant disuse. it is but fair to add that these instances of the cave-crab's eye-stalk and the closely-packed teeth are put forward by mr. spencer with the more immediate object of proving that there is "no concomitant variation in co-operative parts," even when "formed out of the same tissue, like the crab's eye and its peduncle" (pp. - , , ). it escapes his notice, however, that in two out of his three cases it is _disuse_, or _diminished use_, which fails to cause concomitant variation or proportionate variation. the giraffe, and necessity for concomitant variation. having unwittingly shown that lessened use of closely-connected and co-operative parts does not cause concomitant variation in these parts, mr. spencer concludes that the concomitant variation requisite for evolution can only be caused by altered degrees of use or disuse. he elaborately argues that the many co-ordinated modifications of parts necessitated by each important alteration in an animal are so complex that they cannot possibly be brought about except by the inherited effect of the use and disuse of the various parts concerned. he holds, for instance, that natural selection is inadequate to effect the numerous concomitant changes necessitated by such developments as that of the long neck of the giraffe. darwin, however, on the contrary, holds that natural selection alone "would have sufficed for the production of this remarkable quadruped."[ ] he is surprised at mr. spencer's view that natural selection can do so little in modifying the higher animals. thus one of the chief arguments with which mr. spencer supports his theory is so poorly founded as to be rejected by a far greater authority on such subjects. all that is needed is that natural selection should preserve the tallest giraffes through times of famine by their being able to reach otherwise inaccessible stores of foliage. the continual variability of all parts of the higher animals gives scope for innumerable changes, and nature is not in a hurry. mr. spencer, however, says that "the chances against any adequate readjustments fortuitously arising must be infinity to one." but he has also shown that altered degree of use does not cause the needed concomitant variation of co-operative parts. so the chances against a beneficial change in an animal must be, at a liberal estimate, infinity to two. mr. spencer, if he has proved anything, has proved that it is practically impossible that the giraffe can have acquired a long neck, or the elk its huge horns, or that any species has ever acquired any important modification. mr. wallace, in his _darwinism_, answers mr. spencer by a collection of facts showing that "variation is the rule," that the range of variation in wild animals and plants is much greater than was supposed, and that "each part varies to a considerable extent independently" of other parts, so that "the materials constantly ready for natural selection to act upon are abundant in quantity and very varied in kind." while co-operative parts would often be more or less correlated, so that they would tend to vary together, coincident variation is not necessary. the lengthened wing might be gained in one generation, and the strengthened muscle at a subsequent period; the bird in the meanwhile drawing upon its surplus energy, aided (as i would suggest) by the strengthening effect of increased use in the individual. seeing that artificial selection of complicated variations has modified animals in many points either simultaneously or by slow steps, as with otter-sheep, fancy pigeons, &c. (many of the characters thus obtained being clearly independent of use and disuse), natural selection must be credited with similar powers, and mr. wallace concludes that mr. spencer's insuperable difficulty is "wholly imaginary." the extract concerning a somewhat similar "class of difficulties," which mr. spencer quotes from his _principles of biology_, is faulty in its reasoning,[ ] though legitimate in its conclusion concerning the increasing difficulty of evolution in proportion with the increasing number and complexity of faculties to be evolved. but this increasing difficulty of complex evolution is only overcome by _some_ favourably-varying individuals and species--not by all. and as the difficulty increases we find neglect and decay of the less-needed faculties--as with domesticated animals and civilized men, who lose in one direction while they gain in another. the increasing difficulty of complex evolution by natural selection is no proof whatever of use-inheritance[ ] except to those who confound difficulty with impossibility. alleged ruinous effects of natural selection. mr. spencer further contends that natural selection, by unduly developing specially advantageous modifications without the necessary but complex secondary modifications, would render the constitution of a variety "unworkable" (p. ). but this seems hardly feasible, seeing that natural selection must continually favour the most workable constitutions, and will only preserve organisms in proportion as they combine general workableness with the special modification. on the other hand, according to mr. spencer himself, use-inheritance must often disturb the balance of the constitution. thus it tends to make the jaws and teeth unworkable through the overcrowding and decay of the teeth--there being, as his illustrations show, no simultaneous or concomitant or proportional variation in relation to altered degree of use or disuse. adverse case of neuter insects. mr. spencer also holds that most mental phenomena, especially where complex or social or moral, can only be explained as arising from use-inheritance, which becomes more and more important as a factor of evolution as we advance from the vegetable world and the lower grades of animal life to the more complex activities, tastes, and habits of the higher organizations (preface, and p. ). but there happens to be a tolerably clear proof that such changes as the evolution of complicated structures and habits and social instincts _can_ take place independently of use-inheritance. the wonderful instincts of the working bees have apparently been evolved (at least in all their later social complications and developments) without the aid of use-inheritance--nay, in spite of its utmost opposition. working bees, being infertile "neuters," cannot as a rule transmit their own modifications and habits. they are descended from countless generations of queen bees and drones, whose habits have been widely different from those of the workers, and whose structures are dissimilar in various respects. in many species of ants there are two, and in the leaf-cutting ants of brazil there are _three_, kinds of neuters which differ from each other and from their male and female ancestors "to an almost incredible degree."[ ] the soldier caste is distinguished from the workers by enormously large heads, very powerful mandibles, and "extraordinarily different" instincts. in the driver ant of west africa one kind of neuter is three times the size of the other, and has jaws nearly five times as long. in another case "the workers of one caste alone carry a wonderful sort of shield on their heads." one of the three neuter classes in the leaf-cutting ants has a single eye in the midst of its forehead. in certain mexican and australian ants some of the neuters have huge spherical abdomens, which serve as living reservoirs of honey for the use of the community. in the equally wonderful case of the termites, or so-called "white ants" (which belong, however, to an entirely different order of insect from the ants and bees) the neuters are blind and wingless, and are divided into soldiers and workers, each class possessing the requisite instincts and structures adapting it for its tasks. seeing that natural selection can form and maintain the various structures and the exceedingly complicated instincts of ants and bees and wasps and termites in direct defiance of the alleged tendency to use-inheritance, surely we may believe that natural selection, unopposed by use-inheritance, is equally competent for the work of complex or social or mental evolution in the many cases where the strong presumptive evidence cannot be rendered almost indisputable by the exceptional exclusion of the modified animal from the work of reproduction. ants and bees seem to be capable of altering their habits and methods of action much as men do. bees taken to australia cease to store honey after a few years' experience of the mild winters. whole communities of bees sometimes take to theft, and live by plundering hives, first killing the queen to create dismay among the workers. slave ants attend devotedly to their captors, and fight against their own species. forel reared an artificial ant-colony made up of five different and more or less hostile species. why cannot a much more intelligent animal modify his habits far more rapidly and comprehensively without the aid of a factor which is clearly unnecessary in the case of the more intelligent of the social insects? Æsthetic faculties. the modern development of music and harmony (p. ) is undeniable, but why could it only have been brought about by the help of the inheritance of the effects of use? why are we to suppose that "minor traits" such as the "æsthetic perceptions" cannot have been evolved by natural selection (p. ) or by sexual selection? darwin holds that our musical faculties were developed by sexual preference long before the acquisition of speech. he believes that the "rhythms and cadences of oratory are derived from previously developed musical powers"--a conclusion "exactly opposite" to that arrived at by mr. spencer.[ ] the emotional susceptibility to music, and the delicate perceptions needed for the higher branches of art, were apparently the work of natural and sexual selection in the long past. civilization, with its leisure and wealth and accumulated knowledge, perfects human faculties by artificial cultivation, develops and combines means of enjoyment, and discovers unsuspected sources of interest and pleasure. the sense of harmony, modern as it seems to be, must have been a latent and indirect consequence of the development of the sense of hearing and of melody. use, at least, could never have called it into existence. nature favours and develops enjoyments to a certain extent, for they subserve self-preservation and sexual and social preference in innumerable ways. but modern æsthetic advance seems to be almost entirely due to the culture of latent abilities, the formation of complex associations, the selection and encouragement of talent, and the wide diffusion and imitation of the accumulated products of the well-cultivated genius of favourably varying individuals. the fact that uneducated persons do not enjoy the higher tastes, and the rapidity with which such tastes are acquired or professed, ought to be sufficient proof that modern culture is brought about by far swifter and more potent influences than use-inheritance. neither would this hypothetical factor of evolution materially aid in explaining the many other rapid changes of habit brought about by education, custom, and the changed conditions of civilization generally. powerful tastes--as is incontestably shown in the cases of alcohol and tobacco--lie latent for ages, and suddenly become manifest when suitable conditions arise. every discovery, and each step in social and moral evolution, produces its wide-spreading train of consequences. i see no reason why use-inheritance need be credited with any share in the cumulative results of the invention of printing and the steam-engine and gunpowder, or of freedom and security under representative government, or of science and art and the partial emancipation of the mind of man from superstition, or of the innumerable other improvements or changes that take place under modern civilization. mr. spencer suggests an inquiry whether the greater powers possessed by eminent musicians were not mainly due to the inherited effect of the musical practice of their fathers (p. ). but these great musicians inherited far more than their parents possessed. the excess of their powers beyond their parents' must surely be attributed to spontaneous variation; and who shall say that the rest was in any way due to use-inheritance? if, too, the superiority of geniuses proves use-inheritance, why should not the inferiority of the sons of geniuses prove the existence of a tendency which is the exact opposite of use-inheritance? but nobody collects facts concerning the degenerate branches of musical families. only the favourably varying branches are noticed, and a general impression of rapid evolution of talent is thus produced. such cases might be explained, too, by the facts that musical faculty is strong in both sexes, that musical families associate together, and that the more gifted members may intermarry. great musicians are often astonishingly precocious. meyerbeer "played brilliantly" at the age of six. mozart played beautifully at four. are we to suppose that the effect of the _adult_ practice of parents was inherited at this early age? if use-inheritance was not necessary in the case of handel, whose father was a surgeon, why is it needed to account for bach? lack of evidence. the "direct proofs" of use-inheritance are not as plentiful as might be desired, it appears (pp. - ). this acknowledged "lack of recognized evidence" is indeed the weakest feature in the case, though mr. spencer would fain attribute this lack of direct proof to insufficient investigation and to the inconspicuous nature of the inheritance of the modification. but there is an almost endless abundance of conspicuous examples of the effects of use and disuse in the individual. how is it that the subsequent inheritance of these effects has not been more satisfactorily observed and investigated? horse-breeders and others could profit by such a tendency, and one cannot help suspecting that the reason they ignore it must be its practical inefficacy, arising probably from its weakness, its obscurity and uncertainty or its non-existence. inherited epilepsy in guinea-pigs. brown-séquard's discovery that an epileptic tendency artificially produced by mutilating the nervous system of a guinea-pig is occasionally inherited may be a fact of "considerable weight," or on the other hand it may be entirely irrelevant. cases of this kind strike one as peculiar exceptions rather than as examples of a general rule or law. they seem to show that certain morbid conditions may occasionally affect both the individual and the reproductive elements or transmissible type in a similar manner; but then we also know that such prompt and complete transmission of an artificial modification is widely different from the usual rule. exceptional cases require exceptional explanations, and are scarcely good examples of the effect of a general tendency which in almost all other cases is so inconspicuous in its immediate effects. further remarks on this inherited epilepsy can be most conveniently introduced later on in connection with darwin's explanation of the inherited mutilation which it usually accompanies, but which mr. spencer does not mention. inherited insanity and nervous disorders. mr. spencer infers that, because insanity is usually hereditary, and insanity can be artificially produced by various excesses, therefore this artificially-produced insanity must also be hereditary (p. ). direct evidence of this conclusion would be better than a mere inference which may beg the very question at issue. that the liability to insanity commonly runs in families is no proof that strictly non-inherited insanity will subsequently become hereditary. i think that theories should be based on facts rather than facts on theories, especially when those facts are to be the basis or proof of a further theory. mr. spencer also points out that he finds among physicians "the belief that nervous disorders of a less severe kind are inheritable"--a general belief which does not necessarily include the transmission of purely artificially-produced disorders, and so misses the point which is really at issue. he proceeds, however, to state more definitely that "men who have prostrated their nervous systems by prolonged overwork or in some other way, have children more or less prone to nervousness." the following observations will, i think, warrant at least a suspension of judgment concerning this particular form of use-inheritance. ( ) the nervousness is seen in the _children_ at an early age, although the nervous prostration from which it is supposed to be derived obviously occurs in the parent at a much later period of life. this change in time is contrary to the rule of inheritance at corresponding periods; and, together with the unusual promptness and comparative completeness of the inheritance, it may indicate a special injury or deterioration of the reproductive elements rather than true inheritance. the healthy brain of early life has failed to transmit its robust condition. is use-inheritance, then, only effective for evil? does it only transfer the newly-acquired weakness, and not the previous long-continued vigour? ( ) members of nervous families would be liable to suffer from nervous prostration, and by the ordinary law of heredity alone would transmit nervousness to their children. ( ) the shattered nerves or insanity resulting from alcoholic and other excesses, or from overwork or trouble, are evidently signs of a grave constitutional injury which may react upon the reproductive elements nourished and developed in that ruined constitution. the deterioration in parent and child may often display itself in the same organs--those probably which are hereditarily weakest. acquired diseases or disorders thus appear to be transmitted, when all that was conveyed to the offspring was the exciting cause of a lowered vitality or disordered action, together with the ancestral liability to such diseases under such conditions. ( ) francis galton says that "it is hard to find evidence of the power of the personal structure to react upon the sexual elements, that is not open to serious objection." some of the cases of apparent inheritance he regards as coincidence of effect. thus "the fact that a drunkard will often have imbecile children, although his offspring previous to his taking to drink were healthy," is an "instance of simultaneous action," and not of true inheritance. "the alcohol pervades his tissues, and, of course, affects the germinal matter in the sexual elements as much as it does that in his own structural cells, which have led to an alteration in the quality of his own nerves. exactly the same must occur in the case of many constitutional diseases that have been acquired by long-continued irregular habits."[ ] individual and transmissible type not modified alike by the direct effect of changed habits or conditions. mr. spencer finds it hard to believe that the modifications conveyed to offspring are not identical in tendency with the changes effected in the parent by altered use or habit (pp. - , ). but it is perfectly certain that the two sets of effects do not necessarily correspond. the effect of changed habits or conditions on the individual is often very far from coinciding with the effects on the reproductive elements or the transmissible type. the reproductive system is "extremely sensitive" to very slight changes, and is often powerfully affected by circumstances which otherwise have little effect on the individual (_origin of species_, p. ). various animals and plants become sterile when domesticated or supplied with too much nourishment. the native tasmanians have already become extinct from sterility caused by greatly changed diet and habits. if, as mr. spencer teaches, continued culture and brain-work will in time produce lessened fertility or comparative sterility, we may yet have to be careful that intellectual development does not become a species of suicide, and that the culture of the race does not mean its extinction--or at least the extinction of those most susceptible of culture. the reproductive elements are also disturbed and modified in innumerable minor ways. changed conditions or habits tend to produce a general "plasticity" of type, the "indefinite variability" thus caused being apparently irrelevant to the change, if any, in the individual.[ ] a vast number of variations of structure have certainly arisen independently of similar parental modification as the preliminary. whatever first caused these "spontaneous" congenital variations affected the reproductive elements quite differently from the individual. "when a new peculiarity first appears we can never predict whether it will be inherited." many varieties of plants only keep true from shoots, and not from seed, which is by no means acted on in the same way as the individual plant. seeing that such plants have _two_ reproductive types, both constant, it is evident that these cannot both be modified in the same way as the parent is modified. many parental modifications of structure and habit are certainly not conveyed to neuter ants and bees; other modifications, which are not seen in the parents, being conveyed instead. many other circumstances tend to show that the individual and the transmissible type are independent of each other so far as modifications of parts are concerned. it may seem natural to expect the transmission of an enlarged muscle or a cultivated brain, but, on the other hand, why should it be unreasonable to expect that a modification which was non-congenital in origin should still remain non-congenital? why should the non-transmission of that which was not transmitted be surprising? mr. spencer thinks that the non-transmission of acquired modifications is incongruous with the great fact of atavism. but the great law of the inheritance of that which is a development of the transmissible type does not necessarily imply the inheritance of modifications acquired by the individual. because english children may inherit blue eyes and flaxen hair from their anglo-saxon ancestors, it by no means follows that an englishman must inherit his father's sunburnt complexion or smooth-shaven face. of course atavism ultimately adopts many instances of revolt against its sway. but to assume that these changes of type _follow_ the personal change rather than cause it, is to assume the whole question at issue. that like begets like is true as a broad principle, but it has many exceptions, and the non-heredity of acquired characters may be one of them. footnotes: [ ] _principles of biology_, § , footnote. the english jaws are somewhat lighter than the australian jaws, though i could not undertake to affirm that they are really shorter and smaller. in the typical skulls depicted on p. of the official guide to the mammalian galleries at south kensington, the typical caucasian jaw is very much larger than the tasmanian jaw, although the repulsively obtrusive teeth of the latter convey the contrary idea to the imagination. mr. spencer's assumption that the ancient britons had large jaws appears to me erroneous. (see professor rolleston's _scientific papers and addresses_, i. p. .) [ ] romanes, galton, and weismann have made great use of this principle in explaining the diminution of disused organs. weismann has given it the name of _panmixia_,--_all_ individuals being equally free to survive and commingle their variations, and not merely selected or favoured individuals. see his _essays on heredity_, &c., p. (clarendon press). [ ] inclusive in each case of fixed strengthening wire weighing about a sixteenth of an ounce or less. [ ] references of course are to _factors of organic evolution_. [ ] p. ; and _nineteenth century_, february, , p. . [ ] tomes's _dental surgery_, pp. - . tomes observes that it is as yet uncertain in what way civilization predisposes to caries. but he shows that caries is caused by the lime salts in the teeth being attacked by _acids_ from decomposing food in crevices, from artificial drink such as cyder, from sugar, from medicine, and from vitiated secretions of the mouth. it is evident that in civilized races natural selection cannot so rigorously insist on sound teeth, sound constitutions, and _protective alkaline_ saliva. the reaction of the civilized mouth is often acid, especially when the system is disordered by dyspepsia or other diseases or forms of ill-health common under civilization. the main supply of saliva, which is poured from the cheeks opposite the upper molars, is often acid when in small quantities. but the submaxillary and sub-lingual saliva poured out at the foot of the lower incisors and held in the front part of the jaw as in a spoon, "differs from parotid saliva in being more alkaline" (foster's _text book of physiology_, p. ; tomes, pp. , ). one observer says that the reaction near the lower incisors is "never acid." hence (i conclude) the remarkable immunity of the lower incisors and canines from decay, an immunity which extends backwards in a lessening degree to the first and second bicuspids. the close packing of the lower incisors may assist by preventing the retention of decaying fragments of food. sexual selection may promote caries by favouring white teeth, which are more prone to decay than yellow ones. acid vitiation of the mucus might account both for caries and (possibly) for the strange infertility of some inferior races under civilization. [ ] _origin of species_, pp. - ; _variation of animals and plants under domestication_, vol. ii. p. footnote, also p. . [ ] mr. spencer weakly argues that an advantageous attribute (such as swiftness, keen sight, courage, sagacity, strength, &c.) cannot be increased by natural selection unless it is "of greater importance, for the time being, than most of the other attributes"; and that natural selection cannot develop any one superiority when animals are equally preserved by "other superiorities." but as natural selection will simultaneously eliminate tendencies to slowness, blindness, deafness, stupidity, &c., it _must_ favour and improve many points simultaneously, although no one of them may be of greater importance than the rest. of course the more complicated the evolution the slower it will be; but time is plentiful, and the amount of elimination is correspondingly vast. [ ] i venture to coin this concise term to signify _the direct inheritance of the effects of use and disuse in kind_. having a name for a thing is highly convenient; it facilitates clearness and accuracy in reasoning, and in this particular inquiry it may save some confusion of thought from double or incomplete meanings in the shortened phrases which would otherwise have to be employed to indicate this great but nameless factor of evolution. [ ] _origin of species_, pp. - ; bates's _naturalist on the amazons_. darwin is "surprised that no one has hitherto advanced the demonstrative case of neuter insects, against the well-known doctrine of inherited habit, as advanced by lamarck." as he justly observes, "it proves that with animals, as with plants, any amount of modification may be effected by the accumulation of numerous, slight, spontaneous variations, which are in any way profitable, without exercise or habit having been brought into play. for peculiar habits confined to the workers or sterile females, however long they might be followed, could not possibly affect the males and fertile females, which alone leave any descendants." some slight modification of these remarks, however, may possibly be needed to meet the case of "factitious queens," who (probably through eating particles of the royal food) become capable of producing a few male eggs. [ ] _descent of man_, pp. , , and footnote. [ ] _contemporary review_, december, , p. . [ ] see _origin of species_, pp. - . "changed conditions induce an almost indefinite amount of fluctuating variability, by which the whole organization is rendered in some degree plastic" (_descent of man_, p. ). it also appears that "the nature of the conditions is of subordinate importance in comparison with the nature of the organism in determining each particular form of variation;--perhaps of not more importance than the nature of the spark, by which a mass of combustible matter is ignited, has in determining the nature of the flames" (_origin of species_, p. ). darwin's examples. the most formidable cases brought forward by mr. spencer are from darwin. i shall endeavour to show, however, that darwin was probably wrong in retaining the older explanation of these facts, and that the remains of the lamarckian theory of use-inheritance need not any longer encumber the great explanation which has superseded that fallacious and unproven theory and has rendered it totally unnecessary. meanwhile i think it is an excellent sign that mr. spencer has to complain that "nowadays most naturalists are more darwinian than mr. darwin himself"--inasmuch as they are inclined to say that there is "no proof" that the effects of use and disuse are inherited. other excellent signs are the recent issue of a translation of weismann's important essays on this and kindred subjects,[ ] the strong support given to his views by wallace in his _darwinism_, and their adoption by ray lankester in his article on zoology in the latest edition of the _encyclopædia britannica_. so sound and cautious an investigator as francis galton had also in concluded that "acquired modifications are barely, if at all, _inherited_, in the correct sense of that word." darwin's belief in the inheritance of acquired characters was more or less hereditary in the family. his grandfather, erasmus darwin, anticipated lamarck's views in his _zoonomia_, which darwin at one time "greatly admired." his father was "convinced" of the "inherited evil effects of alcohol," and to this extent at least he strongly impressed the belief in the inheritance of acquired characters upon his children's minds.[ ] darwin must also have been imbued with lamarckian ideas from other sources, although dr. grant's enthusiastic advocacy entirely failed to convert him to a belief in evolution.[ ] "nevertheless," he says, "it is probable that the hearing rather early in life such views maintained and praised may have favoured my upholding them under a different form in my _origin of species_"--a remark which refers to lamarck's views on the general doctrine of evolution, but might also prove equally true if applied to darwin's partial retention of the lamarckian explanation of that evolution. professor huxley has pointed out that in darwin's earlier sketch of his theory of evolution ( ) he attached more weight to the inheritance of acquired habits than he does in his _origin of species_ published fifteen years later.[ ] he appears to have acquired the belief in early life without first questioning and rigorously testing it as he would have done had it originated with himself. in later life it appeared to assist his theory of evolution in minor points, and in particular it appeared absolutely indispensable to him as the _only_ explanation of the diminution of disused parts in cases where, as in domestic animals, economy of growth seemed to be practically powerless. he failed to adequately notice the effect of panmixia, or the withdrawal of selection, in causing or allowing degeneracy and dwindling under disuse; and he hardly attached sufficient importance to the fact that rudimentary organs and other supposed effects of use or disuse are quite as marked features in neuter insects which cannot transmit the effects of use and disuse as they are in the higher animals. reduced wings of birds of oceanic islands. darwin himself has pointed out that the rudimentary wings of island beetles, at first thought to be due to disuse, are mainly brought about by natural selection--the best-winged beetles being most liable to be blown out to sea. but he says that in birds of the oceanic islands "not persecuted by any enemies, the reduction of their wings has probably been caused by disuse." this explanation may be as fallacious as it is acknowledged to have been in the case of the island beetles. according to darwin's own views, natural selection _must_ at least have played an important part in reducing the wings; for he holds that "natural selection is continually trying to economize every part of the organization." he says: "if under changed conditions of life a structure, before useful, becomes less useful, its diminution will be favoured, for it will profit the individual not to have its nutriment wasted in building up an useless structure.... thus, as i believe, natural selection will tend in the long run to reduce any part of the organization, as soon as it becomes, through changed habits, superfluous."[ ] if, as darwin powerfully urges (and he here ignores his usual explanation), ostriches' wings are insufficient for flight in consequence of the economy enforced by natural selection,[ ] why may not the reduced wings of the dodo, or the penguin, or the apteryx, or of the cursores generally, be wholly attributed to natural selection in favour of economy of material and adaptation of parts to changed conditions? the great principle of economy is continually at work shaping organisms, as sculptors shape statues, by removing the superfluous parts; and a mere glance at the forms of animals in general will show that it is well-nigh as dominant and universal a principle as is that of the positive development of useful parts. other causes, moreover besides actual economy, would favour shorter and more convenient wings on oceanic islands. in the first place, birds that were somewhat weak on the wing would be most likely to settle on an island and stay there. shortened wings would then become advantageous because they would restrain fatal migratory tendencies or useless and perilous flights in which the birds that flew furthest would be most often carried away by storms and adverse winds. reduced wings would keep the birds near the shelter and the food afforded by the island and its neighbourhood, and in some cases would become adapted to act as fins or flappers for swimming under water in pursuit of fish. the reduced size of the wings of these island birds is paralleled by the remarkable thinness, &c., of the shell of the "gigantic land-tortoise" of the galapagos islands. the changes seen in the carapace can hardly have been brought about by the inherited effects of special disuse. why then should not the reduction of equally useless, more wasteful, and perhaps positively dangerous wings be also due to an economy which has become advantageous to bird and reptile alike through the absence of the mammalian rivals whose places they are evidently being modified to fill? the _complete_ loss of the wings in neuter ants and termites can scarcely be due to the inherited effects of disuse; and as natural selection has abolished these wings in spite of the opposition of use-inheritance, it must clearly be fully competent to reduce wings without its aid. in considering the rudimentary wings of the apteryx, or of the moa, emu, ostrich, &c., we must not forget the frequent or occasional occurrence of hard seasons, and times of drought and famine, when nature eliminates redundant, wasteful, and ill-adapted organisms in so severe and wholesale a fashion. where enemies are absent there would be unrestrained multiplication, and this would greatly increase the severity of the competition for food, and so hasten the elimination of disused and useless parts. drooping ears and deteriorated instincts. mr. galton has pointed out that existing races and existing organs are only kept at their present high pitch of organic excellence by the stringent and incessant action of natural or artificial selection; and the simple relaxation or withdrawal of such selective influences will almost necessarily result in a certain amount of deterioration, independently even of the principle of economy.[ ] i think that this cessation of a previous selective process will account for the drooping--but _not diminished_--ears of various domesticated animals (human preference and increased weight evidently aiding), and also for the inferior instincts seen in them and in artificially-fed caterpillars of the silk-moth, which now "often commit the strange mistake of devouring the base of the leaf on which they are feeding, and consequently fall down." anyhow, i fail to see that anything is proved by this latter case, except that natural instinct may be perverted or aborted under unnatural conditions and a changed method of selection which abolishes the powerful corrective formerly supplied by natural selection. wings and legs of ducks and fowls. the reduced wings and enlarged legs of domesticated ducks and fowls are attributed by darwin and spencer to the inheritance of the effects of use and disuse. but the inference by no means follows. natural selection would usually favour these adaptive changes, and they would also have been aided by an artificial selection which is often unconscious or indirect. birds with diminished power of flight would be less difficult to keep and manage, and in preserving and multiplying such birds man would be unconsciously bringing about structural changes which would easily be regarded as effects of use and disuse. "about eighteen centuries ago columella and varro speak of the necessity of keeping ducks in netted enclosures like other wild fowl, so that at this period there was danger of their flying away."[ ] is it not probable that the best fliers would escape most frequently, or would pine most if kept confined? on the other hand, birds with lessened powers of flight would not be eliminated as under natural conditions, but would be favoured; and natural selection, together with artificial selection of the most flourishing birds, would thicken and strengthen the legs to meet increased demands upon them. the diminution of the duck's wing is not great even in the birds that "never fly," and from this we must deduct the direct effect of disuse on the individual during its lifetime. as weismann suggests, the _inherited_ portion of the change could only be ascertained by comparing the bones, &c., of wild and tame ducks _similarly reared_. if individual disuse diminished the weight of the duck's wing-bones by per cent. there would be nothing left to account for. i suspect that investigation would reveal anomalies inconsistent with the theory of use-inheritance. thus according to darwin's tables of comparative weights and measurements[ ] the leg-bones of the penguin duck have slightly diminished in length, although they have increased per cent. in weight. relatively to the weight of the skeleton, the leg-bones have shortened in the tame breeds of ducks by over per cent. (and in two breeds by over per cent.) although they have increased more than per cent. in proportional weight.[ ] how can increased use simultaneously shorten and thicken these bones? if the relative shortening is attributed to a heavier skeleton, then the apparently reduced weight of the wing-bones is fully accounted for by the same circumstance, and disuse has had no inherited effect. another strange circumstance is that the wing-bones have diminished _in length only_. the shortening is about per cent. more than in the shortened legs, and it amounts to per cent. as compared with the weight of the skeleton. such a shortening should represent a reduction of per cent. in weight, whereas the actual reduction in the weight of the wing-bones relatively to the weight of the skeleton is only per cent. even in the breeds that never fly. independently of shortening, the disused wing-bones have actually thickened or increased in weight. in the aylesbury duck the disproportion caused by these conflicting changes is so great that the wing-bones are per cent. heavier than they should be if their weight had varied proportionally with their length.[ ] the reduction in weight on which darwin relies seems to be entirely due to the shortening, and this shortening appears to be irrelevant to disuse, since the wings of the call duck are similarly shortened in their proportions by per cent., although this bird habitually flies to such an extent that darwin partly attributes the greatly increased weight of its wing-bones to increased use under domestication. we find that _all_ the changes are in the direction of shorter and thicker bones--a tendency which must be largely dependent upon the suspension of the rigorous elimination which keeps the bones of the wild duck _long and light_. the used leg-bones and the disused wing-bones have alike been shortened and thickened, though in different proportions. natural or artificial selection might easily thicken legs without lengthening them, or shorten wings without eliminating strong heavy bones, but it can hardly be contended that use-inheritance has acted in such conflicting ways. the thickening of the wing-bones has actually more than kept pace with any increase of weight in the skeleton, in spite of the effect of individual disuse and of the alleged cumulative effect of ancestral disuse for hundreds of generations. the case of the duck deserves special attention as a crucial one, if only from the fact that in this instance, and in this instance only, has darwin given the weights of the skeletons, thus furnishing the means for a closer examination of his details than is usually possible. if we ignore such factors as selection, panmixia, correlation, and the effects of use and disuse during lifetime, and still regard the case of the domestic duck as a valid proof of the inheritance of the effects of use and disuse, we must also accept it as an equally valid proof that the effects of use and disuse are _not_ inherited. nay, we may even have to admit that, in two points out of four, the _inherited_ effect of use and disuse on successive generations is exactly opposite to the immediate effect on the individual. among fowls the wing-bones have lost much in weight but little or nothing in length--which is the reverse of what has occurred in ducks, although disuse is alleged to be the common cause in both cases. some of the fowls which fly least have their wing-bones as long as ever. in the case of the silk and frizzled fowls--ancient breeds which "cannot fly at all"--and in that of the cochins, which "can hardly fly up to a low perch," darwin observes "how truly the proportions of an organ may be inherited although not fully exercised during many generations."[ ] in four out of twelve breeds the wing-bones had become slightly heavier relatively to the leg-bones. do not these facts tend to show that the changes in fowls' wings are due to fluctuating variability and selective influences rather than to a general law whereby the effects of disuse are cumulatively inherited? pigeons' wings. concerning pigeons' wings darwin says: "as fancy pigeons are generally confined in aviaries of moderate size, and as even when not confined they do not search for their own food, they must during many generations have used their wings incomparably less than the wild rock-pigeon ... but when we turn to the wings we find what at first appears a wholly different and unexpected result."[ ] this unexpected increase in the spread of the wings from tip to tip is due to the feathers, which have lengthened in spite of disuse. excluding the feathers, the wings were shorter in seventeen instances, and longer in eight. but as artificial selection has lengthened the wings in some instances, why may it not have shortened them in others? wings with shortened bones would fold up more neatly than the long wings of the carrier pigeon for instance, and so might unconsciously be favoured by fanciers. the selection of elegant birds with longer necks or bodies would cause a relative reduction in the wings--as with the pouter, where the wings have been greatly lengthened but not so much as the body.[ ] slender bodies, too, and the lessened divergence of the furculum,[ ] would slightly diminish the spread of the wings, and so would affect the measurements taken. as the wing-bones, moreover, are to some extent correlated with the beak and the feet, the artificial selection of shortened beaks might tend to shorten the wing as well as the feet. under these circumstances how can we be sure of the actual efficacy of use-inheritance? surely selection is as fully competent to effect slight changes in the direction of use-inheritance as it undoubtedly is to effect great changes in direct opposition to that alleged factor of evolution. shortened breast-bone in pigeons. the shortening of the sternum in pigeons is attributed to disuse of the flight muscles attached to it. the bone is only shortened by a third of an inch, but this represents a very remarkable reduction in proportional length, which darwin estimates at from one-seventh to one-eighth, or over per cent. this marked reduction, too, quite unlike the slight reduction of the wing-bones to which the other ends of the muscles are attached, was universal in the eleven specimens measured by darwin; and the bone, though acknowledged to have been modified by artificial selection in some breeds, is not so open to observation as wings or legs. even, however, if this relative shortening of the sternum remained otherwise inexplicable, it might still be as irrelevant to use and disuse as is the fact that "many breeds" of fancy pigeons have lost a rib, having only seven where the ancestral rock-pigeon has eight.[ ] but the excessive reduction in the sternum is far from being inexplicable. in the first place darwin has somewhat over-estimated it. instead of comparing the deficiency of length with the increased length which _should_ have been acquired (since the pigeons have increased in average size) he compares it with the length of the breast-bone in the rock-pigeon.[ ] by this method if a pigeon had doubled in dimensions while its breast-bone remained unaltered, the reduction would be put down as per cent., whereas obviously the true reduction would be one-half, or per cent. of what the bone _should be_. avoiding this error and a minor fallacy besides, a sound estimate reduces the supposed reduction of or per cent. to one of · per cent., which is still of course a considerable diminution. part of this reduction must be due to the direct effect of disuse during the lifetime of the individual. another and perhaps very considerable part of the relative change must be attributed to the lengthening of the neck or body by artificial selection, or to other modifications of shape and proportion effected directly or indirectly by the same cause.[ ] the reduction is greatest in the pouter ( - / per cent.) and in the pied scanderoon ( - / per cent.). in the former the body has been greatly elongated by artificial selection and three or four additional vertebræ have been acquired in the hinder part of the body.[ ] in the latter a long neck increases the length of the bird, and so causes, or helps to cause, the relative shortening of the breast-bone. in the english carrier--which experiences the effects of disuse, as it is too valuable to be flown--the relative reduction of per cent. is apparently more than accounted for by the "elongated neck." the dragon also has a long neck. in the pouter, although the breast-bone has been shortened by - / per cent. relatively to the length of the body, it has _lengthened_ by per cent. relatively to the _bulk_ of the body.[ ] darwin forgot to ask whether allowance must not be made for a frequent, or perhaps general, elongation of the neck and the hinder part of the body, and the relative shortening or the throwing forward of the central portion containing the ribs (frequently one less in number) and the sternum. the whole body of the pigeon is so much under the control of artificial selection, that every precaution must be taken to guard against such possible sources of error.[ ] under domestication there would be a suspension of the previous elimination of reduced breast-bones by natural selection (weismann's panmixia), and a diminution of the parts concerned in flying might even be favoured, as lessened powers of _continuous_ flight would prevent pigeons from straying too far, and would fit them for domestication or confinement. such causes might reduce some of the less observed parts affected by flying, while still leaving the wing of full size for occasional flight, or to suit the requirements of the pigeon-fanciers. a change might thus be commenced like that seen in the rudimentary keel of the sternum in the owl-parrot of new zealand, which has lost the power of flight although still retaining fairly-developed wings. shortened feet in pigeons. darwin thinks it highly probable that the short feet of most breeds of pigeons are due to lessened use, though he owns that the effects of correlation with the shortened beak are more plainly shown than the effects of disuse.[ ] but why need the inherited effects of disuse be called in to explain an average reduction of some per cent., when darwin's measurements show that in the breeds where long beaks are favoured the principle of correlation between these parts has lengthened the foot by per cent. in spite of disuse? shortened legs of rabbits. in the case of the domestic rabbit darwin notices that the bones of the legs have (relatively) become shorter by an inch and a half. but as the leg-bones have _not_ diminished in relative weight,[ ] they must clearly have grown _thicker_ or denser. if disuse has shortened them, as darwin supposes, why has it also thickened them? the ears and the tail have been lengthened in spite of disuse. why then may not the ungainly hind-legs have been shortened by human preference independently of the inherited effects of disuse? by relying on apparently favourable instances and neglecting the others it would be easy to arrive at all manner of unsound conclusions. we might thus become convinced that vessels tend to sail northwards, or that a pendulum oscillates more often in one direction than in the other. it must not be forgotten that it would be easy to cite an enormous number of cases which are in direct conflict with the supposed law of use-inheritance. blind cave-animals. weak or defective eyesight is by no means rare as a spontaneous variation in animals, "the great french veterinary huzard going so far as to say that a blind race [of horses] could soon be formed." natural selection evolves blind races whenever eyes are useless or disadvantageous, as with parasites. this may apparently be done independently of the effects of disuse, for certain neuter ants have eyes which are reduced to a more or less rudimentary condition, and neuter termites are blind as well as wingless. in one species of ant (_eciton vastator_) the sockets have disappeared as well as the eyes. in deep caves not only would natural selection cease to maintain good eyesight but it would persistently favour blindness--or the entire removal of the eye when greatly exposed, as in the cave-crab--and as dr. ray lankester has indicated,[ ] there would have been a previous selection of animals which through spontaneous weakness, sensitiveness, or other affection of the eye found refuge and preservation in the cave, and a subsequent selection of the descendants whose fitness for relative darkness led them deeper into the cave or prevented them from straying back to the light with its various dangers and severer competition. panmixia, however, as weismann has shown, would probably be the most important factor in causing blindness. inherited habits. darwin says: "a horse is trained to certain paces, and the colt inherits similar consensual movements."[ ] but selection of the constitutional tendency to these paces, and imitation of the mother by the colt, may have been the real causes. the evidence, to be satisfactory, should show that such influences were excluded. men acquire proficiency in swimming, waltzing, walking, smoking, languages, handicrafts, religious beliefs, &c., but the children only appear to inherit the innate abilities or constitutional proclivities of their parents. even the songs of birds, including their call-notes, are no more inherited than is language by man (_descent of man_, p. ). they are learned from the parent. nestlings which acquire the song of a distinct species, "teach and transmit their new song to their offspring." if use-inheritance has not fixed the song of birds, why should we suppose that in a single generation it has transmitted a newly-taught method of walking or trotting? it is alleged that dogs inherit the intelligence acquired by association with man, and that retrievers inherit the effects of their training.[ ] but selection and imitation are so potent that the additional hypothesis of use-inheritance seems perfectly superfluous. where intelligence is not highly valued and carefully promoted by selection, the intelligence derivable from association with man does _not_ appear to be inherited. lap-dogs, for instance, are often remarkably stupid. darwin also instances the inheritance of dexterity in seal-catching as a case of use-inheritance.[ ] but this is amply explained by the ordinary law of heredity. all that is needed is that the son shall inherit the suitable faculties which the father inherited before him. tameness of rabbits. darwin holds that in some cases selection alone has modified the instincts and dispositions of domesticated animals, but that in most cases selection and the inheritance of acquired habits have concurred in effecting the change. "on the other hand," he says, "habit alone in some cases has sufficed; hardly any animal is more difficult to tame than the young of the wild rabbit; scarcely any animal is tamer than the young of the tame rabbit; but i can hardly suppose that domestic rabbits have often been selected for tameness alone; so that we must attribute at least the greater part of the inherited change from extreme wildness to extreme tameness to habit and long-continued close confinement."[ ] but there are strong, and to me irresistible, arguments to the contrary. i think that the following considerations will show that the greater part, if not the whole, of the change must be attributed to selection rather than to the direct inheritance of acquired habit. ( ) for a period which may cover thousands of generations, there has been an entire cessation of the natural selection which maintains the wildness (or excessive fear, caution, activity, &c.) so indispensably essential for preserving defenceless wild rabbits of all ages from the many enemies that prey upon them. ( ) during this same extensive period of time man has usually killed off the wildest and bred from the tamest and most manageable. to some extent he has done this consciously. "it is very conducive to successful breeding to keep only such as are quiet and tractable," says an authority on rabbits,[ ] and he enjoins the selection of the handsomest and _best-tempered_ does to serve as breeders. to a still greater extent man has favoured tameness unconsciously and indirectly. he has systematically selected the largest and most prolific animals, and has thus doubled the size and the fertility of the domestic rabbit. in consciously selecting the largest and most flourishing individuals and the best and most prolific mothers, he _must_ have unconsciously selected those rabbits whose relative _tameness_ or placidity of disposition rendered it possible for them to flourish and to produce and rear large and thriving families, instead of fretting and pining as the wilder captives would do. when we consider how exceedingly delicate and easily disturbed yet all-important a function is that of maternity in the continually breeding rabbit, we see that the tamest and the least terrified would be the most successful mothers, and so would continually be selected, although man cared nothing for the tameness in itself. the tamest mothers would also be less liable to neglect or devour their offspring, as rabbits commonly do when their young are handled too soon, or even when merely frightened by mice, &c., or disturbed by changed surroundings. ( ) we must remember the extraordinary fecundity of the rabbit and the excessive amount of elimination that consequently takes place either naturally or artificially. where nature preserved only the wildest, man has preserved the tamest. if there is any truth in the darwinian theory, this thorough and long-continued reversal of the selective process _must_ have had a powerful effect. why should it not be amply sufficient to account for the tameness and mental degeneracy of the rabbit without the aid of a factor which can readily be shown to be far weaker in its normal action than either natural or artificial selection? why may not the tameness of the rabbit be transferred to the group of cases in which darwin holds that "habit has done nothing," and selection has done all? ( ) if use-inheritance has tamed the rabbit, why are the bucks still so mischievous and unruly? why is the angora breed the only one in which the males show no desire to destroy the young? why, too, should use-inheritance be so much more powerful in the rabbit than with other animals which are far more easily tamed in the first instance? wild young rabbits when domesticated "remain unconquerably wild," and, although they may be kept alive, they pine and "rarely come to any good." yet the animal which _acquires_ least tameness--or apparently, indeed, none at all--inherits most! it appears, in fact, to inherit that which it cannot acquire--a circumstance which indicates the selection of spontaneous variations rather than the inheritance of changed habits. such variations occasionally occur in animals in a marked degree. of a litter of wolf-cubs, all brought up in the same way, "one became tame and gentle like a dog, while the others preserved their natural savagery." is it not probable that permanent domestication was rendered possible by the inevitable selection of spontaneous variations in this direction? the _excessive_ tameness, too, of the young rabbit, while easily explicable as a result of unconscious selection, is not easily explained as a result of acquired habit. no particular care is taken to tame or teach or domesticate rabbits. they are bred for food, or for profit or appearance, and they are left to themselves most of their time. as sir j. sebright notices with some surprise, the domestic rabbit "is not often visited, and seldom handled, and yet it is always tame." modifications obviously attributable to selection. innumerable modifications in accordance with altered use or disuse, such as the enlarged udders of cows and goats, and the diminished lungs and livers in highly bred animals that take little exercise, can be readily and fully explained as depending on selection. as the fittest for the natural or artificial requirements will be favoured, natural or artificial selection may easily enlarge organs that are increasingly used and economize in those that are less needed. i therefore see no necessity whatever for calling in the aid of use-inheritance as darwin does, to account for enlarged udders, or diminished lungs, or the thick arms and thin legs of canoe indians, or the enlarged chests of mountaineers, or the diminished eyes of moles, or the lost feet of certain beetles, or the reduced wings of logger-headed ducks, or the prehensile tails of monkeys, or the displaced eyes of soles, or the altered number of teeth in plaice, or the increased fertility of domesticated animals, or the shortened legs and snouts of pigs, or the shortened intestines of tame rabbits, or the lengthened intestines of domestic cats, &c.[ ] changed habits and the requisite change of structure will usually be favoured by natural selection; for habit, as darwin says, "almost implies that some benefit great or small is thus derived." similar effects of natural selection and use-inheritance. here we perceive a difficulty which will equally trouble those who affirm use-inheritance and those who deny. broadly speaking, the adaptive effects ascribed to use-inheritance coincide with the effects of natural selection. the individual adaptability (as shown in the thickening of skin, fur, muscle, &c., under the stimulus of friction, cold, use, &c.) is identical in kind and direction with the racial adaptability under natural selection. consequently the alleged inheritance of the advantageous effects of use and disuse cannot readily be distinguished from the similarly beneficial effects of natural selection. the indisputable fact that natural selection imitates or simulates the beneficial effects ascribed to use-inheritance may be the chief source and explanation of a belief which may prove to be thoroughly fallacious. a similar simulation of course occurs under domestication, where natural selection is partly replaced by artificial selection of the best adapted and therefore most flourishing animals, while in disused parts panmixia or the comparative cessation of selection will aid or replace "economy of growth" in causing diminution.[ ] inferiority of senses in europeans. "the inferiority of europeans, in comparison with savages, in eyesight and in the other senses," is attributed to "the accumulated and transmitted effect of lessened use during many generations."[ ] but why may we not attribute it to the slackened and diverted action of the natural selection which keeps the senses so keen in some savage races? short-sight in watchmakers and engravers. darwin notices that watchmakers and engravers are liable to be short-sighted, and that short-sight and long-sight certainly tend to be inherited.[ ] but we must be careful not to beg the question at issue by assuming that the frequent heredity of short sight necessarily covers the heredity of artificially-produced short-sight. elsewhere, however, darwin states more decisively that "there is ground for believing that it may often originate in causes acting on the individual affected, and may thence-forward become transmissible."[ ] this impression may arise ( ) from the facts of ordinary heredity--the ancestral liability being excited in father and son by similar artificial habits, such as reading, and viewing objects closely as among watchmakers and engravers--or by constitutional deterioration from indoor life, &c., acting upon a constitutional liability of the eye to the "something like inflammation of the coats, under which they yield" and so cause shortness of sight by altering the spherical shape of the eye-ball. ( ) panmixia, or the suspension of natural selection, together with altered habits, will account for an increase of short-sight among the population generally. ( ) long-sighted people could not work at watchmaking and engraving so comfortably and advantageously as at other occupations, and hence would be less likely to take to such callings. larger hands of labourers' infants.[ ] these are best explained as the result of natural selection and of the diminution of the hand by sexual selection in the gentry. if the larger hands of labourers' infants are really due to the inherited effects of ancestral use, why does the development occur so early in life, instead of only at a corresponding period, as is the rule? during the first few years of its life, at least, the labourer's infant does no more work than the gentleman's child. why are not the effects of this disuse inherited by the labourer's infant? if the enlargement of the infant's hand illustrates the transference of a character gained later in life, it is evident that the transference must take place in spite of the inherited effects of disuse. thickened sole in infants. darwin also attributes the thickened sole in infants, "long before birth," to "the inherited effects of pressure during a long series of generations."[ ] but disuse should make the infant's sole _thin_, and it is this thinness that should be inherited. if we suppose the inheritance of the thickened soles of later life to be transferred to an earlier period, we have the anomaly of the inherited effects of disuse at that earlier period being overpowered by the untimely inheritance of the effects of use at another. on the other hand, it is clear that natural selection would favour thickened soles for walking on, and might also promote an early development which would ensure their being ready in good time for actual use; for variations in the direction of delay would be cut off, while variations in the other direction would be preserved. anyhow, the mere transference of a character to an earlier period is no proof of use-inheritance. the real question is whether the thickened sole was gained by natural selection or by the inherited effects of pressure, and the mere transference or hastened appearance of the thickening does not in any degree solve this question. it merely excludes the effect of disuse during lifetime, and thus presents a fallacious appearance of being decisive. the thickened sole of the unborn infant, however, like the lanugo or hairy covering, is probably a result of the direct inheritance of ancestral stages of evolution, of which the embryo presents a condensed epitome. while the relative thinness of the infant's sole might be pointed to as the effect of _disuse_ during a long series of generations, its thickness is rather an illustration of atavism still resisting the effects of long-continued disuse. there is nothing to show that the inheritable portion of the full original thickness was not gained by natural selection rather than by the directly inherited effect of use; and the latter, being cumulative and indiscriminative in its action, would apparently have made the sole very much thicker and harder than it is. if natural selection were not supreme in such cases, how could we account for the effects of pressure resulting in hard hoofs in some cases and only soft pads in others? a source of mental confusion. of course in a certain sense this thickening of the sole has resulted from use. in one sense or other, most--or perhaps all--of the results of natural selection are inherited effects of use or disuse. natural selection preserves that which is of use and which is used, while it eliminates that which is useless and is not used. the most confident assertions of the effects of use and disuse in modifying the heritable type, appear to rest on this indefeasible basis. darwin's statements concerning the effects of use and disuse in evolution can frequently be read in two senses. they often command assent as undeniable truisms as they stand, but are of course written in another and more debatable sense. thus in the case of the shortened wings and thickened legs of the domestic duck, i believe equally with darwin and spencer that "no one will dispute that they have resulted from the lessened use of the wings and the increased use of the legs." "use" is at bottom the determining circumstance in evolution generally. the trunk of the elephant, the fin of the fish, the wing of the bird, the cunning hand of man and his complicated brain--and, in short, all organs and faculties whatsoever--can only have been moulded and developed by use--by usefulness and by using--but not necessarily by use-inheritance, not necessarily by directly inherited effects of use or disuse of parts in the individual. so, too, reduced or rudimentary organs are due to disuse, but it by no means follows that the diminution is caused by any direct tendency to the inheritance of the effects of disuse in the individual. the effects of natural selection are commonly expressible as effects of use and disuse, just as adaptation in nature is expressible in the language of teleology. but use-inheritance is no more proven by one of these necessary coincidences than special design is by the other. the inevitable simulation of use-inheritance may be entirely deceptive. darwin thinks that "there can be no doubt that use in our domestic animals has strengthened and enlarged certain parts, and disuse diminished them; and that such modifications are inherited." undoubtedly "such" or _similar_ modifications have often been inherited, but how can darwin possibly tell that they are not due to the simulation of use-inheritance by natural or artificial selection acting upon general variability? of the inevitability of selection and of its generally adaptive tendencies "there can be no doubt," and panmixia would tend to reduce disused parts; so that there _must always_ remain grave doubts of the alleged inheritance of the similar effects of use and disuse, unless we can accomplish the extremely difficult feat of excluding both natural and artificial selection as causes of enlargement, and panmixia and selection as causes of dwindling. weakness of use-inheritance. use-inheritance is normally so weak that it appears to be quite helpless when opposed to any other factor of evolution. natural selection evolves and maintains the instincts of ants and termites in spite of use-inheritance to a more wonderful degree than it evolves the instincts of almost any other animal with the fullest help of use-inheritance. it develops seldom-used horns or natural armour just as readily as constantly-used hoofs or teeth. sexual selection evolves elaborate structures like the peacock's tail in spite of disuse and natural selection combined. artificial selection appears to enlarge or diminish used parts or disused parts with equal facility. the assistance of use-inheritance seems to be as unnecessary as its opposition is ineffective. the alleged inheritance of the effects of use and disuse in our domestic animals must be very slow and slight.[ ] darwin tells us that "there is no good evidence that this ever follows in the course of a single generation." "several generations must be subjected to changed habits for any appreciable result."[ ] what does this mean? one of two things. either the tendency is very weak, or it is non-existent. if it is so weak that we cannot detect its alleged effects till several generations have elapsed, during which time the more powerful agency of selection has been at work, how are we to distinguish the effects of the minor factor from that of the major? are we to conclude that use-inheritance _plus_ selection will modify races, just as voltaire firmly held that incantations, together with sufficient arsenic, would destroy flocks of sheep? is it not a significant fact that the alleged instances of use-inheritance so often prove to be self-conflicting in their details? for satisfactory proof of the prevalence of a law of use-inheritance we require normal instances where selection is clearly inadequate to produce the change, or where it is scarcely allowed time or opportunity to act, as in the immediate offspring of the modified individual. of the first kind of cases there seems to be a plentiful lack. of the latter kind, according to darwin, there appears to be none--a circumstance which contrasts strangely and suspiciously with the many decisive cases in which variation from unknown causes has been inherited most strikingly in the immediate offspring. it must be expected, indeed, that among these innumerable cases some will accidentally mimic the alleged effects of use-inheritance. if darwin had felt certain that the effects of habit or use tended in any marked degree to be conveyed directly and cumulatively to succeeding generations, he could hardly have given us such cautious, half-hearted encouragement of good habits as the following:--"it is not improbable that after long practice virtuous tendencies may be inherited." "habits, moreover followed during many generations probably tend to be inherited."[ ] this is probable, independently of use-inheritance. the "many generations" specified or implied, will allow time for the play of selective as well as of cumulatively-educative influences. there must apparently be a constitutional or inheritable predisposition or fitness for the habits spoken of, which otherwise would scarcely be continued for many generations, except by the favourably-varying branches of a family: which again is selection rather than use-inheritance. where is the necessity for even the remains of the lamarckian doctrine of inherited habit? seeing how powerful the general principle of selection has shown itself in cases where use-inheritance could have given no aid or must even have offered its most strenuous opposition, why should it not equally be able to develop used organs or repress disused organs or faculties without the assistance of a relatively weak ally? selection evolved the remarkable protective coverings of the armadillo, turtle, crocodile, porcupine, hedgehog, &c.; it formed alike the rose and its thorn, the nut and its shell; it developed the peacock's tail and the deer's antlers, the protective mimicry of various insects and butterflies, and the wonderful instincts of the white ants; it gave the serpent its deadly poison and the violet its grateful odour; it painted the gorgeous plumage of the impeyan pheasant and the beautiful colours and decorations of countless birds and insects and flowers. these, and a thousand other achievements, it has evidently accomplished without the help of use-inheritance. why should it be thought incapable of reducing a pigeon's wing or enlarging a duck's leg? why should it be credited with the help of an officious ally in effecting comparatively slight changes, when great and striking modifications are effected without any such aid? footnotes: [ ] weismann's _essays on heredity_, &c. clarendon press, . [ ] _life and letters_, i. p. . darwin's reverence for his father "was boundless and most touching. he would have wished to judge everything else in the world dispassionately, but anything his father had said was received with almost implicit faith; ... he hoped none of his sons would ever believe anything because he said it, unless they were themselves convinced of its truth--a feeling in striking contrast with his own manner of faith" (_life and letters_, i. pp. , ). [ ] _ibid._, i. p. . [ ] _life and letters_, ii. p. . [ ] _origin of species_, pp. , . [ ] _ibid._, p. . [ ] _contemporary review_, december, , pp. , . [ ] _variation of animals and plants under domestication_, i. . [ ] _variation of animals and plants under domestication_, i. - . [ ] to keep pace with this lateral increase in weight, the leg-bones should have lengthened considerably so that their total deficiency in proportional length is per cent.,--a changed proportion which being _linear_ is more excessive than the increase of weight by per cent. so marked is the effect of the combined thickening and shortening that in the aylesbury breed--which is the most typically representative one--the leg-bones have become per cent. heavier than they should be if their thickness had continued to be proportional to their length. [ ] this excessive thickening under disuse appears to be due partly to a positive lateral enlargement or increase of proportional weight of about - / per cent., and partly to a shortening of about per cent. carefully calculated, the reduction of the weight of the wing-bones in this breed is only · per cent. relatively to the whole skeleton, or only per cent. relatively to the skeleton _minus_ legs and wings. the latter method is the more correct, since the excessive weight of the leg-bones increases the weight of the skeleton more than the diminished weight of the wing-bones reduces it. [ ] _variation of animals and plants under domestication_, i. . [ ] _variation of animals and plants under domestication_, i. , . [ ] _ibid._, i. , . [ ] _ibid._, i. . [ ] _variation of animals and plants under domestication_, i. . [ ] _variation of animals and plants under domestication_, i. . i suspect that darwin was in poor health when he wrote this page. he nods at least four times in it. twice he speaks of "twelve" breeds where he obviously should have said eleven. [ ] if a prominent breast is admired and selected by fanciers, the sternum might shorten in assuming a more forward and vertical position. if the shortening of the sternum is entirely due to disuse, it seems strange that darwin has not noticed any similar shortening in the sternum of the duck. but selection has not tended to make the duck elegant, or "pigeon-breasted"; it has enlarged the abdominal sack instead, besides allowing the addition of an extra rib in various cases. [ ] _variation of animals and plants under domestication_, , . [ ] _variation of animals and plants under domestication_, i. . [ ] in the six largest breeds the shortening of the sternum is nearly twice as great as in the three smaller breeds which remain nearest the rock-pigeon in size. we can hardly suppose that use-inheritance especially affects the eight breeds that have varied most in size. if we exclude these, there is only a total shortening of per cent. to be accounted for. [ ] _variation of animals and plants under domestication_, i. , . [ ] _variation of animals and plants under domestication_, i. , ; ii. . [ ] _encyclopædia britannica_, article "zoology." [ ] _variation of animals and plants under domestication_, ii. . [ ] _variation of animals and plants under domestication_, ii. . why then does the cheetah inherit ancestral habits so inadequately that it is useless for the chase unless it has first learned to hunt for itself before being captured? (ii. ). [ ] _descent of man_, p. . [ ] _origin of species_, pp. , . [ ] e. s. delamer on _pigeons and rabbits_, pp. , . for other points referred to, see pages , , , , . [ ] _origin of species_, pp. , ; _descent of man_, pp. - ; _variation of animals and plants under domestication_, ii. , . use or disuse during lifetime of course co-operates, and in some cases, as in that of the canoe indians, may be the principal or even perhaps the _sole_ cause of the change. [ ] for the importance of panmixia as invalidating darwin's strongest evidence for use-inheritance--namely, that drawn from the effects of disuse in highly-fed domestic animals where there is supposed to be no economy of growth--see professor romanes on panmixia, _nature_, april , . [ ] _descent of man_, p. . [ ] _descent of man_, p. . [ ] _variation of animals and plants under domestication_, i., . [ ] _descent of man_, p. . [ ] _descent of man_, p. . [ ] wallace shows that the changes in our domestic animals, if spread over the thousands of years since the animals were first tamed, must be extremely insignificant in each generation, and he concludes that such infinitesimal effects of use and disuse would be swallowed up by the far greater effects of variation and selection (_darwinism_, p. ). professor romanes has replied to him in the _contemporary review_ (august ), showing that this is no disproof of the existence of the minor factor, inasmuch as slight changes in each generation need not necessarily be matters of life and death to the individual, although their cumulative development by use-inheritance might eventually become of much service. but selection would favour spontaneous variations of a similarly serviceable character. the slightest tendency to eliminate the extreme variations in either direction would proportionally modify the average in a breed. use-inheritance appears to be so relatively weak a factor that probably neither proof nor disproof of its existence can ever be given, owing to the practical impossibility of disentangling its effects (if any) from the effects of admittedly far more powerful factors which often act in unsuspected ways. thus wild ducklings, which can easily be reared by themselves, invariably "die off" if reared with tame ones (_variation_, &c., i. , ii. ). they cannot get their fair share in the competition for food, and are completely eliminated. professor romanes fully acknowledges that there is the "gravest possible doubt" as to the transmission of the effects of disuse (letter on panmixia, _nature_, march , ). [ ] _variation of animals and plants under domestication_, ii. - . [ ] _descent of man_, pp. , . inherited injuries. inherited mutilations. the almost universal _non-inheritance_ of mutilations seems to me a far more valid argument _against_ a general law of modification-inheritance than the few doubtful or abnormal cases of such inheritance can furnish in its favour. no inherited effect has been produced by the docking of horses' tails for many generations, or by a well-known mutilation which has been practised by the hebrew race from time immemorial. as lost or mutilated parts are reproduced in offspring independently of the existence of those parts in the parent, there is the less reason to suppose that the particular condition of parental parts transmits itself, or tends to transmit itself, to the offspring. so unsatisfactory is the argument derivable from inherited mutilations that mr. spencer does not mention them at all, and darwin has to attribute them to a special cause which is independent of any general theory of use-inheritance.[ ] darwin's most striking case--and to my mind the only case of any importance--is that of brown-séquard's epileptic guinea-pigs, which inherited the mutilated condition of parents who had gnawed off their own gangrenous toes when anæsthetic through the sciatic nerve having been divided.[ ] darwin also mentions a cow that lost a horn by accident, followed by suppuration, and subsequently produced three calves which had on the same side of the head, instead of a horn, a bony lump attached merely to the skin. such cases may seem to prove that mutilation _associated with morbid action_ is occasionally inherited or repeated with a promptitude and thoroughness that contrast most strikingly with the imperceptible nature of the immediate inheritance of the effects of use and disuse; but they by no means prove that mutilation in general is inheritable, and they are absolutely no proof whatever of a _normal_ and non-pathological tendency to the inheritance of acquired characters. those who accept darwin's special explanation of the supposed inheritance of mutilations, ought to notice that his explanation applies equally well under a theory which is strongly adverse to use-inheritance--namely, galton's idea of the sterilization and complete "using up" of otherwise reproductive matter in the growth and maintenance of the personal structure. darwin's explanation of inherited mutilations--which, as he notes, occur "especially or perhaps exclusively" when the injury has been followed by disease[ ]--is that all the representative gemmules which would develop or repair or reproduce the injured part are attracted to the diseased surface during the reparative process and are there destroyed by the morbid action.[ ] hence they cannot reproduce the part in offspring. this explanation by no means implies that mutilation would _usually_ affect the offspring. on the contrary, in all ordinary cases of mutilation the purely atavistic elements or gemmules would be set free from any modifying influence of the non-existent or mutilated part. the gemmules--as in galton's theory of heredity and with neuter insects--might be perfectly independent of pangenesis and the normal inheritance of acquired characters. such self-multiplying gemmules without pangenesis would enable us to understand both the excessive weakness or non-existence of normal use-inheritance, and the excessive strength and abruptness of the effect of their partial destruction under special pathological conditions. the series of epileptic phenomena that can be excited by tickling a certain part of the cheek and neck of the adult guinea-pig during the growth and rejoining of the ends of the severed nerve, are said to be repeated with striking accuracy of detail in the young who inherit mutilated toes; but as epilepsy is often due to some _one_ exciting cause or morbid condition, the single transmission of a highly morbid condition of the system might easily reproduce the whole chain of consequences and might also have caused the loss of toes. the particulars of the guinea-pig cases are very inadequately recorded,[ ] but the results are so anomalous[ ] that brown-séquard's own conclusion is that the epilepsy and the inherited injuries are _not_ directly transmitted, but that "what is transmitted is the morbid state of the nervous system." he thinks that the missing toes may "possibly" be exceptions to this conclusion, "but the other facts only imply the transmission of a morbid state of the sympathetic or sciatic nerve or of a part of the medulla oblongata." until we can tell what is transmitted, we are not in a position to determine whether there is any true inheritance or only an exaggerated simulation of it under peculiar circumstances. when the actual observers believe that the mutilations and epilepsy are not the cause of their own repetition, and when these observers guard themselves by such phrases as, "if any conclusion can at present be drawn from those facts," we who have only incomplete reports to guide us may well be excused if we preserve an even more pronounced attitude of caution and reserve.[ ] the morbid state of the system may be wholly due to general injury of the germs rather than to specific inheritance. weismann suggests that the morbid condition of the nervous system may be due to some infection such as might arise from microbes, which find a home in the mutilated and disordered nervous system in the parent, and subsequently transmit themselves to the offspring through the reproductive elements, as the infections of various diseases appear to do--the muscardine silkworm disease in particular being known to be conveyed to offspring in this manner. but whether we can discover the true explanation or not, inherited mutilations can hardly be accounted for as the result of a general tendency to inherit acquired modifications. how could a factor which seems to be totally inoperative in cases of ordinary mutilation, and only infinitesimally operative in transmitting the normal effects of use and disuse, suddenly become so powerful as to completely overthrow atavism, and its own tendency to transmit the non-mutilated type of one of the parents and of the non-mutilated type presented by the injured parent in earlier life? does not so striking and abrupt an intensification of its usually insignificant power demand an explanation widely different from that which might account for the extremely slow and slight inheritance of the normal effects of use and disuse? surely it would be better to suspend one's judgment as to the true explanation of highly exceptional and purely pathological cases rather than resort to an hypothesis that creates more difficulties than it solves. the motmot's tail. the narrowing of the long central tail feathers of the motmot is attributed to the inherited effects of habitual mutilation (_descent of man_, pp. , ). but in the specimens at south kensington[ ] the narrowness extends upwards much beyond the habitually denuded part, and the broadened end is the broadest part of the whole feather. if the inherited effect of an inch or two of denudation extends from three to six inches upwards, why has it not also extended two inches downwards so as to narrow the broadened end? the narrowness seems to be a mainly relative or negative effect produced by the broadening out of a long tapering feather at its end under the influence of sexual selection. several other birds have similarly narrowed or spoon-shaped feathers and do not bite them. is it not more feasible to suppose that this attractive peculiarity first suggested its artificial intensification, than to suppose that the bird began nibbling without any definite cause? sexual selection would then encourage the habit. anyhow, it is as impossible to show that the mutilation preceded the narrowing as it is to show that tonsure preceded baldness. other inherited injuries mentioned by darwin. darwin quotes some cases from dr. prosper lucas's "long" but weak and unsatisfactory "list of inherited injuries."[ ] but lucas was somewhat credulous. one of his cases is that many girls were born in london without mammæ through the injurious effect of certain corsets on the mothers. he also gives a long account of a jew who could read through the thick covers of a book, and whose son inherited this "hyperæsthesia" of the sense of sight in a still more remarkable degree (i. - ). evidently lucas's cases cannot be accepted without some amount of reserve. the cases of the three calves which inherited the one-horned condition of the cow, the two sons who inherited a father's crooked finger, and the two sons who were microphthalmic on the same side as their father had lost an eye, may be due to mere coincidence; or an inherited constitutional tendency or liability might lead to somewhat similar results in parent and offspring[ ]--just as the tendency to certain fatal diseases or to suicide may produce similar results in father and son, although the artificially-produced hanging or apoplexy obviously cannot be directly transmitted. that more than one of the offspring was affected does not render the chances against coincidence "almost infinitely great," as darwin mistakenly supposes. it "frequently occurs" that a man's sons or daughters may _all_ exhibit either a latent or a newly-developed congenital peculiarity previously unknown;[ ] and the coincidence may merely be that one of the parents accidentally suffered a similar kind of injury--a kind of coincidence which must of course occasionally occur, and which may have been partly caused by a latent tendency. the chances against coincidence are indeed great, but the cases appear to be correspondingly rare. darwin acknowledges that many supposed instances of inherited mutilation may be due to coincidence; and there is apparently no more reason for attributing inherited scars, &c., to any special form of heredity than to the effect of the mother's imagination on the unborn babe--a popular but fallacious belief in corroboration of which far more alleged instances could be collected than of the inheritance of injuries. as an instance of the coincidences that occur, i may mention that a friend of mine has a daughter who was born with a small hole in one ear, just as if it were already pierced for the earring which she has since worn in it. i suppose, however, that no one will venture to claim this as an instance of the inheritance of a mutilation practised by female ancestors, especially as such holes are not altogether unknown or inexplicable, though very rarely occurring low down in the lobe of the ear.[ ] many cases are known of the inheritance of mutilations or malformations arising congenitally from some abrupt variation in the reproductive elements. in such cases as the one-eared rabbits, the two-legged pigs, the three-legged dogs, the one-horned stags, hornless bulls, earless rabbits, lop-eared rabbits, tailless dogs, &c., if the father or the mother or the embryo had suffered from some accident or disease which might plausibly have been assigned as the cause of the original malformation, these transmitted defects would readily be cited as instances of the inheritance of an accidentally-produced modification. the inheritance of exostoses on horses' legs may be the inheritance of a constitutional tendency rather than of the effect of the parents' hard travelling. horses congenitally liable to such formations would transmit the liability,[ ] and this might readily be mistaken for inheritance of the results of the liability. an apparent increase in this liability might arise from greater attention being now paid to it, or from increased use of harder roads; or a real increase might be due to panmixia and some obscure forms of correlation. quasi-inheritance. of course artificially-caused ill-health or weakness in parents will tend in a general way to injure the offspring. but deterioration thus caused is only a form of quasi-inheritance, as i should prefer to call it. semi-starvation in a new-born babe is _not_ truly inherited from its half-starved mother, but is the direct result of insufficient nourishment. the general welfare of germs--as of parasites--is necessarily bound up with that of the organism which feeds and shelters them, but this is not heredity, and is quite irrelevant to the question whether particular modifications are transmitted or not. another form of quasi-inheritance is seen in the communication of certain infections to offspring. not being transmitted by the action of the organism so much as in defiance of it, such diseases are not truly hereditary, though for convenience' sake they are usually so described. a perversion or prevention of true inheritance is also seen in the action of alcohol, or excessive overwork, or any other cause which by originating morbid conditions in individuals may also injure the reproductive elements. these forms of quasi-inheritance are, of course, highly important so far as the improvement of the race is concerned. so, too, is the fact that improved or deteriorated habits and thoughts are transmitted by personal teaching and influence and are cumulative in their effect. but all this must not be confounded with the inheritance of acquired characters. cases of quasi-inheritance may perhaps be most readily distinguished from cases of true inheritance by the time test. when a modification acquired in adult life is promptly communicated to the child in early life or from birth, it may rightly be suspected that the inheritance, like that of money or title, is not truly congenital, but is extraneous or even anti-congenital in its nature. judged by such a standard, the inherited injuries in brown-séquard's guinea-pigs are only exceptional cases of quasi-inheritance, and are not necessarily indicative of any general rule affecting true inheritance. footnotes: [ ] a very able anatomist of my acquaintance denies the inheritance of mutilations and injuries, although he strongly believes in the inheritance of the effects of use and disuse. [ ] _variation of animals and plants under domestication_, i. - . lost toes were only seen by dr. dupuy in three young out of two hundred. obersteiner found that most of the offspring of his epileptic guinea-pigs were injuriously affected, being weakly, small, paralysed in one or more limbs, and so forth. only two were epileptic, and both were weakly and died early (weismann's _essays_, p. ). a morbid condition of the spinal cord might affect the hind limbs especially (as in paraplegia) and might occasionally cause loss of toes in the embryo by preventing development or by ulceration. brown-séquard does not say that the defective feet were on the same side as in the parents (_lancet_, jan., , pp. , ). [ ] _variation of animals and plants under domestication_, ii. . [ ] _ibid._, ii. . perhaps it might be better to suppose that the _best_ gemmules were sacrificed in repairing the injured _nerve_, and hence only inferior substitutes were left to take their place, and could only imperfectly reproduce the injured part of the nervous system in offspring. [ ] hence perhaps mr. spencer's error in representing the epileptic liability as permanent and as coming on _after_ healing (_factors of organic evolution_, p. ). [ ] it is not claimed that the imperfect foot was on the same side of the body as in the parent, and where parents had lost _all_ the toes of a foot, or the whole foot, the few offspring affected usually had lost only two toes out of the three, or only a part of one or two or three toes. sometimes the offspring had toes missing on _both_ hind feet, although the parent was only affected in _one_. _one_ diseased ear and eye in the parent was "generally" or "always" succeeded by _two_ equally affected ears and eyes in the offspring (cf. _pop. science monthly_, new york, xi. ). the important law of inheritance at corresponding periods was also set aside. gangrene or inflammation commenced in both ears and both eyes soon after birth (pointing possibly to infection of some kind); the epileptic period commenced "perhaps two months or more after birth," while the loss of toes had occurred before birth. in no case, as weismann points out, is the original mutilation of the nervous system ever transmitted. even where an extirpated ganglion was never regenerated in the parent, the offspring always regained the part in an apparently perfect condition. on the whole the conflicting results ought to be as puzzling to those who may attribute them to a universal tendency to inherit the exact condition of parents as they are to those who, like myself, are sceptical as to the existence of such a law or tendency. [ ] the various results need to be fully and impartially recorded, and they should also be well tested and confirmed in proportion as they appear improbable and contrary to general experience. professor romanes has been carrying out the necessary experiments for some time past. [ ] natural history museum, central hall, third recess on the left. [ ] _traité de l'hérédité_, ii. ; _variation of animals and plants under domestication_, i. . if injuries are inherited, why has the repeated rupture of the hymen produced no inherited effect? [ ] compare the three cases of crooked fingers given in _variation of animals and plants under domestication_, ii. , . [ ] _ibid._, i. . thus, where two brothers married two sisters all the seven children were perfect albinos, although none of the parents or their relatives were albinos. in another case the nine children of two sound parents were all born blind (ii. ). [ ] see pp. - , _evolution and disease_, by j. bland sutton, to whom and to our mutual friend dr. d. thurston i am indebted for information on various points. [ ] _variation of animals and plants under domestication_, ii. ; i. . miscellaneous considerations. true relation of parents and offspring. it is difficult to entirely free ourselves from the flattering and almost universal idea that parents are true originators or creators of copies of themselves. but the main truth, if not the whole truth, is that they are merely the transmitters of types of which they and their offspring are alike more or less similarly moulded resultants. a parent is a trustee. he transmits, not himself and his own modifications, but the stock, the type, the representative elements, of which he is a product and a custodian in one. it seems probable that he has no more definite or "particulate" influence over the reproductive elements within him than a mother over the embryo or a vessel over its cargo. parent and offspring are like successive copies of books printed from the same "type." a battered letter in the "type" will display its effects in both earlier and later copies alike, but a purely extraneous or acquired flaw in the first copy is not necessarily repeated in subsequent copies. unlike printer's type, however, the material source of heredity is of a fluctuating nature, consisting of competing elements derived from two parents and from innumerable ancestors. galton compares parent and child to successive pendants on the same chain. weismann likens them to successive offshoots thrown up by a long underground root or sucker. such comparisons indicate the improbability of acquired modifications being transmitted to offspring. that parts are developed in offspring independently of those parts in parents is clear. mutilated parents transmit parts which they do not possess. the offspring of young parents cannot inherit the later stages of life from parents who have not passed through them. cases of remote reversion or atavism show that ancestral peculiarities can transmit themselves in a latent or undeveloped condition for hundreds or thousands of generations. many obvious facts compelled darwin to suppose that vast numbers of the reproductive gemmules in an individual are not thrown off by his own cells, but are the self-multiplying progeny of ancestral gemmules. galton restricts the production of gemmules by the personal structure to a few exceptional cases, and would evidently like to dispense with pangenesis altogether, if he could only be sure that acquired characters are never inherited. weismann entirely rejects pangenesis and the inheritance of acquired characters. this enables him to explain heredity by his theory of the "continuity of the germ-plasm."[ ] parent and offspring are alike successive products or offshoots of this persistent germ-substance, which obviously would not be correspondingly affected by modifications of parts in parents, and so would render the transmission of acquired characters impossible. inverse inheritance. mr. galton contends that the reproductive elements become sterile when used in forming and maintaining the individual, and that only a small proportion of them are so used.[ ] he holds that the next generation will be formed entirely, or almost entirely, from the residue of undeveloped germs, which, not having been employed in the structure and work of the individual, have been free to multiply and form the reproductive elements whence future individuals are derived. hence the singular inferiority not infrequently displayed by the children of men of extraordinary genius, especially where the ancestry has been only of a mediocre ability. the valuable germs have been used up in the individual, and rendered sterile in the structure of his person. hence, too, the "strong tendency to deterioration in the transmission of every exceptionally gifted race." mr. galton's hypothesis "explains the fact of certain diseases skipping one or more generations," and it "agrees singularly well with many classes of fact;" and it is strongly opposed to the theory of use-inheritance. the elements which are used die almost universally without germ progeny: the germs which are _not_ used are the great source of posterity. hence, when the germs or gemmules which achieve development are either better or worse than the residue, the qualities transmitted to offspring will be of an inverse character. if brain-work attracts, develops _and sterilizes_ the best gemmules, the ultimate effect of education on the intellect of posterity may differ from its immediate effect. early origin of the ova. as the ova are formed at as early a period as the rest of the maternal structure, galton notices that it seems improbable that they would be correspondingly affected by subsequent modifications of parental structure. of course it is not certain that this is a valid argument. we know that the paternal half of the reproductive elements does not enter the ovum till a comparatively late stage in its history, and it is quite possible that maternal elements or gemmules may also enter the ovum from without. if reproductive elements were confined to one special part or organ, we should be unable to explain the reproduction of lost limbs in salamanders, and the persistent effect of intercrossing on subsequent issue by the same mother, and the propagation of plants from shoots, or of the begonia from minute fragments of leaves, or the development of small pieces of water-worms into complete animals. marked effects of use and disuse on the individual. these are, to some extent, an argument against the cumulative inheritance of such effects. when a nerve atrophies from disuse, or a duct shrivels, or bone is absorbed, or a muscle becomes small or flabby, it proves, so far, that the average effect of use through enormous ages is _not_ transmitted. when the fibula of a dog's leg thickens by per cent. to a size "equal to or greater than" that of the removed tibia which previously did the work,[ ] it shows that in spite of disuse for countless generations, the "almost filiform" bone has retained a potentiality of development which is fully equal to that possessed by the larger one which has been constantly used. when, after being reared on the ailanthus, the caterpillars of the _bombyx hesperus_ die of hunger rather than return to their natural food, the inherited effect of ancestral habit does not seem to be particularly strong. neither is there any strongly-inherited effect of long-continued ancestral wildness in many animals which are easily tamed. would natural selection favour use-inheritance? if use-inheritance is really one of the factors of evolution, it is certainly a subordinate one, and an utterly helpless one, whenever it comes into conflict with the great ruling principle of selection. would this dominant cause of evolution have favoured a tendency to use-inheritance if such had appeared, or would it have discouraged and destroyed it? we have already seen that use-inheritance is unnecessary, since natural selection will be far more effective in bringing about advantageous modifications; and if it can be shown that use-inheritance would often be an evil, it then becomes probable that on the whole natural selection would more strongly discourage and eliminate it as a hostile factor than it might occasionally favour such a tendency as a totally unnecessary aid. use-inheritance an evil. use-inheritance would crudely and indiscriminately proportion parts to actual work done--or rather to the varying _nourishment and growth_ resulting from a multiplicity of causes--and this in its various details would often conflict most seriously with the real necessities of the case, such as occasional passive strength, or appropriate shape, lightness and general adaptation. if its accumulated effects were not corrected by natural or sexual selection, horns and antlers would disappear in favour of enlarged hoofs. the elephant's tusks would become smaller than its teeth. men would have callosities for sitting on, like certain monkeys, and huge corns or hoofs for walking on. bones would often be modified disastrously. thus the condyle of the human jaw would become larger than the body of the jaw, because as the fulcrum of the lever it receives more pressure. some organs (like the heart, which is always at work) would become inconveniently or unnecessarily large. other absolutely indispensable organs, which are comparatively passive or are very seldom used, would dwindle until their weakness caused the ruin of the individual or the extinction of the species. in eliminating various evil results of use-inheritance, natural selection would be eliminating use-inheritance itself. the displacement of lamarck's theory by darwin's shows that the effects of use-inheritance often differ from those required by natural selection; and it is clear that the latter factor must at least have reduced use-inheritance to the very minor position of comparative feebleness and harmlessness assigned to it by darwin. use-inheritance would be ruinous through causing unequal variation in co-operative parts--of which mr. spencer may accept his own instances of the jaws and teeth, and the cave-crab's lost eyes and persistent eye-stalks, as typical examples. that the variation would be unequal seems almost self-evident from the varying rapidity and extent of the effects of use and disuse on different tissues and on different parts of the general structure. the optic nerve may atrophy in a few months from disuse consequent on the loss of the eye. some of the bones of the rudimentary hind legs of the whale are still in existence after disuse for an enormous period. evidently use-inheritance could not equally modify the turtle and its shell, or the brain and its skull; and in minor matters there would be the same incongruity of effect. thus, if the molar teeth lengthened from extra use the incisors could not meet. unequal and indiscriminate variation would throw the machinery of the organism out of gear in innumerable ways. use-inheritance would perpetuate various evils. we are taught, for instance, that it perpetuates short-sight, inferior senses, epilepsy, insanity, nervous disorders, and so forth. it would apparently transmit the evil effects of over-exertion, disuse, hardship, exposure, disease and accident, as well as the defects of age or immaturity. would it not be better on the whole if each individual took a fresh start as far as possible on the advantageous typical lines laid down by natural selection? through the long stages of evolution from primæval protoplasm upwards, such species as were least affected by use-inheritance would be most free to develop necessary but seldom-used organs, protective coverings such as shells or skulls, and natural weapons, defences, ornaments, special adaptations, and so forth; and this would be an advantage--for survival would obviously depend on the _importance_ of a structure or faculty in deciding the struggle for existence and reproduction, and not on the total amount of its using or nourishment. if natural selection had on the whole favoured this officious ally and frequent enemy, surely we should find better evidence of its existence. without laying undue stress upon the evil effects of use-inheritance, a careful examination of them in detail may at least serve to counter-balance the optimistic _a priori_ arguments for belief in that plausible but unproven factor of evolution. the benefits derivable from use-inheritance are largely illusory. the effects of _use_, indeed, are generally beneficial up to a certain point; for natural selection has sanctioned or evolved organs which possess the property or potentiality of developing to the right extent under the stimulus of use or nourishment. but use-_inheritance_ would cumulatively alter this individual adaptability, and would tend to fix the size of organs by the average amount of ancestral use or disuse rather than by the actual requirements of the individual. of course under changed conditions involving increased or lessened use of parts it might become advantageous; but even here it may prove a decided hindrance to adaptive evolution in some respects as well as an unnecessary aid in others. thus in the case of animals becoming heavier, or walking more, it would _lengthen_ the legs although natural selection might require them to be shortened. in the aylesbury duck and the call duck, if use-inheritance has increased the dimensions of the bones and tendons of the leg, natural selection has had to counteract this increase so far as length is concerned, and to effect per cent. of shortening besides. if use-inheritance thickens bones without proportionally lengthening them, it would hinder rather than help the evolution of such structures as the long light wings of birds, or the long legs and neck of the giraffe or crane. varied effects of use and disuse. the changes which we somewhat roughly and empirically group together as the effects of "use and disuse" are of widely diverse character. thus bone, as the physiological fact, thickens under _alternations_ of pressure (and the consequent increased flow of nourishment), but atrophies under a steadily continued pressure; so that if the use of a bone involved continuous pressure, the effect of such use would be a partial or total absorption of that bone. darwin shows that bone lengthens as well as thickens from carrying a greater weight, while tension (as seen in sailors' arms, which are used in pulling) appears to have an equally marked effect in shortening bones (_descent of man_, p. ). thus different kinds of use may produce opposite results. the cumulative inheritance of such effects would often be mischievous. the limbs of the sloth and the prehensile tail of the spider monkey would continually grow shorter, while the legs of the evolving elephant or rhinoceros might lengthen to an undesirable extent. such cumulative tendencies of use-inheritance, if they exist, are obviously well kept under by natural selection. although the ultimate effect of use is generally growth or enlargement through increased flow of blood, the first effect usually is a loss of substance, and a consequent diminution of size and strength. when the loss exceeds the growth, use will diminish or deteriorate the part used, while disuse would enlarge or perfect it. teeth, claws, nails, skin, hair, hoofs, feathers, &c., may thus be worn away faster than they can renew themselves. but this wearing away usually stimulates the repairing process, and so increases the rate of growth; that is, it will increase the size produced, if not the size retained. which effect of use does use-inheritance transmit in such cases--the increased rate of growth, or the dilapidation of the worn-out parts? we can hardly suppose that both these effects of use will be inherited. would shaving destroy the beard in time or strengthen it? will the continued shearing of sheep increase or lessen the growth of wool? what will be the ultimate effect of plucking geese's quills, and of the eider duck's abstraction of the down from her breast? if the mutilated parts grow stronger or more abundantly, why were the motmot's feathers alleged to be narrowed by the inherited effects of ancestral nibbling? the "use" or "work" or "function" of muscles, nerves, bones, teeth, skin, tendon, glands, ducts, eyes, blood corpuscles, cilia, and the other constituents of the organism, is as widely different as the various parts are from each other, and the effects of their use or disuse are equally varied and complicated. use-inheritance implies pangenesis. how could the transmission of these varied effects to offspring be accounted for? is it possible to believe, with mr. spencer, that the effects of use and disuse on the parts of the personal structure are simultaneously registered in corresponding impressions on the seminal germs? must we not feel, with darwin apparently,[ ] that the _only_ intelligible explanation of use-inheritance is the hypothesis of pangenesis, according to which each modified cell, or physiological unit, throws off similarly-modified gemmules or parts of itself, which ultimately reproduce the change in offspring? if we reject pangenesis, it becomes difficult to see how use-inheritance can be possible. pangenesis improbable. the more important and best-known phenomena of heredity do not require any such hypothesis, and leading facts (such as atavism, transmission of lost parts, and the general non-transmission of acquired characters) are so adverse to it that darwin has to concede that many of the reproductive gemmules are atavistic, and that by continuous self-multiplication they may preserve a practical "continuity of germ-substance," as weismann would term it. the idea that the relationship of offspring to parent is one of direct descent is, as galton tells us, "wholly untenable"; and the only reason he admits some supplementary traces of pangenesis into his "theory of heredity,"[ ] is that he may thus account for the more or less questionable cases of the transmission of acquired characters. but there appears to be no necessity even for this concession. we ought therefore to dispense with the useless and gratuitous hypothesis that cells multiply by throwing off minute self-multiplying gemmules, as well as by the well-known method of self-division. if pangenesis occurs, the transmission of acquired characters ought to be a prominent fact. the size, strength, health and other good or evil qualities of the cells could hardly fail to exercise a marked and corresponding effect upon the size and quality of the reproductive gemmules thrown off by those cells. the direct evidence tends to show that these free gemmules do not exist. transfusion of blood has failed to affect inheritance in the slightest degree. pangenesis, with its attraction of gemmules from all parts of the body into the germ-cells, and the free circulation of gemmules in the offspring till they hit upon or are attracted by the particular cell or cells, with which alone they can readily unite, seems a less feasible theory and less in conformity with the whole of the facts than an hypothesis of germ-continuity which supposes that the development of the germ-plasm and of the successive self-dividing cells of the body proceeds from within. darwin's keen analogy of the fertilization of plants by pollen renders development from without conceivable, but as there are no insects to convey gemmules to their destination, each kind of gemmule would have to be exceedingly numerous and easily attracted from amongst an inconceivable number of other gemmules. arguments against pangenesis can also be drawn from the case of neuter insects--a fact which seems to have escaped darwin's notice, although he had seen how strongly that case was opposed to the doctrine which is the essential basis of the theory of pangenesis. spencer's explanation of use-inheritance. mr. spencer's explanation of the inheritance of the effects of use and disuse (p. ) is that "while generating a modified _consensus_ of functions and of structures, the activities are at the same time impressing this modified _consensus_ on the sperm-cells and germ-cells whence future individuals are to be produced"--a proposition which reads more like metaphysics than science. difficult to understand or believe in ordinary instances, such _consensus_-inheritance seems impossible in cases like that of the hive-bee. can we suppose that the _consensus_ of the activities of the working bee impresses itself on the sperm-cells of the drones and on the germ-cells of the carefully secluded queen? büchner thinks so, for he says: "although the queens and drones do not now work, yet the capacities inherited from earlier times still remain to them, especially to the former, and are kept alive and fresh by the impressions constantly made upon them during life, and they are thus in a position to transmit them to posterity." surely it is better to abandon a cherished theory than to be compelled to defend it by explanations which are as inconsistent as they are inadequate. new capacities are developed as well as old ones kept fresh. the massacre or expulsion of the drones would have to impress itself on the germ-cells of an onlooking queen, and the imprisonment of the queen on the sperm-cells of the drones--and in such a way, moreover, as to be afterwards developed into action in the neuters only. and use-inheritance all the while is being thoroughly overpowered by impression-inheritance--by the full transmission of that which is merely seen in others! if such a law prevails, one may feel cold because an ancestor thought of the frosty caucasus. none of this absurdity would arise if it were clearly seen that a parent is only a trustee--that transmission and development are perfectly distinct--that parental modifications are irrelevant to those transmitted to offspring. footnotes: [ ] _essays on heredity_, p. . weismann's theory is clear, simple and convenient, but incomplete; for, unlike darwin's theory of pangenesis, it scarcely attempts any real explanation of the extremely complex potentialities possessed by the reproductive elements. perhaps we might retain darwin's self-multiplying gemmules without supposing them to be thrown off by the cells, which will no longer be credited with _two_ modes of multiplication. these minute germs or gemmules may have been evolved by natural selection playing upon the sample germs that achieve development; and they may exist either separately, or (preferably but perhaps not invariably) in aggregates to form weismann's germ-plasm. [ ] _contemporary review_, dec., , p. . [ ] _variation of animals and plants under domestication_, ii. . [ ] _variation of animals and plants under domestication_, ii. , , ; _life and letters_, iii. . [ ] _contemporary review_, dec., , pp. , . conclusions. use-inheritance discredited as unnecessary, unproven, and improbable. general experience teaches that acquired characters are not usually inherited; and investigation shows that the apparent exceptions to this great rule are probably fallacious. even the alleged instances of use-inheritance culled by such great and judicious selectors as darwin and spencer break down upon examination; for they can be better explained without use-inheritance than with it. on the other hand, the adverse facts and considerations are almost strong enough to prove the actual non-existence of such a law or tendency. there is no need to undertake the apparently impossible task of demonstrating an absolute negative. it will be enough to ask that the lamarckian factor of use-inheritance shall be removed from the category of accredited factors of evolution to that of unnecessary and improbable hypotheses. the main explanation or source of the fallacy may be found in the fact that natural selection frequently imitates some of the more obvious effects of use and disuse. modern reliance on use-inheritance misplaced. modern philanthropy--so far at least as it ever studies ultimate results--constantly relies on this ill-founded belief as its justification for ignoring the warnings of those who point out the ultimately disastrous results of a systematic defiance or reversal of the great law of natural selection. this reliance finds strong support in mr. spencer's latest teachings, for he holds that the inheritance of the effects of use and disuse takes place universally, and that it is now "the chief factor" in the evolution of civilized man (pp. , , iv)--natural selection being quite inadequate for the work of progressive modification. practically he abandons the hope of evolution by natural selection, and substitutes the ideal of a nation being "modified _en masse_ by transmission of the effects" of its institutions and habits. use-inheritance will "mould its members far more rapidly and comprehensively" than can be effected by the survival of the fittest alone. but could we rely upon the aid of use-inheritance if it really were a universal law and not a mere simulation of one? let us consider some of the features of this alleged factor of evolution, seeing that it is henceforth to be our principal means of securing the improvement of our species and our continued adaptation to the changing conditions of a progressive civilization. it is curiously uncertain and irregular in its action. it diminishes or abolishes some structures (such as jaws or eyes) without correspondingly diminishing or abolishing other equally disused and closely related parts (such as teeth, or eye-stalks). it thickens ducks' leg-bones while allowing them to shorten. it shortens the disused wing-bones of ducks and the leg-bones of rabbits while allowing them to thicken; and yet in other cases it greatly reduces the thickness of bones without shortening them. it transmits tameness most powerfully in an animal which usually cannot acquire it. it aids in webbing the feet of water-dogs, but fails to web the feet of the water-hen or to remove the web in the feet of upland geese.[ ] it allows the disused fibula to retain a potentiality of development fully equal to that possessed by the long-used tibia. it lengthens legs because they are used in supporting the body, and shortens arms because they are used in pulling. whether it enlarges brain if used in one way and diminishes it if used in another, we cannot tell; but it must obviously deaden nervous sensibilities in some cases and intensify them in others. it enlarges hands long before they are used, and thickens soles long before the time for walking on them. at the same time, as if by an oversight, it so delays its transmission of the habit of walking on these thickened soles, that the gradual and tedious acquisition of the non-transmitted habit costs the infant much time and trouble and often some pain and danger. yet where aided by natural selection, as with chickens and foals, it transmits the habit in wonderful perfection and at a remarkably early date. it transmits new paces in horses in a single generation, but fails to perpetuate the songs of birds. it modifies offspring like parents, and yet allows the formation of two reproductive types in plants, and of two or more types widely different from the parents in some of the higher insects. it is said to be indispensable for the co-ordinated development of man and the giraffe and the elk, but appears to be unnecessary for the evolution and the maintenance of wonderful structures and habits and instincts in a thousand species of ants and bees and termites. it is the only possible means of complex evolution and adaptation of co-operative parts, and yet in mr. spencer's most representative case it renders such important parts as teeth and jaws unsuited for each other, and is said to ruin the teeth by the consequent overcrowding and decay. it survives amidst a general "lack of recognised evidence," and only seems to act usefully and healthily and regularly in quarters where it can least easily be distinguished from other more powerful and demonstrable factors of evolution. so little does it care to display its powers where they would be easily verifiable as well as useful that practical breeders ignore it. so slight is its independent power that it seems to allow natural selection or sexual selection or artificial selection to modify organisms in sheer defiance of its utmost opposition, just as readily as they modify organisms in other directions with its utmost help. if it partially perpetuates and extends the pecked-out indentations in the motmot's tail feathers, it on the other hand fails to transmit the slightest trace of mutilation in an almost infinite number of ordinary cases, and even where the mutilation is repeated for a hundred generations; and it apparently repairs rather than transmits the ordinary and oft-repeated losses caused by plucking hair, down and feathers, and the wear and tear of claws, teeth, hoofs and skin. it is often mischievous as well as anomalous in its action. under civilization with its division of labour, the various functions of mind and body are very unequally exercised. there is overwork or misuse of one part and disuse and neglect of others, leading to the partial breakdown or degeneration of various organs and to general deterioration of health through disturbed balance of the constitution. the brain, or rather particular parts of it, are often over-stimulated, while the body is neglected. in many ways education and civilization foster nervousness and weakness, and undermine the rude natural health and spirits of the human animal. alcohol, tobacco, tea, coffee, extra brain work, late hours, dissipation, overwork, indoor life, division of labour, preservation of the weak, and many other causes, all help to injure the modern constitution; so that the prospect of cumulative intensification of these evils by the additional influence of use-inheritance is not an encouraging one. it is true that modern progress and prosperity are improving the people in various respects by their direct action; but if use-inheritance has any share in effecting this improvement it must also transmit increased wants and more luxurious habits, together with such evils as have already been referred to. as depicted by its defenders, use-inheritance transmits evils far more powerfully and promptly than benefits. it transmits insanity and shattered nerves rather than the healthy brain which preceded the breakdown. it perpetuates, and cumulatively intensifies, a deterioration in the senses of civilized men, but it fails to perpetuate the rank vigour of various plants when too well nourished, or the flourishing condition of various animals when too fat or when tamed. it already transmits the short-sight caused by so modern an art as watchmaking, but so fails to transmit the long-practised art of seeing (as it does of walking and talking) that vision is worse than useless to a man until he gradually acquires the necessary but non-transmitted associations of sensation and idea by his own experience. in a well-known case, a blind man on gaining his sight by an operation said that "all objects seemed to touch his eyes, as what he felt did his skin"--so little had the universal experience of countless ages impressed itself on his faculties. under normal healthy conditions use-inheritance is so slow in its action that "several generations" must elapse before it produces any appreciable effect, and then that effect is only precisely what selection might be expected to bring about without its aid. strong for evil and slow for good, it can convey epilepsy promptly in guinea pigs, but transmits the acquirements of genius so poorly that our best student of the heredity of genius has to account for the frequent and remarkable deterioration of the offspring by a theory which is strongly hostile to use-inheritance. it would tend to make organisms unworkable by the excessive differences in its rate and manner of action on co-operative parts, and by adapting these parts to the total amount of nourishment received rather than to occasional necessity or actual usefulness. it would tend to stereotype habits and convert reason into instinct. how then can we rely upon use-inheritance for the improvement of the race? even if it is not a sheer delusion, it may be more detrimental as a positive evil than it is advantageous as an unnecessary benefit; and as a normal modifying agent it is miserably weak and untrustworthy in comparison with the powerful selective influences by which nature and society continually and inevitably affect the species for good or for evil. the effects of use and disuse--rightly directed by education in its widest sense--must of course be called in to secure the highly essential but nevertheless _superficial, limited, and partly deceptive_ improvement of individuals and of social manners and methods; but as this artificial development of already existing potentialities does not directly or readily tend to become congenital, it is evident that some considerable amount of natural or artificial selection of the more favourably varying individuals will still be the only means of securing the race against the constant tendency to degeneration which would ultimately swallow up all the advantages of civilization. the selective influences by which our present high level has been reached and maintained may well be modified, but they must not be abandoned or reversed in the rash expectation that state education, or state feeding of children, or state housing of the poor, or any amount of state socialism or public or private philanthropy, will prove permanently satisfactory substitutes. if ruinous deterioration and other more immediate evils, are to be avoided, the race must still be to the swift and the battle to the strong. the healthy individualism so earnestly championed by mr. spencer must be allowed free play. open competition, as darwin teaches, with its survival and multiplication of the fittest, must be allowed to decide the battle of life independently of a foolish benevolence that prefers the elaborate cultivation and multiplication of weeds to the growth of corn and roses. we are trustees for the countless generations of the future. if we are wise we shall trust to the great ruling truths that we assuredly know, rather than to the seductive claims of an alleged factor of evolution for which no satisfactory evidence can be produced. the end. richard clay and sons, limited, london and bungay. footnotes: [ ] professor romanes had casts made of the feet of upland geese, and could not detect any diminution as compared with the web of other geese in relation to the toes. nature series. popular lectures and addresses on various subjects in physical 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f.r.s., _macdonald professor of physics, mcgill university_. _price $ . net; postage cents extra._ experimental and theoretical applications of thermodynamics to chemistry. _by_ dr. walther nernst, _professor and director of the institute of physical chemistry in the university of berlin_. _price $ . net; postage cents extra._ the problems of genetics. _by_ william bateson, m.a., f.r.s., _director of the john innes horticultural institution, merton park, surrey, england_. _price $ . net; postage cents extra._ stellar motions. with special reference to motions determined by means of the spectrograph. _by_ william wallace campbell, sc.d., ll.d., _director of the lick observatory, university of california_. _price $ . net; postage cents extra._ theories of solutions. _by_ svante august arrhenius, ph.d., sc.d., m.d., _director of the physico-chemical department of the nobel institute, stockholm, sweden_. _price $ . net; postage cents extra._ irritability. a physiological analysis of the general effect of stimuli in living substances. _by_ max verworn, _professor at bonn physiological institute_. _price $ . net; postage cents extra._ the evolution of modern medicine. _by_ sir william osler, bart., m.d., ll.d., sc.d., _regius professor of medicine, oxford university_. _price $ . net; postage cents extra._ problems of genetics by william bateson, m.a., f.r.s. director of the john innes horticultural institution, hon. fellow of st. john's college, cambridge, and formerly professor of biology in the university _with illustrations_ [illustration] new haven: yale university press london: humphrey milford oxford university press mcmxiii copyright, by yale university first printed august, , copies [** transcriber's note: underscores "_" before and after a word or phrase indicate italics in the original text. hyphenation was used inconsistently by the author and has been left as in the original text. ] the silliman foundation in the year a legacy of about eighty-five thousand dollars was left to the president and fellows of yale college in the city of new haven, to be held in trust, as a gift from her children, in memory of their beloved and honored mother, mrs. hepsa ely silliman. on this foundation yale college was requested and directed to establish an annual course of lectures designed to illustrate the presence and providence, the wisdom and goodness of god, as manifested in the natural and moral world. these were to be designated as the mrs. hepsa ely silliman memorial lectures. it was the belief of the testator that any orderly presentation of the facts of nature or history contributed to the end of this foundation more effectively than any attempt to emphasize the elements of doctrine or of creed; and he therefore provided that lectures on dogmatic or polemical theology should be excluded from the scope of this foundation, and that the subjects should be selected rather from the domains of natural science and history, giving special prominence to astronomy, chemistry, geology, and anatomy. it was further directed that each annual course should be made the basis of a volume to form part of a series constituting a memorial to mrs. silliman. the memorial fund came into the possession of the corporation of yale university in the year ; and the present volume constitutes the fifth of the series of memorial lectures. preface this book gives the substance of a series of lectures delivered in yale university, where i had the privilege of holding the office of silliman lecturer in . the delay in publication was brought about by a variety of causes. inasmuch as the purpose of the lectures is to discuss some of the wider problems of biology in the light of knowledge acquired by mendelian methods of analysis, it was essential that a fairly full account of the conclusions established by them should first be undertaken and i therefore postponed the present work till a book on mendel's principles had been completed. on attempting a more general discussion of the bearing of the phenomena on the theory of evolution, i found myself continually hindered by the consciousness that such treatment is premature, and by doubt whether it were not better that the debate should for the present stand indefinitely adjourned. that species have come into existence by an evolutionary process no one seriously doubts; but few who are familiar with the facts that genetic research has revealed are now inclined to speculate as to the manner by which the process has been accomplished. our knowledge of the nature and properties of living things is far too meagre to justify any such attempts. suggestions of course can be made: though, however, these ideas may have a stimulating value in the lecture room, they look weak and thin when set out in print. the work which may one day give them a body has yet to be done. the development of negations is always an ungrateful task apt to be postponed for the positive business of experiment. such work is happily now going forward in most of the centers of scientific life. of many of the subjects here treated we already know more than we did in . the delay in production has made it possible to incorporate these new contributions. the book makes no pretence at being a treatise and the number of illustrative cases has been kept within a moderate compass. a good many of the examples have been chosen from american natural history, as being appropriate to a book intended primarily for american readers. the facts are largely given on the authority of others, and i wish to express my gratitude for the abundant assistance received from american colleagues, especially from the staffs of the american museum in new york, and of the boston museum of natural history. in connexion with the particular subjects personal acknowledgments are made. dr. f. m. chapman was so good as to supervise the preparation of the coloured plate of _colaptes_, and to authorize the loan of the plate representing the various forms of _helminthophila_, which is taken from his _north american warblers_. i am under obligation to messrs. macmillan & co., for permission to reproduce several figures from _materials for the study of variation_, illustrating subjects which i wished to treat in new associations, and to m. leduc for leave to use fig. . in conclusion i thank my friends in yale for the high honour they did me by their invitation to contribute to the series of silliman lectures, and for much kindness received during a delightful sojourn in that genial home of learning. table of contents. chapter page i. introductory. the problem of species and variety ii. meristic phenomena iii. segmentation, organic and mechanical iv. the classification of variation and the nature of substantive variation note to chapter iv v. the mutation theory note to chapter v vi. variation and locality vii. local differentiation--_continued_. overlapping forms viii. locally differentiated forms--_continued_. climatic varieties ix. the effects of changed conditions x. the effects of changed conditions--_continued_. the causes of genetic variation xi. the sterility of hybrids. concluding remarks appendix to chapter x index problems of genetics chapter i introductory the purpose of these lectures is to discuss some of the familiar phenomena of biology in the light of modern discoveries. in the last decade of the nineteenth century many of us perceived that if any serious advance was to be made with the group of problems generally spoken of as the theory of evolution, methods of investigation must be devised and applied of a kind more direct and more penetrating than those which after the general acceptance of the darwinian views had been deemed adequate. such methods obviously were to be found in a critical and exhaustive study of the facts of variation and heredity, upon which all conceptions of evolution are based. to construct a true synthetic theory of evolution it was necessary that variation and heredity instead of being merely postulated as axioms should be minutely examined as phenomena. such a study darwin himself had indeed tentatively begun, but work of a more thorough and comprehensive quality was required. in the conventional view which the orthodoxy of the day prescribed, the terms variation and heredity stood for processes so vague and indefinite that no analytical investigation of them could be contemplated. so soon, however, as systematic inquiry into the natural facts was begun it was at once found that the accepted ideas of variation were unfounded. variation was seen very frequently to be a definite and specific phenomenon, affecting different forms of life in different ways, but in all its diversity showing manifold and often obvious indications of regularity. this observation was not in its essence novel. several examples of definite variation had been well known to darwin and others, but many, especially darwin himself in his later years, had nevertheless been disposed to depreciate the significance of such facts. they consequently then lapsed into general disparagement. upon more careful inquiry the abundance of such phenomena proved to be far greater than was currently supposed, and a discussion of their nature brought into prominence a consideration of greater weight, namely that the differences by which these definite or discontinuous variations are constituted again and again approximate to and are comparable with the class of differences by which species are distinguished from each other. the interest of such observations could no longer be denied. the more they were examined the more apparent it became that by means of the facts of variation a new light was obtained on the physiological composition and capabilities of living things. genetics thus cease to be merely a method of investigating theories of evolution or of the origin of species but provide a novel and hitherto untried instrument by which the nature of the living organism may be explored. just as in the study of non-living matter science began by regarding the external properties of weight, opacity, colour, hardness, mode of occurrence, etc., noting only such evidences of chemical attributes and powers as chance spontaneously revealed; and much later proceeded to the discovery that these casual manifestations of chemical properties, rightly interpreted, afford a key to the intrinsic nature of the diversity of matter, so in biology, having examined those features of living things which ordinary observations can perceive, we come at last to realize that when studied for their own sake the properties of living organisms in respect of heredity and variation are indications of their inner nature and provide evidences of that nature which can be obtained from no other source. while such ideas were gradually forming in our minds, came the rediscovery of mendel's work. investigations which before had only been imagined as desirable now became easy to pursue, and questions as to the genetic inter-relations and compositions of varieties can now be definitely answered. without prejudice to what the future may disclose whether by way of limitation or extension of mendelian method, it can be declared with confidence and certainty that we have now the means of beginning an analysis of living organisms, and distinguishing many of the units or factors which essentially determine and cause the development of their several attributes. briefly put, the essence of mendelism lies in the discovery of the existence of unit characters or factors. for an account of the mendelian method, how it is applied and what it has already accomplished, reference must be made to other works.[ ] with this part of the subject i shall assume a sufficient acquaintance. in these lectures i have rather set myself the task of considering how certain problems appear when viewed from the standpoint to which the application of these methods has led us. it is indeed somewhat premature to discuss such questions. the work of mendelian analysis is progressing with great rapidity and anything i can say may very soon be superseded as out of date. nevertheless a discussion of this kind may be of at least temporary service in directing inquiry to the points of special interest. the problem of species and variety nowhere does our new knowledge of heredity and variation apply more directly than to the problem what is a species and what is a variety? i cannot assert that we are already in a position to answer this important question, but as will presently appear, our mode of attack and the answers we expect to receive are not those that were contemplated by our predecessors. if we glance at the history of the scientific conception of species we find many signs that it was not till comparatively recent times that the definiteness of species became a strict canon of the scientific faith and that attempts were made to give precise limits to that conception. when the diversity of living things began to be accurately studied in the sixteenth and seventeenth centuries names were applied in the loosest fashion, and in giving a name to an animal or a plant the naturalists of those times had no ulterior intention. names were bestowed on those creatures about which the writer proposed to speak. when gesner or aldrovandi refer to all the kinds of horses, unicorns, dogs, mermaids, etc., which they had seen or read of, giving to each a descriptive name, they do not mean to "elevate" each named kind to "specific rank"; and if anyone had asked them what they meant by a species, it is practically certain that they would have had not the slightest idea what the question might imply, or any suspicion that it raised a fundamental problem of nature. spontaneous generation being a matter of daily observation, then unquestioned, and supernatural events of all kinds being commonly reported by many witnesses, transmutation of species had no inherent improbability. matthioli,[ ] for instance, did not expect to be charged with heresy when he declared _stirpium mutatio_ to be of ordinary occurrence. after giving instances of induced modifications he wrote, "tantum enim in plantis naturae germanitas potest, ut non solum saepe praedictos praestet effectus, sed etiam ut alteram in alteram stirpem facile vertat, ut cassiam in cinnamomum, sisymbrium in mentham, triticum in lolium, hordeum in avenam, et ocymum in serpyllum." i do not know who first emphasized the need for a clear understanding of the sense in which the term species is to be applied. in the second half of the seventeenth century ray shows some degree of concern on this matter. in the introduction to the _historia plantarum_, , he discusses some of the difficulties and lays down the principle that varieties which can be produced from the seed of the same plant are to be regarded as belonging to one species, being, i believe, the first to suggest this definition. that new species can come into existence he denies as inconsistent with genesis , in which it is declared that god finished the work of creation in six days. nevertheless he does not wholly discredit the possibility of a "transmutation" of species, such that one species may as an exceptional occurrence give rise by seed to another and nearly allied species. of such a phenomenon he gives illustrations the authenticity of which he says he is, against his will, compelled to admit. he adds that some might doubt whether in the cases quoted the two forms concerned are really distinct species, but the passage is none the less of value for it shews that the conception of species as being distinct unchangeable entities was not to ray the dogma sacrosanct and unquestionable which it afterwards became.[ ] in the beginning of the eighteenth century marchant,[ ] having observed the sudden appearance of a lacinated variety of _mercurialis_, makes the suggestion that species in general may have arisen by similar mutations. indeed from various passages it is manifest that to the authors of the seventeenth and early eighteenth centuries species appeared simply as groups more or less definite, the boundaries of which it was unnecessary to determine with great exactitude. such views were in accord with the general scientific conception of the time. the mutability of species is for example sometimes likened (see for instance sharrock, loc. cit.) to the metamorphoses of insects, and it is to be remembered that the search for the philosopher's stone by which the transmutation of metals was to be effected had only recently fallen into discredit as a pursuit. the notion indeed of a peculiar, fixed meaning to be attached to species as distinct from variety is i think but rarely to be found categorically expressed in prae-linnaean writings. but with the appearance of the _systema naturae_ a great change supervened. linnaeus was before all a man of order. foreseeing the immense practical gain to science that must come from a codification of nomenclature, he invented such a system. it is not in question that linnaeus did great things for us and made natural history a manageable and accessible collection of facts instead of a disorderly heap; but orderliness of mind has another side, and inventors and interpreters of systems soon attribute to them a force and a precision which in fact they have not. the systematist is primarily a giver of names, as ray with his broader views perceived. linnaeus too in the exordium to the _systema naturae_ naively remarks, that he is setting out to continue the work which adam began in the golden age, to give names to the living creatures. naming however involves very delicate processes of mind and of logic. carried out by the light of meagre and imperfect knowledge it entails all the mischievous consequences of premature definition, and promotes facile illusions of finality. so was it with the linnaean system. an interesting piece of biological history might be written respecting the growth and gradual hardening of the conception of species. to readers of linnaeus's own writings it is well known that his views cannot be summarized in a few words. expressed as they were at various times during a long life and in various connexions, they present those divers inconsistencies which commonly reflect a mind retaining the power of development. nothing certainly could be clearer than the often quoted declaration of the _philosophia botanica_, "species tot numeramus quot diversae formae in principio sunt creatae," with the associated passage "varietates sunt plantae ejusdem speciei mutatae a caussa quacunque occasionali." those sayings however do not stand alone. in several places, notably in the famous dissertation on the peloric _linaria_ he explicitly contemplates the possibility that new species may arise by crossing, declaring nevertheless that he thinks such an event to be improbable. in that essay he refers to marchant's observation on a laciniate _mercurialis_, but though he states clearly that that plant should only be regarded as a variety of the normal, he does not express any opinion that the contemporary genesis of new species must be an impossibility. in the later dissertation on hybrid plants he returns to the same topic. again though he states the belief that species cannot be generated by cross-breedings, he treats the subject not as heretical absurdity but as one deserving respectful consideration. the significance of the aphorisms that precede the lectures on the natural orders is not easy to apprehend. these are expressed with the utmost formality, and we cannot doubt that in them we have linnaeus's own words, though for the record we are dependent on the transcripts of his pupils. the text of the first five is as follows: . creator t. o. in primordio vestiit vegetabile _medullare_ principiis constitutivis diversi _corticalis_ unde tot difformia individua, quot _ordines_ naturales prognata. . _classicas_ has ( ) plantas omnipotens miscuit inter se, unde tot _genera_ ordinum, quot inde plantae. . _genericas_ has ( ) miscuit natura, unde tot _species_ congeneres quot hodie existunt. . _species_ has miscuit casus, unde totidem quot passim occurrunt, _varietates_. . suadent haec ( - ) creatoris leges a simplicibus ad composita. naturae leges generationis in hybridis. hominis leges ex observatis a posteriori. i am not clear as to the parts assigned in the first sentence respectively to the "_medulla_" and the "_cortex_," beyond that linnaeus conceived that multiformity was first brought about by diversity in the "_cortex_." the passage is rendered still more obscure if read in connection with the essay on "_generatio ambigena_," where he expresses the conviction that the _medulla_ is contributed by the mother, and the _cortex_ by the father, both in plants and animals.[ ] but however that may be, he regards this original diversity as resulting in the constitution of the natural orders, each represented by one individual. in the second aphorism the omnipotent is represented as creating the genera by intermixing the individual _plantae classicae_, or prototypes of the natural orders. the third statement is the most remarkable, for in it he declares that species were formed by the act of nature, who by inter-mixing the genera produced _species congeneres_, namely species inside each genus, to the number which now exist. lastly, chance or accident, intermixing the species, produced as many varieties as there are about us. linnaeus thus evidently regarded the intermixing of an originally limited number of types as the sufficient cause of all subsequent diversity, and it is clear that he draws an antithesis between _creator_, _natura_, and _casus_, assigning to each a special part in the operations. the acts resulting in the formation of genera are obviously regarded as completed within the days of the creation, but the words do not definitely show that the parts played by nature and chance were so limited. recently also e. l. greene[ ] has called attention to some curious utterances buried in the _species plantarum_, in which linnaeus refers to intermediate and transitional species, using language that even suggests evolutionary proclivities of a modern kind, and it is not easy to interpret them otherwise. whatever linnaeus himself believed to be the truth, the effect of his writings was to induce a conviction that the species of animals and plants were immutably fixed. linnaeus had reduced the whole mass of names to order and the old fantastical transformations with the growth of knowledge had lapsed into discredit; the fixity of species was taken for granted, but not till the overt proclamation of evolutionary doctrine by lamarck do we find the strenuous and passionate assertions of immutability characteristic of the first half of the nineteenth century. it is not to be supposed that the champions of fixity were unacquainted with varietal differences and with the problem thus created, but in their view these difficulties were apparent merely, and by sufficiently careful observation they supposed that the critical and permanent distinctions of the true species could be discovered, and the impermanent variations detected and set aside. this at all events was the opinion formed by the great body of naturalists at the end of the eighteenth and beginning of the nineteenth centuries, and to all intents and purposes in spite of the growth of evolutionary ideas, it remains the guiding principle of systematists to the present day. there are 'good species' and 'bad species' and the systematists of europe and america spend most of their time in making and debating them. in some of its aspects the problem of course confronted earlier naturalists. parkinson for instance ( ) in introducing his treatment of _hieracium_ wrote, "to set forth the whole family of the hawkeweedes in due forme and order is such a world of worke that i am in much doubt of mine own abilitie, it having lyen heavie on his shoudiers that hath already waded through them ... for such a multitude of varieties in forme pertaining to one herbe is not to be found againe in _rerum natura_ as i thinke," and the same idea, that the difficulty lay rather in man's imperfect powers of discrimination than in the nature of the materials to be discriminated, is reflected in many treatises early and late. it was however with the great ouburst of scientific activity which followed linnaeus that the difficulty became acute. simultaneously vast masses of new material were being collected from all parts of the world into the museums, and the products of the older countries were re-examined with a fresh zeal and on a scale of quantity previously unattempted. but the problem how to name the forms and where to draw lines, how much should be included under one name and where a new name was required, all this was felt, rather as a cataloguer's difficulty than as a physiological problem. and so we still hear on the one hand of the confusion caused by excessive "splitting" and subdivisions, and on the other of the uncritical "lumpers" who associate together under one name forms which another collector or observer would like to see distinguished. in spite of darwin's hopes, the acceptance of his views has led to no real improvement--scarcely indeed to any change at all in either the practice or aims of systematists. in a famous passage in the _origin_ he confidently declares that when his interpretation is generally adopted "systematists will be able to pursue their labours as at present; but they will not be incessantly haunted by the shadowy doubt whether this or that form be a true species. this, i feel sure, and i speak after experience, will be no slight relief. the endless disputes whether or not some fifty species of british brambles are good species will cease." those disputes nevertheless proceed almost exactly as before. it is true that biologists in general do not, as formerly, participate in these discussions because they have abandoned systematics altogether; but those who are engaged in the actual work of naming and cataloguing animals and plants usually debate the old questions in the old way. there is still the same divergence of opinion and of practice, some inclining to make much of small differences, others to neglect them. not only does the work of the systematists as a whole proceed as if darwin had never written but their attitude towards these problems is but little changed. in support of this statement i may refer to several british museum catalogues, much of the _biologia centrali-americana_, ridgway's _birds of north america_, the _fauna hawaiensis_, indeed to almost any of the most important systematic publications of england, america, or any other country. these works are compiled by the most proficient systematists of all countries in the several groups, but with rare exceptions they show little misgiving as to the fundamental reality of specific differences. that the systematists consider the species-unit as of primary importance is shown by the fact that the whole business of collection and distribution of specimens is arranged with regard to it. almost always the collections are arranged in such a way that the phenomena of variation are masked. forms intermediate between two species are, if possible, sorted into separate boxes under a third specific name. if a species is liable to be constantly associated with a mutational form, the mutants are picked out, regardless of the circumstances of their origin, from the samples among which they were captured, and put apart under a special name. only by a minute study of the original labels of the specimens and by redistributing them according to locality and dates, can their natural relations be traced. the published accounts of these collections often take no notice of variations, others make them the subject of casual reference. very few indeed treat them as of much importance. from such indications it is surely evident that the systematists attach to the conception of species a significance altogether different from that which darwin contemplated. i am well aware that some very eminent systematists regard the whole problem as solved. they hold as darwin did that specific diversity has no physiological foundation or causation apart from fitness, and that species are impermanent groups, the delimitations of which are ultimately determined by environmental exigency or "fitness." the specific diversity of living things is thus regarded as being something quite different in nature from the specific diversity of inorganic substances. in practice those who share these opinions are, as might be anticipated, to be found among the 'lumpers' rather than among the 'splitters.' in their work, certainly, the darwinian theory is actually followed as a guiding principle; unanalysed inter-gradations of all kinds are accepted as impugning the integrity of species; the underlying physiological problem is forgotten, and while the product is almost valueless as a contribution to biological research, i can scarcely suppose that it aids greatly in the advances of other branches of our science. but why is it that, with these exceptions, the consequences of the admittedly general acceptance of a theory of evolution are so little reflected in the systematic treatment of living things? surely the reason is that though the systematist may be convinced of the general truth of the evolution theory at large, he is still of opinion that species are really distinct things. for him there are still 'good' species and 'bad' species and his experience tells him that the distinction between the two is not simply a question of degree or a matter of opinion. to some it may seem that this is mere perversity, a refusal to see obvious truth, a manifestation of the spirit of the collector rather than of the naturalist. but while recognising that from a magnification of the conception of species the systematists are occasionally led into absurdity i do not think the grounds for their belief have in recent times been examined with the consideration they deserve. the phenomenon of specific diversity is manifested to a similar degree by living things belonging to all the great groups, from the highest to the lowest, vertebrates, invertebrates, protozoa, vascular plants, algae, and bacteria, all present diversities of such a kind that among them the existence of specific differences can on the whole be recognised with a similar degree of success and with very similar limitations. in all these groups there are many species quite definite and unmistakable, and others practically indefinite. the universal presence of specificity, as we may call it, similarly limited and characterised, is one of its most remarkable features. not only is this specificity thus universally present among the different forms of life, but it manifests itself in respect of the most diverse characteristics which living things display. species may thus be distinguished by peculiarities of form, of number, of geometrical arrangement, of chemical constitution and properties, of sexual differentiation, of development, and of many other properties. in any one or in several of these features together, species may be found distinguished from other species. it is also to be observed that the definiteness of these distinctions has no essential dependence on the nature of the characteristic which manifests them. it is for example sometimes said that colour-distinctions are of small systematic importance, but every systematist is familiar with examples (like that of the wild species of _gallus_) in which colours though complex, show very little variation. on the other hand features of structure, sexual differentiation, and other attributes which by our standards are estimated as essential, may be declared to show much variation or little, not according to any principle which can be detected, but simply as the attention happens to be applied to one species or group of species, or to another. in many groups of animals and plants observers have hit upon characters which were for a time thought to be finally diagnostic of species. the lepidoptera and diptera for instance, have been re-classified according to their neuration. through a considerable range of forms determinations may be easily made on these characters, but as is now well known, neuration is no more immune from variation than any other feature of organisation, and in some species great variability is the rule. again it was once believed by some that the genitalia of the lepidoptera provided a basis of final determination--with a similar sequel. in some groups, for example the lycaenidae, or the hesperidae, there are forms almost or quite indistinguishable on external examination, but a glance at the genitalia suffices to distinguish numerous species, while on the contrary among pieridae a great range of species show scarcely any difference in these respects: and again in occasional species the genitalia show very considerable variations. the proposition that animals and plants are on the whole divisible into definite and recognisable species is an approximation to the truth. such a statement is readily defensible, whereas to assert the contrary would be palpably absurd. for example, a very competent authority lately wrote: "in the whole lepidopterous fauna of england there is no species of really uncertain limits."[ ] others may be disposed to make certain reservations, but such exceptions would be so few as scarcely to impair the validity of the general statement. the declaration might be extended to other orders and other lands. we know, of course, that the phenomenon of specific diversity is complicated by local differentiation: that, in general, forms which cannot disperse themselves freely exhibit a multitude of local races, and that of these some are obviously adaptative, and that a few even owe their peculiarity to direct environmental effects. every systematist also is perfectly aware that in dealing with collections from little explored countries the occurrence of polymorphism or even of sporadic variation may make the practical business of distinguishing the species difficult and perhaps for the time impossible; still, conceding that a great part of the diversity is due to geographical differentiation, and that some is sporadic variation, our experience of our own floras and faunas encourages the belief that if we were thoroughly familiar with these exotic productions it would usually be possible to assign their specific limitations with an approach to certainty. for apart from any question of the justice of these wider inferences, if we examine the phenomenon of specificity as it appears in those examples which are nearest to hand, surely we find signs in plenty that specific distinction is no mere consequence of natural selection. the strength of this proposition has lain mainly in the appeal to ignorance. steadily with the growth of knowledge has its cogency diminished, and such a belief could only have been formulated at a time when the facts of variation were unknown. in darwin's time no serious attempt had been made to examine the manifestations of variability. a vast assemblage of miscellaneous facts could formerly be adduced as seemingly comparable illustrations of the phenomenon "variation." time has shown this mass of evidence to be capable of analysis. when first promulgated it produced the impression that variability was a phenomenon generally distributed amongst living things in such a way that the specific divisions must be arbitrary. when this variability is sorted out, and is seen to be in part a result of hybridisation, in part a consequence of the persistence of hybrids by parthenogenetic reproduction, a polymorphism due to the continued presence of individuals representing various combinations of mendelian allelomorphs, partly also the transient effect of alteration in external circumstances, we see how cautious we must be in drawing inferences as to the indefiniteness of specific limits from a bare knowledge that intermediates exist. conversely, from the accident of collocation or from a misleading resemblance in features we deem essential, forms genetically distinct are often confounded together, and thus the divergence of such forms in their other features, which we declare to be non-essential, passes as an example of variation. lastly, and this is perhaps the most fertile of all the sources of confusion, the impression of the indefiniteness of species is created by the existence of numerous local forms, isolated geographically from each other, forms whose differences may be referable to any one of the categories i have enumerated. the advance has been from many sides. something has come from the work of systematists, something from cultural experiments, something from the direct study of variation as it appears in nature, but progress is especially due to experimental investigation of heredity. from all these lines of inquiry we get the same answer; that what the naturalists of fifty years ago regarded as variation is not one phenomenon but many, and that what they would have adduced as evidence against the definiteness of species may not in fact be capable of this construction at all. if we may once more introduce a physical analogy, the distinctions with which the systematic naturalist is concerned in the study of living things are as multifarious as those by which chemists were confronted in the early days of their science. diversities due to mechanical mixtures, to allotropy, to differences of temperature and pressure, or to degree of hydration, had all to be severally distinguished before the essential diversity due to variety of chemical constitution stood out clearly, and i surmise that not till a stricter analysis of the diversities of animals and plants has been made on a comprehensive scale, shall we be in a position to declare with any confidence whether there is or is not a natural and physiological distinction between species and variety. as i have said above, it is in the cases nearest to hand that the problem may be most effectively studied. comparison between forms from dissimilar situations contributes something; but it is by a close examination of the behaviour, especially the genetic behaviour, of familiar species when living in the presence of their nearest allies that the most direct light on the problem is to be obtained. i cannot understand the attitude of those who, contemplating such facts as this examination elicits, can complacently declare that specific difference is a mere question of degree. with the spread of evolutionary ideas to speak much of the fixity of species has become unfashionable, and yet how striking and inscrutable are the manifestations of that fixity! consider the group of species composing the _agrestis_ section of the genus _veronica_, namely _tournefortii_, _agrestis_, and _polita_. these three grow side by side in my garden, as they do in suitable situations over a vast area of the temperate regions. i have for years noticed them with some care and become familiar with their distinctions and resemblances. never is there any real doubt as to the identity of any plant. the species show some variability, but i have never seen one which assumed any of the distinguishing features of the others. a glance at the fruits decides at once to which species a plant belongs. i find it impossible to believe that the fixity of these distinctions is directly dependent on their value as aids in the struggle for existence. the mode of existence of the three forms in so far as we can tell is closely similar. by whatever standard we reckon systematic affinity i suppose we shall agree that these species come very near indeed to each other. bentham even takes the view that _polita_ is a mere variety of _agrestis_. now in such cases as this it has been argued that the specific features of the several types have been separately developed in as many distinct localities, and that their present association is due to subsequent redistribution. of these veronicas indeed we know that one, _tournefortii_ (= _buxbaumii_) is as a matter of fact a recent introduction from the east.[ ] but this course of argument leads to still further difficulties. for if it is true that the peculiarities of the several species have been perfected and preserved on account of their survival-value to their possessors, it follows that there must be many ways of attaining the same result. but since sufficient adaptation may be ensured in so many ways, the disappearance of the common parent of these forms is difficult to understand. obviously it must have been a plant very similar in general construction to its modern representatives. like them it must have been an annual weed, with an organisation conformable to that mode of life. why then, after having been duly perfected for that existence should it have been entirely superseded in favour of a number of other distinct contrivances for doing the same thing, and--if a gradual transition be predicated--not only by them, but by each intermediate stage between them and the original progenitor? surely the obvious inference from such facts is that the burden cast upon the theory of gradual selection is far greater than it can bear; that adaptation is not in practice a very close fit, and that the distinctions between these several species of veronica have not arisen on account of their survival-value but rather because none of their diversities was so damaging as to lead to the extermination of its possessor. when we see these various veronicas each rigidly reproducing its parental type, all comfortably surviving in competition with each other, are we not forced to the conclusion that _tolerance_ has as much to do with the diversity of species as the stringency of selection? certainly these species owe their continued existence to the fact that they are each good enough to live, but how shall we refer the distinctions between them directly or indirectly to the determination of natural selection? the control of selection is loose while the conformity to specific distinction is often very strict and precise, and no less so even when several closely related species co-exist in the same area and in the same circumstances. the theory of selection fails at exactly the point where it was devised to help: _specific_ distinction. let us examine a somewhat different set of facts in the case of another pair of nearly allied species _lychnis diurna_ and _vespertina_. the two plants have much in common. both are dioecious perennials, with somewhat similar flowers, the one crimson, the other white. each however has its peculiarities which are discernible in almost any part of its structure, whether flower, leaf, fruit or seed, distinctions which would enable a person thoroughly familiar with the plants to determine at once from which species even a small piece had been taken. there is so much resemblance however as readily to support the surmise that the two were mere varieties of one species. bentham, following linnaeus, in fact actually makes this suggestion, with what propriety we will afterwards consider. now this case is typical of many. the two forms have a wide distribution, occurring sometimes separately, sometimes in juxtaposition. _l. diurna_ is a plant of hedgerows and sheltered situations. _l. vespertina_ is common in fields and open spaces, where _diurna_ is hardly ever found; but not rarely _vespertina_ occurs in association with _diurna_ in the places which that plant frequents. in this case i do not doubt that we have to do with organisms of somewhat different aptitudes. that _l. vespertina_ has powers which _diurna_ has not is shown very clearly by the fact that _diurna_ is sometimes entirely absent from areas where _vespertina_ can abound.[ ] but in order to understand the true genetic relations of the two plants to each other it is necessary to observe their behaviour when they meet as they not unfrequently do. if the _lychnis_ population of such a locality be examined it will be found to consist of many undoubted and unmodified _diurna_, a number--sometimes few, sometimes many--of similarly unmodified _vespertina_, and an uncertain but usually rather small proportion of plants obviously hybrids between the two. how is it possible to reconcile these facts with the view that specific distinction has no natural basis apart from environmental exigency? darwinian orthodoxy suggests that by a gradual process of natural selection either one of these two types was evolved from the other, or both from a third type. i cannot imagine that anyone familiar with the facts would propose the first hypothesis in the case of _lychnis_, nor can i conceive of any process, whether gradual or sudden, by which _diurna_ could have come out of _vespertina_, or _vespertina_ out of _diurna_. both however may no doubt have been derived from some original third type. it is conceivable that _lychnis macrocarpa_ of boissier, a native of southern spain and morocco, may be this original form. this species is said to combine a white flower (like that of _l. vespertina_), with capsule-teeth rolled back (like those of _diurna_).[ ] but whatever the common progenitor may have been, if we are to believe that these two species have been evolved from it by a gradual process of natural selection based on adaptation, enormous assumptions must be made regarding the special fitness of these two forms and the special unfitness of the common parent, and these assumptions must be specially invoked and repeated for each several feature of structure or habits distinguishing the three forms. why, if the common parent was strong enough to live to give rise to these two species, is it either altogether lost now, or at least absent from the whole of northern europe? its two putative descendants, though so distinct from each other, are, as we have seen, able often to occupy the same ground. if they were gradually derived from a common progenitor--necessarily very like themselves--can we believe that this original form should always, in all the diversities of soil and situation which they inhabit, be unable to exist? some one may fancy that the hybrids which are found in the situations occupied by both forms are this original parental species. but nothing can be more certain than that these plants are simply heterozygous combinations made by the union of gametes bearing the characters of _diurna_ and _vespertina_.[ ] for they may be reproduced exactly in f_{ } or in later generations of that cross when it is artificially made; when bred from their families exhibit palpable phenomena of segregation more or less complex; and usually, if perhaps not always, they are partially sterile.[ ] in a locality on the norfolk coast that i know well, there is a strip of rough ground chiefly sand-bank, which runs along the shore. this ground is full of _vespertina_. not a hundred yards inland is a lane containing _diurna_, and among the _vespertina_ on the sand-bank are always some of the hybrid form, doubtless the result of fertilisation from the neighbouring _diurna_ population. seed saved from these hybrids gave _vespertina_ and hybrids again, having obviously been fertilised by other _vespertina_ or by other hybrids, and i have no doubt that such hybrid plants if fertilised by _diurna_ would have shown some _diurna_ offspring. the absence of _diurna_ in such localities may fairly be construed as an indication that _diurna_ is there at a real disadvantage in the competition for life. but if, admitting this, we proceed to consider how the special aptitude of _vespertina_ is constituted, or what it is that puts _diurna_ at a disadvantage, we find ourselves quite unable to show the slightest connexion between the success of one or the failure of the other on the one hand, and _the specific characteristics_ which distinguish the two forms on the other. the orthodox selectionist would, as usual, appeal to ignorance. we ask what can _vespertina_ gain by its white flowers, its more lanceolate leaves, its grey seeds, its almost erect capsule-teeth, its longer fruits, which _diurna_ loses by reason of its red flowers, more ovate leaves, dark seeds, capsule-teeth rolled back, and shorter fruits? we are told that each of these things _may_ affect the viability of their possessors. we cannot assert that this is untrue, but we should like to have evidence that it is true. the same problem confronts us in thousands upon thousands of examples, and as time goes on we begin to feel that speculative appeals to ignorance, though dialectically admissible, provide an insufficient basis for a proposition which, if granted, is to become the foundation of a vast scheme of positive construction. one thing must be abundantly clear to all, that to treat two forms so profoundly different as one, because intermediates of unknown nature can be shown to exist between them, is a mere shirking of the difficulties, and this course indeed creates artificial obstacles in the way of those who are seeking to discover the origin of organic diversity. in the enthusiasm with which evolutionary ideas were received the specificity of living things was almost forgotten. the exactitude with which the members of a species so often conform in the diagnostic, specific features passed out of account; and the scientific world by dwelling with a constant emphasis on the fact of variability, persuaded itself readily that species had after all been a mere figment of the human mind. without presuming to declare what future research only can reveal, i anticipate that, when variation has been properly examined and the several kinds of variability have been successfully distinguished according to their respective natures, the result will render the natural definiteness of species increasingly apparent. formerly in such a case as that of the two _lychnis_ species, the series of "intermediates" was taken to be a palpable proof that _vespertina_ "graded" to _diurna_. it is this fact, doubtless, upon which bentham would have relied in suggesting that both may be one species.[ ] genetic tests, though as yet imperfectly applied, make it almost certain that these inter-grading forms are not in any true sense variations from either species in the direction of the other, but combinations of elements derived from both. the points in which very closely allied species are distinguished from each other may be found in the most diverse features of their organisation. sometimes specific difference is to be seen in a character which we can believe to be important in the struggle, but at least as often it is some little detail that we cannot but regard as trivial which suffices to differentiate the two species. even when the diagnostic point is of such a nature that we can imagine it to make a serious difference in the economy we are absolutely at a loss to suggest why this feature should be a necessity to species a and unnecessary to species b its nearest ally. the house sparrow (_passer domesticus_) is in general structure very like the tree sparrow (_p. montanus_). they differ in small points of colour. for instance _montanus_ has a black patch on the cheek which is absent in _domesticus_. the presence in the one species and the absence in the other are equally definite, and in both cases we are equally unable to suggest any consideration of utility in relation to these features. the two species are distinguished also by a characteristic that may well be supposed to be of great significance. in _domesticus_ the two sexes are strongly differentiated, the cock being more ornate than the hen. on the other hand the two sexes in _montanus_ are alike, and, if we take a standard from _domesticus_, we may fairly say that in _montanus_ the hen has the colouration of the male. it is not unreasonable to suppose that such a distinction may betoken some great difference in physiological economy, but the economical significance of this perhaps important distinction is just as unaccountable as that of the seemingly trivial but equally diagnostic colour-point. i have spoken of the fixed characteristics of the two species. if we turn to a very different feature, their respective liability to albinistic variation, we find ourselves in precisely similar difficulty. _passer domesticus_ is a species in which individuals more or less pied occur with especial frequency, but in _p. montanus_ such variation is extremely rare if it occurs at all. the writer of the section on birds in the _royal natural history_ (iii., - , p. ) calls attention to this fact and remarks that in that species he knows no such instance. the two species therefore, apart from any differences that we can suppose to be related to their respective habits, are characterised by small fixed distinctions in colour-markings, by a striking difference in secondary sexual characters, and by a difference in variability. in all these respects we can form no surmise as to any economic reason why the one species should be differentiated in the one way and the other in the other way, and i believe it is mere self-deception which suggests the hope that with fuller knowledge reasons of this nature would be discovered. the two common british wasps, _vespa vulgaris_ and _vespa germanica_, are another pair of species closely allied although sharply distinguished, which suggest similar reflexions. both usually make subterranean nests but of somewhat different materials. _v. vulgaris_ uses rotten wood from which the nest derives a characteristic yellow colour, while _v. germanica_ scrapes off the weathered surfaces of palings and other exposed timber, material which is converted into the grey walls of the nest. the stalk by which the nest is suspended (usually to a root) in the case of _germanica_ passes freely through a hole in the external envelope, but _vulgaris_ unites this external wall solidly to the stalk. in bodily appearance and structure the two species are so much alike that they have often been confounded even by naturalists, and to the untrained observer they are quite indistinguishable. there are nevertheless small points of difference which almost though not quite always suffice to distinguish the two forms. for example the yellow part of the sinus of the eyes is emarginate in _vulgaris_ but not emarginate in _germanica_. _v. vulgaris_ often has black spots on the tibiae while in _germanica_ the tibiae are usually plain yellow. in both species there is a horizontal yellow stripe on the thorax, but whereas in _vulgaris_ this is a plain narrow stripe, it is in _germanica_ enlarged downwards in the middle. these and other apparently trivial details of colouration, though not absolutely constant, are yet so nearly constant that irregularities in these respects are quite exceptional. lastly the genitalia of the males, though not very different, present small structural points of distinction which are enough to distinguish the two species at a glance.[ ] in considering the meaning of the distinctions between these two wasps we meet the old problem illustrated by the sparrows. the two species have somewhat different habits of life and we should readily expect to find differences of bodily organisation corresponding with the differences of habits. but is that what we do find? surely not. to suppose that there is a correspondence between the little points of colour and structure which we see and the respective modes of life of the two species is perfectly gratuitous. we have no inkling of the nature of such a correspondence, how it can be constituted, or in what it may consist. is it not time to abandon these fanciful expectations which are never realised? everywhere both among animals and plants does the problem of specific difference reiterate itself in the same form. in view of such facts as i have related and might indefinitely multiply, the fixity of specific characters cannot readily be held to be a measure of their economic importance to their possessors. the incidence of specific fixity is arbitrary and capricious, sometimes lighting on a feature or a property which can be supposed to matter much, but as often is it attached to the most trifling of superficial peculiarities. the incidence of _variability_ is no less paradoxical, and without investigation of the particular case no one can say what will be found to show much or little variability. the very characteristic which in one species may exhibit extreme variability may in an allied species show extreme constancy. illustrations will occur to any naturalist, but nowhere is this truth more strikingly presented than in the british noctuid moths. many are so variable that, in the common phrase, "scarcely two can be found alike," while others show comparatively slight variation. it need scarcely be remarked that, in the instances i have in mind, the evidence of great variability is in no way due to the abundance with which the particular species occurs, for common species may show constancy, and less abundant species may show great variability. the polymorphism seems to be now at least a general property of the variable species, as the fixity is a property of the fixed species. in illustration i may refer to the following examples. _dianthoecia capsincola_ is a common and widely distributed moth which feeds on _lychnis_. it shows little variation. _dianthoecia carpophaga_ is another species which feeds chiefly on _silene_. its habits are very similar to those of _capsincola_. like that species it has a wide geographical range and is abundant in its localities, but in contrast to the fixity of _capsincola_, _carpophaga_ exhibits a complex series of varieties. _agrotis suffusa_ (= _ypsilon_) is a moth widely spread through the southern half of england. it is very constant in colour and markings. _agrotis segetum_ and _tritici_ are excessively variable both in ground colour and markings, being found in an immense profusion of dissimilar forms throughout their distribution. of these and several other species of _agrotis_ there are many named varieties, some of which have by various writers been regarded as specifically distinct. of the genus _noctua_ many species (e. g. _festiva_) show a similar polymorphism, but _n. triangulum_, though showing some variation in certain respects, is usually very constant to its type, and the same is true of _n. umbrosa_. in several species of _taeniocampa_, especially _instabilis_, the multiplicity of forms is extreme, while _cruda_ (= _pulverulenta_) is a comparatively constant species. the genus _plusia_ contains a number of constant species, but in _plusia interrogationis_ we meet the fact that the central silvery mark undergoes endless variation. "truly no two are alike," says mr. tutt, "and to look down a long series of _interrogationis_ is something like looking at a series of chinese characters." in contrast to this we have the fact that in _plusia gamma_ the very similar silvery mark is by no means variable. i have taken this series of cases from the noctuid moths, but it would be as easy to illustrate the same proposition from the geometridae or the micro-lepidoptera.[ ] i have a long series of _peronea cristana_, for example, which was given to me by mr. w. h. b. fletcher, of bognor. all were beaten out of the same hedge, and their polymorphism is such that no one unaccustomed to such examples could suppose that they belonged to a single species. another common form, _p. schalleriana_, which lives in similar circumstances, exhibits comparatively slight variability. it should be expressly noted that the variation of which i am speaking is a genuine polymorphism. several of the species enumerated exhibit also geographical variation, possessing definite and often strikingly distinct races peculiar to certain localities; but apart from the existence of such local differentiation, stands out the fact upon which i would lay stress, that some species are excessively variable while others are by comparison constant, in circumstances that we may fairly regard as comparable. this fact is difficult to reconcile with the conventional view that specific type is directly determined by natural selection and that the precision with which a species conforms to its pattern is an indication of the closeness of that control. anyone familiar with the characteristics of moths will agree that the noctuids, geometrids and tortricids are creatures whose existence depends in some degree on the success with which they can escape detection by their enemies in the imaginal state. we are therefore not surprised to find that some species of these orders exhibit definite geographical variation in conformity with the character of the ground, which may reasonably be supposed to aid in their protection. if this were all, there would be nothing to cause surprise. we might even be disposed to allow that variability might contribute to the perpetuation of animals so situated, on the principle that among a variety of surroundings some would probably be in harmony with the objects on which they rest. but we cannot admit the plausibility of an argument which demands on the one hand that the extreme precision with which species a adheres in the minutest details of its colour and pattern to a certain type shall be ascribed to the protective fitness of those details, and on the other hand that the abundant variability of species b shall be ascribed to the same determination. if it is absolutely necessary for a to conform to one type how comes it that b may range through some twenty distinct forms, any two of which differ more from each other than the regular species of many other genera? the only reply i can conceive is a suggestion that there _may_ be some circumstance which differentiates the various classes of cases, that the exigencies of the fixed species _may_ be different from those of the variable. those who make such appeals to ignorance do not always perhaps realise whither this course of reasoning may lead. if admissible here the same argument would lead us to suggest that because albino moles have for an indefinite period occurred on a certain land near bath there may be something in the soil or in the conditions of life near bath which requires a proportion of albinos in its mole population. or again, because the butterfly _thais rumina_ in one locality, digne in the south of france, has a percentage of individuals of the variety _honoratii_ (with certain normally yellow spots on the hind wing coloured bright red) and nowhere else throughout its distribution, that therefore we may suggest that there is some difference in the condition of life at digne which makes the continuance of _honoratii_ there possible and beneficial. a polymorphism offering a parallel to that of the variable moths is afforded by the breeding plumage of the ruff, the male of _machetes pugnax_. the variety of plumage which these cocks exhibit is such that the statement that no two can be found alike is only a venial exaggeration. newton remarks[ ] "that all this wonderful 'show' is the consequence of the polygamous habit of the ruff can scarcely be doubtful"; but even if it be conceded that the great external differentiation of the cocks may be a result of sexual selection, the problem of their _polymorphism_ remains unsolved, for, as we are well aware, polygamy is not usually associated with polymorphism of the male. the black cock (_tetrao tetrix_), for example, is as polygamous as the ruff, but in that and countless other cases, both sexes are constant to one type of plumage. when we thus compare the polymorphism of one species with the fixity of another, and attempt to determine the causes which have led to these extraordinary contrasts, two distinct lines of argument are open to us. we may ascribe the difference either to causes external to the organisms, primarily, that is to say, to a difference in the exigencies of adaptation under natural selection; or on the other hand we may conceive the difference as due to innate distinctions in the chemical and physiological constitutions of the fixed and the variable respectively. there is truth undoubtedly in both conceptions. if the mole were physiologically incapable of producing an albino that variety would not have come into being, and if the albino were totally incapable of getting its living it would not be able to hold its own. were _plotheia frontalis_ constructed on a chemical plan which admitted of no variation, the countless varieties would not have been produced; and if one of its varieties had an overwhelming success out of all proportion to that of the rest, then the species would soon become monomorphic again. we cannot declare that natural selection has no part in the determination of fixity or variability; nevertheless looking at the whole mass of fact which a study of the incidence of variation provides, i incline to the view that the variability of polymorphic forms should be regarded rather as a thing tolerated than as an element contributing directly to their chances of life; and on the other hand that the fixity of the monomorphic forms should be looked upon not so much as a proof that natural selection controls them with a greater stringency, but rather as evidence of a natural and intrinsic stability of chemical constitution. compare the condition of a variable form like the male ruff (or in a less degree the red grouse in both its sexes) with that of the common pheasant which is comparatively constant. in the pheasant no doubt variations do occur as in other wild birds, but apart from the effects of mongrelisation the species is unquestionably uniform. could it seriously be proposed that we should regard the constancy of the pheasant's plumage in this country as depending on the special fitness of that type of colouration? even if the pheasant be not an alien in western europe, it has certainly been protected for centuries, and for a considerable period has existed in a state of semi-domestication. such conditions should give good opportunity for polymorphism to be produced. in some coverts various aberrations do of course occur and persist, yet there is nothing indicative of a general relaxation of the fixity of the specific type, and the pheasant remains substantially a fixed species.[ ] the common pheasant (_phasianus colchicus_) even shows little of that disposition to form local races which appears in the species of further india. are we not then on safer ground in regarding the fixity of our species as a property inherent in its own nature and constitution? just as in ages of domestication no rose has ever given off a blue variety so has the pheasant never broken out into the polymorphism of the ruff. as soon as it is realised how largely the phenomena of variation and stability must be an index of the internal constitution of organisms, and not mere consequences of their relations to the outer world, such phenomena acquire a new and more profound significance. footnotes: [ ] in _mendel's principles of heredity_ (cambridge university press, ) i have dealt with this subject, giving an account of the principal facts discovered up to the beginning of . [ ] matthioli opera, ed. , p. , originally published . [ ] ray's instances relate to kales, and in most of these examples we can see that there was no question of mutation or transmutation at all, but that the occurrence was due either to mistake or to cross-fertilisation. sharrock, to whom ray refers, was inclined to discredit stories of transmutation, but he has also this passage (_history of the propagation and improvement of vegetables by the concurrence of art and nature_, oxford, , p. ): "it is indeed growen to be a great question, whether the transmutation of a species be possible either in the vegetable, animal, or minerall kingdome. for the possibility of it in the vegetable; i have heard _mr. bobart_ and his _son_ often report it, and proffer to make oath that the crocus and gladiolus, as likewise the leucoium, and hyacinths by a long standing without replanting have in his garden changed from one kind to the other: and for satisfaction about the curiosity in the presence of _mr. boyle_ i tooke up some bulbs of the very numericall roots whereof the relation was made, though the alteration was perfected before, where we saw the diverse bulbs growing as it were on the same stoole, close together, but no bulb half of the one kind, and the other half of the other: but the changetime being past it was reason we should believe the report of good artists in matters of their own faculty." robert sharrock was a fellow of new college, oxford. both the bobarts were professional botanists, the father was author of a catalogue of the plants in the hortus medicus at oxford, and the son was afterwards curator of the oxford garden. [ ] _mém. ac. roy. des sci._ for ( ), p. . [ ] _amoen. acad._, , vol. . i do not know whether attention has been called to the curious mistake which linnaeus makes in the course of this argument. he cites the differences between the mule and the hinny in illustration of his thesis, pointing out that the mule is externally more like a horse and the hinny more like an ass. this, he says, is because the mule has the horse for a father, and the hinny the ass, thus inverting the actual facts! [ ] _proc. washington ac. sci._, , xi, pp. - . [ ] j. w. tutt, in _ent. rec._, , xxi, p. . [ ] e. lehmann (_bull. l'herb. boissier_, ser. , viii, , p. ) has published an admirable paper on the interrelationships of these species and has instituted cultural experiments which will probably much elucidate the nature of their specific distinctness. as regards the existence of intermediate forms he comes to the conclusion that two only can be so regarded. the first was described by kuntze from specimens found on a flower-pot on board a caspian steamer, from which lehmann proposes the new specific name _siaretensis_. this comes between _polita_ and _filiformis_, a close ally of _tournefortii_. the other, which combines some of the features of both _polita_ and _tournefortii_, was found in the province of asterabad. [ ] in cambridgeshire for example _vespertina_ is common but _diurna_ is absent. whether this absence is connected with the general presence of chalk i cannot say. when introduced artificially _diurna_ establishes itself, for a time at least, without any apparent difficulty and occasionally escapes from the garden on to the neighbouring roadside. [ ] conceivably however it may be a segregated combination. for an account of this plant see boissier, _voy. bot. midi de l'espagne_, , ii, . [ ] a discussion of this subject with references to literature is given by rolfe, in an excellent paper on "hybridisation viewed from the standpoint of systematic botany" (_jour. r. hort. soc._, xxiv, , p. ). he concludes: "the simple fact is that the two plants (_l. diurna_ and _vespertina_) are thoroughly distinct in numerous particulars, and affect such different habitats that in some localities one or the other of them is completely wanting. but when their stations are adjacent they hybridise together very readily, and it is here that these intermediate forms occur which have puzzled botanists so much." the same paper contains valuable information concerning several cognate illustrations. [ ] in only two cases have i seen such plants (both females) completely sterile. [ ] as is well known, in an even more notorious example, he proposed to unite _primula vulgaris_, _p. elatior_, and _p. acaulis_, similarly relying on the existence of "intermediates," which we now well know to be mongrels between the species. [ ] for an account of the distinctions between _vespa vulgaris_ and _germanica_ see ch. janet, _Ã�tudes sur les fourmis, les guêpes et les abeilles_, ^e, note. sur _vespa germanica_ et _v. vulgaris_. limoges (ducourtieux), ; and r. du buysson, monographie des guêpes, _ann. soc. ent. france_, , vol. lxxii, p. , pl. viii. [ ] the statements made above are for the most part taken from barrett, c. g., _lepidoptera of the british islands_, and from tutt, j. w., _the british noctuae and their varieties_. the reader who is unfamiliar with the amazing polymorphism exhibited by some of these moths should if possible take an opportunity of looking over a long series in a collection, or, if that be impossible, refer to the admirable coloured plates published by barrett. it may not be superfluous to observe that plenty of similar examples are known in other countries. for instance _plotheia frontalis_, a noctuid which often abounds in ceylon, shows an equally bewildering wealth of forms. if a dozen specimens of such a species were to be brought home from some little known country, each individual would almost certainly be described as the type of a distinct species. (see the coloured plate published by sir g. hampson, cat. brit. mus., heterocera, vol. ix.) [ ] _dict. of birds_, p. . it would be interesting and profitable to attempt in a long series of ruffs to determine the mendelian factors which by their combinations give rise to this complex assemblage of varietal forms. a few such factors both of colour and pattern can be at once distinguished, and it is noticeable that some of the resulting types of barring, spangling and penciling show a perceptible correspondence with some of the types of colouration found in the breeds of domestic fowls. [ ] howard saunders (_illust. manual of british birds_, , p. ) states that there is evidence that the pheasant had become naturalized in the south of england before the norman invasion. he adds, "little, if any, deviation from the typical _p. colchicus_ took place up to the end of last century, when the introduction of the chinese ring-necked _p. torquatus_ commenced, which has left almost indelible marks, especially with regard to the characteristic white collar." chapter ii meristic phenomena twenty years ago in describing the facts of variation, argument was necessary to show that these phenomena had a special value in the sciences of zoology and botany. this value is now universally understood and appreciated. in spite however of the general attention devoted to the study of variation, and the accumulation of material bearing on the problem, no satisfactory or searching classification of the phenomena is possible. the reason for this failure is that a real classification must presuppose knowledge of the chemistry and physics of living things which at present is quite beyond our reach. it is however becoming probable that if more knowledge of the chemical and physical structure of organisms is to be attained, the clue will be found through genetics, and thus that even in the uncoordinated accumulation of facts of variation we are providing the means of analysis applicable not only to them, but to the problems of normality also. the only classification that we can yet institute with any confidence among the phenomena of variation is that which distinguishes on the one hand variations in the processes of division from variations in the nature of the substances divided. variations in the processes of division are most often made apparent by a change in the number of the parts, and are therefore called _meristic_ variations, while the changes in actual composition of material are spoken of as _substantive_ variations. the meristic variations form on the whole a natural and fairly well defined group, but the substantive variations are obviously a heterogeneous assemblage. though this distinction does not go very far, it is useful, and in all probability fundamental. it is of value inasmuch as it brings into prominence the distinct and peculiar part which the process of division, or, more generally, repetition of parts, plays in the constitution of the forms of living things. that there may be a real independence between the meristic and the substantive phenomena is evident from the fact both that meristic changes may occur without substantive variation, and that the substances composing an organism may change without any perceptible alteration in its meristic structure. when the distinction between these two classes of phenomena is perceived it will be realised that the study of genetics has on the one hand a physical, or perhaps more strictly a mechanical aspect, which relates to the manner in which material is divided and distributed; and also a chemical aspect, which relates to the constitution of the materials themselves. somewhat as the philosophers of the seventeenth and eighteenth centuries were awaiting both a chemical and a mechanical discovery which should serve as a key to the problems of unorganised matter, so have biologists been awaiting two several clues. in mendelian analysis we have now, it is true, something comparable with the clue of chemistry, but there is still little prospect of penetrating the obscurity which envelops the mechanical aspect of our phenomena. to make clear the application of the terms chemical and mechanical to the problem of genetics the nature of that problem must be more fully described. in its most concrete form this problem is expressed in the question, how does a cell divide? if the organism is unicellular, and the single cell is the whole body, then the process of heredity is accomplished in the single operation of cell-division. similarly in animals and plants whose bodies are made up of many cells, the whole process of heredity is accomplished in the cell-divisions by which the germ-cells are formed. when therefore we see a cell dividing, we are witnessing the process by which the form and the properties of the daughter-cells are determined. now this process has the two aspects which i have called mechanical and chemical. the term "_entwicklungsmechanik_" has familiarised us with the application of the word mechanics to these processes, but on reflexion it will be seen that this comprehensive term includes two sorts of events which are sometimes readily distinguishable. there is the event by which the cell _divides_, and the event by which the two halves or their descendants are or may be _differentiated_. it is common knowledge that in some cell-divisions two similar halves, indistinguishable in appearance, properties, and subsequent fate, may be produced, while in other divisions daughter-cells with distinct properties and powers are formed. we cannot imagine but that in the first case, when the resulting cells are identical, the division is a mechanical process by which the mother-cell is simply cut in two; while in order that two differentiated halves may be produced, some event must have taken place by which a chemical distinction between the two halves is effected.[ ] in any ordinary mendelian case we have a clear proof that such a chemical difference may be established between germ-cells. the facts of colour-inheritance for instance prove that germ-cells, otherwise identical, may be formed _possessing_ the chromogen-factor which is necessary to the formation of colour in the flowers, or _destitute_ of that factor. similarly the germ-cells may possess the ferment which, by its action on the chromogenic substance, produces the colour, or they may be without that ferment. the same line of argument applied to a great range of cases. nevertheless, though differences in chemical properties are often thus constituted by cell-divisions, and though we are thus able to make a quasi-chemical analysis of the individual by determining and enumerating these properties, yet it is evident that the distribution of these factors is not itself a chemical process. this is proved by the fact that similar divisions may be effected between halves which are exactly alike, and also by the fact that the numbers in which the various types of germ-cells are formed negative any suggestion of valency between them. the recognition of the unit-factors may lead--indeed must lead--to great advances in chemical physiology which without that clue would have been impossible, but in causation the chemical phenomena of heredity must be regarded as secondary to the physical or mechanical phenomena by which the cells and their constituents are divided and separated. when therefore we speak of the _essential_ phenomena of heredity we mean the mechanics of division, especially, though not, as we shall see, exclusively, of _cell_-division; and in the relation between the two halves of the dividing cell we have the problem presented in what seems to be its simplest form. in attempting to form some conception of the processes by which bodily characteristics are transmitted, or--to avoid that confusing metaphor of "transmission"--how it comes about that the offspring can grow to resemble its parent, continuity of the germ-substance which in some animals is a visible phenomenon,[ ] gives at least apparent help. an egg for example on becoming adult develops in certain parts a particular pigment. the eggs of that adult when they reach the appropriate age develop the same pigment. we have no clear picture of the mechanism by which this process is effected, but when we realise that the pigment results from the interaction of certain substances, and that since all the eggs are in reality pieces of the same material, it seems, unless we inquire closely, not unnatural that the several pieces of the material should exhibit the same colours at the same periods of their development. the continuity of the material of the germs suggests that there is a continuity of the materials from which the pigment is formed, and that thus an actual bit of those substances passes into each egg ready at the appropriate moment to generate the pigment. the argument thus outlined applies to all _substantive_ characteristics. in each case we can imagine, if we will, the appearance of that characteristic as due to the contribution of its rudiment from the germ tissues. when we consider more critically it becomes evident that the aid given by this mental picture is of very doubtful reality, for even if it were true that any predestined particle actually corresponding with the pigment-forming materials is definitely passed on from germ to germ, yet the power of increase which must be attributed to it remains so incomprehensible that the mystery is hardly at all illuminated. when however we pass from the substantive to the meristic characters, the conception that the character depends on the possession by the germ of a particle of a specific material becomes even less plausible. hardly by any effort of imagination can we see any way by which the division of the vertebral column into _x_ segments or into _y_ segments, or of a medusa into segments or into , can be determined by the possession or by the want of a material particle. the distinction must surely be of a different order. if we are to look for a physical analogy at all we should rather be led to suppose that these differences in segmental numbers corresponded with changes in the amplitude or number of dividing waves than with any change in the substance or material divided. phenomena of division i have said that in the division of a cell we seem to see the problem in its simplest form, but it is important to observe that the problem of division may be presented by the bodies of animals and plants in forms which are independent of the divisions between cells. the existence of pattern implies a repetition of parts, and repetition of parts when developed in a material originally homogeneous can only be created by division. cell-division is probably only a special case of a process similar to that by which the pattern of the skeleton is laid down in a unicellular body such as that of a radiolarian or foraminiferan. attempts have lately been made to apply mathematical treatment to problems of biology. it has sometimes seemed to me that it is in the geometrical phenomena of life that the most hopeful field for the introduction of mathematics will be found. if anyone will compare one of our animal patterns, say that of a zebra's hide, with patterns known to be of purely mechanical production, he will need no argument to convince him that there must be an essential similarity between the processes by which the two kinds of patterns were made and that parts at least of the analysis applicable to the mechanical patterns are applicable to the zebra stripes also. patterns mechanically produced are of many and very diverse kinds. one of the most familiar examples, and one presenting some especially striking analogies to organic patterns, is that provided by the ripples of a mackerel sky, or those made in a flat sandy beach by the wind or the ebbing tide. with a little search we can find among the ripple-marks, and in other patterns produced by simple physical means, the closest parallels to all the phenomena of striping as we see them in our animals. the forking of the stripes, the differentiation of two "faces," the deflections round the limbs and so forth, which in the body we know to be phenomena of division, are common both to the mechanical and the animal patterns. we cannot tell what in the zebra corresponds to the wind or the flow of the current, but we can perceive that in the distribution of the pigments, that is to say, of the chromogen-substances or of the ferments which act upon them, a rhythmical disturbance has been set up which has produced the pattern we see; and i think we are entitled to the inference that in the formation of patterns in animals and plants mechanical forces are operating which ought to be, and will prove to be, capable of mathematical analysis. the comparison between the striping of a living organism and the sand-ripples will serve us yet a little farther, for a pattern may either be formed by actual cell-divisions, and the distribution of differentiation coincidently determined, or--as visibly in the pigmentation of many animal and plant tissues--the pattern may be laid down and the pigment (for example) distributed through a tissue across or independently of the cell-divisions of the tissue. our tissues therefore are like a beach composed of sands of different kinds, and different kinds of sands may show distinct and interpenetrating ripples. when the essential analogy between these various classes of phenomena is perceived, no one will be astonished at, or reluctant to admit, the reality of discontinuity in variation, and if we are as far as ever from knowing the actual causation of pattern we ought not to feel surprised that it may arise suddenly or be suddenly modified in descent. biologists have felt it easier to conceive the evolution of a striped animal like a zebra from a self-coloured type like a horse (or of the self-coloured from the striped) as a process involving many intergradational steps; but so far as the _pattern_ is concerned, the change may have been decided by a single event, just as the multitudinous and ordered rippling of a beach may be created or obliterated at one tide. [illustration: fig. . tusk of indian elephant, showing an abnormal segmentation.] this point is well illustrated by the tusk of an indian elephant which i lately found in a london sale-room. this tusk is by some unknown cause, presumably a chronic inflammation, thrown up into thirteen well-marked ridges which closely simulate a series of segments (fig. ). whatever the cause the condition shows how easily a normally unsegmented structure may be converted into a series of repeated parts. the spread of segmentation through tissues normally unsegmented is very clearly exemplified in the skates' jaws shown in fig. . the right side of the upper figure shows the normal arrangement in the species _rhinoptera jussieui_, but the structure on the left side is very different. the probable relations of the several rows of teeth to the normal rows is indicated by the lettering, but it is evident that by the appearance of new planes of division constituting separate centers of growth, the series has been recast. the pattern of the left side is so definite that had the variation affected the right side also, no systematist would have hesitated to give the specimen a new specific name. the other two drawings show similar variations of a less extensive kind, the nature of which is explained by the lettering of the rows of teeth. [illustration: fig. . jaws of skates (_rhinoptera_) showing meristic variation. (for a detailed discussion see _materials for the study of variation_, p. .)] this power to divide is a fundamental attribute of life, and of that power cell-division is a special example. in regard to almost all the chief vital phenomena we can say with truth that science has made some progress. if i mention respiration, metabolism, digestion, each of these words calls to mind something more than a bare statement that such acts are performed by an animal or a plant. each stands for volumes of successful experiment and research, but the expression cell-division, the fundamental act which typifies the rest, and on which they all depend, remains a bare name. we can see with the microscope the outward symptoms of division, but we have no surmise as to the nature of the process by which the division is begun or accomplished. i know nothing which to a man well trained in scientific knowledge and method brings so vivid a realisation of our ignorance of the nature of life as the mystery of cell-division. what is a living thing? the best answer in few words that i know is one which my old teacher, michael foster, used to give in his lectures introductory to biology. "a living thing is a vortex of chemical and molecular change." this description gives much, if not all, that is of the essence of life. the living thing is unlike ordinary matter in the fact that, through it, matter is always passing. matter is essential to it; but, provided that the flow in and out is unimpeded, the life-process can go on so far as we know indefinitely. yet the living "vortex" differs from all others in the fact that it can divide and throw off other "vortices," through which again matter continually swirls. we may perhaps take the parallel a stage further. a simple vortex, like a smoke-ring, if projected in a suitable way will twist and form two rings. if each loop as it is formed could grow and then twist again to form more loops, we should have a model representing several of the essential features of living things. it is this power of spontaneous division which most sharply distinguishes the living from the non-living. in the excellent book dealing with the problems of development, lately published by mr. jenkinson a special emphasis is very properly laid on the distinction between the processes of division, and those of differentiation. too often in discussions of the developmental processes the distinction is obscured. he regards differentiation as the "central difficulty." "growth and division of the nucleus and the cells," he tells us, are side-issues. this view is quite defensible, but i suspect that the division _is_ the central difficulty, and that if we could get a rationale of what is happening in cell-division we should not be long before we had a clue to the nature of differentiation. it may be self-deception, but i do not feel it impossible to form some hypothesis as to the mode of differentiation, but in no mood of freest speculation are we ever able to form a guess as to the nature of the division. we see differentiations occurring in the course of chemical action, in some phenomena of vibration and so forth: but where do we see anything like the spontaneous division of the living cell? excite a gold-leaf electroscope, and the leaves separate, but we know that is because they were double before. in electrolysis various substances separate out at the positive and negative poles respectively. now if in cell-division the two daughter-cells were always dissimilar--that is to say, if differentiation always occurred--we could conceive some rough comparison with such dissociations. but we know the dissimilarity between daughter-cells is not essential. in the reproduction of unicellular organisms and many other cases, the products formed at the two poles are, so far as we can tell, identical. any assumption to the contrary, if we were disposed to make it, would involve us in difficulties still more serious. at any rate, therefore, if differentiation be really the central difficulty in development, it is division which is the essential problem of heredity. sir george darwin and professor jeans tell us that "gravitational instability" consequent on the condensation of gases is "the primary agent at work in the actual evolution of the universe," which has led to the division of the heavenly bodies. the greatest advance i can conceive in biology would be the discovery of the nature of the instability which leads to the continual division of the cell. when i look at a dividing cell i feel as an astronomer might do if he beheld the formation of a double star: that an original act of creation is taking place before me. enigmatical as the phenomenon seems, i am not without hope that, if it were studied for its own sake, dissociated from the complications which obscure it when regarded as a mere incident in development, some hint as to the nature of division could be found. it is i fear a problem rather for the physicist than for the biologist. the sentiment may not be a popular one to utter before an assembly of biologists, but looking at the truth impersonally i suspect that when at length minds of first rate analytical power are attracted to biological problems, some advance will be made of the kind which we are awaiting. the study of the phenomena of bodily symmetry offers perhaps the most hopeful point of attack. the essential fact in reproduction is cell-division, and the essential basis of hereditary resemblance is the symmetry of cell-division. the phenomena of twinning provide a convincing demonstration that this is so. by twinning we mean the production of equivalent structures by division. the process is one which may affect the whole body of an animal or plant, or certain of its parts. the term twin as ordinarily used refers to the simultaneous birth of two individuals. those who are naturalists know that such twins are of two kinds, ( ) twins that are not more alike than any other two members of the same family, and ( ) twins that are so much alike that even intimate friends mistake them. these latter twins, except in imaginative literature, are always of the same sex. it is scarcely necessary for me to repeat the evidence from which it has been concluded that without doubt such twins arise by division of the same fertilised ovum. there is a perfect series of gradations connecting them with the various forms of double monsters united by homologous parts. they have been shown several times to be enclosed in the same chorion, and the proofs of experimental embryology show that in several animals by the separation of the two first hemispheres of a dividing egg twins can be produced. lastly we have recently had the extraordinarily interesting demonstration of loeb, to which i may specially refer. herbst some years ago found that in sea water, from which all lime salts had been removed, the segments of the living egg fall apart as they are formed. using this method loeb has shown that a temporary immersion in lime-free sea water may result in the production of per cent. of twins. we are therefore safe in regarding the homologous or "identical" twins as resulting from the divisions of one fertilised egg, while the non-identical or "fraternal" twins, as they are called, arise by the fertilisation of two separate ova.[ ] in the resemblance of identical twins we have an extreme case of hereditary likeness[ ] and a proof, if any were needed, that the cause of individual variation is to be sought in the differentiation of germ-cells. the resemblance of identical twins depends on two circumstances, first, since only two germ-cells take part in their production, difference between the germ cells of the same individual cannot affect them. secondly the division of the fertilised ovum, the process by which they became two instead of one, must have been a symmetrical division. the structure of twins raises however one extremely significant difficulty, which as yet we cannot in any way explain. the resemblance between twins is a phenomenon of symmetry, like the resemblance between the two sides of a bilaterally symmetrical body. not only is the general resemblance readily so interpreted, but we know also that in double monsters, namely unseparated twins, various anatomical abnormalities shown by the one half-body are frequently shown by the other half-also.[ ] the two belong to one system of symmetry. how then does it happen that the body of one of a pair of twins does not show a transposition of viscera? we know that the relation of right and left implies that the one should be the mirror-image of the other. such a relation of images may be maintained even in minute details. for example if the same pattern of finger-print is given by the fingers of the two hands, one is the reverse of the other. in double monsters, namely unseparated twins, there is evidence that an inversion of viscera does occur with some frequency. evidence from such cases is not so clear and simple as might be expected, because as a matter of fact, the heart and stomach, upon which the asymmetry of the viscera chiefly depend, are usually common to the two bodies. duplicity generally affects either the anterior end alone, or the posterior end alone. the division is generally _from the heart forwards_, giving two heads and two pairs of anterior limbs on a common trunk, or _from the heart backwards_, giving two pairs of posterior limbs with the anterior body common. in either case, though the bodies may be grouped in a common system of symmetry, neither can be proved to show definite reversal of the parts. to see that reversal recourse must be had to more extreme duplications, such as the famous siamese twins. they, as a matter of fact, were an excellent instance of the proposition that twins are related as mirror-images, for both of them had eleven pairs of ribs instead of the normal twelve, and one of them had a partial reversal of viscera.[ ] (küchenmeister, _verlagerung_, etc., p. .) if anyone could show how it is that neither of a pair of twins has transposition of viscera the whole mystery of division would, i expect, be greatly illuminated.[ ] at present we have simply to accept the fact that twins, by virtue of their detachment from each other, have the power of resuming the polarity which is proper to any normal individual. it was nevertheless with great interest that i read wilder's recent observation[ ] that occasionally in identical twins the finger-print of one or both the index-fingers may be reversed, showing that there is after all some truth in the notion that reversal should occur in them. there is another phenomenon by twinning which, if we could understand it, might help. i refer to the free-martin, the subject of one of john hunter's masterpieces of anatomical description. in horned cattle twin births are rare, and when twins of opposite sexes are born, the male is perfect and normal, but the reproductive organs of the female are deformed and sterile, being known as a free-martin. the same thing occasionally happens in sheep, suggesting that in sheep also twins may be formed by the division of one ovum; for it is impossible to suppose that mere development in juxtaposition can produce a change of this character. i mention the free-martin because it raises a question of absorbing interest. it is conceivable that we should interpret it by reference to the phenomenon of gynandromorphism, seen occasionally in insects, and also in birds as a great rarity. in the gynandromorph one side of the body is male, the other female. a bullfinch for instance has been described with a sharp line of division down the breast between the red feathers of the cock on one side and the brown feathers of the hen on the other. (poll, h., _sb. ges. nat. fr._, berlin, , p. .) in such cases neither side is sexually perfect. if the halves of such a gynandromorph came apart, perhaps one would be a free-martin. the behaviour of homologous twinning in heredity has been little studied. it does not exist as a normal feature in any animal which is amenable to experiment, and we cannot positively assert that a comparable phenomenon exists in plants; for in them--the orange, for example--polyembryony may evidently be produced by a parthenogenetic development of nucellar tissue. it is possible that in man twinning is due to a peculiarity of the mother, not of the father. it may and not rarely does descend from mother to daughter, but whether it can be passed on through a male generation to a daughter again, there is not sufficient evidence to show. the facts as far as they go are consistent with the inference which may be drawn from loeb's experiment, that the twinning of a fertilized ovum may be determined not by the germ-cells which united to form it, but by the environment in which it begins to develop. the opinion that twinning may descend through the male directly has been lately expressed by dr. j. oliver in the _eugenics review_ ( ), on the evidence of cases in which twins had occurred among the relations of fathers of twins, but i do not know of any comprehensive collection of evidence bearing on the subject. besides twinning of the whole body a comparable duplicity of various parts of the same body may occur. such divisions affect especially those organs which have an axis of bilateral symmetry, such as the thumb, a cotyledon, a median petal, the frond of a fern or the anal fin of a fish. from the little yet known it is clear that the genetic analysis of these conditions must be very difficult, but evidence of any kind regarding them will be valuable. we want especially to know whether these divisions are due to the _addition_ of some factor or power which enables the part to divide, or whether the division results from the _absence_ of something which in the normal body prevents the part from dividing. breeding experiments, so far as they go, suggest that the less divided state is usually dominant to the more divided.[ ] the two-celled tomato fruit is dominant to the many-celled type. the manx cat's tail, with its suppression of caudal segmentation is a partial dominant over the normal tail. the tail of the fowl in what is called the "rumpless" condition is at least superficially comparable with that of the manx cat, and though the evidence is not wholly consistent, davenport obtained facts indicating that this suppressed condition of the caudal vertebrae is an imperfect dominant.[ ] some evidence may also be derived from other examples of differences which at first sight appear to be substantive though they are more probably meristic in ultimate nature. the distinction between the normal and the "angora" hair of the rabbit is a case in point. we can scarcely doubt that one of the essential differences between these two types is that in the angora coat the hair-follicles are more finely divided than they are in the normal coat, and we know that the normal, or less-divided condition, is dominant to the angora, or more finely divided. [illustration: fig. . _i_, _ii_, _iii_, various degrees of syndactyly affecting the medius and annularis in the hand; _iv_, syndactyly affecting the index and medius in the foot. (after annandale.)] in the case of the solid-hoofed or "mule-footed" swine, the evidence shows, as spillman has lately pointed out,[ ] that the condition behaves as a dominant. the essential feature of this abnormality is that the digits iii and iv are partially united. the union is greatest peripherally. sometimes the third phalanges only are joined to form one bone, but the second and even the first phalanges may also be compounded together. here the variation is obviously meristic and consists in a failure to divide, the normal separation of the median digits of the foot being suppressed. [illustration: fig. . case of complete syndactyly in the foot. _ii_ and _iii_, digit apparently representing the index and medius. _c_^{ } + _c_^{ }, bone apparently representing the middle and external cuneiform; _cb_, cuboid; _c_^{ }, internal cuneiform. (after gruber.)] webbing between the digits, in at least some of its manifestations, is a variation of similar nature. the family recorded by newsholme[ ] very clearly shows the dominance of this condition. the case is morphologically of great interest and must undoubtedly have a bearing on the problems of the mechanics of division. in discussing the phenomena of syndactylism i pointed out some years ago that the digits most frequently united in the human hand are iii and iv, while in the foot, union most frequently takes place between ii and iii.[ ] in newsholme's family the union was always between ii and iii of the foot, except in the case of one male who had the digits iii and iv of the right _hand_ alone webbed together. there can be little doubt that the geometrical system on which the foot is planned has an axis of symmetry passing between the digits ii and iii, while the corresponding axis in the hand passes between iii and iv. union between such digits may therefore be regarded as comparable with any non-division or "coalescence" of lateral structures in a middle line, and when as in these examples such a condition is shown to be a dominant we cannot avoid the inference that some concrete factor has the power of suppressing or inhibiting this division. figs. and illustrate degrees of union between digits in the human hand and foot. it is not in question that various other forms of irregular webbing and coalescence of digits exist, and respecting the genetic behaviour of these practically nothing is as yet known. such a case is described by walker,[ ] in which the first and second metacarpals of both feet were fused in mother and daughter, and several more are found in literature. contrasted with these phenomena we have the curious fact that in the pigeon, staples-browne found webbing of the toes a _recessive_ character. the question thus arises whether this webbing is of the same nature as that shown to be a dominant in man, and indeed whether the phenomenon in pigeons is really meristic at all. there is some difference perceptible between the two conditions; for in man there is not so much a development of a special web-like skin uniting the digits as a want of proper division between the digits themselves, and in extreme cases two digits may be represented by a single one. in the pigeon i am not aware that a real union of this kind has ever been observed, and though the web-like skin may extend the whole length of the digits and be so narrow as to prevent the spread of the toes, it may, i think, be maintained that the unity of the digits is unimpaired. for the present the nature of this variation in the pigeon's feet must be regarded as doubtful, and we should note that if it is actually an example of a more perfect division being dominant to a less perfect division, the case is a marked exception to the general rule that non-division is dominant to division. reference must also be made to the phenomenon of fasciation in the stems of plants. as mendel showed in the case of _pisum_ this condition is often a recessive. the appearances suggest that the difference between a normal and a fasciated plant consists in the inability of the fasciated plant to separate its lateral branches. the nature of the condition is however very obscure and it is equally likely that some multiplication of the growing point is the essential phenomenon.[ ] stockard's interesting experiments[ ] illustrate this question. he showed that by treating the embryos of a fish (_fundulus heteroclitus_) with a dilute solution of magnesium salts, various cyclopian monstrosities were frequently produced. these have been called cases of _fusion_ of the optic vesicles. i would prefer to regard them as cases of a division suppressed or restricted by the control of the environment. conversely, the splendid discovery of loeb, that an unfertilised egg will divide and develop parthenogenetically without fertilisation, as a consequence of exposure to various media, may be interpreted as suggesting that the action of those media releases the strains already present in the ovum, though i admit that an interpretation based on the converse hypothesis, that the medium acts as a stimulus, is as yet by no means excluded. in these cases we come nearest to the direct causation or the direct inhibition of a division, but the meaning of the evidence is still ambiguous. i incline to compare loeb's parthenogenesis with the development (and of course accompanying cell-division) of dormant buds on stems which have been cut back. it is interesting to note that sometimes as an abnormality, the faculty of division gets out of hand and runs a course apparently uncontrolled. a remarkable instance of this condition is seen in _begonia_ "_phyllomaniaca_", which breaks out into buds at any point on the stem, petioles, or leaves, each bud having, like other buds, the power of becoming a new plant if removed. we would give much to know the genetic properties of _b. phyllomaniaca_, and in conjunction with mr. w. o. backhouse i have for some time been experimenting with this plant. it proved totally sterile. its own anthers produce no pollen, and all attempts to fertilise it with other species failed though the pollen of a great number of forms was tried. recently however we have succeeded in making plants which are in every respect _begonia phyllomaniaca_, so far as the characters of stems and leaves are concerned. these plants, of which we have sixteen, were made by fertilising _b. heracleifolia_ with _b. polyantha_. they are all beginning to break out in "phyllomania." as yet they have not flowered, but as they agree in all details with _phyllomaniaca_ there can be little doubt that the original plant bearing that name was a hybrid similarly produced. the production of "phyllomania" on a hybrid begonia has also been previously recorded by duchartre.[ ] in this case the cross was made between _b. incarnata_ and _lucida_. the synonymy of the last species is unfortunately obscure, and i have not succeeded in repeating the experiment. [illustration: fig. . piece of petiole of _begonia phyllomaniaca_. the proximal end is to the right of the figure.] from these facts it seems practically certain that the condition is one which is due to the meeting of complementary factors. at first sight we may incline to think that the phyllomania is in some way due to the sterility. this however cannot be seriously maintained; for not only is sterility in plants not usually associated with such manifestations, but we know a begonia called "wilhelma" which is exactly _phyllomaniaca_ and equally sterile, though it has no trace of phyllomania. this plant arose in the nurseries of mm. p. bruant of poitiers, and has generally been described as a seedling of _phyllomaniaca_, but from the total sterility of that form this account of its origin must be set aside. [illustration: fig. . two right hind feet of polydactyle cats. _ii_ shows the lowest development of the condition yet recorded. the digit, _d_^{ }, which stands as hallux is fully formed and has three phalanges. both it and the digit marked _d_^{ } are formed as _left_ digits. in the normal hind foot of the cat the hallux is represented by a rudiment only. _i_ shows a further development of the condition. in this foot there are _six_ digits. _d_^{ } has two phalanges, but both it and _d_^{ } and _d_^{ } are shaped as left digits. thus _d_^{ }, which in the normal foot would be shaped as a right digit, is transformed so as to look like a _left_ digit.] the phenomenon in this case can hardly be regarded as due to the excitation of dormant buds, for it is apparent on examination that the new growths are not placed in any fixed geometrical relation to the original plant. they arise on the petiole, for example, as small green outgrowths each of which gradually becomes a tiny leaf. the attitude of these leaves is quite indeterminate, and they may point in any direction, some having their apices turned peripherally, some centrally, and others in various oblique or transverse positions (fig. ). these little leaves are thus comparable with seedlings, in that their polarity is not related to, or consequent upon that of the parent plant. they have in fact that "individuality," which we associate with germinal reproduction. there are many curious phenomena seen in the behaviour of parts normally repeated in bilateral symmetry which may some day guide us towards an understanding of the mechanics of division. a part like a hand, which needs the other hand to complete its symmetry, cannot twin by mere division, yet by proliferation and special modifications on the radial side of the same limb, even a hand may be twinned. in the well known polydactyle cats a change of this kind is very common and indeed almost the rule. when extra digits appear at the inner (tibial) side of the limb, they are shaped as digits of the other side, and even the normal digit ii (index) is usually converted into the mirror-image of its normal self. the limb then develops a new symmetry in itself. nevertheless it is not easy to interpret these facts as meaning that there has been some interruption in the control which one side of the body exercises over the other. the heredity of polydactylism is complex but there is little doubt that the condition familiar in the cat is a dominant. in some human cases also the descent is that of a dominant, but irregularities are so frequent that no general rule can yet be perceived. the dominance of such a condition is an exception to the principle that the less-divided is usually dominant to the more-divided, a fact which probably should be interpreted as meaning that divisions are of more than one kind. among ordinary somatic divisions, whether of organs, cells, or patterns of differentiation, the control of symmetry is usually manifested. there is however one class of somatic differentiations which are exceptionally interesting from the fact that they may show a complete independence of such geometrical control. the most familiar examples of these geometrically uncontrolled variations are to be seen in bud-sports. the normal differentiation of the organs of a plant is arranged on a definite geometrical system, which to those who have never given special attention to such things before, will often seem surprisingly precise. the arrangement of the leaves on uninjured, free-growing shoots can generally be seen to follow a very definite order, just as do the flowers or the parts of the flowers. if however bud sports occur, then though the parts included in the sports show all the geometrical peculiarities proper to the sport-variety, yet the sporting-buds themselves are not related to each other according to any geometrical plan. a very familiar illustration is provided by the distribution of colour in those carnations that are not self-coloured. the pigment may, as in picotees, be distributed peripherally with great regularity to the edges of the petals; or, as in bizarres and flakes, it may be scattered in radial sectors which show no geometrical regularity. now in this case the pigments are the same in both types of flower, and the chemical factors concerned in their production must surely be the same. the difference must lie in the mechanical processes of distribution of the pigment. in the picotee we see the orderly differentiation which we associate with normality; in the bizarre we see the disorderly differentiation characteristic of bud-sports. the distribution of colour in this case lies outside the scheme of symmetry of the plant. such a distribution is characteristic of bud-sports, and of certain other differentiations in both plants and animals, which i cannot on this occasion discuss. now reflexion will show that these facts have an intimate bearing on the mechanical problems of heredity. for first in the bud-sports we are witnessing the distribution of factors which distinguish genetic varieties. we do not know the physical nature of those factors, but if we must give them a name, i suppose we should call them "ferments" exactly as boyle did in . he is discussing how it comes about that a bud, budded on a stock, becomes a branch bearing the fruit of its special kind. he notes that though the bud inserted be "not so big oftentimes as a pea," yet "whether by the help of some peculiar kind of strainer or by the operation of some powerful ferment lodged in it, or by both these, or some other cause," the sap is "so far changed as to constitute a fruit quite otherwise qualify'd."[ ] we can add nothing to his speculation, and we believe still that by a differential distribution of "ferments" the sports are produced. all the factors are together present in the normal parts; some are left out in the sport. in an analogous case however, that of a variegated _pelargonium_ which has green and also albino shoots, baur proved that the shoots pure in colour are also pure in their posterity. there can be no doubt that the sports of carnations, azaleas, chrysanthemums, etc., would behave in the same way. the well-known azaleas perle de ledeburg, president kerchove, and _vervaeana_ are familiar illustrations. perle de ledeburg is predominantly white, but it has red streaks in some of its flowers. it not very rarely gives off a self-red sport. this is evidently due to the development of a bud in a red-bearing area of the stem. the red in this plant is not under "geometrical control." many plants have white flowers with no markings, but if the red markings are geometrically ordered differentiations, no self-coloured sports are formed. the case of _vervaeana_ is a good illustration of this proposition. it has white flowers with red markings arranged in an orderly manner on the lower parts of the petals, especially on the dorsal petals. this is one of the azaleas most liable to have red sports, and at first sight it might seem that the sport represented the red of the central marks. examination however of a good many flowers shows that irregular red streaks like those of perle de ledeburg occur, about as commonly as in that variety. _vervaeana_ in fact is perle de ledeburg with _definite_ red markings added, and its red sports obviously are those branches the germs of which came in a patch of the stem bearing these red elements. that this is the true account is rendered quite obvious by the fact that the red of the sport is a colour somewhat different from that of the definite marks, and that these marks are still present on the red ground of the sporting flowers. it will be understood that these remarks apply to those cases in which the production of sports is habitual or frequent, and i imagine in all such examples it will be found that there are indications of irregularity in the distribution of the differentiations such as to justify the view that they are not under that geometrical control which governs the normal differentiation of the parts. the question next arises whether these considerations apply also to the production of a bud-sport as a rare exception, but by the nature of the case it is not possible to say positively whether the appearance of an exceptional sport is due to the unsuspected presence of a pre-existing fragment of material having a special constitution, or to the origin, _de novo_, of such a material. for instance one of the garden forms of _pelargonium_ known as _altum_ is liable perhaps once in some hundreds of flowers to have one or two magenta petals. the normal colour is a brilliant red; and as we may be fairly sure that this red is recessive to magenta the interpretation would be quite different according as the appearance of the magenta is regarded as due to the presence of small areas endowed with magentaness, or to the spontaneous generation of the factor for that pigment. either interpretation is possible on the facts, but the view that the whole plant has in it scarce mosaic particles of magenta seems on the whole more consistent with present knowledge. in _pelargonium altum_ the enzyme causing the magenta colours must be distributed in very small areas, but a case in which the magenta is similarly arranged in a much coarser patchwork may be seen in the _pelargonium_ "don juan," which often bears whole trusses or branches of red flowers upon plants having the normal dominant magenta trusses. in most cases there is little doubt that though the magenta flowered parts can "sport" to red, the red parts could not produce the magenta flowers. the asymmetrical, or to speak more precisely, the disorderly, mingling of the colours in the somatic parts is thus an indication of a similarly disorderly mixing of the factors for those colours in the germ-tissues, so that some of the gametes bear enough of the colour-factors to make a self-coloured plant, while others bear so little that the plant to which they give rise is a patchwork. if this view is correct we may extend it so far as to consider whether the fineness or coarseness of the mixture visible in the flowers or leaves may not give an indication of the degree to which the factors are subdivided among the germ-cells. we know very little about the genetic properties of striped varieties. in both _antirrhinum_ and _mirabilis_ it has been found that the striped may occasionally and irregularly throw self-coloured plants, and therefore the striping cannot be regarded simply as a recessive character. on the other hand in _primula sinensis_ there are well-known flaked varieties which ordinarily at least breed true. whether these ever throw selfs i do not know, but if they do it must be quite exceptionally. the power of these flaked plants to breed true is, i suspect, connected with the fact that in their flowers the coloured and white parts are _intimately_ mixed, this intimate mixture thus being an indication of a similarly intimate mixture in the germ-cells. it would be important to ascertain whether self-fertilised seed from the occasional flowers in which the colour has run together to join a large patch gives more self-coloured plants than the intimately flaked flowers do. the next fact may eventually prove of great importance. we have seen that in bud-sports the differentiation is of the same nature as that between pure types, and also that in the sporting plant this differentiation is distributed without any reference to the plant's axis, or any other consideration of symmetry. now among the germ-cells of a mendelian hybrid exactly such characters are being distributed allelomorphically, and there again we have strong evidence for believing that the distribution obeys no pattern. for example, we can in the case of seeds still _in situ_ perceive how the characters were distributed among the germ-cells, and there is certainly no obvious pattern connecting them, nor can we suppose that there is an actual pattern obscured. of this one illustration is especially curious. individual plants of the same species are, as regards the decussations of their leaves and in other respects, _either rights or lefts_. the fact is not emphasized in modern botany and is in some danger of being forgotten. when, as in the flowers of arum, some _gladioli_, _exacum_, _st. paulia_, or the fruits of _loasa_, rights and lefts occur on the same stem, they come off alternately. but if, as in the seedlings of barley the twist of the first leaf be examined, it will be seen to be either a right-or left-handed screw. an ear of barley, say a two-row barley, is a definitely symmetrical structure. the seeds stand in their envelopes back to back in definite positions. each has its organs placed in perfectly definite places. _if these seeds were buds_ their differentiations would be grouped into a common plan. one might expect that the differentiations of these embryos would still fall into the pattern; but they do not, and so far as i have tested them, any one may be a right or a left, just as each may carry any of the mendelian allelomorphs possessed by the parent plant, without reference to the differentiation of any other seed. the fertilisation may be responsible, but our experience of the allelomorphic characters suggest that the irregularity is in the egg-cells themselves.[ ] _germ cells thus differ from somatic cells in the fact that their differentiations are outside the geometrical order which governs the differentiation of the somatic cells._ i can think of possible exceptions, but i have confidence that the rule is true and i regard it as of great significance. the old riddle, what is an individual, finds at least a partial solution in the reply that an individual is a group of parts differentiated in a geometrically interdependent order. with the germ-cell a new geometrical order, with independent polarity is almost if not quite always, begun, and with this geometrical independence the power of rejuvenescence may possibly be associated. the problems thus raised are unsolved, but they do not look insoluble. the solution may be nearer than we have thought. in a study of the geometry of differentiation, germinal and somatic, there is a way of watching and perhaps analyzing what may be distinguished as the mechanical phenomena of heredity. if any one could in the cases of the picotee and the bizarre carnation, respectively, detect the real distinction between the two types of distribution, he would make a most notable advance. any one acquainted with mechanical devices can construct a model which will reproduce some of these distinctions more or less faithfully. the point i would not lose sight of is that the analogy with such models must for a long way be a true and valuable guide. i trust that some one with the right intellectual equipment will endeavor to follow this guide; and i am sanguine enough to think that a comprehensive study of the geometrical phenomena of differentiation will suggest to a penetrative mind that critical experiment which may one day reveal the meaning of spontaneous division, the mystery through which lies the road, perhaps the most hopeful, to a knowledge of the nature of life. footnotes: [ ] in saying this we make no assumption as to the particular cell-division at which differentiation occurs. this may be one of the maturation-divisions, or it may perhaps be much earlier. [ ] from the recent discoveries of erwin baur we are led to surmise that in the flowering plants the sub-epidermal layer, or some of its elements, may legitimately be regarded as a similar germ-substance, continuous in weismann's sense. [ ] these fraternal twins, which show no special resemblance to each other, are like the multiple births of other animals, and there is no disposition for them to be of the same sex. in the sheep, for example, statistics show that the frequency of pairs of twins, male and female, is approximately double that of the frequency of pairs, both male or both female, as it should be if the sex-distribution were fortuitous. for instance bernadin (_la bergerie de rambouillet_, , p. ) gives the following figures for twin-lambs in merinos: both male, ; both female, ; sexes mixed, . the -banded armadillo (_dasypus novemcinctus_), in which the young born in one litter are said to be always of one sex, is the only known exception in vertebrates, and is presumably a genuine case of normal polyembryony (see especially, rosner, _bull. ac. soc. cracovie_, , p. , and newman and patterson, _biol. bull._, xvii, , p. ), and an important paper lately published by h. h. newman and j. t. patterson, _jour. morph._, , xxii, p. . [ ] a good collection of evidence as to disease in homologous twins was lately published by e. a. cockayne, _brit. jour. child. diseases_, nov., . [ ] cp. windle, b. c. a., _jour. anal. phys._, xxvi, p. . [ ] mr. e. nettleship tells me that in the course of collecting pedigrees of families containing colour-blind members he has discovered two cases (shortly to be published) of pairs of twins, which on account of their very close resemblances must be deemed homologous, one of each pair being colour-blind and the other normal. such a distinction between closely similar twins is most curious and unexpected. [ ] another paradoxical phenomenon of the same nature occurs in the narwhal the males normally have the _left_ tusk alone developed, the corresponding right tusk remaining as an undeveloped rudiment in its socket. the left tusk is a left-handed screw. occasionally the right tusk is also developed and grows to the same length as that of the left side, but in such specimens the right tusk is also a left-hand screw like the tusk of the other side, instead of being reversed as we should certainly have expected. it need scarcely be remarked that in the case of the horns of antelopes, and in other examples of spiral organs arranged in pairs, that of one side of the body is the mirror image of that on the other side. the narwhal's tusks in being both twisted in the same direction are thus highly anomalous, and are comparable with pairs of twins. [ ] wilder, h. h., _amer. jour. anat._, , iii, p. . [ ] polydactylism which is often a dominant and the web-foot of pigeons which is recessive should be remembered as possible exceptions (see p. ). [ ] davenport inclined at first to regard rumplessness as a recessive, but in his latest publication on the subject he definitely concludes that it is an imperfect dominant. this conclusion accords well with evidence quoted by darwin (_an. and plts._, ii, ed. , p. ) that rumpless fowls may throw tailed offspring. (_amer. nat._, , xliv, p. .) [ ] spillman, w. j., _amer. breeders mag._, , i, p. . [ ] newsholme, _lancet_, december , , p. . [ ] _materials for the study of variation_, , p. . [ ] walker, g., _johns hopkins hospital bulletin_, xii, , p. . [ ] cp. r. h. compton, _new phytologist_, , p. . [ ] _arch. f. entwickelungsmech._, , xxiii, p. . [ ] bull. soc. bot. de france, xxxiv, , p. . [ ] r. boyle, _the origine of formes and qualities_, oxford, . [ ] remarkable experiments on this question have lately been carried out by r. h. compton (_camb. phil. soc._, xv, , p. ), showing that in a certain barley, "plumage corn," the average ratio of left to right is about . . a fuller paper has since been published by compton, _jour. genetics_, , ii, i, p. . chapter iii segmentation, organic and mechanical models may be and often have been devised imitating some of the phenomena of division, but none of them have reproduced the peculiarity which characterises divisions of living tissues, that _the position of chemical differentiation_ is _determined by those divisions_. for example, models of segmentation, whether radial or linear, may be made by the vibration of plates as in the familiar chladni figures of the physical laboratory, or by the bowing of a tube dusted on the inside with lycopodium powder, and in various other ways. the sand or the powder will be heaped up in the nodes or regions of least movement, and the patterns thus formed reproduce many of the geometrical features of segmentation. but in the segmentations of living things the nodes and internodes, once determined by the dividing forces, would each become the seat of appropriate and distinct chemical processes leading to the differentiation of the parts, and the deposition of the bones, petals, spines, hairs, and other organs in relation to the meristic ground-plan. the "ripples" of meristic division not merely divide but differentiate, and when a "ripple" forks the result is not merely a division but a reduplication of the organ through which the fork runs. an example illustrating such a consequence is that of the half-vertebrae of the python. on the left side the vertebra is single (fig. ) and bears a single rib, but on the right side a division has occurred with the result that two half-vertebrae, each bearing a rib, are formed, one standing in succession to the other. we cannot, indeed, imagine any operation of physiological division carried out in such an organ as a vertebra, passing through a plane at right angles to the long axis of the body, which does not necessarily involve the further process of reduplication. as the meristic system of distribution spreads through the body, chemical differentiations follow in its track, with segmentation and pattern as the visible result. could we analyse these simultaneous phenomena and show how it is that the places of chemical differentiation are determined by the system of division, progress would then be rapid. it is here that all speculation fails. [illustration: figs. and . two examples of imperfect division in the vertebræ of a python. _i_, the vertebræ - from the right side, showing imperfect division between the th and th. the condition on the left side of this vertebra was the same. _ii_, the dorsal surface of vertebræ - . on the right side the th is double and bears two ribs, but on the left side it is normal and has one rib only.] many attempts have been made to interpret the processes of division and repetition, in terms of mechanics, or at least to refer them to their nearest mechanical analogies, so far with little success. the problem is beset with difficulties as yet insurmountable and of these one must be especially noticed. in the living thing the process by which repetition and patterns come into being consists partly in division but partly also in growth. we have no means of studying the phenomena of pattern-formation except in association with that of growth. growth soon ceases unless division takes place, and if growth is impossible division soon ceases also. in consequence of this fact that the final pattern is partly a product of growth, it can never be used as unimpeachable evidence of the primary geometrical relations of the members as laid down in the divisions. in the last chapter in referring to the problem of repetition i introduced an analogy, comparing the patterns of the organic world with those produced in unorganised materials by wave-motion. in the preliminary stage of ignorance, having no more trustworthy clue, i do not think it wholly unprofitable to consider the applicability of this analogy somewhat more fully. it possesses, as i hope to show, at least so much validity as to encourage the belief that morphology may safely discard one source of long-standing error and confusion. those who have studied the structure of parts repeated in series will have encountered the old morphological problem of "serial homology," which has absorbed so much of the attention of naturalists and especially of zoologists at various periods. this problem includes two separate questions. the first of these is the origin in evolution of the resemblance between two organs occurring in a repeated series, of which the fore and hind limbs of vertebrates are the prerogative instance. from the fact that these resemblances can be traced very far, often into minute details of structure, many anatomists have inclined to the opinion that the resemblance must originally have been still more complete, and that the two limbs, for instance, must have acquired their present forms by the differentiation of two identical groups of parts. similar questions arise whenever parts are repeated in series, whether the series be linear or radial, and, though less obviously, even when the repetition is bilateral only. in each such example the question arises, is the resemblance between the parts the remains of a still closer resemblance, or is differentiation original? sometimes the view that these parts have arisen by the differentiation of a series of identical parts is plausible enough, as for example when the peculiarities of various appendages of a decapod crustacean are referred to modifications of the phyllopod series. in application to other cases however we soon meet with difficulty, and the suggestion that the segments of a vertebrate were originally all alike is seen at once to be absurd, for the reason that a creature so constituted could not exist, and that, differentiation of at least one anterior and one posterior segment, is an essential condition of a viable organism consisting of parts repeated in a linear series. between these two terminal segments it is possible to imagine the addition of one segment, or of a series of approximately similar segments; but when once it is realised that the terminals must have been differentiated from the beginning, it will be seen that the problem of the origin of the resemblance between segments is not rendered more comprehensible by the suggestion that even the intervening members were originally alike. seeing indeed that some differentiation must have existed primordially it is as easy to imagine that the original body was composed of a series grading from the condition of the anterior segment to that of the posterior, as any other arrangement. the existence of a linear or successive series in fact postulates a polarity of the whole, and in such a system the conception of an ideal segment containing all the parts represented in the others has manifestly no place. the introduction of that conception though sanctioned by the great masters of comparative anatomy, has, as i think, really delayed the progress of a rational study of the phenomena of division. the same notion has been applied to every class of repetition both in animals and plants, generally with the same unhappy results. in the cruder forms in which this doctrine was taught thirty years ago it is now seldom expressed, but modified presentations of it still survive and confuse our judgments. the process of repetition of parts in the bodies of organisms is however a periodic phenomenon. this much, provided we remain free from prejudice as to the nature and causation of the period or rhythm, we may safely declare, and a comparison may thus be instituted between the consequences of meristic repetition in the bodies of living things and those repetitions which in the inorganic world are due to rhythmical processes. of such processes there is a practically unlimited diversity and we have nothing to indicate with which of them our repetitions should rather be compared. [illustration: fig. . osmotic growths simulating segmentation. (after leduc.)] in some respects perhaps the best models of living organisms yet made are the "osmotic growths" produced by leduc.[ ] these curious structures were formed by placing a fragment of a salt, for instance calcium chloride, in a solution of some colloidal substance. as the solid takes up water from the solution a permeable pellicle or membrane is formed around it. the vesicle thus enclosed grows by further absorption of water, often extending in a linear direction, and in many examples this growth occurs by a series of rhythmically interrupted extensions. some of the growths thus formed are remarkably like organic structures, and might pass for a series of antennary segments or many other organs consisting of a linear series of repeated parts. in admitting the essential resemblance between these "osmotic growths" and living bodies or their organs i lay less stress on the general conformation of the growths, which often as leduc points out, recall the forms of fungi or hydroids, but rather on the fact that the interruptions in the development of these systems are so closely analogous to the segmentations or repetitions of parts characteristic of living things (fig. ). in the same way i am less impressed by leduc's models of karyokinesis, wonderful as they nevertheless are, for the division is here imitated by putting separate drops on the gelatine film. what we most want to know is how in the living creature one drop becomes two. the models of linear segmentation have the remarkable merit that they do in some measure imitate the process of actual division or repetition. so in a somewhat modified method leduc, by causing the diffusion of a solution in a gelatine film, produced rhythmical or periodic precipitations strikingly reminiscent of various organic tissues, for here also the process of periodic repetition is imitated with success. it is a feature common to these and to all other rhythmical repetitions produced by purely mechanical forces that there is resemblance between the members of the series, and that this similarity of conformation may be maintained in most complex detail. when however in the mechanical series some of the members differ from the rest we have no difficulty in recognising that these differences--which correspond with the differentiations of the organic series--are due to special heterogeneity in the conditions or in the materials, and it never occurs to us to suppose that all the members must have been primordially alike. for example, in the case of ripple-marks on the sand, which i choose as one of the most familiar and obvious illustrations of a repeated series due to mechanical agencies, if we notice one ripple different in form from those adjacent to it, we do not suppose that this variation must have been brought about by deformation of a ripple which was at first formed like the others, but we ascribe it to a difference in the sand at that point, or to a difference in the way in which the wind or the tide dealt with it. we may press the analogy further by observing that in as much as such a series of waves has a beginning and an end, it possesses polarity like that of the various linear series of parts in organisms, and even the formation of each member must influence the shape of its successor. since in an organism the beginning and end of the series are always included, some differentiation among the repetitions must be inevitable. if therefore it be conceded, as i think it must, that segmentation and pattern are the consequence of a periodic process we realize that it is at least as easy to imagine the formation of such a series of parts having family likeness combined with differentiation as it would be to conceive of their arising primordially as a series of identical repetitions. the suggestion that the likenesses which we now perceive are the remains of a still more complete resemblance is a substitution of a more complex conception for a simpler one. the other question raised by the problem of serial homology is how far there is a correspondence between individual members of series when the series differ from each other either in the number of parts, or in the mode of distribution of differentiation among them. students, for example, of vertebrate morphology debate whether the _n_th vertebra which carries the pelvic girdle in lizard a is individually homologous with the _n_ + _x_th vertebra which fulfils this function in lizard b, or whether it is not more truly homologous with the vertebra standing in the _n_th ordinal position, though that vertebra in lizard b is free. in various and more complex aspects the same question is debated in regard to the cranial and spinal nerves, the branches of the aorta, the appendages of arthropoda, and indeed in regard to all such series of differentiated parts in linear or successive repetition. persons exercised with these problems should before making up their minds consider how similar questions would be answered in the case of any series of rhythmical repetitions formed by mechanical agencies. in the case of our illustration of the ripples in the sand, given the same forces acting on the same materials in the same area, the number of ripples produced will be the same, and the _n_th ripple counting from the end of the series will stand in the same place whenever the series is evoked. if any of the conditions be changed, the number and shapes can be changed too, and a fresh "distribution of differentiation" created. stated in this form it is evident that the considerations which would guide the judgment in the case of the sand ripples are not essentially different from those which govern the problem of individual homology in its application to vertebrae, nerves, or digits. the fact that the unit of repetition is also the unit of growth is the source of the obscurity which veils the process. when we compare the skeleton of a long-tailed monkey with that of a short-tailed or tailless ape we see at once how readily the additional series of caudal segments may be described as a consequence of the propagation of the "waves" of segmentation beyond the point where they die out in the shorter column, and we see that with an extension of the series of repetitions there is growth and extension of material. the considerations which apply to this example will be found operating in many cases of the variation of terminal members of linear series. some of these series, like the teeth of the dog, end in a terminal member of a size greatly reduced below that of the next to it. even when there is thus a definite specialisation of the last member of the series it not infrequently happens that the addition, by variation, of a member beyond the normal terminal, is accompanied by a very palpable increase in size of the member which stands numerically in the place of the normal terminal.[ ] so also with variation in the number of ribs, when a lumbar vertebra varies homoeotically into the likeness of the last dorsal and bears a rib, the rib placed next in front of this, which in the normal trunk is the last, shows a definite increase in development. the consequences of such homoeoses are sometimes very extensive, involving readjustments of differentiation affecting a long series of members, as may easily be seen by comparing the vertebral columns of several individual sloths[ ] (whether _bradypus_ or _choloepus_) to take a specially striking example. it may be urged that no feature as yet enables us to perceive wherein lies the primary distinction which determines such variation, whether it is due to a difference in the dividing forces or in the material to be divided. if for instance we were to imitate such a series of segments by pressing hanging drops of a viscous fluid out of a paint-tube by successive squeezes, the number of times the tube is contracted before it is empty will give the number of the segments, but their size may depend either on the force of the contractions or on the capacity of the tube, or on various other factors. nevertheless in the case of the variation of terminal members, whatever be the nature of the rhythmical impulse which produces the series of organs, the elevation of the normally terminal member in correspondence with the addition of another is what we should expect. if the organism acquired its full size first and the delimitation of the parts took place afterwards, there might be some hope that the resemblance between living patterns and those mechanically caused by wave-motion might be shown to be a consequence of some real similarity of causation, but in view of the part played by growth, appeal to these mechanical phenomena cannot be declared to have more than illustrative value. similarly in as much as living patterns appear, and almost certainly do in reality come into existence by a rhythmical process, comparisons of these patterns with those developed in crystalline structures, and in the various fields of force are, as it seems to me, inadmissible, or at least inappropriate. however their intermittence be determined, the rhythms of division must be looked upon as the immediate source of those geometrically ordered repetitions universally characteristic of organic life. in the same category we may thus group the segmentation of the vertebrates and of the arthropods, the concentric growth of the lamellibranch shells or of fishes' scales, the ripples on the horns of a goat, or the skeletons of the foraminifera or of the heliozoa. in the case of plant-structures church[ ] has admirably shown, with an abundance of detail, how on analysis the definiteness of phyllotaxis is an expression of such rhythm in the division of the apical tissues, and how the spirals and "orthostichies" displayed in the grown plant are its ultimate consequences. the problem thus narrows itself down to the question of the mode whereby these rhythms are determined. it is natural that we should incline to refer them to a chemical source. if we think of the illustration just given, of the segmentation of a viscous fluid into drops by successive contractions of a soft-walled tube we can, i think, conceive of such rhythmic contractions as due to summations of chemical stimuli, somewhat as are the beats of the heart. but when we recognize the vast diversity of materials the distribution of which is determined by an ostensibly similar rhythmic process it seems hopeless to look forward to a directly chemical solution. that the chemical degradation of protoplasm or of materials which it contains is the source of the energy used in the divisions cannot be in dispute, but that these divisions can be themselves the manifestations of chemical action seems in the highest degree improbable. we may therefore insist with some confidence on the distinction between the meristic and the substantive constitution of organisms, between, that is to say, the system according to which the materials are divided and the essential composition of the materials, conscious of the fact that the energy of division is supplied from the materials, and that in the ontogeny the manner in which the divisions are effected must depend secondarily on the nature of the substances to be divided. the mechanical processes of division remain a distinguishable group of phenomena, and variations in the substances to be distributed in division may be independent of variations in the system by which the distribution is effected. modern genetic analysis supplies many remarkable examples of this distinction. when formerly we compared the leaves of a normal palmatifid chinese primula with the pinnatifid leaves[ ] of its fern-leaved variety we were quite unable to say whether the difference between the two types of leaf was due to a difference in the material cut up in the process of division or to a difference in that process itself. knowledge that the distinction is determined by a single segregable factor tends to prove that the critical difference is one of substance. so also in the silky fowl we know that the condition of its feathers is due to the absence of some one factor present in the normal form. we may conceive such differences as due to change of form in the successive "waves" of division, but we cannot yet imagine segregation otherwise than as acting by the removal or retention of a material element. future observation by some novel method may suggest some other possibility, but such cases bring before us very clearly the difficulties by which the problem is beset. [illustration: fig. . the palm-and fern type of leaf in _primula sinensis_. the palm is dominant and the fern is recessive.] in another region of observation phenomena occur which as it seems to me put it beyond question that the meristic forces are essentially independent of the materials upon which they act, save, in the remoter sense, in so far as these materials are the sources of energy. the physiology of those regenerations and repetitions which follow upon mutilation supplies a group of facts which both stimulate and limit speculation. no satisfactory interpretations of these extraordinary occurrences has ever been found, but we already know enough to feel sure that in them we are witnessing indications which should lead to the discovery of the true mechanics of repetition and pattern. the consequences of mutilation in causing new growth or perhaps more strictly in enabling new growth to take place, are such that they cannot be interpreted as responses to chemical stimuli in any sense which the word chemical at present connotes. powers are released by mutilation of which in the normal conditions of life no sign can be detected. all who have tried to analyse the phenomena of regeneration are compelled to have recourse to the metaphor of equilibrium, speaking of the normal body as in a state of strain or tension (morgan) which when disturbed by mutilation results in new division and growth. the forces of division are inacessible to ordinary means of stimulation. applications, for example, of heat or of electricity excite no responses of a positive kind unless the stimuli are so violent as to bring about actual destruction.[ ] these agents do not, to use a loose expression, come into touch with the meristic forces. changes in the chemical environment of cells may, as in the experiments of loeb and of stockard produce definite effects, but the facts suggest that these effects are due rather to alterations in the living material than to influence exerted directly on the forces of division themselves. by destruction of tissue however the forces both of growth and of division also may often be called into action with a resulting regeneration. interruption of the solid connexion between the parts may produce the same effects, as for example when the new heads or tails grow on the divided edges of planarians (morgan), or when from each half embryo partially separated from its normally corresponding half, a new half is formed with a twin monster as the result. often classed with regenerations but in reality quite distinct from them are those special and most interesting examples where the growth of a _paired_ structure is excited by a simple wound. some of the best known of these instances are presented by the paired extra appendages of insects and crustacea. some years ago i made an examination of all the examples of such monstrosities to which access was to be obtained, and it was with no ordinary feeling of excitement that i found that these supernumerary structures were commonly disposed on a recognizable geometrical plan, having definite spatial relations both to each other and to the normal limb from which they grew. the more recent researches of tornier[ ] and especially his experiments on the frog have shown that a cut into the posterior limb-bud induces the outgrowth of such a _pair_ of limbs at the wounded place. few observations can compare with this in novelty or significance; and though we cannot yet interpret these phenomena or place them in their proper relations with normal occurrences, we feel convinced that here is an observation which is no mere isolated curiosity but a discovery destined to throw a new light on biological mechanics. the supernumerary legs of the frog are evidently grouped in a system of symmetry similar to that which those of the arthropods exhibit, and though in arthropods paired repetitions have not been actually produced by injury under experimental conditions we need now have no hesitation in referring them to these causes as przibram has done. at this point some of the special features of the supernumerary appendages become important. first they may arise at any point on the normal limb, being found in all situations from the base to the apex. nor are they limited as to the surface from which they spring, arising sometimes from the dorsal, anterior, ventral, or posterior surfaces, or at points intermediate between these principal surfaces. with rare and dubious exceptions, the parts which are contained in these extra appendages are only those which lie _peripheral to their point of origin_. thus when the point of origin is in the apical joint of the tarsus, the extra growth if completely developed consists of a double tarsal apex bearing two pairs of claws. if they arise from the tibia, two complete tarsi are added. if they spring from the actual base of the appendage then two complete appendages may be developed in addition to the normal one. we must therefore conclude that in any point on a normal appendage the power exists which, if released, may produce a bud containing in it a paired set of the parts peripheral to this point. [illustration: fig. . diagrams of the geometrical relations which are generally exhibited by extra pairs of appendages in arthropoda. the sections are supposed to be those of the apex of a tibia in a beetle. _a_, anterior, _p_, posterior, _d_, dorsal, _v_, ventral. _m_^{ }, _m_^{ } are the imaginary planes of reflexion. the shaded figure is in each case a limb formed like that of the other side of the body, and the outer unshaded figures are shaped like the normal for the side on which the appendages are. on the several radii are shown the extra pairs in their several possible relations to the normal from which they arise. the normal is drawn in thick lines in the center.] next the geometrical relations of the halves of the supernumerary pair are determined by the position in which they stand in regard to the original appendage. these relations are best explained by the diagram (fig. ), from which it will be seen that the two supernumerary appendages stand as images of each other; and, of them, that which is adjacent to the normal appendage forms an image of it. thus if the supernumerary pair arise from a point on the dorsal surface of the normal appendage, the two _ventral_ surfaces of the extra pair will face each other. if they arise on the anterior surface of the normal appendage, their morphologically posterior surfaces will be adjacent, and so on. these facts give us a view of the relations of the two halves of a dividing bud very different from that which is to be derived from the exclusive study of normal structures. ordinary morphological conceptions no longer apply. the distribution of the parts shows that the bud or rudiment which becomes the supernumerary pair may break or open out in various ways according to its relations to the normal limb. its planes of division are decided by its geometrical relations to the normal body. especially curious are some of the cases in which the extra pair are imperfectly formed. the appearance produced is then that of two limbs in various stages of coalescence, though in reality of course they are stages of imperfect separation. the plane of "coalescence" may fall anywhere, and the two appendages may thus be compounded with each other much as an object partially immersed in mercury "compounds" with its optical image reflected from the surface. supernumerary paired structures are not usually, if ever, formed when an appendage is simply amputated. cases occasionally are seen which nevertheless seem to be of this nature. borradaile,[ ] for example, described a crab (_cancer pagurus_) having in place of the right chela three _small_ chelae arising from a common base, where the appearances suggested that the three reduced limbs replaced a single normal limb. from the details reported however it seems still possible that one of the chelae (that lettered f. i in borradaile's figure) may be the normal one, and the other two an extra pair. the chela which i suspect to be the normal is in several respects deformed as well as being reduced in size, and this deformity may perhaps have ensued as a consequence of the same wound which excited the growth of the extra pair. its reduced size may be due to the same injury, which may quite well have checked its growth to full proportions. admitting doubt in these ambiguous cases it seems to be a general rule that for the production of the extra pair the normal limb should persist in connexion with the body. moreover it is practically certain that in no case can a _single_, viz. an unpaired, duplicate of the normal appendage grow from it. many examples have been described as of this nature, but all of them may be with confidence regarded as instances of a supernumerary pair in which only the two morphologically anterior or the two morphologically posterior surfaces are developed. we have thus the paradox that a limb of one side of the body, say the right, has in it the power to form a pair of limbs, right and left, as an outgrowth of itself, but cannot form a second left limb alone. a very interesting question arises whether it is strictly correct to describe the extra pair as a right and a left, or whether they are not rather two lefts or two rights of which one is reversed. this question did not occur to me when in former years i studied these subjects. it was suggested to me by dr. przibram. the answer might have an important bearing on biological mechanics, but i know no evidence from which the point can be determined with certainty. in order to decide this question it would be necessary to have cases in which the paired repetition affected a limb markedly differentiated on the two sides of the body, and of course the development of the extra parts in order to be decisive must be fairly complete. one example only is known to me which at all satisfies these requirements, that of the lobster's chela figured (after van beneden) in _materials for the study of variation_, p. , fig. , iii. here the drawing distinctly suggests that one of the extra dactylopodites, namely that lettered r, is differentiated as a left and not merely a reversed right. for the teeth on this dactylopodite are those of a cutting claw, not of a crushing claw, whereas the dactylopodites r' and l' bear crushing teeth. the figure makes it fairly certain also that the limb affected was a crushing claw. accepting this interpretation, we reach the remarkable conclusion that the bud of new growth consisted of halves differentiated into cutter and crusher as the normal claws are, and that the extra crusher is geometrically a left but physiologically a right. though shaped as a left in respect of the direction in which it points, the extra crusher is really an optically reversed right, while the dactylopodite r, which is placed pointing like a right, is really a reversed left (fig. ). [illustration: fig. . right claw of lobster bearing a pair of extra dactylopodites (after van beneden). the fine toothing on r suggests that this is part of a cutting claw, though the limb bearing it is a crusher.] if these indications are reliable[ ] and are established by further observation we shall be led to the conclusion that the bud which becomes an extra pair of limbs does not merely contain the parts proper to the side on which it grows, but is comparable with the original zygotic cell, and consists not simply of two halves, but of two halves differentiated as a right and a left like the two halves of the normal body. phenomena of this kind, evoked by mutilation or injury, together with the cognate observations on regeneration throw very curious lights on the nature of living things. to an understanding of the nature of the mechanics of living matter and its relation to matter at large they offer the most hopeful line of approach. i allude especially to the examples in which it has been established that the part which is produced after mutilation is a structure different from that which was removed. the term "regeneration" was introduced before such phenomena were discovered, and though every one recognizes its inapplicability to these remarkable cases, the word still misleads us by presenting a wrong picture to the mind. the expression "heteromorphosis" (loeb) has been appropriately applied to various phenomena of this kind, and morgan has given the name "morphallaxis" to another group of cases in which the renewal occurs by the transformation of a previously existing part.[ ] but we must continually remember that all these occurrences which we know only as abnormalities and curiosities must in reality be exemplifications of the normal mechanics of division and growth. the conditions needed to call them forth are abnormal, but the responses which the system makes are evidences of its normal constitution. when therefore, for example, the posterior end of a worm produces a reversed tail from its cut end we have a proof that there must be in the normal body forces ready to cause this outgrowth. the new structure is not an ill-shaped head-end, for, as morgan shows, the nephridial ducts have their funnels perforating the segments in a reversed direction. the "tension" of growth is actually reversed.[ ] so also when in a planarian amputation of the body immediately behind the head leads to the formation of a new reversed head at the back of the normal head, while amputation further back leads to the regeneration of a new tail, these responses give indications of forces normally present in the body of the planarian. such facts open up a great field of speculation and research. especially important it would be to determine where the critical region may be at which the one response is replaced by the other. i suppose it is even possible that there is some neutral zone in which neither kind of response is made. physical parallels to the phenomena of regeneration are not easy to find and we still cannot penetrate beyond the empirical facts. przibram has laid stress on the general resemblance between the new growth of an amputated part in an animal and the way in which a broken crystal repairs itself when placed in the mother-solution. that the two processes have interesting points of likeness cannot be denied. it must however never be forgotten that there is one feature strongly distinguishing the two; for i believe it is universally recognized by physicists that all the phenomena of geometrical regularity which crystals display are ultimately dependent on the forms of the particles of the crystalline body. this cannot in any sense be supposed to hold in regard to protoplasm or its constituents. the definiteness of crystals is also an unlikely guide for the reason that it is absolute and perfect, or in other words because this kind of regularity cannot be disturbed at all without a change so great that the substance itself is altered; whereas we know that the forms of living things are capable of such changes, great and small, that we must regard perfection of form, whether manifested in symmetry or in number, as an ideal which will only be produced in the absence of disturbance. the symmetry of the living things is like the symmetry of the concentric waves in a pool caused by a splash. perfect circles are made only in the imaginary case of mathematical uniformity, but the system maintains an approximate symmetry though liable to manifold deformation. since the geometrical order of the living body cannot be a direct function of the materials it must be referred to some more proximate control. in renewing a part the body must possess the power of seizing particles of many dissimilar kinds, and whirl them into their several and proper places. the action in renewal, like that of original growth, may be compared--very crudely--with the action of a separator which simultaneously distributes a variety of heterogeneous materials in an orderly fashion; but in the living body the thing distributed must rather be the _appetency_ for special materials, not the materials themselves. if the analogy of crystals be set aside and we seek for other parallels to regeneration there are none very obvious. i have sometimes wondered whether it might not be possible to institute a fruitful comparison between the renewal of parts and the reformation of waves of certain classes after obliteration. in several respects, as i have already said, some curious resemblances with the repetitions formed by wave-motion are to be traced in our organic phenomena, and though admitting that i cannot develop these comparisons, i think nevertheless they may be worth bearing in mind. when, after obliteration, an eddy in a stream, or a ripple-mark (a more complex case of eddy-formation) in blown sand is re-formed, we have an example in which pattern is reconstituted and growth takes place not by virtue of the composition of the materials--in this case the water or the sand--but by the way in which they are acted upon by extraneous forces. a feature in the actual mode by which ripple-marks are reconstituted may not be without interest in connexion with our phenomena of regeneration. when, for example, the wind is blowing steadily over a surface of fine, dry sand, the familiar ripple-marks are formed by a heaping of the sand in lines transverse to the direction of the wind. the heaping is due to the formation of eddies corresponding with positions of instability. when the wind is steady and the sand homogeneous, the distances between the ripples, or wave-lengths, are sensibly equal. if while the wind continues to blow, the ripples are obliterated with a soft brush they will quickly be re-formed over the whole area, but i have noticed that at first their wave-length is approximately half that of the ripples in the undisturbed parts of the system.[ ] the normal wave-length is restored by the gradual accentuation of alternate ripples. of course the sand-ripples are in reality slowly travelling forward in the direction towards which the wind is blowing, and for this our living segmentations afford no obvious parallel, but the appearances in the area of reformation, and especially the forking of the old ridges where they join the new ones, are curiously reminiscent of the irregularities of segmentation seen in regenerated structures. the value of the considerations adduced in the chapter is, i admit, very small. the utmost that can be claimed for them is that mechanical segmentations, like those seen in ripple-mark, or in leduc's osmotic growths, show how by the action of a continuous force in one direction, repeated and serially homologous divisions can be produced having features of similarity common to those repetitions by which organic forms and patterns are characterised. the analogy supplies a vicarious picture of the phenomena which in default of one more true may in a slight degree assist our thoughts. it suggests that the rhythms of segmentation may be the consequence of a single force definite in direction and continuously acting during the time of growth. the polarity of the organism would thus be the expression of the fact that this meristic force is definitely directed after it has once been excited, and the reversal seen in some products of regeneration suggest further that it is capable of being reflected. this polarity cannot be a property of the material, as such, but is determined by a force acting on that material, just as the polarity of a magnet is not determined by the arrangement of its particles, but by the direction in which the current flows. to some it may appear that even to embark on such discussions as this is to enter into a perilous flirtation with vitalistic theories. how, they may ask, can any force competent to produce chemical and geometrical differentiation in the body be distinguished from the "entelechy" of driesch? let me admit that in this reflexion there is one element of truth. if those who proclaim a vitalistic faith intend thereby to affirm that in the processes by which growth and division are effected in the body, a part is played by an orderly force which we cannot _now_ translate into terms of any known mechanics, what observant man is not a vitalist? driesch's first volume, putting as it does into intelligible language that positive deduction from the facts--especially of regeneration--should carry a vivid realisation of this truth to any mind. if after their existence is realised, it is desired that these unknown forces of order should have a name, and the word entelechy is proposed, the only objection i have to make is that the adoption of a term from aristotelian philosophy carries a plain hint that we propose to relegate the future study of the problem to metaphysic. from this implication the vitalist does not shrink. but i cannot find in the facts yet known to us any justification of so hopeless a course. it was but yesterday that the study of _entwicklungsmechanik_ was begun, and if in our slight survey we have not yet seen how the living machine is to be expressed in terms of natural knowledge that is poor cause for despair. driesch sums up his argument thus:[ ] "it seems to me that there is only one conclusion possible. if we are going to explain what happens in our harmonious-equipotential systems by the aid of causality based upon the constellation of single chemical factors and events, there _must_ be some such thing as a machine. now the assumption of the existence of a machine proves to be absolutely absurd in the light of the experimental facts. _therefore there can be neither any sort of a machine nor any sort of causality based upon constellation underlying the differentiation of harmonious-equipotential systems._" "for a machine, typical with regard to the three chief dimensions of space, cannot remain itself if you remove parts of it or if you rearrange its parts at will." to the last clause a note is added as follows: "the pressure experiments and the dislocation experiments come into account here; for the sake of simplicity they have not been alluded to in the main line of our argument." i doubt whether any man has sufficient knowledge of all possible machines to give reality to this statement. in spite also of the astonishing results of experiments in dislocation, doubt may further be expressed as to whether they have been tried in such variety or on such a scale as to justify the suggestion that the living organism remains itself if its parts are rearranged at will. all we know is that it can "remain itself" when much is removed, and when much rearrangement has been affected, which is a different thing altogether. i scarcely like to venture into a region of which my ignorance is so profound, but remembering the powers of eddies to re-form after partial obliteration or disturbance, i almost wonder whether they are not essentially machines which remain themselves when parts of them are removed. real progress in this most obscure province is not likely to be made till it attracts the attention of physicists; and though they for long may have to forego the application of exact quantitative methods, i confidently anticipate that careful comparison between the phenomena of repetition formed in living organisms and the various kinds of segmentation produced by mechanical agencies would be productive of illuminating discoveries. footnotes: [ ] stéphane leduc, _théorie physico-chymique de la vie_, paris, . [ ] _materials for the study of variation_, no. , p. ; and p. . [ ] _materials_, p. . [ ] church, a. h., _on the relation of phyllotaxis to mechanical laws_, london, . [ ] it is a question whether the dominance of the palmatifid leaf over the pinnatifid is not really an example of the dominance of a lower number of segmentations over a higher. from the uncertainty whether two given leaves of two separate plants are actually comparable one cannot institute quite satisfactory numerical comparisons, but i think the view that the "fern" leaf has more lobes than an otherwise similar "palm" leaf may be fairly maintained. if this be admitted, the "palm" leaf represents the dominant low number and its round shape is a consequence of the greater powers of growth which are so often possessed by the members of a shorter series. [ ] it is perhaps of importance to remember that in certain species of bacteria (e. g. _bacillus anthracis_) division may cease where the organism is cultivated under certain artificial conditions though growth continues. in this way very long unsegmented threads are produced. [ ] _arch. f. entwm._, xx, , p. ; _sitzungsb. d. ges. naturf._, berlin, , p. , etc. [ ] borradaile, l. a., _jour. marine zool._, , no. . [ ] dr. przibram, i should mention, concludes that on the whole the facts are against this interpretation, but as more evidence is certainly required, i call attention to the possibility. [ ] morgan, t. h., _regeneration_, . [ ] it would be interesting to know whether growth continues at the original posterior end after the new "posterior" end has been formed in front. [ ] in the actual case observed, the ripples unsmoothed had a wave-length of about - / inches; and when the new ones were first formed, there were about ridges in the length originally traversed by or . [ ] _the science and philosophy of the organism_; gifford lectures, . london, , p. . chapter iv the classification of variation and the nature of substantive factors we have now seen that among the normal physiological processes the phenomena of division form a recognisable, and in all likelihood a naturally distinct group. variations in these respects may thus be regarded as constituting a special class among variations in general. the substantive variations have only one property in common--the negative one that they are not meristic. the work of classifying them and distinguishing them according to their several types demands a knowledge of the chemistry of life far higher than that to which science has yet attained. in reference to some of the simplest variations garrod has introduced the appropriate term "chemical sports." the condition in man known as alkaptonuria in which the urine is red is due especially to the absence of the enzyme which decomposes the excretory substance, alkapton. the "chemical sport" here consists in the inability to break up the benzene ring. the chemical feature which distinguishes and is the proximate cause of several colour-varieties can now in a few cases be declared. the work of miss wheldale has shown that colour-varieties may be produced by the absence of the chromogen compound the oxidation of which gives rise to sap-colours, by differences in the completeness of this process of oxidation, and by a process of reduction supervening on or perhaps suppressing the oxidation. some of these processes moreover may be brought about by the combined action of two bodies, the one an enzyme, for example an oxygenase, and the other a substance regarded as a peroxide, contributing the oxygen necessary for the oxidation to take place. variation in colour may thus be brought about by the addition or omission of any one of the bodies concerned in the action. similar variations, or rather similar series of variations will undoubtedly hereafter be identified in reference to all the various kinds of chemical processes upon which the structure and functions of living things depend. the identification of these processes and of the bodies concerned in them will lead to a real classification of substantive variations. to forecast the lines on which such classification will proceed is to look too far ahead. we may nevertheless anticipate with some confidence that future analysis will recognise among the contributing elements, some which are intrinsic and inalienable, and others which are extrinsic and superadded. we already know that there may be such interdependence among the substantive characters that to disentangle them will be a work of extreme difficulty. the mere fact that in our estimation characters belong to distinct physiological systems is no proof of their actual independence. in illustration may be mentioned the sap-colour in stocks and the development of hoariness on the leaves and stems, which miss saunders's experiments have shown to be intimately connected, so that in certain varieties no hoariness is produced unless the elements for sap-colour are already present in the individual plant. the first step in the classification of substantive variations is therefore to determine which are due to the addition of new elements or factors, and which are produced by the omission of old ones. _a priori_ there is no valid criterion by which this can be known, and actual experiments in analytical breeding can alone provide the knowledge required. some very curious results have by this method been obtained, which throw an altogether unexpected light on these problems. for example, in order that the remarkable development of mesoblastic black pigment characteristic of the silky fowl should be developed, it is practically certain that two distinct variations from such a type as _gallus bankiva_ must have occurred. i assume, as is reasonable, that _g. bankiva_ has genetic properties similar to those of the brown leghorn breed which has been used in the experiments which mr. punnett and i have conducted. _gallus bankiva_ was not available but the brown leghorn agrees with it very closely in colouration, and probably in the general physiology of its pigmentation. setting aside the various structural differences between the two breeds, the silky is immediately distinguished from the leghorn by the fact that the skin of the whole body including that of the face and comb appears to be of a deep purplish colour. the face and comb of the leghorn are red and the skin of the body is whitish yellow. on examination it is found that the purple colour of the silky is in reality due to the distribution of a deep black pigment in the mesoblastic membranes throughout the body. the somatopleura, the pleura, _pia mater_, the dermis, and in most organs the connective tissue and the sheaths of the blood-vessels, are thus impregnated with black. no such pigmentation exists in the leghorn. as the result of an elaborate series of experimental matings we have proved that the distinction between the leghorn and the silky consists primarily in the fact that the silky possesses a pigment-producing factor, _p_, which is not present in the leghorn. this variation must undoubtedly have been one of _addition_. but besides this there is another difference of an altogether dissimilar nature; for the brown leghorn possesses a factor which has the power of partially or completely restricting the operation of the pigment-producing factor, _p_. moreover in respect of this pigment-restricting factor which we may call _d_, the sexes of the brown leghorn differ, for the male is homozygous or _dd_, but the female is heterozygous, _dd_. thus in order that the black-skinned breed could be evolved from such a type as a brown leghorn it must be necessary _both_ that _p_ should be added and that _d_ should drop out. we have not the faintest conception of the process by which either of these events have come to pass, but there is no reasonable doubt that in the evolution of the silky fowl they did actually happen. we may anticipate that numerous interdependences of this kind will be discovered. before any indisputable progress can be made with the problem of evolution it is necessary that we should acquire some real knowledge of the genesis of that class of phenomena which formed the subject of the last chapter. so long as the process of division remains entirely mysterious we can form no conception even of the haziest sort as to the nature of living organisms, or of the proximate causes which determine their forms, still less can we attempt any answer to those remoter questions of origin and destiny which form the subject of the philosopher's contemplation. it is in no spirit of dogmatism that i have ventured to indicate the direction in which i look for a solution, though i have none to offer. it may well be that before any solution is attained, our knowledge of the nature of unorganised matter must first be increased. for a long time yet we may have to halt, but we none the less do well to prepare ourselves to utilise any means of advance that may be offered, by carefully reconnoitering the ground we have to traverse. the real difficulty which blocks our progress is ignorance of the nature of division, or to use the more general term, of repetition. let us turn to the more familiar problem of the causes of variation. now since variation consists as much in meristic change as in alteration in substance or material, there is one great range of problems of causation from which we are as yet entirely cut off. we know nothing of the causation of division, and we have scarcely an observation, experiment or surmise touching the causes by which the meristic processes may be altered. of the way in which variations in the substantive composition of organisms are caused we have almost as little real evidence, but we are beginning to know in what such variations must consist. these changes must occur either by the addition or loss of factors. we must not lose sight of the fact that though the factors operate by the production of enzymes, of bodies on which these enzymes can act, and of intermediary substances necessary to complete the enzyme-action, yet these bodies themselves can scarcely be themselves genetic factors, but consequences of their existence. what then are the factors themselves? whence do they come? how do they become integral parts of the organism? whence, for example, came the power which is present in a white leghorn of destroying--probably reducing--the pigment in its feathers? that power is now a definite possession of the breed, present in all its germ-cells, male and female, taking part in their symmetrical divisions, and passed on equally to all as much as is the protoplasm or any other attribute of the breed. from the body of the bird the critical and efficient substance could in all likelihood be isolated by suitable means, just as the glycogen of the liver can be. but even when this extraction has been accomplished and the reducing body isolated, we shall know no more than we did before respecting the mode by which the power to produce it was conferred on the fowl, any more than we know how the walls of its blood-vessels acquired the power to form a fibrin-ferment. it is when the scope of such considerations as this are fully grasped that we realise the fatuousness of the conventional treatment which the problem of the causes of variation commonly receives. environmental change, chemical injury, differences in food supply, in temperature, in moisture, or the like have been proposed as "causes." admitting as we must do, that changes may be produced--usually inhibitions of development--by subjecting living things to changes in these respects, how can we suppose it in the smallest degree likely that very precise, new, and adaptative powers can be conferred on the germs by such treatment? reports of positive genetic consequences observed comparable with those i have mentioned, become from time to time current. we should i think regard them with the gravest doubt. few, so far as i am aware, have ever been confirmed, though clear and repeated confirmation should be demanded before we suffer ourselves at all to build upon such evidence. in a subsequent chapter some of these cases will be considered in detail. in no class of cases would the transmission of an acquired character superficially appear so probable as in those where power of resisting the attack of a pathogenic organism is acquired in the lifetime of the zygote. the possession of such a power is moreover a distinction comparable with those which differentiate varieties and species. it is due to the development in the blood of specific substances which pervade the whole fluid. this development is exactly one of those "appropriate responses to stimuli" which naturalists who incline to regard adaptation as a direct consequence of an environmental influence might most readily invoke as an illustration of their views. and yet all evidence is definitely unfavourable to the suggestion of an inheritance of the acquired power of resistance. such change as can be perceived in the virulence of the attacks on successive generations may be most easily regarded as due to the extermination of the more susceptible strains, and perhaps in some measure to variation in the invading organisms themselves, an "acquired character" of quite different import. the specific "anti-body" may have been produced in response to the stimulus of disease, but the power to produce it without this special stimulus is not included in the germ-cells any more than a pigment. all that they bear is the _power to produce_ the anti-bodies when the stimulus is applied. if we could conceive of an organism like one of those to which disease may be due becoming actually incorporated with the system of its host, so as to form a constituent of its germ-cells and to take part in the symmetry of their divisions, we should have something analogous to the case of a species which acquires a new factor and emits a dominant variety. when we see the phenomenon in this light we realise the obscurity of the problem. the appearance of recessive varieties is comparatively easy to understand. all that is implied is the omission of a constituent. how precisely the omission is effected we cannot suggest, but it is not very difficult to suppose that by some mechanical fault of cell-division a power may be lost. such variation by unpacking, or analysis of a previously existing complex, though unaccountable, is not inconceivable. but whence come the new dominants? whether we imagine that they are created by some rearrangement or other change internal to the organism, or whether we try to conceive them as due to the assumption of something from without we are confronted by equally hopeless difficulty. the mystery of the origin of a dominant increases when it is realised that there is scarcely any recent and authentic account of such an event occurring under critical observation, which can be taken as a basis for discussion. the literature of horticulture for example abounds in cases alleged, but i do not think anyone can produce an illustration quite free from doubt. such evidence is usually open to the suspicion that the plant was either introduced by some accident, or that it arose from a cross with a pre-existing dominant, or that it owed its origin to the meeting of complementary factors. in medical literature almost alone however, there are numerous records of the spontaneous origin of various abnormal conditions in man which habitually behave as dominants, and of the authenticity of some of these there can be no doubt. when we know that such conditions as hereditary cataract or various deformities of the fingers behave as dominants, we recognize that those conditions must be due to the addition of some element to the constitution of the normal man. in the collections of pedigrees relating to such pathological dominants there are usually to be found alleged instances of the origin of the condition _de novo_. not only do these records occur with such frequency that they cannot be readily set aside as errors, but from general considerations it must be obvious that as these malformations are not common to normal humanity they must at some moment of time have been introduced. the lay reader may not be so much impressed with the difficulty as we are. he is accustomed to regard the origin of _any_ new character as equally mysterious, but when once dominants are distinguished from recessives the problem wears a new aspect. thus the appearance of high artistic gifts, whether as an attribute of a race or as a sporadic event among the children of parents destitute of such faculties, is not very surprising, for we feel fairly sure that the faculty is a recessive, due to the loss of a controlling or inhibiting factor; but the _de novo_ origin of brachydactylous fingers in a child of normal parents is of quite a different nature, and must indicate the action of some new specific cause. whether such evidence is applicable to the general problem of evolution may with some plausibility be questioned; but there is an obvious significance in the fact that it is among these pathological occurrences that we meet with phenomena most nearly resembling the spontaneous origin of dominant factors, and i cannot see such pedigrees as these without recalling virchow's aphorism that every variation owes its origin to some pathological accident. in the evolution of domestic poultry, if _gallus bankiva_ be indeed the parent form of all our breeds, at least some half dozen new factors must have been added during the process. in _bankiva_ there is, for example, no factor for rose comb, pea comb, barring on the feathers, or for the various dominant types of dark plumage. whence came all these? it is, i think, by no means impossible that some other wild species now extinct did take part in the constitution of domestic poultry. it seems indeed to me improbable that the heavy breeds descend from _bankiva_. both in regard to domestic races of fowls, pigeons, and some other forms, the belief in origin within the period of human civilization from one simple primitive wild type seems on a balance of probabilities insecurely founded, but allowing something for multiplicity of origin we still fall far short of the requisite total of factors. elements exist in our domesticated breeds which we may feel with confidence have come in since their captivity began. such elements in fowls are dominant whiteness, extra toe, feathered leg, frizzling, etc., so that even hypothetical extension of the range of origin is only a slight alleviation of the difficulty. somehow or other, therefore, we must recognize that dominant factors do arise. whether they are created by internal change, or whether, as seems to me not wholly beyond possibility, they obtain entrance from without, there is no evidence to show. if they were proved to enter from without, like pathogenic organisms, we should have to account for the extraordinary fact that they are distributed with fair constancy to half the gametes of the heterozygote. in proportion as the nature of dominants grows more clear so does it become increasingly difficult to make any plausible suggestion as to their possible derivation. on the other hand the origin of a recessive variety by the loss of a factor is a process so readily imagined that our wonder is rather that the phenomenon is not observed far more often. some slip in the accurate working of the mechanical process of division, and a factor gets left out, the loss being attested by the appearance of a recessive variety in some subsequent generation. consistently with this presentation of the facts we find that, as in our domesticated animals and plants, a diversity of recessives may appear within a moderately short period, and that when variations come they often do not come alone. witness the cultural history of the sweet pea, _primula sinensis_, _primula obconica_, _nemesia strumosa_ and many such examples in which variation when it did come was abundant. the fact cannot be too often emphasized that in the vast proportion of these examples of substantive variation under domestication, as well as of substantive variation in the natural state, the change has come about by omission, not by addition. to take, for example, the case of the potato, in which so many spontaneous bud-variations have been recorded, east after a careful study of the evidence has lately declared his belief that all are of this nature, and the opinion might be extended to many other groups of cases whether of bud or seminal variation. morgan draws the same conclusion in reference to the many varieties he has studied in _drosophila_. in the sweet pea, a form which is beyond suspicion of having been crossed with anything else, and has certainly produced all the multitude of types which we now possess by variations from one wild species, there is only one character of the modern types which could, with any plausibility, be referred to a factor not originally forming part of the constituents of the wild species. this is the waved edge, so characteristic of the "spencer" varieties; for the cross between a smooth-edged and a waved type gives an intermediate not unfrequently. nevertheless there is practically no doubt that this is merely an imperfection in the dominance of the smooth edge, and we may feel sure that any plant homozygous for smooth edge would show no wave at all. hence it is quite possible that even the appearance of the original waved type, countess spencer, was due to the loss of one of the factors for smooth edge at some time in the history of the sweet pea. in the case of the chinese primrose (_primula sinensis_) one dominant factor has been introduced in modern times, probably within the last six years at most. this is the factor which causes suppression of the yellow eye, giving rise to the curious type known as "queen alexandra." mr. r. p. gregory's experiments proved that this was a very definite dominant, and the element responsible for this development is undoubtedly an addition to the original ingredient-properties, with which the species was endowed. unfortunately, as happens in almost every case of the kind, the origin of this important novelty appears to be lost. its behaviour, however, when crossed with various other types is that of a simple dominant giving an ordinary : ratio. there is therefore no real doubt that it came into existence by the definite addition of a new factor, for if it was simply a case of the appearance of a new character made by combination of two previously existing complementary factors we should expect that when queen alexandra was self-fertilised a : ratio would be a fairly common result, which is not in practice found. in _oenothera_ gates[ ] has observed the appearance, in a large sowing of about , _oenothera rubrinervis_, of a single individual having considerably more red pigment in the calyx than is usual in _rubrinervis_. the whole of the hypanthium in the flowers of this plant was red instead of green as in _rubrinervis_, and the whole of the sepals were red in the bud-stage, except for small green areas at the base. this type behaved as a dominant over _rubrinervis_, but so far a pure-breeding individual was not found. admittedly the variation of this plant from the type of _rubrinervis_ can be represented as one of degree, though there is a very sensible gap in the series between the new form which gates names "_rubricalyx_" and the reddest _rubrinervis_ seen in his cultures. it must certainly be recognised as a new dominant. gates, rightly as i consider, regards the distinction between _rubrinervis_ and _rubricalyx_ as a quantitative one, and the same remark applies to certain other types differing in the amount of anthocyanin which they produce. i do not understand the argument which gates introduces to the effect that the difference between such quantitative types cannot be represented in terms of presence and absence. we are quite accustomed to the fact that in the rabbit self-colour segregates from the dutch-marked type. these two types differ in a manner which we may reasonably regard as quantitative. it is no doubt possible that the self-coloured type contains an ingredient which enables the colour to spread over the whole body, but it is, i think, perhaps more easy to regard the dutch type as a form from which a part of the colour is absent. it may be spoken of in terms i have used, as a _subtraction-stage_ in colour. following a similar method we may regard _rubricalyx_ as an addition-stage in colour-variation. the fact that crosses between _rubrinervis_, or _rubricalyx_ and _lamarckiana_ give a mixture of types in f_{ }, does not i think show, as gates declares, that there is any system here at work to which a factorial or mendelian analysis does not apply; but that question may be more fitly discussed in connexion with the other problems raised by the behaviour of _oenothera_ species in their crosses. i do, however, feel that, interesting as this case must be admitted to be, we cannot quite satisfactorily discuss it as an illustration of the _de novo_ origin of a dominant factor. the difference between the novelty and the type is quantitative, and it is not unreasonable to think of such a difference being brought about by some "pathological accident" in a cell-division. recognition of the distinction between dominant and recessive characters has, it must be conceded, created a very serious obstacle in the way of any rational and concrete theory of evolution. while variations of all kinds could be regarded as manifestations of some mysterious instability of organisms this difficulty did not occur to the mind of evolutionists. to most of those who have taken part in genetic analysis it has become a permanent and continual obsession. with regard to the origin of recessive variations, there is, as we have seen, no special difficulty. they are negative and are due to absences, but as soon as it is understood that dominants are caused by an addition we are completely at a loss to account for their origin, for we cannot surmise any source from which they may have been derived. just as when typhoid fever breaks out in his district the medical officer of health knows for certain that the bacillus of typhoid fever has by some means been brought into that district so do we know that when first dominant white fowls arose in the evolution of the domestic breeds, by some means the factor for dominant whiteness got into a bird, or into at least one of its germ-cells. whence it came we cannot surmise. whether we look to the outer world or to some rearrangement within the organism itself, the prospect of finding a source of such new elements is equally hopeless. leaving this fundamental question aside as one which it is as yet quite unprofitable to discuss, we are on safe ground in foreseeing that the future classification of substantive variations, which genetic research must before long make possible, will be based on a reference to the modes of action of the several factors. some will be seen to produce their effects by oxidation, some by reduction, some by generating substances of various types, sugars, enzymes, activators, and so forth. it may thus be anticipated that the relation of varieties to each other and to types from which they are derived will be expressible in terms of definite synthetical formulae. clearly it will not for an indefinite time be possible to do this in practice for more than a few species and for characters especially amenable to experimental tests, but as soon as the applicability of such treatment is generally understood the influence on systematics must be immediate and profound, for the nature of the problem will at length be clear and, though the ideal may be unattainable, its significance cannot be gainsaid. * * * * * _note._--with hesitation i allow this chapter to appear in the form in which it was printed a year ago, but in passing it for the press after that interval i feel it necessary to call attention to a possible line of argument not hitherto introduced. in all our discussions we have felt justified in declaring that the dominance of any character indicates that some factor is present which is responsible for the production of that character. where there is no definite dominance and the heterozygote is of an intermediate nature we should be unable to declare on which side the factor concerned was present and from which side it was absent. the degree of dominance becomes thus the deciding criterion by which we distinguish the existence of factors. but it should be clearly realized that in any given case the argument can with perfect logic be inverted. we already recognize cases in which by the presence of an inhibiting factor a character may be suppressed and purely as a matter of symbolical expression we might apply the same conception of inhibition to any example of factorial influence whatever. for instance we say that in as much as two normal persons do not have brachydactylous children, there must be some factor in these abnormal persons which causes the modification. our conclusion is based on the observed fact that the modification is a dominant. but it may be that normal persons are homozygous in respect of some factor _n_, which prevents the appearance of brachydactyly, and that in any one heterozygous, _nn_, for this inhibiting factor, brachydactyly can appear. similarly the round pea we say contains _r_, a factor which confers this property of roundness, without which its seeds would be wrinkled. but here we know that the wrinkled seed is in reality one having compound starch-grains, and that the heterozygote, though outwardly round enough, is intermediate in that starch-character. if we chose to say that the compoundness of the grains is due to a factor _c_ and that two doses of it are needed to make the seed wrinkled, i know no evidence by which such a thesis could be actually refuted. that such reasoning is seemingly perverse must be conceded; but when we consider the extraordinary difficulties which beset any attempt to conceive the mode of origin of a new dominant factor, we are bound to remember that there is this other line of argument which avoids that difficulty altogether. in the case of the "alexandra"-eye in _primula_, or the red calyx in gates's _oenothera_, inverting the reasoning adopted in the text, we may see that only the _primula_ homozygous for the yellow eye can develop it and that two doses of the factor for the _rubrinervis_ calyx are required to prevent that part of the plant from being red. we may proceed further and extend this mode of reasoning to all cases of genetic variation, and thus conceive of all alike as due to loss of factors present in the original complex. until we can recognize factors by means more direct than are provided by a perception of their effects, this doubt cannot be positively removed. for all practical purposes of symbolic expression we may still continue to use in our analyses the modes of representation hitherto adopted, but we must not, merely on the ground of its apparent perversity, refuse to admit that the line of argument here indicated may some day prove sound. footnotes: [ ] gates, r. r., _zts. f. abstammungslehre_, , iv, pp. and . chapter v the mutation theory when with the thoughts suggested in the last chapter we contemplate the problem of evolution at large the hope at the present time of constructing even a mental picture of that process grows weak almost to the point of vanishing. we are left wondering that so lately men in general, whether scientific or lay, were so easily satisfied. our satisfaction, as we now see, was chiefly founded on ignorance. every specific evolutionary change must represent a definite event in the construction of the living complex. that event may be a disturbance in the meristic system, showing itself in a change in the frequency of the repetitions or in the distribution of differentiation among them, or again it may be a chemical change, adding or removing some factor from the sum total. if an attempt be made to apply these conceptions to an actual series of allied species the complexity of the problem is such that the mind is appalled. ideas which in the abstract are apprehended and accepted with facility fade away before the concrete case. it is easy to imagine how man was evolved from an _amoeba_, but we cannot form a plausible guess as to how _veronica agrestis_ and _veronica polita_ were evolved, either one from the other, or both from a common form. we have not even an inkling of the steps by which a silver wyandotte fowl descended from _gallus bankiva_, and we can scarcely even believe that it did. the wyandotte has its enormous size, its rose comb, its silver lacing, its tame spirit, and its high egg production. the tameness and the high egg production are probably enough both recessives, and though we cannot guess how the corresponding dominant factors have got lost, it is not very difficult to imagine that they were lost somehow. but the rose comb and the silver colour are _dominants_. the heavy weight also appears in the crosses with leghorns, but we need not at once conclude that it depends on a simple dominant factor, because the big size of the crosses may be a consequence of the cross and may depend on other elements. now no wild fowl known to us has these qualities. may we suppose that some extinct wild species had them? if so, may we again make the same supposition in all similar cases? to do so is little gain, for we are left with the further problem, whence did those lost wild species acquire those dominants? suppositions of this kind help no more than did the once famous conjecture as to the origin of living things--that perhaps they came to earth on a meteorite. the unpacking of an original complex, the loss of various elements, and the recombination of pre-existing materials may all be invoked as sources of specific diversity. undoubtedly the range of possibilities thus opened up is large. it will even cover an immense number of actual examples which in practice pass as illustrations of specific distinction. the indian rock pigeon which has a blue rump may quite reasonably be regarded as a geographically separated recessive form of our own _columba livia_, for as staples-browne has shown the white rump of _livia_ is due to a dominant factor. the various degrees to which the leaves of indian cottons are incised have, as leake says, been freely used as a means of classification. the diversities thus caused are very remarkable, and when taken together with diversities in habit, whether sympodial or monopodial, the various combinations of points of difference are sufficiently distinctive to justify any botanist in making a considerable number of species by reference to them alone. nevertheless leake's work goes far to prove that all of these forms represent the re-combinations of a very small number of factors. the classical example of _primula sinensis_ and its multiform races is in fact for a long way a true guide as to the actual interrelations of the species which systematists have made. that they did make them was due to no mistake in judgment or in principle, but simply to the want of that extended knowledge of the physiological nature of the specific cases which we now know to be a prime necessity. but will such analysis cover all or even most of the ordinary cases of specific diversity between near allies? postponing the problem of the interrelations of the larger divisions as altogether beyond present comprehension, can we suppose, that in general, closely allied species and varieties represent the various consequences of the presence or absence of allelomorphic factors in their several combinations? the difficulty in making a positive answer lies in the fact that in most of the examples in which it has been possible to institute breeding experiments with a view to testing the question, a greater or less sterility is encountered. where, however, no such sterility is met with, as for instance in the crosses made by e. baur among the species of _antirrhinum_ there is every reason to think that the whole mass of differences can and will eventually be expressed in terms of ordinary mendelian factors. baur has for example crossed species so unlike as _antirrhinum majus_ and _molle_, forms differing from each other in almost every feature of organisation.[ ] the f_{ } generation from this cross presents an amazingly motley array of types which might easily if met with in nature be described as many distinct species. yet all are fertile and there is not the slightest difficulty in believing that they can all be reduced to terms of factorial analysis. if allowance be made for the complicating effects of sterility, is there anything which prevents us from supposing that such good species as those of _veronica_ or of any other genus comprising well-defined forms may not be similarly related? i do not know any reason which can be pointed to as finally excluding such a possibility. nevertheless it has been urged with some plausibility that good species are distinguished by _groups_ of differentiating characters, whereas if they were really related as the terms of a mendelian f_{ } family are, we should expect to find not groups of characters in association, but rather series of forms corresponding to the presence and absence of the integral factors composing the groups of characters. i am not well enough versed in systematic work to be able to decide with confidence how much weight should be attached to this consideration. some weight it certainly has, but i cannot yet regard it as forming a fatal objection to the application of factorial conceptions on the grand scale. it may be recalled that we are no longer under any difficulty in supposing that differences of all classes may be caused by the presence or absence of factors. it seemed at first for example that such characters as those of leaf shape might be too subtle and complex to be reducible to a limited number of factors. but first the work of gregory on _primula sinensis_ showed that several very distinct types of leaves were related to each other in the simplest way. in that particular example, intermediates are so rare as to be negligible, but subsequently shull dealing with such a complicated example as _capsella_, and leake in regard to cottons, both forms in which intergrades occur in abundance, have shown that a simple factorial scheme is applicable. we need not therefore, to take an extreme case, doubt that if it were possible to examine the various forms of fruit seen in the squashes by really comprehensive breeding tests, even this excessive polymorphism in respect of structural features would be similarly reducible to factorial order. it must always be remembered also that in a vast number of cases, nearly allied forms which are distinct, occupy distinct ground. moreover, by whatever of the many available mechanisms that end be attained, it is clear that nature very often does succeed in preventing intercrossing between distinct forms so far that the occurrence of that phenomenon is a rarity under natural conditions. the facts may, i think, fairly be summarized in the statement that species are on the whole distinct and not intergrading, and that the distinctions between them are usually such as might be caused by the presence, absence, or inter-combination of groups of mendelian factors; but that they are so caused the evidence is not yet sufficient to prove in more than a very few instances. the alternative, be it explicitly stated, is not to return to the view formerly so widely held, that the distinctions between species have arisen by the accumulation of minute or insensible differences. the further we proceed with our analyses the more inadequate and untenable does that conception of evolutionary change become. if the differences between species have not come about by the addition or loss of factors one at a time, then we must suppose that the changes have been effected by even larger steps, and variations including groups of characters, must be invoked. that changes of this latter order are really those by which species arise, is the view with which de vries has now made us familiar by his writings on the mutation theory. in so far as mutations may consist in meristic changes of many kinds and in the loss of factors it is unnecessary to repeat that we have abundant evidence of their frequent occurrence. that they may also more rarely occur by the addition of a factor we are, i think, compelled to believe, though as yet the evidence is almost entirely circumstantial rather than direct. the evidence for the occurrence of those mutations of higher order, by which new species characterized by several distinct features are created, is far less strong, and after the best study of the records which i have been able to make, i find myself unconvinced. the facts alleged appear capable of other interpretations. the most famous and best studied examples are of course the forms of _oenothera_ raised by de vries from _oenothera lamarckiana_ in circumstances well known to all readers of genetic literature. whatever be the true significance of these extraordinary "mutations" there can be no question about the great interest which attaches to them, and the historical importance which they will long preserve. apart also from these considerations it is becoming more and more evident that in their peculiarities they provide illustrations of physiological phenomena of the highest consequence in the study of genetics at large. de vries found, as is well known, that _oenothera lamarckiana_ gives off plants unlike itself. these mutational forms are of several distinct and recognizable types which recur, and several of them breed true from their first appearance. the obvious difficulty, which in my judgment should make us unwilling at present to accept these occurrences as proof of the genesis of new species by mutation, is that we have as yet no certainty that the appearance of the new forms is not an effect of the recombination of factors, such as is to be seen in so many generations of plants derived from a cross involving many genetic elements. the first question is what is _oenothera lamarckiana_? is it itself a plant of hybrid origin? to this fundamental question no satisfactory answer has yet been given. all attempts to find it as a wild plant in america have failed. it existed in europe in the latter half of the eighteenth century. whence it came is still uncertain, but the view that it came into existence in europe and perhaps in paris, seems on the whole the most probable. the question has been debated by macdougal, gates, and davis. from historical sources there is little expectation of further light. those who favour the notion of a hybrid origin look on _oenothera biennis_ as one of the putative parents. it has been conjectured that a species called _grandiflora_ lately re-discovered on the alabama river was the other parent. experiments have been instituted by davis to discover whether _lamarckiana_ can be made artificially by crossing these two species. the results so far have shown that while plants approximating in various respects to _lamarckiana_ have thus been produced, none agree exactly with that form. davis, to whom reference should be made for a full account of the present state of the enquiry, points out that there are many strains of _biennis_ in existence and that it is by no means impossible that by using others of these strains a still closer approximation can be made. none of davis's artificial productions as yet breed at all true, as _lamarckiana_ on the whole does. in such a case, however, where several characters are involved, this is perhaps hardly to be expected. one feature of the _oenotheras_ is very curious. not only _lamarckiana_, but all the allied species so far as i am aware, have a considerable proportion of bad and shrivelled pollen grains. this is undoubtedly true of species living in the wild state as well as of those in cultivation. i have had opportunities of verifying this for myself in the united states. no one looking at the pollen of an _oenothera_ would doubt that it was taken from some hybrid plant exhibiting partial sterility. on the other hand, it is difficult to suppose that numbers, perhaps all, of the "species" of the genus are really hybrids, and many of them breed substantially true. i regard this constant presence of bad pollen grains as an indication that the genetic physiology of _oenothera_ is in some way abnormal, and as we shall presently see, there are several other signs which point in the same direction. discussion of the whole series of phenomena is rendered exceedingly difficult first, by reason of the actual nature of the material. the characteristics of many of the types which de vries has named are evasive. a few of these types, for instance, _gigas_, _nanella_, _albida_, _brevistylis_, and perhaps a few more are evidently clear enough, but we have as yet no figures and descriptions precise enough to enable a reader to appreciate exactly the peculiarities of the vast number of forms which have now to be considered in any attempt to gain a comprehensive view of the whole mass of facts. it is also not in dispute that the forms are susceptible of great variations due simply to soil and cultural influences. the fact that no mendelian analysis has yet been found applicable to this group of _oenotheras_ as a whole is perhaps largely due to the fact that until recently such analysis has not been seriously attempted. following the system which he had adopted before the rediscovery of mendelism, or at all events, before the development of that method of analysis, de vries has freely applied _names_ to special combinations of characters and has scarcely ever instituted a factorial analysis. before we can get much further this must be attempted. it may fail, but we must know exactly where and how this failure comes about. there are several indications that such a recognition of factorial characters, could be carried some way. for example, the height, the size of the flowers, the crinkling of the leaves, the brittleness of the stems, perhaps even the red stripes on stems and fruits, and many more, are all characters which may or may not depend on distinct factors, but if such characters are really transmitted in unresolved groups, the limitations of those groups should be carefully determined. the free use of names for the several forms, rather than for the characters, has greatly contributed to deepen the obscurity which veils the whole subject. i do not mean to suggest that these _oenotheras_ follow a simple mendelian system. all that we know of them goes to show that there are curious complications involved. one of these, probably the most important of all, has lately been recognized by de vries himself, namely, that in certain types the characters borne by the female and the male germ-cells of the same plant are demonstrably different. there can be little doubt that further research will reveal cognate phenomena in many unsuspected places. the first example in which such a state of things was proved to exist is that of the stocks investigated by miss saunders.[ ] by a long course of analysis she succeeded in establishing in the fact that if a plant of _matthiola_ is of that eversporting kind which gives a large proportion of double-flowered plants among its offspring (produced by self-fertilisation), then the egg-cells of such a plant are mixed in type, but the pollen of the same plant is homogeneous. some of the egg-cells have in them the two factors for singleness, but some of them are short of one or both of these factors. the pollen-grains, however, are all recessives, containing neither of these factors. the egg-cells, in other words, are mixed, "singles" and "doubles," while the pollen-grains are all "doubles." the same is true of the factor differentiating "white," or colourless plastids from cream-coloured plastids in _matthiola_, the egg-cells being mixed "whites" and "creams," while the pollen-grains are all "creams," viz: recessives. later in the same year ( ) de vries[ ] announced a remarkable case which will be discussed in detail subsequently. it relates to certain _oenotheras_ heterozygous for dwarfness, in which (p. ) the ovules were mixed, tails and dwarfs, while the pollen is all dwarf. again in _petunia_ miss saunders's[ ] work has shown that a somewhat similar state of things exists, but with this remarkable difference, that though the egg-cells are mixed, singles and doubles, the pollen-grains are all _singles_, viz: dominants. all the _petunias_ yet examined have been in this condition, including some which in botanic gardens pass for original species. whether actual wild plants from their native habitats are in the same state, is not yet known, but it is by no means improbable. the case may be compared with that of the moth _abraxas grossulariata_ studied by doncaster and raynor, in which the females are all heterozygous, or we may almost say "hybrids" of _grossulariata_ and the variety _lacticolor_. similarly we may say that at least garden petunias are heterozygous in respect of singleness. the proof of this is of course that when fertilised with the pollen of doubles they throw a mixture of doubles and singles. the statements which de vries has published regarding the behaviour of several of the _oenotheras_ go far to show that they must have a somewhat similar organisation. on the present evidence it is still quite impossible to construct a coherent scheme which will represent all the phenomena in their interrelations, and among the facts are several which, as will appear, seem mutually incompatible. the first indication that the _oenotheras_ may have either mixed ovules or mixed pollen appears in the fact that _lamarckiana_ and several of its "mutants" used as males, with several other forms as females, give a mixed offspring. for example, de vries ( ) found that _biennis_ [f] Ã� _lamarckiana_ [m] _biennis cruciata_ [f] Ã� _lamarckiana_ [m] _muricata_ [f] Ã� _lamarckiana_ [m] _biennis_ [f] Ã� _rubrinervis_ [m] _biennis cruciata_ [f] Ã� _rubrinervis_ [m] all give a mixture of two distinct types which he names _laeta_ and _velutina_, consisting of about equal numbers of each. on account of the fact that the two forms are produced in association de vries has called these forms "twin hybrids," a designation which is not fortunate, seeing that it is impossible to imagine that any kind of twinning is concerned in their production. the distinction between these two seems to be considerable, _laeta_ having leaves broader, bright green in colour, and flat, with pollen scanty, while _velutina_ has leaves narrower, grayish green, more hairy, and furrow-shaped, with pollen abundant. we next meet the remarkable fact that these two forms, _laeta_ and _velutina_ breed true to their respective types, and do not reproduce the parent-types among their offspring resulting from self-fertilisation. this statement must be qualified in two respects. when _muricata_ [m] is fertilised by _brevistylis_ the forms _laeta_ and _velutina_ are produced, but each of them subsequently throws the short-styled form as a recessive (de vries, , p. ). it may be remembered that de vries's previous publications had already shown that the short style of _brevistylis_, one of the _lamarckiana_ "mutants," behaves as a recessive habitually (_mutationstheorie_, ii, p. , etc.). also when _nanella_, the dwarf "mutant" of _lamarckiana_ is used as male on _muricata_ as female, _laeta_ and _velutina_ are produced, but one only of these, namely, _velutina_, subsequently throws dwarfs on self-fertilisation. the dwarfs thus thrown are said to form about per cent. of the families in which they occur (de vries, , p. ). the fact that the two forms, _laeta_ and _velutina_, are produced by many matings in which _lamarckiana_ and its mutant _rubrinervis_ are used as males is confirmed abundantly by honing, who has carried out extensive researches on the subject. after carefully reading his paper, i have failed to understand the main purport of the argument respecting the "double nature" of _lamarckiana_ which he founds on these results, but i gather that in some way _laeta_ is shown to partake especially of the nature of _lamarckiana_, while _velutina_ is a form of _rubrinervis_. the paper contains many records which will be of value in subsequent analysis of these forms. before considering the possible meaning of these facts we must have in our minds the next and most novel of the recent extensions of knowledge as to the genetic properties of the _oenotheras_. in the previous statement we have been concerned with the results of using either _lamarckiana_ itself or one of its "mutants" _rubrinervis_, _brevistylis_, or _nanella_ as male, on one of the species _biennis_ or _muricata_. the new experiments relate to crosses between the two species _biennis_ and _muricata_ themselves. de vries found: . that the reciprocal hybrids from these two species differed, _biennis_ Ã� _muricata_ producing one type of f_{ } and _muricata_ Ã� _biennis_ producing another. each f_{ } resembled the father more than the mother. . that each of the hybrids so produced breeds true on self-fertilisation. . that if we speak of the hybrid from _biennis_ Ã� _muricata_ as _bm_ and of the reciprocal as _mb_, then _bm_ Ã� _mb_ gives exclusively offspring of _biennis_ type but that _mb_ Ã� _bm_ gives exclusively offspring of _muricata_ type. evidently, apart from all controversy as to the significance of the "mutants" of _lamarckiana_, we have here a series of observations of the first importance. the fact that reciprocal crossings give constantly distinct results must be taken to indicate that the male and female sides of one, if not of both, of the parents are different in respect of characters which they bear. this is de vries's view, and he concludes rightly, i think, that the evidence from all the experiments shows that both _biennis_ and _muricata_ are in this condition, having one set of characters represented in their pollen-grains and another in their ovules. the plants breed true, but their somatic structures are compounded of the two sets of elements which pass into them from their maternal and paternal sides respectively. this possibility that species may exist of which the males really belong to one form and the females to another, is one which it was evident from the first announcement of the discovery of mendelian segregation might be found realised in nature.[ ] _oe. biennis_ and _muricata_ were crossed reciprocally with each other and with a number of other species, and the behaviour of each, when used as mother, was consistently different from its behaviour when used as father. de vries is evidently justified by the results of this series of experiments in stating that the "bild," as he terms it, or composition of the male and female sides of these two species, _biennis_ and _muricata_, are distinct. on the evidence before us it is not, however, possible to form a perfectly clear idea of each, and until details are published, a reader without personal knowledge of the material cannot do more than follow the general course of the argument. for fuller comprehension a proper analysis of the characters with a clear statement of how they are distributed among the several types and crosses is absolutely necessary. according to de vries the female of _biennis_ possesses a group of characters which he defines as "_conica_" in allusion to the shape of the flower-buds. besides the conical buds, this group of features includes imperfect development of wood, rendering the plant very liable to attacks of _botrytis_, and comparatively narrow leaves. the female of _muricata_ carries a group of features which he calls "_frigida_," and, though this is not quite explicitly stated in a definition of that type, it is to be inferred[ ] that its characteristics are regarded as greater height, strong development of wood with comparative resistance to _botrytis_, and broad leaves. the characters borne by the male parts of the two species are in general those by which they are outwardly distinguished. for example, the leaves of _oe. biennis_ are comparatively broad and are bright green, while those of _muricata_ are much narrower and of a glaucous green, and i understand that de vries regards these properties as contributed by the male side in each case and to be carried by the male cells of each species. the suggestion as regards _biennis_ and _muricata_ comes near the conception often expressed by naturalists in former times (_e. g._, linnaeus) and not rarely entertained by breeders at the present day, that the internal structure is contributed by the mother and the external by the father. on the other hand, the offspring of each species when used as mother is regarded as possessing in the main the features of the maternal "bild," but the matter is naturally complicated by the introduction of features from the father's side, and it is here especially that the account provided is at present unsatisfactory and inconclusive. there seems, however, to be no serious doubt that _biennis_ and _muricata_ each in their outward appearance exhibit on the whole the features which their pollens respectively carry, and that the features borne by their ovules are in many respects distinct. the _types_ are thus "hybrids" which breed true. the results of intercrossing them each way are again "hybrids" which breed true. it will be remembered that on former occasions de vries has formulated a general rule that _species_-hybrids breed true, but that the cross-breds raised by interbreeding _varieties_ do not. one of these very cases was quoted[ ] as an illustration of this principle, viz: _muricata_ Ã� _biennis_. the grounds for this general statement have always appeared to me insufficient, and with the further knowledge which the new evidence provides we are encouraged to hope that when a proper factorial analysis of the types is instituted we shall find that the phenomenon of a constant hybrid will be readily brought into line with the systems of descent already worked out for such cases as that of the stocks, and others already mentioned. in further discussion of these facts de vries makes a suggestion which seems to me improbable. since the egg-cells of _muricata_, for instance, bear a certain group of features which are missing on the male side, and conversely the pollen bears features absent from the female side, he is inclined to regard the _bad pollen grains_ as the bearers of the missing elements of the male side and to infer that there must similarly be defective ovules representing the missing elements of the female side. no consideration is adduced in support of this view beyond the simple fact that the characters borne by male and female are dissimilar, whereas it would be more in accord with preconception if the same sets of combinations were represented in each--as in a normal mendelian case. there is as yet no instance in which the absence of any particular class of gametes has been shown with any plausibility to be due to defective viability, though there are, of course, cases in which certain classes of zygotes do not survive owing to defective constitution (_e. g._, the albinos of _antirrhinum_ studied by baur, and the homozygous yellow mice). i am rather inclined to suppose that in these examples of hybrids breeding true we shall find a state of things comparable with that to which we formerly applied the terms "coupling" and "repulsion." in these cases certain of the possible combinations of factors occur in the gametic series with special frequency, being in excess, while the gametes representing other combinations are comparatively few. in a recent paper on these cases professor punnett and i have shown that these curious results vary according to the manner in which the factors are grouped in the parents. if _a_ and _b_ are two factors which exhibit these phenomena we find that the gametic series of the double heterozygote differs according as the combination is made by crossing _ab Ã� ab_, or by crossing _ab Ã� ab_. in a normal mendelian case the f_{ } form, _aabb_, produces gametes _ab_, _ab_, _ab_, _ab_, in equal numbers; but in these peculiar cases those gametes which contain gametic series number of number of --------------------- gametes zygotes ab ab ab ab in series formed partial repulsion { (n- ) (n- ) n n^{ } from zygote { of form { abÃ�ab { { partial coupling { from zygote { of form { abÃ�ab { { { (n- ) (n- ) n n^{ } nature of zygotic series --------------------------------------- ab ab ab ab partial repulsion { n^{ }+ n^{ }- n^{ }- from zygote { of form { abÃ�ab { { { partial coupling { from zygote { of form { abÃ�ab { { n^{ }-( n - ) n- n- n^{ }-( n- ) the _parental combinations_ are in excess. this excess almost certainly follows the system indicated by the accompanying table. in the general expressions _n_ is half the number of gametes required to express the whole system. now if we imagine that sex-factors are involved with the others concerned in such a relationship as this we have a system of distribution approximating to that found in _biennis_ and _muricata_. the difference in reciprocals is represented in a not improbable way. it cannot yet be said that the rarer terms in the series are formed at all, and perhaps they are not. as we pointed out in our discussion of these phenomena, the peculiar distribution of factors in these cases must be taken to mean that the planes of division at some critical stage in the segregation are determined with reference to the parental groups of factors, or in other words, that the whole system has a polarity, and that the distribution of factors with reference to this polarity differs according to the grouping of factors in the gametes which united in fertilization to produce the plant. subsequent proliferation of cells representing certain combinations would then lead to excess of the gametes bearing them. it is on similar lines that i anticipate we shall hereafter find the interpretation of the curious facts discovered by de vries, though it is evident that a long course of experiment and analysis must be carried through before any certainty is reached. the work must be begun by a careful study of the descent of some single factor, for example, that causing the broader leaf of _biennis_, and we may hope that the study of _oenothera_ by proper analytical methods will no longer be deferred. we have now to return to the relations of _laeta_ and _velutina_. these two forms, it will be remembered are frequently produced when _lamarckiana_ or one of its derivatives is used as male, and the most unexpected feature in their behaviour is that _both breed true as regards their essential characteristics, on self-fertilisation_. if one only bred true the case might, in view of the approximate numerical equality of the two types, be difficult to interpret on ordinary lines, but as both breed true it must be clear that some quite special system of segregation is at work. what this may be cannot be detected on the evidence, but with the results from the _biennis-muricata_ experiments before us, it is natural to suspect that we may here again have to recognise a process of allocation of different factors to the male and female sides in _laeta_ and _velutina_. that some such system is in operation becomes the more probable from the new fact which de vries states in describing the group of characters which he calls _conica_, namely that this type is the same as that of _velutina_. there are many collateral observations recorded both by de vries and others which have a bearing on the problems, but they do not yet fall into a coherent scheme. for example, we cannot yet represent the formation of _laeta_ and _velutina_ from the various species fertilised by _lamarckiana_ [m]. that this is not due to any special property associated with the pollen of _lamarckiana_ is shown by the fact that a species called _hookeri_ gives _laeta_ and _velutina_ in both its reciprocal crosses with _lamarckiana_ (de vries, , p. ), and also by the similar fact that _lamarckiana_ [f] fertilised by the pollen of a peculiar race of _biennis_ named _biennis chicago_ throws the same types. before these very complicated phenomena can be usefully discussed particulars must be provided as to the individuality of the various plants used. this criticism applies to much of the work which de vries has lately published, for, as we now know familiarly, plants to which the same name applies can be quite different in genetic composition. attention should also be called to one curiously paradoxical series of results. when the dwarf "mutant" of _lamarckiana_ which de vries names "_nanella_" is used as father on _muricata_, f_{ } consists of _laeta_ and _velutina_ in approximately equal numbers. both forms breed true to their special characteristics, but _velutina_ throws dwarfs of its own type, while _laeta_ does not throw dwarfs. subsequent investigation of the properties of these types has led to some remarkable conclusions, and it was in a study of these plants that de vries first came upon the phenomena of dissimilarity between the factors borne by the male and female cells of the same plant, a condition which had been recently detected in the stocks as a result of miss saunders's investigations. the details are very remarkable. we have first the fact that _muricata_ [f] Ã� dwarf _nanella_ [m] gives about per cent. _laeta_ and about per cent. of _velutina_. as regards _velutina_ it was shown that: talls, dwarfs, per cent. per cent. . velutina selfed gave {velutina [f] Ã� dwarf nanella [m] gave .{ do. Ã� do. gave { do. Ã� dwarf [m] derived from velutina gave . dwarfs Ã� velutina [m] gave -- all dwarfs the three experiments taken together prove, as de vries says, that the ovules of _velutina_ are mixed, talls and dwarfs, and that the pollen is all dwarf. the condition is almost the same as that of the stocks. it may be noted also that in the stocks the egg-cells of the "double" type are in excess, being approximately to of the "single" type, but de vries regards the two types in _velutina_ as probably equal in number. the figures ( : ) rather suggest some excess of the recessives, perhaps : , and the point would be worth a further investigation. as regards _laeta_, by self-fertilisation _no dwarfs were produced_, but in all other respects it behaved almost exactly like _velutina_. the ovules are evidently mixed talls and dwarfs, and whether fertilised by dwarfs or by the pollen of _velutina_, which is already proved to be all dwarf, the result was a steady per cent. of talls and per cent. of dwarfs. the pollen of _laeta_ used on dwarfs gives nothing but dwarfs, and in three series of such experiments dwarfs were produced. we are thus faced with this difficulty. since the egg-cells of _laeta_ are evidently mixed, talls and dwarfs, and the pollen used on dwarfs gives all dwarfs, why does not self-fertilisation give a mixed result, talls and dwarfs, instead of _all talls_? de vries regards the result of self-fertilisation as showing the real nature of the pollen, and declares it to be all talls, while he represents the behaviour of the same pollen used on dwarfs by stating that in these combinations the dwarf character dominates. this does not seem to me a natural interpretation. i should regard the pollen of _laeta_ as identical with that of _velutina_, namely dwarf, and i suspect the difficulty is really created by the behaviour of _laeta_ on self-fertilisation. until a proper analysis is made in which the identity of the different individuals used is recorded, no further discussion is possible.[ ] other results of a complicated kind involving production of _laeta_ and _velutina_ together with a third form have been published by de vries in his paper on "triple hybrids." to these also the same criticism applies. some of the observations seem capable of simple factorial representation and others are conflicting. taking the work on _oenothera_ as a whole we see in it continually glimpses of order which further on are still blocked by difficulties and apparent inconsistencies. through such a stage all the successful researches in complicated factorial analysis have passed and i see no reason for supposing that with the application of more stringent methods this more difficult set of problems will be found incapable of similar solutions. to return to the original question whether in _oenothera_ we can claim to see a special contemporaneous output of new species in actual process of creation, it will be obvious that while the interrelation of the several types is still so little understood, such a claim has no adequate support. it is true that many of the "mutants" of _lamarckiana_ can well pass for species, but this is equally true of many new combinations of pre-existing factors as we have seen in _primula sinensis_ and other cases. still less can it be admitted that these facts of uncertain import supply a justification for the conception which has played a prominent part in the scheme of the _mutationstheorie_, namely that there are special periods of mutation, when the parent-species has peculiar genetic properties. to conclude: the impression which the evidence leaves most definitely on the mind is that further discussion of the bearing which the _oenotheras_ may have on the problem of evolution should be postponed until we have before us the results of a searching analysis applied to a limited part of the field. in such an analysis it is to be especially remembered that we have now a new clue in the well-ascertained fact that the genetic composition of the male and female germ-cells of the same individual may be quite different. when with this possibility in view the behaviour of the types is re-examined i anticipate that many of the difficulties will be removed. outside the evidence from _oenothera_, which, as we have seen, is still ambiguous, i know no considerable body of facts favourable to that special view of mutation which de vries has promulgated. of variation, or if we will, mutation, in respect of some one character, or resulting from recombination, there is proof in abundance; but of that simultaneous variation in several independent respects to which de vries especially attributes the origin of new specific types i know only casual records which have yet to undergo the process of criticism. * * * * * besides de vries's "_mutationstheorie_" and "species and varieties" the chief publications relating to the subject of the behaviour of _oenothera_ are the following: (many other papers relating especially to the cytology of the forms have appeared.) davis, b. m. genetical studies on _oenothera_, i. _amer. nat._, xliv, , p. . genetical studies on _oenothera_, ii. _ibid._, xlv, , p. . gates, r. r. an analytical key to some of the segregates of oenothera. _twentieth annual report of the missouri botanical garden_, . studies on the variability and heritability of pigmentation in _oenothera_. _ztsch. f. abstammungslehre_, , iv, p. . honing, j. a. die doppelnatur der _oenothera lamarckiana_. _ztsch. f. abstammungslehre_, , iv, p. . macdougal, d. t. (with a. m. vail, g. h. shull, and j. k. small). mutants and hybrids of the _oenotheras_. _carnegie institution's publication_, no. , . macdougal, d. t., vail, a. m., shull, j. h. mutations, variations and relationships of the _oenotheras_. _carnegie institution's publication_, no. , . de vries, h. on atavistic variation in _oenothera cruciata_. _bull. torrey club_, , vol. , p. . on twin hybrids, _bot. gaz._, vol. , , p. . ueber die zwillingsbastarde von _oenothera nanella_. _ber. deut. bot. ges._, , xxvi, _a_, p. . bastarde von _oenothera gigas_. _ibid._, p. . on triple hybrids. _bot. gaz._, , vol. , p. . ueb. doppeltreziproke bastarde von _oenothera biennis_ l. und _oenothera muricata_ l. _biol. cbltt._, , xxxi, p. . zeijlstra, h. h. _oenothera nanella_ de vries, eine krankhafte pflanzenart. _biol. cbltt._, , xxxi, p. . note. since this chapter was written two contributions of special importance have been made to the study of the _oenothera_ problems. the first is that of heribert-nilsson.[ ] the author begins by giving a critical account of the evidence for de vries's interpretation of the nature of the mutants. in general this criticism pursues lines similar to those sketched in the foregoing chapter, concluding, as i have done, that the chief reason why factorial analysis has been declared to be inapplicable to the _oenothera_ mutants is because no one has hitherto set about this analysis in the right way. he has also himself made a valuable beginning of such an analysis and gives good evidential reasons for the belief that at least the red veining depends on a definite factor which also influences the size of certain parts of the plant. he argues further that many of the distinctions between the mutants are quantitative in nature. with great plausibility he suggests that the system of cumulative factors which nilsson-ehle discovered in the case of wheat (subsequently traced by east in regard to maize) may be operating also in these _oenotheras_. according to this system several factors having similar powers may coexist in the same individual, and together produce a cumulative effect. scope would thus be given for the production of the curious and seemingly irregular numbers so often recorded in the "mutating" families. another remarkable observation relating to the crosses of _muricata_ and _biennis_ has been published by goldschmidt.[ ] he finds that in the formation of this cross the female pronucleus takes no part in the development of the zygotic cell, but that when the male pronucleus enters, the female pronucleus is pushed aside and degenerates. as de vries observed, the reciprocal hybrids are in each case very like the father ("_stark patroklin_"), a consequence which finds a natural explanation in the phenomenon witnessed by goldschmidt. the results of the subsequent matings can also be readily interpreted on the same lines. indications of maternal characters are nevertheless mentioned by de vries, and if goldschmidt's account of the cytology is confirmed, these must presumably be referred to the influence of the maternal cytoplasm. clearly this new work opens up lines of exceptional interest. the interpretation i have offered above must probably be reconsidered. the distinction between the male and female cells of the types may no doubt be ultimately factorial, but it is difficult to regard such a distinction as created by a differential distribution of the ordinary factors. footnotes: [ ] see lotsy and baur, rep. genetics conf., paris, , pp. - . compare lecoq on _mirabilis jalapa_ Ã� _longiflora_, fécondation des végétaux, , p. . [ ] _rep. evol. ctee. r. s._, iv, , p. . [ ] _ber. deut. bot. ges._, , xxvi, _a_, p. . [ ] _jour. genetics_, , , p. . [ ] in rep. to evol. committee, , p. , attention was called to this possibility, though of course at that date it was in sexual animals alone that it was supposed to exist. it had not occurred to me that even a hermaphrodite plant might be in this condition. [ ] from the description of the offspring of _muricata_ used as mother. [ ] de vries, _species and varieties_, , p. . [ ] zeijlstra in a recent paper announces that many _nanella_ plants are the subject of a bacterial disease to which he attributes their dwarfness. i gather that this does not apply to all _nanella_ plants and that some are dwarfs apart from disease. the matter may no doubt be further complicated from this cause. [ ] _zts. f. abstamm._, , viii. [ ] _arch. f. zellforschung_, , ix, p. . chapter vi variation and locality in all discussions of the modes of evolution the phenomena of geographical distribution have been admitted to be of paramount importance. first came the broad question, were the facts of distribution consistent with the doctrine of descent? i suppose all naturalists are now agreed that they are thus consistent, and that though some very curious and as yet inexplicable cases remain to be accounted for, the distribution of animal and plant life on the face of the earth is much what we might expect as a result of a process of descent with modification. passing from this general admission to the more particular question whether the facts of distribution favour one special conception of the mode of progress of evolution rather than another, no agreement has yet been reached. one outstanding feature is hardly in dispute, namely that prolonged isolation is generally followed by greater or less change in the population isolated. groups of individuals which from various causes are debarred from free intermixture with other groups almost always exhibit peculiarities, but on the other hand, cosmopolitan types which range over wide areas are on the whole uniform, or nearly so throughout their distribution. examples of these two categories will be familiar to all naturalists. the barriers to intercourse may be seas, deserts, prairies, mountain-chains, or circumstances of a much less obvious character which isolate quite as effectually. the local unit is not necessarily an island, a district, or an area of special geological formation, but may, as every collector knows, be a valley, a pond, a creek, a "bank" in the sea, a clump of trees, a group of rocks in a bay, or a particular patch of ground on a mountain side. all the great groups provide examples of such specially isolated forms. the botanist knows them well; the conchologist, the entomologist, the ornithologist and the student of marine life are all equally aware that special varieties or special species come from special places and from nowhere else. in one remarkable case the season of appearance plainly acts as the isolating barrier. _tephrosia bistortata_ is a small geometrid moth which has two broods, appearing in _march_ and _july_ respectively. it is closely allied to _t. crepuscularia_ which emerges in _may_ and _june_. from the fact that occasional specimens cannot be quite certainly referred to one or other of the two, many have held that the two are one species. nevertheless, in general they present distinctions which are plain enough. some localities have one form only, but in several woods they co-exist. experiment has shown that the two can be crossed, and that the cross-breds can breed _inter se_ and with at least one of the parent stocks.[ ] some diminution in fertility was observed, but perhaps not more than is commonly encountered when wild forms are bred in captivity. in such a case it can scarcely be doubted that the distinctness of the two forms in the places where they co-exist is maintained by the seasonal isolation. just as the consequences of isolation are to be seen in the most different forms of life so may they also affect the most diverse features of organisation, such as size, colour, sculpture, shape, or number of parts. in the sloth (_choloepus_) the geographical races differ in the number of cervical vertebrae--or in other words, in the distribution of vertebral differentiation. the geographical races of _cistudo_ differ in the number of claws and phalanges.[ ] in shetland, the males of _hepialus humuli_ (the ghost moth) are not sharply differentiated in colour from the females, as they are elsewhere, but in varying degrees resemble them.[ ] no such males are found in other localities, and even in the other scottish islands they are normal. in the island of waigiu the converse phenomenon has been observed in _phalanger maculatus_. generally the male is spotted with white, and the female is without spots, but in waigiu the females are spotted like the males.[ ] the following striking illustration was pointed out to me by dr. w. d. miller. _euphonia elegantissima_ as it occurs in mexico and central america has the two sexes very distinct from each other. the male has the lower parts orange and the upper parts a dark indigo blue, with a bright turquoise-blue head and neck. the female, except for the head, is of a bright olive green. a form in which the sexes are similarly differentiated exists in porto rico and is known as _e. sclateri_. but in many of the other west indian islands the representative "species" (_e. flavifrons_) has the two sexes closely resembling the _female_ of _e. elegantissima_. this form is found in antigua, barbados, st. vincent, and guadeloupe, from which localities the british museum has specimens. all three so-called species are very much alike otherwise. in the genus _pyrrhulagra_ (_loxigilla_) to which mr. outram bangs called my attention, several distinct and alternative possibilities occur. the genus has many local species occurring on the various west indian islands. these species are characterized by differences in size, colour, and the shape of the bill. the colours have a narrow range, being black or greyish, with or without chestnut marks about the head and throat. in most of the islands the males are in general colour a full black, and the females are distinctly grey. they are thus found in san domingo, jamaica, bahama, and most of the lesser antilles. in porto rico we meet the peculiarity that the hens are almost as black as the males (ridgway describes the black of the hens as slightly less intense). this form is called _portoricensis_. a larger type, known as _grandis_, similarly coloured, inhabits st. kitt's. then, on the contrary, in barbados, _both sexes_ are a dull blackish grey, like the hens of the lesser antilles in general. the local species of _agelaius_ show similarly capricious distinctions. _a. phoeniceus_ is a widely spread species, found over a great part of north america. the male is black with red-orange bars on the wings, but the female is somewhat thrush-like in colour. in the island of porto rico there is a form called _xanthomus_, in which _both sexes_ are like the males of the mainland. a similar species called _humeralis_, also with both sexes male-like, lives in cuba. the island of cuba, curiously enough, has also a distinct species named _assimilis_, in which the female is a dull black all over, though the male is like the mainland type. so also may local races differ in respect of variability. _argynnis paphia_, the silver washed fritillary, through a great part of its distribution has only one female form. in the english new forest a second female form, _valesina_, co-exists with the ordinary _paphia_ female. but in the southern valleys of the alps the _valesina_ female is much the commoner of the two, and indeed in some localities where the species is abundant, i have seen no _paphia_ females in many days collecting. the beetle _gonioctena variabilis_ furnishes an illustration of a comparable phenomenon affecting the male sex. in and i studied the curious colour variations of this species especially in the neighbourhood of granada, and mr. doncaster ten years later repeated the observations on the same ground, and also collected the insect in other places in the south of spain. the distinctions are not easy to give in words and the reader is referred to the colour plate accompanying my paper.[ ] the essential fact is that the males commonly have the elytra _red with black spots_ and the females for the most part have greenish grey elytra with black stripes. in some localities a large minority of males closely resemble the female type, being identical in colour and then only distinguishable by structural differences. in two granada localities i found the proportion of such males quite different. in the darro valley about per cent. (in ) were of this feminine type, but on the hills some feet above only per cent. (in , ) were like the females. at castillejo, not far from toledo i found no such male in specimens. mr. doncaster collected from several localities, especially from two areas near malaga, about miles apart. in one of these the female-like males were, as usual, in a minority, but in the other these were actually in great excess, amounting to about per cent. in the taken. doncaster found a doubtful indication that the composition of the population varies with the season, which is quite possible, but it is most interesting to note that in my chief locality after the lapse of ten years he found the proportions very much the same as i had done at the same season, for where i had per cent. of the female-like males his collecting gave per cent. in other respects also, his statistics corresponded very closely with mine.[ ] the various forms of _heliconius erato_ are well known to entomologists. they are strikingly distinguished by the colours of the strong comb-like marking on the hind wing, which may be red, yellow, green or blue. in various parts of the distribution in south america sometimes two and sometimes three of these distinct types co-exist.[ ] the distribution of the varieties of _noctua castanea_ typifies a large range of cases. the form which is reckoned the normal of the species has red fore-wings. it is practically restricted to great britain and germany, according to tutt. the other common form, _neglecta_, has grey fore-wings, and in this pattern it ranges through west central europe from north italy to germany. in the british isles it extends up to orkney. in britain this grey form is by far the commoner, occurring wherever the species is found. the red form is much scarcer in england, and does not occur at all in many localities where the grey form is common. mr. woodforde, from whom this account is taken,[ ] states that in august, , he saw considerably over a hundred of the grey in the new forest at sugar, but only two red ones. in staffordshire however the red is proportionately more numerous and he estimates them as per cent. of the population. lastly a form has been taken in staffordshire as a rarity in which the red is replaced by yellow, and this has hitherto been seen nowhere else. it is beyond our immediate purposes to discuss the genetic relationships of such forms, but the details of this case are interesting as making fairly clear the fact that the distinctions between _castanea_ and _neglecta_ are due to combinations of the presence of and absence of two pairs of factors, of which one produces a red pigment in the ground colour of the forewing and the other irrorates the same region with black scales. mr. woodforde states that all intermediates exist, and that in staffordshire the greys always have a pinkish tinge. the yellow is doubtless another recessive to the red. species which are uniform in some localities may be polymorphic in others. such a phenomenon is well exemplified by the orchid _aceras hircina_. of this species distinct varieties had previously been known in germany, but gallé[ ] has lately given a detailed account of a number of most diverse forms found growing in a district of eastern france. without reference to his plates it is impossible to give any adequate conception of the profusion of types which the flowers of the species there assume. in some the lip is elongated to many times its usual length, twisting and dividing in a fashion suggesting some of the strangest of the tropical orchids. in others the labellum and the lateral petals are all comparatively short and wide (fig. ). intermediates, combining these qualities in various degrees, were abundant, and the condition of the species, which was the only representative of the genus in the locality, recalls the extreme polymorphism of many of the noctuid moths. [illustration: fig. . various forms of _aceras hircina_. (after gallé.) this figure only shows a few of the more striking forms illustrated in gallé's plates.] somewhat comparable variability has been seen in another orchid genus _ophrys_. in great britain the species _apifera_, _aranifera_ and _muscifera_ though variable are fairly distinct, but moggridge has published two series of plates[ ] showing a very different state of things as regards the _ophrys_ population of the riviera. here the outward diversity is such that the ordinary specific names cannot be applied with any confidence and the limits of the species are quite uncertain. it may well be supposed that these riviera plants are interbreeding, and indeed we may safely assume that they are. it is, however, to be remembered that darwin showed _apifera_ in this country to be habitually self-fertilised, so that the different behaviour on the riviera may itself constitute a local peculiarity. moreover it is to be gathered from moggridge's account that in the districts which he examined the condition was not to be described by the statement that our three types were there co-existing and hybridising, but rather we should say that the population was polymorphic, containing these three types amongst others. conchologists are aware that on the dogger bank _modiola_ attains a size unparalleled elsewhere. the same is true of the sponges _grantia compressa_ and _grantia ciliata_ in the estuary of the orwell.[ ] conversely, as we know so well in the case of man, dwarf races occur in several special localities. such examples may be multiplied indefinitely. the relation of local forms to species has often been discussed from many points of view, but i know no treatment of the subject clearer or more comprehensive than an excellent account of some of the various manifestations of local differentiation as they appear in helicidæ published by coutagne[ ] and a reader interested in the problem which they raise would do well to make himself acquainted with the original from which the following notes are taken. he speaks for example of _helix lapicida_. this is on the whole a constant form ranging up to the altitude of , m., common all over france except at great heights and in the olive regions where it is restricted to moist places. though subjected to such diverse conditions it shows only trivial variations in colour and other respects throughout its distribution, excepting that on both sides of the pyrenees it has a very distinct sporadic variety called _andorrica_ or _microporus_. this variety occurs here and there, together with the type-form sometimes in colonies (pp. - and ). _bulimus detritus_ though more restricted in geographical range is a much more variable form. it exhibits great variations in colour, form, and size, and as coutagne well insists, these are independent of each other. foreshadowing the methods of factorial analysis he suggests that distinctions in each respect, the "modes" as he calls them, should be denoted by a letter, or if desired, by a name, and the several combinations of differences might thus be most logically and usefully expressed. of such combinations he says there are at least , all of which can be found. the whole possible series does not necessarily occur in the same place, and various localities are characterised by the presence or absence of certain of the combinations as coutagne calls them, and by the relative frequency with which they occur. the ideas thus enunciated are much in advance of the ordinary practice of systematists, who give names to forms which are nothing but accidental combinations of factors, just as the horticulturists for practical reasons give names to similar combinations, which as we now know are merely specially noticeable terms in a long series of possibilities. in each case it is rather the _factors_ which should be named than the forms which are constituted by their casual collocation. in this special example of _bulimus detritus_ the forms are made by the combinations of three pairs of independent factors. besides these combinations which may occur anywhere or almost anywhere in the distribution there are two more distinct local forms, each of which is regarded by coutagne as probably constituting a fresh "mode," perhaps compatible with the others. _helix striata_ (draparnauld)[ ] is truly polymorphic; and its various forms have been described under various specific names. it abounds in the calcareous hills of provence and languedoc, disappearing in the alluvial lowlands and equally in the upper levels at about - , m. from this district it extends through regions of similar altitude over a great part of france (details given). locard in his monograph of this group, which he calls collectively the group of _helix heripensis_, tabulates distinct named forms. the characteristics in which these forms differ have been reckoned as , and as several of these vary in degree of development, the number of modes may be increased to . for practical purposes however coutagne considers that the various developments of characteristics in their several combinations are enough to express the various forms, and he gives examples of this method of definition. as he observes, though names may be required to define the modes, no one need be alarmed at that, for the same names of modes will be applicable to a great range of distinct species, and the formulae expressing their combinations will replace the varietal names. this particular example of polymorphism is but little limited by locality. occasional colonies present some special physiognomy which may in a given place seem almost invariable, though in this very respect the colonies found elsewhere may be highly variable, but such limitations are exceptional for _h. striata_. some distinct and obvious susceptibilities to the influence of soil and climate are however noticeable. for example on siliceous ground the shells are thinner, while on calcareous soils they are thicker; similarly those from the northern districts attain a larger size than those from further south. moreover those subjected to curtailed development, whether from drought, heat or cold often show a shortening of the spire. in contrast with this case coutagne describes the varieties of _helix caespitum_, which he says are for the most part localised, quoting many illustrative cases. another remarkable case in which locality plays a curious part is provided by the two species _helix trochoides_ and _pyramidata_. in france generally they are distinct enough from each other, _trochoides_ being smaller and having a characteristic keel. coutagne says that after having collected these species from more than a score of localities he came upon a colony of _trochoides_ on the island of pomègues in which the shells were relatively enormous, most of them having only a slight keel, and a few none at all. on the other hand he received a consignment of _pyramidata_ from four localities in sicily, all small, and one of them exactly like the _trochoides_ from pomègues. judging by the samples received from sicily, _trochoides_ is there not more variable than it is in provence, while the sicilian _pyramidata_ is protean. the relations of the two species _helix nemoralis_ and _hortensis_ provide an illustration of another kind of manifestation of local peculiarity. _h. hortensis_ and _nemoralis_ as usually met with, are two very distinct forms. _h. hortensis_ is smaller and duller, and its peristome is white. _h. nemoralis_ is larger and more shiny, and its peristome is brown. in several anatomical points, moreover, especially in the shape of the dart, there are great differences. for a full account of these peculiarities of the two forms and a discussion of their inter-relations the reader is referred to the elaborate work of a. lang[ ] who has studied them extensively and has also succeeded in experimentally raising hybrids between them. these hybrids were in a slight degree fertile with both the parent species, but up to the time of publication no young had been reared from hybrids _inter se_. coutagne describes the result of collections made in french localities. some had exclusively _hortensis_, some exclusively _nemoralis_, and in some the two were found in association. he gives details of five of these collections from which i take the following summary of the more essential facts, omitting much that is almost equally significant. _locality a_, near honfleur. both forms present, each sharply and normally distinguished, without any intermediates. they are thus found in many places. coutagne instances müller's observations in denmark, his own series from the jura, etc. _locality b._ vonges (côte d'or), _hortensis_ taken at random, showed with light peristomes (either more or less pinkish or quite white) and with dark _brown_ peristomes; together with _nemoralis_ all with the usual brown peristomes. of the _hortensis_ were in ground-colour _opalescens_ and _roseus_; and in shape were _umbilicatus_. _locality c_, about kilometres from _b_. there were found _hortensis_, of which had light peristomes and brown; together with _nemoralis_. of the _hortensis_ none were _opalescens_; were _roseus_ and none has the shape of _umbilicatus_. _locality d_, about , metres from _b_. _hortensis_, of which had light peristomes and had brown. no _nemoralis_ were found. none of the _hortensis_ were _opalescens_ or _roseus_, but were _umbilicatus_. in these localities intermediates of every grade existed between the well-characterised _opalescens_, _roseus_, or _umbilicatus_, and the other forms, but there were no intergrades between the other _nemoralis_ and the smaller _hortensis_, about which there was no hesitation. in the next locality a very different state of things was found. _locality e._ banks of the yvette at orsay (seine-et-oise). the actual numbers are not given, but we are told that per cent. were _hortensis_, per cent. _nemoralis_, and per cent. intermediate. as at honfleur, the _hortensis_ had white peristomes, and the _nemoralis_ brown. coutagne's visits to this locality were in and , and he calls attention to the fact that pascal found similar intermediates in the same neighbourhood in . the two species, in coutagne's view, when they occur together, can generally be sorted from each other with perfect confidence, and it is only in exceptional localities that these intermediates occur. whether they are hybrids, or whether sometimes the species in their variations transgress their usual limitations is regarded both by coutagne and by lang as a question not yet answerable with certainty. coutagne moreover lays stress on the fact that although each species may be easily known from the other _in its own district_, yet when shells from different districts are brought together it is sometimes impossible to sort them. he mentions an example of such casual intermixture occurring under natural conditions on an island in the rhone, to which it may well be supposed that floods had brought immigrants from miscellaneous localities. this population contained a very large number of uncertain specimens, and as he says, it was much as if he were to mix the shells from his localities, after which it would certainly be impossible to separate the two species again.[ ] further evidence is given in the same treatise as to other examples of polymorphism, especially in the genus _anodonta_, of which locard made species for france alone. here again are cases like those already given, and many forms or "modes" are found restricted to special localities, while occasionally in the same locality dissimilar forms are found, collectively forming a colony, without intermediates. taken as a whole the evidence shows the following conclusions to be true. local races, whether of animals or plants, may be distinguished by characters which we are compelled to regard as trivial, or again by features of such magnitude that if they were known to us only as the characteristics of a uniform species they would certainly be assumed without hesitation to be essential for its maintenance. local forms may be sharply differentiated from the corresponding populations of other localities or they may be connected with them by numbers of intermediates. not rarely also we find a fact which has always seemed to me of special significance, that the peculiarity of the local population or colony may show itself in a special liability to variation, and this variability may show itself in one of many degrees, either in the constant possession of a definite aberration, in a dimorphism, or in an extreme polymorphism. at this stage attention should be called to two points. first, that when the details of the geographical distribution of any variable species are studied in that thorough and minute fashion which is necessary for any true knowledge of the interrelations of the several forms, the conception of a species invented by the popular expositions of evolution under selection is found to be rarely if ever realised in nature. a species in this generalised sense is an aggregate of individuals, none exactly alike, but varying round a normal type, the characters of which are fixed in so far as they are adapted to environmental exigency. in nature, however, the occurrence of the varieties, and even the occurrence of the variability is sporadic. in one place a population may be perfectly uniform. in another it may be again uniform but distinct. in others the two forms may occur together, sometimes with and sometimes without intergrades. in some localities a sporadic variety may be an element of the population, persisting through long periods of time. in other localities there may be several such aberrations occurring together which are absent elsewhere. secondly, i would remind the reader that in the light of genetic analysis we know that intergrades, when they do occur, cannot be assumed to represent conditions through which the species must pass or has passed on its way to the extreme and definite forms. often, perhaps generally, they are nothing but heterozygous forms, and often also they are conditions corresponding with the presence of factors in their reduction-stages. a broad survey of the facts shows beyond question that it is impossible to reconcile the mode of distribution of local forms with any belief that they are on the whole adaptational. their peculiarities are occasionally the result of direct environmental influence, as we shall hereafter notice in certain cases, but none can attribute such sporadic and irregular phenomena to causes uniformly acting. writers on systematics, especially those of former generations often conjecture or assert that local distinctions are caused by "differences of climate, soil, food, etc.," in vague general terms. it is usually safe to assume that these remarks do not represent conclusions drawn from actual evidence, for only rarely can they be translated into more precise language. so thoroughly have the biological sciences become permeated with the belief that all distinctions are dependent upon adaptation, that the mere existence of definite distinctions is felt by many to be sufficient ground to warrant an assumption that these distinctions are directly or indirectly due to special local conditions. for example, dr. j. a. allen, who has done so much careful and valuable work in delimiting the local forms of the united states fauna, writes of the ground squirrels (tamias)[ ] as follows:-- "from the extreme susceptibility of this plastic group to the influences of environment, it is one of the most instructive and fascinating groups among north american mammals. no one can doubt its comparatively recent differentiation from a common stock, and its dispersion from some common centre. whether the type originated at some point in north america, or in the northern part of eurasia, it is perhaps idle to speculate, but that it has increased, multiplied, spread, and become differentiated to a wonderful degree in north america is beyond question; as it is found from the arctic regions to the high mountain ranges of central mexico, and has developed some twenty to thirty very palpable local phases." "some of them easily take rank as species, others as subspecies. probably a more striking illustration of evolution by environment cannot be cited." he proceeds to point out that the habits of these creatures are such as lead to isolation. this may well be admitted, and indeed no exception can possibly be taken to the passage as a whole, save in the one respect that there is no real proof that the local diversity is due to "evolution by environment" or an indication of "susceptibility to the influences of environment." dr. allen does indeed adduce the fact that california "extending through miles of latitude, with numerous sharply contrasted physiographic regions, has apparently no less than six strongly differentiated forms, while the region east of the rocky mountains from a little below the northern boundary of the united states northward to the limit of trees--a slightly diversified region of at least ten times the area of california--has only one"! but when one comes to ask how the various forms are adaptational, and how the influences of environment have led to their production, only conjectures of a preliminary and tentative character could be expected in reply. desert forms are no doubt pallid as in so many instances, and forest forms are more fully coloured, and we may readily enough accept such facts as indications of a connection between bodily features and the conditions of life, but further than that no one can go; so that when we find size, length of ears or of tail, the number of dorsal stripes, the pattern of the colours, not to speak of differences in the pigments themselves, all exhibiting large modifications, we cannot refer these peculiarities to the causation of environmental difference, save as a simple expression of faith. i incline far more to agree with gulick who, after years of study of the local variations of the achatinellidae, came to the conclusion that it was useless to expect that such local differentiation can be referred to adaptation in any sense.[ ] even the most convinced selectionist must hesitate before such facts as those related by a. g. mayer regarding the distribution of _partula otaheitana_, one of these achatinellidae. the island of tahiti has been scored by erosion so that a series of separated valleys radiate to the coast. from four successive valleys mayer collected the species, and found that in the first (tipaerui) valley all the shells were dextral ( , containing young); in the second valley (fautaua) per cent. of adults and . per cent. of the young contained were sinistral; in the third valley (hamuta) per cent. of adults and per cent. of young contained in them were sinistral; and lastly, in the fourth valley (pirae) all the shells ( , containing young) were sinistral.[ ] in connection with these observations i may mention the fact that in a certain pond in the north of england[ ] the sinistral form of _limnaea peregra_ has been known to occur for about fifty years. visiting it lately i found the left-handed shells to be about per cent. of the population. the species is the commonest british freshwater shell, but left-handed specimens are exceedingly rare. will anyone ask us to suppose that the persistence of a percentage of this rarity in the same place is an indication of some specially favouring circumstance in the waters of that pond? it is a horse-pond to all appearances exactly like any other horse-pond; and i believe that in perfect confidence we may accept the suggestion of common sense, which teaches us that there is nothing particular in the circumstances which either calls such varieties into existence or contributes in any direct way to their survival. had the phenomenon of local variation been studied in detail before darwin wrote, the attempt to make selection responsible for fixity wherever found, could never have been made. the proposition that not only the definiteness of local forms but their variability also is sporadic, can be established by countless illustrations taken from any group of either the animal or the vegetable kingdoms. only exceptionally can the fixed differences be even suspected of contributing to adaptation, and sporadic variability, which is a no less positive fact, must manifestly lie outside the range of such suspicions. it is open to any one to suggest speculatively that the persistence of special varieties or of special variability in special places is an indication that in those places the conditions of life are such that the forms in question are tolerated though elsewhere the same types are exterminated; but that consideration, even if it could be proved to be well founded, is not one which lends much force to the thesis that definiteness of type is a consequence of natural selection. on the contrary, recourse to such reasoning implies the inevitable but very damaging admission that the stringency of selection is frequently so far relaxed that two or more equally definite forms of the same species can persist side by side. there is no doubt that this is the simple truth, but when once that truth is perceived it is useless to invoke the control of selection as the factor to which definiteness of type in general must be referred. the genetic relations of local forms to each other cannot in the absence of actual breeding experiments be often ascertained. standfuss formerly enunciated as a general principle that when two forms co-exist in the same locality and are able to interbreed, they do not produce intermediates; but that when the forms are geographically separated as local races, crosses between them result in a series of intermediates.[ ] in this aphorism there is a good deal of truth, but if in the light of mendelian principles we examine the two statements we see now that the first is in reality only another way of saying that the distinctness of an aberrational form co-existing with another is due to segregation, accompanied by some degree of dominance of one type. whether, however, one geographically isolated race will give intermediates when bred with another must depend entirely on the genetic physiology of the special case, and no general rule can be laid down. it may well be that, inasmuch as the distinctness of the variety is maintained by isolation, the difference in factorial composition between it and the representative form in another area is neither simple nor sharp; but when two varieties co-exist, though interbreeding, it is now clear that their differences must depend on the segregation of simple factors. plainly such aberrations may in one place co-exist with another type, and elsewhere be separated from it as local races. excellent illustrations of these two stages in evolution are provided by the melanic varieties of british lepidoptera. the fact that black or blackish varieties of many species especially of geometridae have come into existence in recent years is well known to british collectors, and it is not in dispute that they have in several instances replaced the older type more or less completely in certain districts. in the year the evolution committee of the royal society instituted a collective inquiry as to the contemporary distribution of these dark varieties. as the change had happened within living memory and had greatly progressed in recent years it was hoped that a record of the existing distribution would serve as a point of departure for future comparison. the records thus obtained were tabulated by mr. l. doncaster.[ ] from that account and from the statements in barrett's british lepidoptera[ ] this description of some of the more notable cases is taken. the most striking and familiar case is that of _amphidasys betularia_, of which only the ordinary type was known in any locality until about - , when the totally black var. _doubledayaria_ first appeared in the neighbourhood of manchester. this black form was subsequently recorded in huddersfield between and ; kendal about ; cannock chase, ; berkshire, ; norfolk, essex and cambridge about ; suffolk, ; london, . for the southern counties of england, except in the london district, there are still very few records. it cannot of course be asserted positively that the variety spread from its place of first appearance into the other localities, and that it did not arise _de novo_ in them, but there can be little doubt that the process was one of colonisation. on the european continent the first records are from hanover in , belgium and , crefeld -, berlin , dresden about the same date. as regards the increase of the variety we have the fact that in lancashire, cheshire and the west riding of yorkshire the black is now the prevalent form; and in some places, as for example, huddersfield, the black alone is now found, though it was unknown there till between and . about at newport, monmouth, the two forms were in about equal numbers, but a few years later the type had almost vanished. similarly in crefeld, where the black form was still very rare in the eighties, it now forms about per cent. of the population. in the london district the black remains scarce and at the date of the report it was still very scarce. from ireland there is only one record and there are hardly any from scotland. _boarmia repandata_ is another species which is behaving in a somewhat similar way. unlike _betularia_, however, the species is a variable one, and has several colour-forms, amongst them the banded var. _conversaria_, and many others. in addition to these there is a black form in the north of england which seems to be spreading. in huddersfield the black was first recorded in , and in - per cent. were black. at rotherham the black or very dark are now prevalent and have increased in the last years. from the midlands, east anglia and southern counties the returns show only the light and medium forms. of _odontoptera bidentata_ several intergrading dark forms exist, and these are found exclusively in the north and the midlands. unicolorous blacks have been found recently in the lancashire mosses and at wakefield. at huddersfield years ago the light forms were prevalent, but now a rather dark brown, not infrequently suffused with black, is the commonest. in southern counties only light forms are known. _phigalia pilosaria_ in south england is always light, but in the north the prevalent form is darker. about years ago a form with unicolorous sooty fore-wings and dull grey hind wings was first seen in yorkshire and a similar form is now taken regularly in south wales. in the following cases the dark varieties were found originally only in the south. _boarmia rhomboidaria_ gave rise about years ago to a unicolorous smoky variety called _perfumaria_. this was at first peculiar to the london district, but it has since been taken in birmingham and other large cities. more lately coal-black specimens have been found at norwich, and others similar but hardly so dark were taken in the south of scotland and at cannock chase. _eupithecia rectangulata_ is a similar case. formerly the light forms were prevalent but within sixty years they have almost entirely been replaced in the south of london by a nearly black form. _tephrosia_ (_boarmia_) _consortaria_ and _tephrosia consonaria_ are exceptionally interesting, for they have both given off dark forms in the same wood near maidstone, which is far from the usual "centres of melanism." they were discovered in this locality by mr. e. goodwin. that of _consortaria_ is a dark grey, but that of _consonaria_ is a full black, and nothing like either has been found anywhere else. these examples are all taken from the geometridae but others, though of a less conspicuous kind, could be given from the noctuidae or the micro-lepidoptera. _acronycta psi_, for instance, has a suffused form which is believed to be becoming more frequent in the london district. _polia chi_ has two dark forms, _olivacea_, a yellowish grey with dark markings, and _suffusa_ which is a darker, blackish-slate colour. both occur in the north of england, sometimes together, sometimes separately, or mixed with the type and many intermediates. the distribution is peculiarly irregular. at huddersfield, where the very dark form appeared suddenly about , some per cent. are said to be now dark and about - per cent. very dark, but at saddleworth, miles away, only the pale forms occur. several questions of interest arise in regard to this evidence. this progressive melanism has arisen in certain families only, and may be confined to certain species only, within those families. as in almost all other examples in which variation has been much observed, its incidence is capricious and specific. a collateral line of inquiry relates to the degree of discontinuity which the variation manifests. here again there is no rule. generally speaking, in _a. betularia_, to take the case most fully studied, the variation is discontinuous. real intermediates between _betularia_ and _doubledayaria_ are in most localities absent or rare. the black spots of _betularia_ may often be larger or more numerous than in the normal, but this variation has nothing to do with _doubledayaria_, and is not an intermediate stage towards it, though sometimes wrongly so described. _doubledayaria_ owes its characteristic appearance to a factor which blurs the surface of the wings with a layer of black. sometimes this blurring is slighter than in the real _doubledayaria_, and these forms are real intermediates. occasionally the fore-wings alone are thus blurred. these intermediates are clearly due to reduction-stages of the _doubledayaria_ factor, and are related to it as a blue mouse is to a black, or a dutch rabbit to a self-colour. it cannot positively be asserted that the full _doubledayaria_ existed before the intermediate, but it almost certainly did. in certain places as for instance in belgium, there is evidence that intermediates have at various times been fairly abundant, but they have never become common, nor are they known to exist in the absence of _doubledayaria_. when the black variety and the light type breed together they do not usually have intermediates among their offspring, and the evidence is consistent with the view that the black is a complete dominant. the same is probably true of _tephrosia consonaria_. in some of the other species we know that the darkest forms did not appear first. for example in _phigalia pilosaria_ and _boarmia rhomboidaria_ dark forms existed and are believed to have increased in number before the darkest made its appearance. _hybernia progemmaria_ is said to have become darker gradually both in cheshire and in the west riding, and a uniformly smoky variety appeared in south yorkshire less than years ago which has spread to neighbouring counties. the dark medium has become the commonest form in huddersfield district, where the very dark variety is now about per cent. of the population, though the light form is still common. taking the evidence together we find it consistent with the view that dark forms have appeared sporadically, in some species the very dark appearing first and intermediates later, in others the moderately dark came first and the darkest later in time. it is practically certain that the change has in general come about not by a gradual change supervening on the population at large, but by the sporadic appearance of dark specimens as a new element in the population, and strains derived from these dark individuals have gradually superseded the normal type more or less completely. if it could be shown that these melanic novelties had a definite advantage in the struggle for existence they would provide an instance of evolution proceeding much in the way which darwin contemplated. the whole process would differ from that conceived by him as the normal method of evolution only in so far as the change has come about with great rapidity and in some instances largely by the appearance and success of discontinuous varieties. the question, however, must be asked whether the dark form can reasonably be supposed to have an advantage by reason of their darkness. some naturalists believe that the darkness of the colours does thus definitely contribute to their protection by making the insects less conspicuous and thus more likely to escape the search of birds. in support of this view it may be pointed out that it is in the manufacturing districts of lancashire and yorkshire, and again in the london area that the melanics have attained their greatest development. consistently with this argument also, it is in the neighbourhood of crefeld and essen, the black country of germany, that they have chiefly established themselves on the continent, and _phigalia pilosaria_ in the black form is now at home in south wales. thus superficially regarded, the evidence looks rather strong, but it is difficult to apply the reasoning in detail. we have first the difficulty that the black form of _betularia_ for instance has established itself in thoroughly rural districts, notably near king's lynn in norfolk, and in the neighbourhood of kendal and windermere. the black form of _consonaria_ and the dark _consortaria_ appeared in a wood near maidstone, far from town smoke, and the black _rhomboidaria_ was first found at norwich, which, as towns go, is clean. then again the spread of the melanics is very irregular and unaccountable. the black _pilosaria_ is found both in the west riding and in the swansea district, but not yet elsewhere. it rapidly increased at huddersfield, but made no noticeable progress at sheffield though recorded there for ten years. it is also a remarkable fact that no similar melanic development has been observed in america, and, so far as i am aware, comparable melanic varieties have not appeared on the european continent except in the case of the few sorts which possibly may have come from england. the whole subject is beset with complications. it must not be forgotten that in a few species of moths there is an obvious and recognised conformity between the colours of the perfect insect and that of the soil on which they live, comparable with that which is so striking in the case of some oedipodidae and other grasshoppers. of this phenomenon the clearest example is _gnophos obscurata_, which is a most variable species with many local forms. of these a well-known dark variety lives on the peaty heaths of the new forest and other districts, but on the chalk hills of kent, sussex and surrey various light varieties are found, of which one is a bright silvery white, very near in colour to the colour of a chalky bank. this case does not seem to be one of direct environmental action,[ ] for poulton found no change induced by rearing larvae among either white or black surrounding objects. no one however can doubt that there is some indirect connection between the colour of the ground and that of the moths. to my mind there is a serious objection to the theory of protective resemblance in application to such a case as that of the _betularia_ forms, which arises from the fact that the black _doubledayaria_ is a fairly conspicuous insect anywhere except perhaps on actually black materials, which are not common in any locality. tree trunks and walls are dirty in smoky districts but they are not often black, and i doubt whether in the neighbourhood of rotherham, for instance, which is one of the great melanic centres, _doubledayaria_ can be harder for a bird to find than _betularia_ would be. after all, too, many of the species much affected are not urban insects. they live in country places between the towns, and the general tone of these places even in lancashire and the west riding is not very different from that of similar places elsewhere. as against the objection that the black varieties are much blacker than the case requires it may be replied that we know nothing of the senses of birds, and that perhaps to their eyes blackness does constitute a disguise even though the surroundings are much less dark. this is undeniable, but recourse to such an argument is dangerous; for if the sight of the insect-eating birds is so dull that it does not distinguish dark things from dingy grey, we cannot subsequently regard the keen sight of birds as the sufficient control which has led to the minute and detailed resemblance of many insects to their surroundings. those who see in such cases examples of the omnipotence of selection must frequently find themselves in this dilemma. taking the evidence as a whole, we may say that it fairly suggests the existence of some connection between modern urban developments and the appearance and rise of the melanic varieties. more than that we cannot yet affirm. it is a subject in which problems open up on every side, and all of them are profitable subjects for investigation. unhappily such animals are difficult to rear successfully in captivity for many generations, owing to their extreme liability to disease. not the least interesting feature of the melanics is the fact that the black varieties provide about the best and clearest example of a new dominant factor attaching itself to a wild species in recent times. none of the cases are satisfactorily recorded or analysed as yet, but the evidence is clear that _doubledayaria_ is a dominant to its type, and in several other dark varieties, though the pigment deposited is not black, the records show that the increased amount of the pigment almost certainly is due to a positive factor. of this, _hemerophila abruptaria_ is a good example.[ ] there are some irregularities in the results, but taken together they leave little doubt that the dark brown variety is a dominant and the light, yellowish brown a recessive. a curious parallel to the rise of the melanic moths in england is provided by the case of the honey-creepers or sugar-birds, in certain west indian islands.[ ] these birds of the genus _coereba_ (_certhiola_) range from southern mexico to the northern parts of south america and through the whole chain of the west indian islands and bahamas except cuba. there are numerous local forms, and many of the islands have types peculiar to themselves, as is usual in such cases. some of the types or species range through several islands, but according to austin clark[ ] no island has more than one of them. cory[ ] reckoned twelve such species within the antillean region. they are small birds about the size of a nuthatch with a general colouring of black, yellow, and white. from the island of st. vincent the smithsonian institution received in the late seventies of last century several completely black specimens in addition to two of the usual type of colouring. the black were described by w. n. lawrence as _atrata_, and those marked with the usual yellow and white were called _saccharina_. the collector (mr. f. a. ober) reported that the black form was common, and that the _saccharina_ form was rarer. lawrence remarks, "had there been only a single example (of the black form) i should have considered it as probably a case of abnormal colouring, but it seems to be a representative form of the genus in this island."[ ] there is of course no doubt of the correctness of the view taken by austin clark that "_atrata_" is a black variety. the black bird is in every respect, other than colour, identical with _saccharina_, and it is even possible to detect a greenish colour in the areas which would normally be yellow, showing plainly enough the yellow pigment obscured by the black. we have next the interesting fact that like our melanic moths the dark form is replacing the "type." at the time of ober's visit the type was already in a minority, but now it is nearly or perhaps actually extinct, though the black form is one of the commonest birds on the island. austin clark found no specimen when he collected there in - , though formerly it was not uncommon in the vicinity of kingston and in the immediate windward district of st. vincent. the grenadines are geographically just south of st. vincent, though separated by a deep channel. in these islands no black forms have yet been taken, but grenada, the next island to the south, has both normals and blacks. there are trifling differences of size between the grenada birds and those from st. vincent, the grenada specimens being slightly smaller and for this reason they have received distinct names, the form marked with yellow and white being called _godmani_ (cory) and the black, _wellsi_ (cory), but this merely introduces a useless complication. there is evidence that in grenada, as in st. vincent, the black is gradually ousting the original type, but the process has not gone so far as in st. vincent. austin clark very properly compares this case of the sugar-birds with that of _papilio turnus_, which as is well-known, has a black female in the southern parts of its distribution, in addition to a female of the yellow type, but in the northern states the black female does not occur. during the present year p. r. lowe, who lately studied _coerebas_ on a large scale in the west indies, has published an important paper on the subject.[ ] he calls attention to the fact that cory recently found a black form of _coereba_ on los roques islands, and he himself discovered another on the testigos islands. both localities are on the coast of venezuela, far from st. vincent and grenada. the whole problem is thus further complicated by the fact that the black varieties have, as we are almost driven to admit, arisen independently in remote places. improbable as this conclusion may be, it is still more difficult to regard all the black forms as derived from one source. for first, they present definite small differences from each other; and secondly we have to remember a consideration of greater importance, that the very fact that each island has its own type must be accepted as proving that the localities are effectively isolated from each other, and that migration must be a very rare event. the rarity of such illustrative cases is, i believe, more apparent than real. it is probably due to the extreme reluctance of systematists to admit that such things can be, and of course to the almost complete absence of knowledge as to the genetic behaviour of wild animals and plants. only in such examples as this of the _coereba_, where colour constitutes the sole difference, or that of the moths which have been minutely studied by many collectors, does the significance of the facts appear. the arrangement of catalogues and collections is such that much practical difficulty of a quite unnecessary kind is introduced. for example, in this very case of _coereba_, i find the british museum has a fine series from grenada including normals and black, and also blacks from st. vincent. if the black specimens from grenada were put with the normals which are almost certainly nothing but a recessive form of the same bird, the variation would strike the eye on even a superficial glance at the drawer. but following the notions so naively expressed in the passage quoted above from w. n. lawrence, the blacks from grenada are put apart together with the other blacks from st. vincent, though two of them were shot on the same date as one of the normals. footnotes: [ ] for the evidence see tutt, j. w., _trans. ent. soc._, , p. . compare the remarkable case given by gulick (_evolution racial and habitudinal_, p. ) of the two races of _cicada_, which are separated by reason of their life-cycles, one having a period of , the other years. [ ] for references see _materials_, p. , and also g. baur, _amer. nat._, , july, p. . [ ] jenner weir, _entomologist_, , xiii, p. . [ ] jentink, _notes leyden mus._, , vii, p. . specimens illustrating this peculiarity are in the british museum. [ ] _proc. zool. soc._, , p. . plate. many points beyond that mentioned above are involved in this remarkable case. for example, not only are there males like females, but a small proportion of females resemble the ordinary male type. the stripes are not merely the spots produced, for they occupy different anatomical positions. the spots almost always go with a black ventral surface, but the striped forms nearly always have that region testaceous. _spartium retama_, the food-plant, will not grow in england, but if it could be naturalised in america the whole problem might be investigated there and results of exceptional interest would almost certainly be attained. [ ] doncaster, l., _proc. zool. soc._, , ii, p. . [ ] i am not aware that the details of this striking case have ever been worked out. it should be noted that the green and blue forms are not due to simple modification of the red pigment; for these colours, due to interference, fork over the area occupied by the red lines. the distinctions between these forms cannot therefore be simply chemical, as we may suppose them to be, for instance, in the case of many red and yellow forms, and the genetic relationships of the _heliconid_ varieties would raise many novel problems and be well worth studying experimentally. [ ] woodeforde, f. c., _trans. north staffordshire field club_, xxxv, , plate. [ ] e. gallé, _compte rendus du congres internat. de bot. a l'expos. univ._, , p. . [ ] flora of mentone, - , _nova acta acad. caes._, xxxv, . [ ] i owe these facts to canon a. m. norman, who showed me illustrative specimens. they were originally described by bowerbank (_monogr. brit. spongiadae_, vol. ii, pp. and xx; vol. iii, pls. i and iii). a specimen of _g. compressa_ measured inches, with a greatest width of - / in. _g. ciliata_ was found measuring in. long and / in. wide. these dimensions are many times those of normal specimens. [ ] coutagne, g., _recherches sur le polymorphisme des mollusques de france_, _annales soc. d'agric. sci. et industr. lyon_, . [ ] as to the synonymy and references see coutagne, p. . [ ] a. lang, _die bastarde von h. hortensis muller h. nemoralis l._ jena, g. fischer, ; with a fine coloured plate showing the varieties of the species and their hybrids. [ ] with this evidence compare that given by a. delcourt in his valuable papers lately published relating to the variations of _notonecta_. see especially _bull. sci. fr. belg._, , xliii, p. ; and _c. r. soc. biol._, , lxvi, p. . [ ] allen, j. a., _bull. amer. mus. n. h._, iii, , pp. - . [ ] j. t. gulick, _evolution, racial and habitudinal_, carnegie institution, publication no. , . [ ] a. g. mayer, _mem. mus. comp. anat. harvard_, vol. xxvi, , p. . from the tables given i cannot ascertain the actual numbers from the two intermediate valleys, but they were considerable. [ ] to which i was very kindly guided by mr. c. t. trechmann. [ ] standfuss, _handbuch d. paläarkt gross-schmet_, , p. . [ ] _ent. rec._, xviii, no. , . [ ] this evidence was largely collected by mr. g. t. porritt, who has given much attention to the subject. [ ] such direct action has of course been proved to occur in the case of several dimorphic larvae (_e. g._, _a. betularia_, itself) and pupae. [ ] see harris, _proc. ent. soc. london_, , p. lxxii, and , p. lxiii; also hamling, _trans. city of london ent. soc._, , p. . [ ] i am indebted to mr. outram bangs of the harvard museum for calling my attention to this remarkable case. [ ] _auk_, , vi, p. . [ ] _ann. n. y. acad. sci._, , i, p. . [ ] _ann. n. y. acad. sci._, , i, p. . [ ] _ibid_, , pp. - . chapter vii local differentiation. _continued_ overlapping forms the facts of the distribution of local forms on the whole are consistent with the view that these forms come into existence by the sporadic appearance of varieties in a population, rather than by transformation of the population as a whole. of such sporadically occurring varieties there are examples in great abundance, though by the nature of the case it can be but rarely that we are able to produce evidence of a previous type being actually superseded by the variety. when the two forms are found co-existing in the same area they are usually recorded as one species if intergrades are observed, and as two species if the intergrades are absent. on the other hand when two forms are found occupying separate areas, when, that is, the process of replacement is completed in one of the areas, then forthwith each is named separately either as species or subspecies. successive observations carried out through considerable periods of time would be necessary to establish beyond question that the history proceeds in one way rather than another. such continuity of observation has for the most part never been attempted. the kind of information wanted has indeed only been lately recognized, and really critical collecting is a thing of only the last few decades. the methods of the older collectors, who aimed at bringing together a few typical specimens of all distinct forms, are of little service in this class of inquiry, which is better promoted by the indiscriminate collection of large numbers of common forms from many localities. when this has been done on a comprehensive scale we shall be in a position to form much more confident judgments as to the general theory of evolution. some little work of the kind has however been done and the results are already of great value. seeing that the differentiation of local forms is only made possible by isolation, it necessarily happens that the collector finds one form in one locality and another in a distinct locality, and there is no evidence as to the behaviour which the two representative species might exhibit if they came into touch with each other. in the most familiar examples of such distinction each inhabits an island, completely occupying it to the exclusion of any other similar form. it can only be when the two representative species occupy parts of a continental area connected with each other by regions habitable for the organism in question, that there is a chance of seeing the two forms in contact. often also, even where this condition is satisfied, the habits, social organisation, or some other special cause may act as a barrier which prevents the distinguishable forms from ever coming into such complete contact as to interbreed or to behave as a genetically continuous race. when genetic continuity is ensured by a constant diffusion of the population over the whole area which they inhabit there will manifestly be no formation of local races. the practical uniformity, for example, of so many species of birds which inhabit widely extended ranges of western europe is doubtless maintained by such constant diffusion. when, as in the case of the falcons, many localities have peculiar forms, the fact may be taken as conclusive evidence that there is little or no diffusion; and when we find in such a species as the goldfinch that in spite of migratory fluctuations there are nevertheless geographical races fairly well differentiated, it may similarly be inferred that these fluctuations habitually move up and down on paths which do not intermingle. there are however a few examples of animals, not given to much irregular wandering, which occupy a wide and continuous range of diversified country and are differentiated as local races in two or more districts, though the distinct races meet in intervening areas. of these the most notorious illustration which has been investigated with any thoroughness is that of the species of _colaptes_ (woodpeckers) known in the united states as flickers. the study of the variations of these forms, made by j. a. allen[ ] is an admirable piece of work, with which every student of variation and evolutionary problems should make himself familiar. the two forms with which we are most concerned are known as _c. auratus_ and _c. cafer_, and are very strikingly different in appearance. in size, proportions, general pattern of colouration, habits, and notes, the two are alike, but they differ in the following seven respects as stated by allen. _auratus_ _cafer_ . quills _yellow_. . quills _red_. . male with a _black_ malar . male with a _red_ stripe. malar stripe. . adult female with _no_ . adult female with usually a malar stripe. brown malar stripe. . _a scarlet nuchal crescent . no nuchal crescent in in both sexes._ either sex. . throat and fore neck . throat and fore neck _brown_. _grey_. . whole top of head and hind neck . whole top of neck and hind _grey_. neck _brown_. . general plumage with an . general plumage with a _olivaceous_ cast. _rufescent_ cast. these differences are illustrated in the accompanying coloured plate, which has been most kindly prepared for me under the instructions of dr. f. m. chapman of the american museum of natural history. before going further it is worth considering the nature of these differences a little more closely. all but the last are large differences which no one would overlook even in a hasty glance at the birds. if the only distinction lay in the colour of the quills we might feel fairly sure that _auratus_ was a recessive form of _cafer_, and so probably it is in this respect. similarly the black malar stripe of _auratus_ is in all probability recessive to the red malar stripe of _cafer_ and i imagine the pigments concerned are comparable with those in the gouldian finch (_poephila gouldiae_) of australia. both sexes in that species may have the head black, red, or, less often, yellow, and though it is not any longer in question that birds may breed in either plumage, i believe that the young are always black-headed and i imagine that those which become red-headed possess a dominant factor absent from the permanently black-headed birds.[ ] yellow as a recessive form of a red is certainly very common, but red and black as variants of the same pigment are less usual. in the gouldian finch we seem to have a case where a pigment can assume all three forms. it would be interesting to know whether the red of the malar stripes in _colaptes_ is a pigment of the same nature as the red of the quills. both in _colaptes_ and in _poephila gouldiae_ i have seen specimens intermediate between the black and the red, and the appearance of the part affected was exactly alike in the two cases, red feathers coming up among the black ones, and many feathers containing both red and black pigments mixed together. the development of the scarlet nuchal crescent in _auratus_ and the absence of this conspicuous mark in _cafer_ constitute from the physiological point of view the most remarkable pair of differences. when the red crescent is not formed, the feathers which would bear it are exactly like the rest, and no special pigment is visible in them which one can regard as ready to be modified into red. if the crescent is due to a factor it must therefore be supposed that this factor has the power of modifying the pigment of the neck in one special place alone. dr. w. d. miller called my attention to the fact that a similar variation occurs in another american woodpecker, the sapsucker, _sphyropicus varius_.[ ] i do not suggest that such variations are without parallel: indeed in _p. gouldiae_ the factor which turns the black of the head into scarlet affects one special region of the black only, being sharply distinct from the unmodified black of the throat. these regions of the head are however often the seat of special colours in birds.[ ] so also may be instanced the variety of the common guillemot (_uria troile_) which has a white line round the eyes and at the sides of the head where the normal has no such mark; but this line is formed in a very special place, the groove joining the eye to the ear, whereas the feathers of the nuchal crescent are not ostensibly distinguished from those adjacent.[ ] the transposition of the brown and the grey on the back and front of the neck also constitutes a very remarkable difference. if either grey or brown depends on a factor then it must be supposed that _auratus_ has one of these factors and _cafer_ the other. from these several considerations it is quite clear that if _auratus_ and _cafer_ are modifications of the same type produced by presence or absence of factors, several independent elements must be concerned, and to unravel their inter-relations would be most difficult even if it were possible to breed the types under observation, which is of course quite beyond present possibilities. the distribution of the two is as follows. on the east side of the continent _c. auratus_, relatively pure, occupies the whole of canada and the states from the north to galveston. westward it extends across the whole continent in the more northern region to alaska, but in its pure form it only reaches down the pacific coast to about the northern border of british columbia. its southern and western limit is thus roughly a line drawn from north of vancouver, southeast to north dakota and then south to galveston. _c. cafer_ in the comparatively pure form inhabits mexico, arizona, california (except lower california and the opposite coast), central and western nevada, utah, oregon, and is bounded on the east by a line drawn from the pacific south of washington, south and eastward through colorado to the mouth of the rio grande or the gulf of mexico. between the two lines thus roughly defined is a band of country about , - , miles long and - miles wide, which contains some normal birds of each type, but chiefly birds exhibiting the characters of both, mixed together in various and irregular ways. even in the areas occupied by the pure forms occasional birds are recorded with more or less indication of characteristics of the other form, but within the area in which the two forms are conterminous, the mixed birds are in the majority. the condition of these birds of mixed character is described by allen as follows: "as has been long known--indeed, as shown by baird in --the 'intermediates' or 'hybrids' present ever-varying combinations of the characters of the two birds, from individuals of _c. auratus_ presenting only the slightest traces of the characters of _c. cafer_, or, conversely--individuals of _c. cafer_ presenting only the slightest traces of the characters of _c. auratus_--to birds in which the characters of the two are about equally blended. thus we may have _c. auratus_ with merely a few red feathers in the black malar stripe, or with the quills merely slightly flushed with orange, or _c. cafer_ with either merely a few black feathers in the red malar stripe, or a few red feathers at the sides of the nape, or an incipient, barely traceable scarlet nuchal crescent. where the blending of the characters is more strongly marked, the quills may be orange-yellow or orange-red, or of any shade between yellow and red, with the other features of the two birds about equally blended. but such examples are exceptional, an unsymmetrical blending being the rule, the two sides of the same bird being often unlike. the quills of the tail, for example, may be part red and part yellow, the number of yellow or red feathers varying in different individuals, and very often in the opposite sides of the tail in the same bird. the same irregularity occurs also, but apparently less frequently, in the quills of the wings. in such cases the quills may be mostly yellow with a few red or orange quills intermixed, or red with a similar mixture of yellow. a bird may have the general colouration of true _cafer_ combined with a well-developed nuchal crescent, or nearly pure _auratus_ with the red malar stripes of a _cafer_. sometimes the body plumage is that of _c. auratus_ with the head nearly as in pure _cafer_, or exactly the reverse may occur. or we may have the general plumage as in _cafer_ with the throat and crown as in _auratus_, and the malar stripe either red or black, or mixed red and black, and so on in almost endless variations, it being rare to find, even in birds of the same nest, two individuals alike in all their features of colouration. usually the first trace of _cafer_ seen in _auratus_ manifests itself as a mixture of red in the black malar stripe, either as a few red feathers, or as a tipping of the black feathers with red, or with merely the basal portion of the feathers red. sometimes, however, there is a mixture of orange or reddish quills, while the malar stripe remains normal. in _c. cafer_ the traces of _auratus_ are usually shown by a tendency to an incipient nuchal crescent, represented often by merely a few red-tipped feathers on the sides of the nape; at other times by a slight mixture of black in the red malar stripe." such a state of things accords very imperfectly with expectations under any received theory of evolution. as in some of the instances discussed in the first chapter we have here two fairly definite forms, nearly allied, which on any evolutionary hypothesis must have been evolved either the one from the other, or both from a third form at a time not very remote from the present, as time must be measured in evolution. yet though intermediates exist in some quantity, no one can for a moment suggest that they are that definite intermediate from which _auratus_ and _cafer_ descend in common. one cannot imagine that the immediate ancestor of these birds was a mosaic, made up of asymmetrical patches of each sort: but that is what many of the intermediates are. it is not much easier to suppose the ancestor to have been a nondescript, with a compromise between the developed characters of each, with quills buff, malar stripes neither black nor red, with a trace of nuchal crescent, and so on. such frankenstein-monsters have played, a considerable part in the imaginations of evolutionary philosophers, but if it were true that there was once a population of these monsters capable of successful existence, surely they should now be found as a population occupying the neutral zone between the two modern forms. yet, though much remains to be done in clearing up the facts, one thing is certain, namely that the neutral zone has not a definite and normally intermediate population, but on the contrary it is peopled by fragments of the two definite types and miscellaneous mongrels between them. on the other hand, one cannot readily suppose that either form was the parent of the other. the process must have involved both addition and loss of factors, for whatever hypothesis be adopted, such changes must be supposed to have occurred. a careful statistical tabulation of the way in which the characters are distributed in the population of the mixed zone would be of great value, and till that has been done there is little that can be said with certainty as to the genetics of these characters. in the collection of dr. bishop of new haven i was very kindly allowed to examine a sample, all taken at random, near together, in saskatchewan. there were females adult, young; males adult and young. this number, though of course insufficient, is enough to give some guide as to the degree of definiteness which the characters generally show in their variations. of the birds, had simply yellow quills; had red; was almost red but had one yellow tail-quill; were intermediate and was buff. as regards the malar patch, which can only be determined properly in the adult males, was red, was approximately red, intermediate. as to nuchal crescent females had none, females very slight; males had it, had only a slight crescent, and had none. in point of quills therefore were definite out of ; in point of crescent, were definite out of ; and in point of malar patch only was definite out of . the last is a feature directly dependent on age and so counts for less, but as regards the other two features there is some indication that the factors show definiteness in their behaviour. it must be remembered that we have no knowledge what the heterozygous form may be, and in the case of red and yellow it is probably a reddish buff. the patch-works are no doubt to be compared with other well-known pied forms, and in these we must suppose the active factor broken up, which it probably can be very easily. the asymmetry, which allen notices as so marked a feature, in the distribution of the red and yellow quills of the tail especially, recalls that of the black markings in the pied canaries. as is well known to students of variations _some_ pigment-factors in _some_ animals are apparently uncontrolled by symmetry, while in other specific cases symmetry is the rule. on the other hand the blackness or redness of the malar patches is, i think, as a rule nearly symmetrical. it should be mentioned that two of dr. bishop's young birds belonged to the same nest, one a female with _red_ quills, the other a male with _yellow_. both are without crescent. as to the question whether certain combinations of characters occur with special frequency, the evidence is insufficient to give a definite answer. among all the birds i have seen in america or in england i have not yet found one having the malar patches black without any nuchal crescent. of dr. bishop's adults not one, however, showed the combination of the three chief features normal for _auratus_ or for _cafer_. besides the two forms that we have hitherto considered, several other local types exist, and these throw some further light on the problem. of these the most important in this connexion is _chrysoides_, which inhabits the whole of southern california and the mainland opposite. this remarkable form is as allen says, very different from _auratus_ except that it has the quills yellow like _auratus_, not red like _cafer_. so that we find here in the extreme west of the whole distribution a type agreeing in one of its chief features with the eastern type. between this and _cafer_ intergrades have, according to allen, not been found. the relations of this _chrysoides_ are, allen thinks, rather with _mexicanoides_, a southern, smaller race with colours more intense, which inhabits guatemala, but however that may be, it must be regarded as a _cafer_ which has lost its red quills. the island of guadeloupe off lower california has an island form. beyond the other side of the continent there is also an island form of _auratus_, inhabiting cuba, so that clearly the yellow quills can extend into the tropics. the above account is in many respects incomplete, but it suffices to give an outline of the chief facts. the whole problem is complicated by the undoubted effects of an uncertain amount of migration, and in many, perhaps all, districts, the winter population differs from the summer population of the same localities. the existence of these seasonal ebbs and flows is now well known to ornithologists, and most of the bird species of temperate regions are subject to them. difficult as it may be to conceive the actual process of origin of the two types _auratus_ and _cafer_, it is i think still harder to suggest any possible circumstance which can have determined their development as distinct races, or which can maintain that distinctness when created. some will no doubt be disposed to appeal once more to our ignorance and suggest that if we only knew more we should see that the yellow quills, the black "moustache" and the red crescent, specially qualify _auratus_ for the north and eastern region, and the red quills, red "moustache" and absence of crescent fit _cafer_ to the conditions of its homes. each can judge for himself, but my own view is that this is a vain delusion, and that to cherish it merely blunts the receptivity of the mind, which if unoccupied with such fancies would be more ready to perceive the truth when at last it shall appear. think of the range of conditions prevailing in the country occupied by _auratus_--a triangle with its apex in florida and its base the whole arctic region of north america. is it seriously suggested that there is some element common to the "conditions" of such an area which demands a nuchal crescent in the flickers, though the birds of the _cafer_ area, almost equally varied, can dispense with the same character? curiously enough, the geographical variation of _sphyropicus varius_, another though a very different woodpecker[ ] shows that conversely the nuchal crescent can be dispensed with in the eastern form though it is assumed by the western.[ ] allen points out the interesting additional fact that superposed upon each of the two distinct forms, _auratus_ and _cafer_, are many geographical variations which can very naturally be regarded as climatic. each decreases in size from the north southward, as so many species do.[ ] they become paler in the arid plains, and show the ordinary phases which are seen in other birds having the same distribution. such differences we may well suppose to be determined directly or indirectly, by environment, and we may anticipate with fuller knowledge it will be possible to distinguish variations of this nature as in the broad sense environmental, from the larger differences separating the two main types of _colaptes_, which i surmise are altogether independent of such influences. it is generally supposed that phenomena like those now so well established in the case of _colaptes_ are very exceptional, and as has already been stated a number of circumstances must combine in order that they may be produced. i suspect however that the examples are more numerous than is commonly thought. in all likelihood the three forms _sphyropicus varius_, _nuchalis_ and _ruber_ are in a very similar condition though the details have not, so far as i know, been worked out. a complex example which is closely parallel to the case of _colaptes_ was described by f. m. chapman[ ] at the same date as allen's work. this is the case of _quiscalus_, the grackles, which in the north american continent have three fairly distinct forms which chapman speaks of as _q. aeneus_, _q. quiscula_, and _q. quiscula aglaeus_. the birds are all, so far as pigment is concerned, dark blackish brown, but the head and mantle have superposed a metallic sheen of interference-colours which in the various forms take different tints, bluish green, bronze green, or bronze purple. the details are complicated and difficult to appreciate without actual specimens, but the two common types are sufficiently distinct. the birds inhabit the whole area east of the rockies, _quiscula aglaeus_ occupying florida and the southern states southwest of a band of country about a hundred miles broad extending roughly from connecticut to the mouth of the mississippi; and _aeneus_ taking the area north and west of this band. in discussing this case chapman expresses the same view as allen does in the _colaptes_ case, that there are two distinct populations, substantially fixed, and that the band of country in which they meet each other has a mongrel population, with no consistent type, but showing miscellaneous combinations of the character of the two chief types. the warblers of the genus _helminthophila_ provide another illustration which has points of special interest. the two chief species are _h. pinus_, which has a yellow mantle and lower parts, white bars on the wings, a black patch behind the eyes and a broad black mark on the throat; and _h. chrysoptera_ with dark grey mantle and pale whitish grey lower parts, yellow bars on the wings, and grey marks on cheeks and throat where _pinus_ has black. these two birds are exceeding distinct, and in addition their songs are quite unlike. _h. pinus_ ranges through the eastern united states up to connecticut and iowa. _h. chrysoptera_ is a northern form extending down to connecticut and new jersey. both are migrants. in these two states, where the two types overlap, certain forms have been repeatedly found which have been described as two distinct species, _lawrencei_ and _leucobronchialis_. dr. l. b. bishop and mr. brewster showed me two long series of _helminthophila_ containing various intergrades between the four named kinds, and details regarding these may be found in chapman's _north american warblers_ and in dr. bishop's paper in auk, , xxii. though the characters evidently break up to some extent, the series can be represented as due to recombinations of definite factors more easily than the others which i have described. the differentiating characters are: _pinus_ . mantle and lower parts _yellow_ (y^ ). . wing-bars _white_ (y^ ). . cheek and throat _not black_ (b). _chrysoptera_ . mantle and lower parts _grey_ (y^ ). . wing-bars _yellow_ (y^ ). . cheek and throat _black_ (b). the grey pigment of the mantle is common to both, but is masked by the yellow in _pinus_, the net result being an olive-green.[ ] i am much indebted to dr. f. m. chapman for the loan of the coloured plate in which these distinctions are shown. it first appeared in his book, _north american warblers_. we cannot tell whether _yellow_ or _not-yellow_ is due to the presence of a factor, but we may suppose that one or other gives the special colour to the parts. the black of character is no doubt a dominant. thus _pinus_ becomes y^{ }y^{ }b and _chrysoptera_ in y^{ }y^{ }b. the _lawrencei_ which has the underparts _yellow_, wing-bars _white_, and _black_ patches is y^{ }y^{ }b and _leucobronchialis_ which has mantle and underparts _not-yellow_, wing-bars _yellow_ and _no black patches_ is y^{ }y^{ }b. this representation, it should be clearly understood, is tentative and approximate only. the characters are not really sharp, for there is much grading; but allowing for the effects of heterozygosis and for some actual breaking-up of factors i believe it gives a fairly correct view of the case. in particular we can see how it meets the difficulty which chapman felt in accepting _leucobronchialis_ as in any sense derived from _pinus_ which has a yellow breast, and _chrysoptera_ which has a black throat, seeing that _leucobronchialis_ has neither. we now recognize at once that this form could be produced by ordinary re-combination of the absence of y^{ } with the absence of b. i note also with great interest that the modern observers agree that the so-called hybrids may have the song either of the one species, or of the other, or a song intermediate between the two. it may also be added that these two types have several times been seen, in the breeding season, paired with each other or with one of the other combinations. [illustration: fig. . _helminthophila pinus_, male. fig. . _helminthophila pinus_, female. fig. . "lawrence's warbler," male; one of the integrading forms. fig. . "brewster's warbler," male; another of the integrading forms. fig. . _helminthophila chrysoptera_, male. fig. . _helminthophila chrysoptera_, female.] allen[ ] has described another excellent american example, the tits of the group _baeolophus bicolor-atricristatus_. the form _bicolor_ belongs to the eastern states and ranges from the atlantic coast to the great plains, and _atricristatus_, of east mexico, extends from vera cruz to central texas. in southern and central texas the breeding ranges adjoin, and in this country various intermediates occur. the chief types differ in two main points. _b. bicolor_ forehead varies from deep _black_ to dull black, suffused with rusty brown. crown and crest _grey_, slightly darker than the back. _b. atricristatus_ forehead _white_ to buffish white. crown and crest _black_, abruptly contrasting with the back. the intergrades between the two have, as usual, received specific names. a detailed description is given by allen, from which it appears that the gradation is very complete. in one case a series of adults were all intermediates. it is not stated whether the collector took these at random, but from the local lists it is clear that the types are found not far away from the place where the intergrades were shot. another very striking case is that of the tanagers, of the genus _rhamphocoelus_. in this group there are several local forms which are related to each other in remarkable ways. the forms known as _passerinii_ and _icteronotus_ exhibit the clearest phenomena of intergradation. the species _passerinii_ has a brilliant scarlet and black male, and it inhabits honduras and nicaragua. proceeding southwards along the isthmus we find next _costaricensis_ which has a male like that of _passerinii_ (but a female with more orange than the olive-grey female of _passerinii_). next we come to panama which is occupied by _icteronotus_, sharply distinguished from _passerinii_ by the fact that the _scarlet is replaced by lemon-yellow_. this same _icteronotus_ occurs again as a pure type in ecuador and many other parts of south america; but colombia, _between panama and ecuador_, contains scarlets like _passerinii_, yellows like _icteronotus_, and various intergrades of several shades of orange. the _passerinii_ males from nicaragua are indistinguishable from those of colombia, and the _icteronotus_ of ecuador are the same as those in panama. the orange intergrades, doubtless heterozygous forms, though collected at the same locality (medellin in colombia) as several pure yellows and pure scarlets, are in the british museum series sorted out as a separate species under the name _chrysonotus_! complications are introduced by the relations of these forms to another named type, _flammigerus_, but we may for our purpose leave that out of consideration, and say that the order of geographical sequence from honduras to ecuador is ( ) scarlet, ( ) yellow, ( ) mixture of types, scarlet, yellow, orange, ( )yellow. similar examples exist in the birds of the old world, but i do not know of any that have been studied so fully as those of america. the best known is that of the two rollers, _coracias indicus_ which spreads from asia minor through persia, baluchistan, the indian peninsula and ceylon, and _affinis_ which ranges from nepal, through assam, tenasserim and the indo-chinese countries. the two types are very different and may be distinguished as follows: _c. indicus_ _mantle_ drab brown-chestnut. _breast_ chestnut. _throat_ purplish, streaked with white. _upper tail-coverts_ indigo. _c. affinis_ dark olive-green. dull purple brown. purple, streaked with blue. turquoise. the wings are the same in both. in the provinces of nepal, sikhim, and darjiling the two species coexist, with the result that intergrades have been frequently recorded. the line of intergradation extends to the coast, and birds showing various combinations of the two types from the calcutta district exist in collections.[ ] the case is interesting inasmuch as like that of _quiscalus_ it shows a series of combinations of various metallic colours. some of these are probably evoked by the development of pigment behind striations or other interferences already existing, but in the present state of knowledge it would be quite impossible to suggest what the actual factors producing these appearances may be. there are, naturally, many other cases among birds which are suspected of being in reality comparable, but in most of them the evidence is still inadequate. among lepidoptera also there are a few of these; perhaps the most striking is that of _basilarchia "proserpina."_[ ] the genus is well known to european collectors under the name _limenitis_, of which we in england have one species, _l. sibylla_, the "white admiral." a species very like _sibylla_ in general appearance is common in the northern parts of the united states, ranging through canada and northern new england, but rarely south of boston. this species has the conspicuous white bands across both wings like our _sibylla_. there is also a more southern type known as _astyanax_, which is very different in its appearance, being without the white bands and having a broad irroration of blue scales on the posterior border of the hind wings. the two are so distinct that one would not be tempted to suspect any very close relation between them. in its distribution _astyanax_ is described by field as replacing arthemis south of latitude °. about boston it is much more common than _arthemis_. the two forms encroach but little on each other's territory, but where they do coexist, a third form, known as _proserpina_, is found which is almost intermediate, with the white bands much reduced. there is now no doubt that this _proserpina_ is a heterozygous form, resulting from a combination of the characters of _arthemis_ and _astyanax_. field succeeded in rearing a brood of from a _proserpina_ mother caught wild which laid eggs, and of these, nine (five males, four females) resembled the mother, being _proserpina_, and seven (four males, three females) were _arthemis_. there can be no question therefore that the mother had been fertilised by a male _arthemis_ and that _no-white-band_ is a factor partially dominant over the _white band_. another point of interest which field observed was that the _proserpina_ female refused to lay on birch, poplar or willow, but accepted wild cherry (_prunus serotina_) a species on which _astyanax_ can live, though that tree is not known to be eaten by _arthemis_. incidentally also the observations show that sterility cannot be supposed to be the bar which maintains the distinctness of _arthemis_ and _astyanax_. in this connection _papilio oregonia_ and _bairdii_ should be mentioned.[ ] _p. oregonia_ is one of the numerous forms like _machaon_, but rather paler. it is a northern insect, inhabiting british colombia east of the cascade range, and reaching to colorado. _p. bairdii_ is a much darker butterfly, representing the _asterias_ group of the genus _papilio_. like _asterias_ it has the abdomen spotted at the sides, not banded as in the _machaon_ group. it belongs to arizona and utah extending into colorado. from colorado the form _brucei_ is described, more or less intermediate, like _bairdii_ but with the abdomen banded as in _oregonia_. w. h. edwards records the results of rearing the offspring of the _bairdii_-like and of the _oregonia_-like mothers. each was found able to have offspring of both kinds, that is to say, _bairdii_ females gave both forms, and _oregonia_ females gave both forms. it is not possible to say which is dominant, since the fathers were unknown. on general grounds one may expect that the _bairdii_ form will be found to dominate, but this is quite doubtful. from this particular discussion i omit reference to those examples in which the permanently established types are obviously associated with special conditions of life. where considerable climatic differences exist between localities, or when we pass from south to north, or from the plains into alpine levels we often find that in correspondence with the change of climate there is a change in the characteristics of a species common to both. when i say "species" in such a connection i am obviously using the term in the inclusive sense. some would prefer to say that in the two sets of conditions two _representative species_ exist. whichever expression be preferred it is plain that such examples present another phase of the problem we have been just considering, and in them also we have an opportunity of observing the consequences of the overlap of two closely related types, but there are advantages in considering them separately. in the examples hitherto given, with the possible exception of the papilios,[ ] the two fixed types severally range over so extensive a region that it may fairly be supposed that in the different parts they are subject to considerable diversities of climate. there is no outstanding difference that we know distinguishing the habitats of the two forms; but in comparing alpine with lowland forms, or essentially northern with essentially southern forms we do know an external circumstance, temperature, that may reasonably be supposed to have an influence, direct or indirect, on the population. footnotes: [ ] j. a. allen, _the north american species of the genus colaptes, considered with special reference to the relationships of c. auratus and c. cafer_. bull. am. mus. nat. hist., iv, . [ ] for a case in which a red-headed female Ã� a black-headed male gave a black-headed female and a red-headed male, see _avian mag._, n. s., iv, pp. and [ ] the other variations of this bird are also interesting and important. the normal male has a red head and a red throat. the female has a red head and a white throat, but varieties of the female are known with a black head, thus again illustrating the change from black to red. it should be noted that this is not a mere retention of a juvenile character, but, as the birds mature, the red feathers come up, or as an exception, the black. there is also a western species, _ruber_, in which both sexes have a great extension of red, and are alike. the male of _nuchalis_ intergrades with this type, but the female does not. [ ] dr. w. brewster, for example, has a remarkable specimen of the teal (_nettion carolinense_) with a white collar strongly developed at the front and sides of the neck, in a place where the normal has no such mark. [ ] this variety is spoken of as the ringed guillemot and is sometimes regarded as a distinct species to which the name _ringvia_ was given by brünnich. in support of this view dr. william brewster, to whom i am indebted for much assistance in regard to the variation of birds, called my attention to observations of his own and also of maynard's, that the ringed birds were sometimes mated together, though in a small minority (see brewster, _proc. boston soc. n. h._, xxii, , p. ). it would however be possible to produce many instances of varieties mated together though surrounded by a typical population (_e. g._, two varying blackbirds, _zoologist_, p. ; two varying nightjars, _ibid._, p. ). i am inclined to believe that in nature matings between brothers and sisters are frequent in many species of animals, and that the production of sporadically varying colonies is thus greatly assisted. [ ] the sap-suckers feed on trees and somewhat resemble our spotted woodpeckers in general appearance. _colaptes_ feeds on the ground and corresponds perhaps rather with the european green woodpecker. [ ] for an introduction to this example i am indebted to mr. w. d. miller of the american museum of natural history. some account of the facts is given by baird, brewer, and ridgway (_a hist. of n. amer. birds_. , ii, pp. , , etc.). _s. varius_ occupies the whole country in suitable places from the atlantic to the eastern slopes of the rockies, and all mexico to guatemala. _s. nuchalis_ was first known from the southern rockies only, but many were afterwards taken in utah. _s. ruber_ is restricted to the pacific coast. in ridgway's opinion all three are geographical forms of one species. in _ruber_ the sexes are alike having both a great extension of the red in the throat, and a red crescent. the male of _nuchalis_ grades to the _ruber_ form, but the female does not. this female has some red in the throat like the male of _varius_, whereas the female of _varius_ has a whitish throat. [ ] not only vertebrates but the marine crustacea and mollusca illustrate this curious "principle" of variation, as canon norman formerly pointed out to me with abundant illustrations. there are of course cases to the contrary also. [ ] chapman, f. m., _bull. amer. mus._, iv, , p. ; see also ridgway, _birds of north and middle america_, , part ii, p. . [ ] it would aid greatly in factorial analysis if the descriptive term "green" could be avoided in application to cases where the green effect is due only to a mixture of black and yellow pigments. the absence of yellow is the sole difference between the mantle and underparts of _pinus_ and _chrysoptera_. [ ] _bull. amer. mus. nat. hist._, xxiii, , p. . [ ] references on this subject will be found in _brit. mus. cat. birds_, xvii, p. . [ ] for these facts i am indebted to mr. w. l. w. field, who has lately published an account of his observations and experiments. see especially, _psyche_, , xvii, no. , where full references to previous publications are given. [ ] for the facts and further references see w. h. edwards, _butterflies of n. america_, d series, papilio vii and x; d series, , papilio iv, _can. entom._, , xxvii, p. . [ ] i think this case is fairly included because the _machaon_ type is so widespread that it cannot be regarded as a product of a northern climate, nor can _asterias_ be claimed as especially a warm country form, seeing that _brevicauda_, which is scarcely distinguishable from _asterias_, inhabits newfoundland (having a curious phase there in which the yellow is largely replaced by red). chapter viii locally differentiated forms. _continued._ climatic varieties in this chapter we will examine certain cases which illustrate phenomena comparable with those just considered, though as i have already indicated, they form to some extent a special group. the outstanding fact that emerges prominently from the study of the local forms is that when two definite types, nearly allied, and capable of interbreeding with production of fertile offspring, meet together in the region where their distributions overlap, though intergrades are habitually found, there is no normally or uniformly intermediate population occupying the area of intergradation. such phenomena as these must, i think, be admitted to have great weight in any attempt to construct a theory of evolution. true we must hesitate in asserting their positive significance, but i see no escape from the conclusion that they throw grave doubt on conventional views. again and again the same question presents itself. if _a_ and _b_ lately emerged from a common form why is that common form so utterly lost that it does not even maintain itself in the region of overlapping? almost equally difficult is it, in the cases which i have numerated, to apply concrete suggestions based on any factorial scheme. we may see that in _heliconius erato_ the type with the red mark on the hind wing probably contains a dominant factor, and that where the red mark is absent the metallic colours are exposed; and that similarly the green metallic colour may have another factor which distinguishes it from the blue. in this way we can fairly easily represent the various types of _erato_ on a factorial system as the result of the various possible combinations of two pairs of factors. but there we stop, and we are quite unable to suggest any reason why one area should have the red and the green type while another should have the blue also. so again with _colaptes_ or the warblers. by application of a factorial system, admittedly in a somewhat lax fashion, the genetic interrelations of the types can be represented; but how it comes about that each type maintains a high degree of integrity in its own region we can only imagine. each has in actual fact a stability which the intermediate forms have not, but we cannot yet analyse the nature of that stability. mendelian conceptions show us how by segregation the integrity of the factors can be in some degree maintained, but not why certain combinations of factors should be exceptionally stable. all that is left us to fall back on is the old unsatisfying suggestions that some combinations _may_ have greater viability than others, that there _may_ be a tendency for like to mate with like, and so forth. these difficulties acquire more than ordinary force in those cases in which the two fixed types inhabit regions differing in some respect so obvious and definite that we are compelled to regard each type as climatic and as specially adapted to the conditions. when for example an animal has a distinct type never met with except in arctic or alpine conditions, and another type proper to the plains and temperate regions, what are the characteristics of the population of intermediate latitudes or at intermediate levels? some of the examples discussed in the last chapter may be instances of this very nature, but even if they are not, others are forthcoming which certainly are. the evidence of these cases leads to the suspicion that with further knowledge they will be found to consist of two classes, some in which the observer as he passes from the one climate to the other will find the intermediate area actually occupied by a population of intermediate character, and others in which, though we may presume the maintenance of intermediate conditions in the transitional area, there is no definite transitional population. this interrupted or discontinuous distribution seems, so far as i have means of judging, to be by far the more common of the two. i do not doubt that by sufficient search individuals representing every or almost every transitional form can be found, but it is apparently rare that _populations_ corresponding to these several grades can be seen. the question has in few if any cases been studied with precision sufficient to provide a positive answer; but i suspect that real and complete continuity, in the sense thus defined, will only be found where the character of the local populations depends _directly_ on the conditions of life, and shows an immediate response to changes in them apart from that postponed response which we suppose to be achieved by selection. obviously the character must be one, like size for instance, capable of sensibly complete gradation. the only example i have met with of the phenomenon of anything like a complete intergradation between local types really distinct in kind is that provided by the butterfly _pararge egeria_. it is well known to entomologists that this insect exists in two very different types, a northern one, the "speckled wood" of england, in which the spots are a pale whitish yellow, and a southern type having the full fulvous colour that we know as characteristic of _megaera_, the "gatekeeper." it appears that linnaeus gave the name _egeria_ to the southern type,[ ] and our own is now called _egerides_. broadly speaking, so far as great britain, france, and the spanish peninsula are concerned, the tawny-coloured _egeria_ occupies spain and western france up to the latitude of poitiers and the pale yellow _egerides_ extends from scotland, where it has a scanty distribution, through southern england, where in suitable localities it is common, and the north of france to paris.[ ] the two types when placed side by side are strikingly different from each other, and are an excellent illustration of what is meant by climatic variation. the insect is not a great traveller and probably scarcely ever wanders far from its home. it should therefore be possible by collecting from north to south to find out how the transition is effected, whether suddenly or gradually. this at various times i have endeavoured to do, but i am still without exact information as to the population in certain critical areas. in addition to the information derived from specimens which i have collected or seen in the collections of others there is a good account of the general distribution in europe given by the speyers,[ ] who evidently paid more attention to the subject than most lepidopterists have done, and many more recent records. in particular oberthür[ ] has published many details as to the distribution in western france and i am especially indebted to mr. h. rowland-brown for a long series of notes as to the distribution in france generally, and to mr. h. e. page and dr. t. a. chapman, mr. oberthür prof. arrigoni degli oddi, mr. h. williams and other correspondents, for showing me forms from many localities. the butterfly is attached for the most part to woods of deciduous trees and to country abounding in tall hedges or rough scrub. it is not usually to be found in highly cultivated districts or in very dry regions. hence there is necessarily some want of continuity in the distribution at the present time and i should think a mile or two of arable land without big hedges would constitute a barrier hardly ever passed. the larva feeds on several coarse grasses, especially _dactylis glomerata_. barrett mentions also _triticum repens_. in this country the winter is usually passed in the larval stage, but i have found that in captivity, at least, there is much irregularity. the larvæ feed whenever the weather is not very cold and may pupate, but if sharp cold comes on when they are pupating or nearly full-grown they often get killed unless protected. some writers speak of a difference between the early and later broods, but i have never noticed this, and i do not think that the general tone of the yellow is affected by the seasons (see tutt, _ent. rec._, ix, , p. ).[ ] beginning at the south of spain the thoroughly fulvous type _egeria_ is common at gibraltar in the cork woods, at granada, and doubtless generally. lederer is said to have found only this type in spain (speyer), and though i have no precise information as to other places in the peninsula north of jaen i feel tolerably sure that there is no change from south to north.[ ] immediately north of the pyrenees we still meet _egeria_ exclusively, and up to poitiers at least there is no noticeable change. but somewhere between poitiers and the bottom of the loire valley at tours, the genuine southern type comes to an end, and the whole population begins at the loire to be of an intermediate type, easy to distinguish both from _egeria_ and from _egerides_. as to the exact condition of the species in the fifty miles separating st. savin on the vienne from places on the loire i have no adequate information. i have only one small sample from there, but it does contain insects both of the southern and intermediate types taken on the same day, in a wood near preuilly. oberthür also states that at nantes the true southern form exists in company with the northern. from this i infer that the southern form extends up the coast further than it does inland, but i imagine the representative spoken of as northern would be of usual brittany or intermediate type. the vienne river joins the loire, so the true southern type reaches over into the basin of the loire. from the loire (tours, corméry) north to calvados (balleroy) only the intermediate is found, so far as i know, and the same type extends over brittany.[ ] in general, however, the woods near paris have the thoroughly northern type _egerides_, but at st. germain-en-laye and at etampes (oberthür) the population approaches the intermediate type. on the whole the intermediate type is certainly less homogeneous than either of the extremes, and females with the two central spots either paler or more fulvous than the rest are not uncommon, but i have never taken one on the loire or in brittany which i should class with either of the extreme types. before speaking of the distribution in other parts of france and in europe generally i will briefly state the results of my breeding experiments. the work was done many years ago before we had the mendelian clue, and it is greatly to be hoped that some one will find opportunities of repeating it. crossing the english and the thoroughly southern type the families produced agree entirely with the intermediates of brittany and the loire. reciprocals are alike. of f_{ } i only succeeded in raising very few and of those that i had (about ) nearly all were intermediate in character, though perhaps rather less uniform than f_{ }. one family alone, containing only specimens, had one _egerides_, and three fulvous intermediates. as the case stands alone i hesitate whether or not to suppose it due to some mistake. moreover from f_{ } crossed back with the respective parental types i had fairly long series, especially from f_{ } Ã� the southern type, and looking at these families i cannot see any clear evidence of segregation. on the contrary, i think that though there are slight irregularities, they would, taken as a whole, be classed as coming between the intermediate type and the extreme form used as the second parent. this at least is true when the second parent was of the southern type. on this evidence i have regarded the case as one in which there is no good evidence of segregation and as conforming most nearly with the conventional view of gradual transition in response to climatic influences. such influence must however be indirect; for i reared five generations of the northern type in england, and these, though they included several abnormal-looking specimens in the last generation and then died out, did not show any noticeable change from the fulvous colour of the wild type. merrifield[ ] also found that heat applied to pupae of the northern type produced no approach to the southern type. looking at the facts now in the light of more experience it seems to me just possible that the case may be one in which, as in nilson-ehle's wheats, the dominant differs from the recessive in having two pairs of factors with similar effects. the fulvous type for example may have two or more elements in separate pairs which together produce the full effect, and the intermediate may have one of these. if this were so, some segregation should of course eventually be observable, but the proportion of the various fulvous and fulvous-intermediate individuals would be large, and the reappearance of actual representatives of the northern type might be rare. i admit that this is a somewhat strained interpretation of the facts, and as yet it is not entitled to serious consideration. nevertheless i am led to form some such expectation partly from the great difficulty in the way of any other, partly from the evidence of the small mixed sample found at preuilly and partly from the statements given by oberthür. there are moreover other features in the general distribution of the species which make it improbable that the dependence on climate can after all be so close. published lists are unfortunately of little use in deciding which form occurs at a particular place, because, since the name _meone_ has ceased to be used for the southern form, there is no complete unanimity among authors as to the application of the names _egeria_ and _egerides_, and unless more particulars are given, either name may be used for either form. besides this, difficulty arises from the fact that the intermediate type is not generally distinguished at all, and english collectors finding it, may easily record it as the southern type. from staudinger's note on the distribution, i gather that he, on the contrary, reckoned the intermediate with the northern type, as do the speyers also. the late mr. j. w. tutt was careful to distinguish the three forms and has left several useful records. easy therefore as it might seem to be to make out the distribution of such a familiar insect in its various modifications, there are serious practical difficulties, and until long series are brought together with this special object in view many obscurities will remain. with only the series from england, the west of france, and spain before one it would be easy to regard the successive series of tones as a fair measure of climate; the brighter the colour, the hotter might one expect the locality to be. such rough correspondence is often to be observed in butterflies and birds. it becomes impossible to take these simple views in the light of more complete knowledge. beginning with france the fulvous _egeria_ occupies the lower valley of the rhone, probably from well above lyon, though i have no exact information respecting the country above avignon. according to speyer it also takes the department of lozère. the same authority says that puy-de-dôme has "_egeria_," meaning perhaps the intermediate form, with the fulvous form much less commonly. next comes the curious fact that though the lower rhone (avignon, tarascon, nîmes) has the true fulvous form, hyères, cannes, grasse, nice, digne, and alassio have _the intermediate_. savoy has the intermediate (chambéry) and even _egerides_ perhaps, though in the same latitude on the west of france there is nothing but the fulvous type. at chalseul and besançon (doubs) the ordinary northern type is found. switzerland generally, i believe, has the northern type, but staudinger gives _egeria_ for valais and the intermediate occurs in vaud.[ ] the south side of the alps has probably colonies of the pale _egerides_, and of intermediates. orta, with a very hot summer, has the english type (tutt, _ent. rec._, xii, , p. ). locarno has the intermediate (_ibid._, xv, , p. ). north italy in general and western piedmont have the intermediate; but further south _egeria_ begins, at what region i do not know. speyer gives on his own authority the remarkable statement that at florence both extremes occur, but chiefly intermediates between the two. mr. r. verity however kindly informs me that in his experience this is not so, and that neither the real southern type nor the northern occur there. sardinia, sicily, crete all have the southern type. greece probably has various types. staudinger (_hor. ross._, vii, , p. ) says intermediates resembling nice types common everywhere, but from "greece" the british museum has a series that would pass for english specimens; and the same type occurs near constantinople. the island of corfu has a pale intermediate, distinct from _egerides_ but approaching it. in roumania all three forms are recorded from various places: _egeria_ in the dobrutscha; not quite typical (presumably an intermediate) at bukharest; intermediate in various mountainous localities as well as in macedonia and dalmatia; but _egerides_ in azuga at about , feet.[ ] hungary has the true _egerides_ also. (cf. caradja, _deut. ent. zt._, ix, p. .) mathew records the same from gallipoli (_e. m. m._, , p. ). staudinger does not distinguish the intermediates from the northern, but he gives "_egerides_" for armenia and fergana (central asia). as against the mere proximity of a great mountain chain being the influence which keeps the riviera population intermediate may be mentioned the fact that the northern foothills of the pyrenees have the pure southern type, and the climate of cambo must surely be far cooler than that of nice. the exact locality of the greek specimens is not given, but there can be no part of greece which is not much hotter in summer than brittany, or calvados, which have the intermediate, not the english type. in face of these facts it can scarcely be maintained that average temperature is the efficient cause of the particular tone of colour which the butterfly shows in a given region. nevertheless it is clear that climate counts for much in determining the distribution. it is noticeable that though the pale _egerides_ can be established in a warm climate we never find _egeria_ in cold climates, and even the intermediate is not found in places that have a hard winter. i suspect that the distribution of the broods through the year and the condition of the animal at the onset of hard frost are features which really determine whether a strain can live in a particular place or not. though the truth of the suggestion cannot be tested by experiments in captivity, which at once introduce disturbances, i incline to the idea that _egeria_ has not got the right periodicity for northern climates. if it could arrange its life so that the population consisted either of young larvae, or perhaps of thoroughly formed pupae[ ] at the onset of winter, it might, for any obvious reason to the contrary, be able to live in england. it is irregularly "polyvoltine," as the silk-worm breeders say, and as soon as a little warmth encourages it, a new generation starts into being, which if the frost comes at an untimely moment, is immediately destroyed. many species are continually throwing off individuals which feed up fast[ ] and emerge at once if the temperature permits, and i imagine a species of satyrid wholly or largely represented by such individuals could scarcely survive in a country which had a hard winter. for such a climate some definite periodicity in the appearance of the broods may well be indispensable. but assuming that _egeria_ is cut off from cold climates for such a reason, there is nothing yet to connect these habits with the fulvous colour, and until breeding can be carried out on a satisfactory scale there is no more to be said. from time to time records appear of individual specimens more or less fulvous being caught in southern england, especially in the new forest.[ ] it would be interesting to know what offspring such individuals might produce. from the evidence now given some notion both of the strength and the weakness of the case considered as one of continuous climatic variation can be formed. i know no other equally satisfactory. whether or not definite mixture of the intermediates with either of the extremes will be proved to occur, the case differs materially from those considered in the last chapter in the fact that at all events there is no general overlapping of forms. in a species so little given to wandering, overlapping could indeed scarcely be expected to occur. it is this circumstance which makes the species preeminently suitable as a subject for the study of climatic influences, and i trust that entomologists with the right opportunities may be disposed to explore the facts further. just as many species, like _egeria_, have varieties which can be regarded as adapted to northern and southern regions, so there are also several which have lowland and alpine forms quite distinct from each other. every such case presents an example of the problem we have been considering. as the collector passes from the plains to the alpine region, how will he find the transition from one form to the other effected? does the lowland form give place to the alpine form suddenly, with a region in which the two are mixed, or will he find a zone inhabited by an intermediate population? i have spent a good deal of time examining the facts in the case of _pieris napi_ and its alpine female variety _bryoniae_, and though there are many complications which still have to be cleared up, no doubt is possible as to the main lines of the answer. if in any valley in the alps inhabited by both _napi_ and _bryoniae_ the collector catches every specimen he can, beginning at the bottom and working up to , feet, he will at first get nothing but _napi_. at about , feet, he may catch an occasional _bryoniae_ flying with the _napi_. after , feet _napi_ usually ceases, and only _bryoniae_ are found. as an exception a colony of _napi_ may be met with at much greater heights. i once found them in numbers at about , feet.[ ] not only were they free from any trace of modification in the direction of _bryoniae_, but they were of the thoroughly southern type of _napi_, being a late brood of that large and very pale kind (_meridionalis_) almost destitute both of dark veining above and of green veining below, which are common on the shores of lago maggiore and in other hot southern localities. not far off at the same level were typical _bryoniae_ in fair abundance. occasionally an intermediate may be met with. i have taken a few, for example, at macugnaga and at fobello. these, however, in my experience are rarities in the alps. fleck[ ] gives notes on the distribution in roumania which shows the same state of things. the lowland form is not transformed though found at great heights, and at azuga (nearly , feet) _bryoniae_ occurs with only occasional "_flavescens_," viz., intermediates of the second brood. if this were all the evidence we should be satisfied that the lowland and alpine types keep practically distinct, overlapping occasionally, but rarely interbreeding. the problem would remain, how is the distinctness of the two types maintained in the region of overlapping? nowadays, i suppose, we should incline to answer this question by reference to segregation, and perhaps by an appeal to selective mating. the suggestion that segregation does take place is certainly true to some extent. there are, however, difficulties in the way, and the whole subject is one of great complexity. my own experiments were made in pre-mendelian times and were not arranged with the simplicity which we now know to be essential. the results are neither extensive enough nor clear enough to settle the many collateral questions which have to be considered, and the work ought to be done again. nevertheless, some notes of the observations may have a suggestive value. when i began, i did not sufficiently appreciate that the "_napi_" group, omitting the north american forms, and the asiatic representatives, has at least three chief types in western europe. the differences we have to deal with are manifested by the females only, so in this account particulars as to the males are omitted for the most part. these are ( ) our own british _napi_; ( ) the form found in the south, from the loire downwards, and in the italian alps, which i think may be spoken of as _meridionalis_; ( ) _bryoniae_, which is a form clearly recognizable in the _female_ only, and is found only in the arctic regions and in the alps above , feet. the first two have several broods, two, three, or more, according to opportunity, and the first brood is different from the later ones. in _napi_ the markings on the upper surface are a dark grey but in _meridionalis_ they are a pale silvery grey and much less extensive. in the later broods of _napi_ there is much less general irroration of the veins, and the spots stand out as more defined and blacker. these differences vary greatly in degree of emphasis. in _meridionalis_ the later broods are entirely different from the first. instead of having silvery markings they have the ground colour quite white, with the spots large and a full black. on the under side of the hind wings the usual green veins are almost absent, and i have seen individuals which could scarcely be distinguished from _rapae_. to these later broods the term _napaeae_ is sometimes applied, but i here use _meridionalis_ for the southern race in general as applicable to all broods. the female _bryoniae_ is totally unlike the others. the ground colour is a full yellow, and each nervure is thickly irrorated with a brown pigment often spreading so far as to hide the ground almost entirely in the fore-wings. the males corresponding with these females are not certainly distinguishable from those of our own _napi_. both sexes have the green veining of the underside of the hind wing fully developed, rather more than is usual in the lowland races, but this is not really diagnostic of the variety. the first serious difficulty arises in regard to the second brood of _bryoniae_. it is stated that there is only one brood,[ ] but i feel fairly sure that a second brood is sometimes produced, and that the females with a yellow ground and diminished irroration of the veins, not very uncommon in the italian alps in july to august, are generally representatives of it. such insects would of course be classed with _bryoniae_ in collections. my experiments began with eggs of true _bryoniae_ females caught at about , feet early in july. these emerged in august-september as intermediates with yellow ground and about half as much black on the upper surface as _bryoniae_. they are exactly like the intermediates usually found in nature and in the light of later experience i regard them as natural f_{ } forms, and i think the mothers had been fertilised by _napi_ males, though i admit that in view of the rarity of natural intermediates there is a difficulty in this suggestion. three of these females were mated with males raised from thorough _meridionalis_ females, and three families were produced. two of them showed distinct evidence of segregation, some being yellow and some white with various intergrades, some being no blacker than _meridionalis_ and some ranging up to a dark intermediate type. part emerged in the same autumn; and part overwintered, emerging as the spring _meridionalis_ or as the peculiar type which i afterwards learnt to know as the spring f_{ } form. the distinctions were fairly sharp between the several forms. but the offspring of the third female gave a series practically continuous from _meridionalis_ to the f_{ } type. the work of subsequent years gave results similarly irregular which could only be described adequately at great length. the outcome may however be summed up in the statement that there is evidence that both the yellow ground and the dark veining are due to factors, but that there are several of these and that imperfect segregation is not uncommon, producing various reduction-stages. the yellow ground may be due to one factor, and the several shades may be the result of irregularities in dominance, but the black markings when fully developed cannot i think be the result of less than three factors, one for the basal darkening, one for general irroration, and one for the margins. probably also the enlargement of the spots is produced by a fourth factor. there was not, in my experience any great difficulty in getting the various forms to pair in captivity. some attempts were made to see whether individuals of either type selected mates of their own type in preference to those of the other, but the results were inconclusive. there were some indications of such a preference; though, from the impossibility of judging how much of this may be due to other circumstances, i could not come to a positive conclusion on the rather meagre evidence. recently schima[ ] has given a careful and detailed account of all the forms found in lower austria which he enumerates under distinct varietal names. he gives full references to previous accounts, especially to the beautiful plates lately published by roger verity.[ ] examination of these and of my own specimens strongly suggests that the several forms are due to the recombination of the factors i have named. among those which i have bred are representatives of most if not all the types enumerated by schima in addition to other curious forms. for example i have _bryoniae_ markings on a ground practically white; the dark veins with spots almost obsolete; _meridionalis_ on a yellow ground; the intermediate amount of black on a white ground, etc. the last-named may occur wild and i have one from macugnaga as well as one given me by mr. f. gayner from lulea (lapmark). to obtain really exact knowledge of the number of factors and their properties it would be necessary to repeat the work. after the beginning, i made a mistake in using british _napi_ instead of _meridionalis_ and the results were much confused thereby. the contrast between _meridionalis_ and the various dark forms is much greater and classification of the types would have been therefore easier. the british form is presumably _meridionalis_ plus the factor for the basal pigmentation. the problem is greatly complicated by the differentiation of the seasonal forms. the first point to be determined is whether _bryoniae_ is capable of producing a second brood when it is thoroughly pure-bred, and whether such a second brood is, as i suspect, normally intermediate in character. in the alps generally there is no definitely intermediate population; nor i believe, is any such population met with in the north where the arctic _bryoniae_ meets _napi_, but as to this i have no precise information. one curious fact, however, must be mentioned, namely that there is a population that can probably be so described with fairness established at mödling near vienna. this is not in any sense an alpine locality, and does not, as i am told, differ in any obvious way from the other suburbs of vienna. dr. h. przibram was so good as to send me a set taken at this place, representing a second brood, and they were decidedly heterogeneous, ranging from an intermediate form such as _bryoniae_ fertilised by _napi_ usually produces, to a light yellowish second-brood type with little dark pigment. there are also two actual _bryoniae_. whether true _napi_ also occur there i do not know, but i have no doubt they do. it would be well worth while to investigate the mödling population statistically, and to breed from the intermediates which might not impossibly prove to be heterozygotes. there are also records of such intermediates being occasionally found in some parts of ireland, in the north of scotland, and in south wales,[ ] but i do not know of any regular colony of these forms. we can scarcely avoid the inference that one or more of the factors which make up _bryoniae_ may be carried by these intermediates. it is not clear why their interbreeding does not produce actual _bryoniae_ occasionally. if this occurred, the probability is that the fact would be known to collectors, at least in the british localities. the absence of true _bryoniae_ must, i think, be taken to mean that some essential factor is absent from these intermediates. to sum up the evidence, the facts that are clear may be thus enumerated: . _napi_ and _bryoniae_, or in the italian alps, _napaeae_ and _bryoniae_ frequently meet each other. . they cross without difficulty, producing fertile offspring. . but in the levels at which they overlap there is no intermediate population, and only occasional intermediate individuals. . in certain parts of the distribution of _napi_ similar intermediates sometimes occur, and at one place (mödling) they are so frequent as apparently to constitute a colony. . as to the genetic relations of the two forms there is no complete certainty. indications of segregation have been observed in some cases, but there are several factors concerned and they are liable to some disintegration. another form in which i tried to investigate the same problem is _coenonympha arcania_, which has one alpine form known as _darwiniana_, and another, _satyrion_. in calling _satyrion_ a form of _arcania_ i follow staudinger and other authorities, but i have never been quite satisfied that it should be so regarded. the differences between _arcania_ and _darwiniana_ are essentially differences of degree; _c. arcania_ occurs in places where there is cover, and reaches up the valleys usually as high as the mixed woods of deciduous trees, which is about , feet. the variety _darwiniana_, on the contrary, is an insect of treeless hillsides, and i regard it as a dwarf and possibly a stunted form. it would not greatly surprise me to find that with the application of good conditions _arcania_ could be raised from _darwiniana_ eggs, or that if _arcania_ larvae were starved they might give rise to _darwiniana_ butterflies. i have been unsuccessful in trying to rear the species, having lost the larvae by disease. usually one does not catch _arcania_ and _darwiniana_ on the same ground, and as _festuca ovina_--a typically hill-side grass--is a common food-plant of _darwiniana_ there can be little doubt that _arcania_ feeds on some other grass, probably woodland species. colonies of _arcania_ of varying size and brightness are commonly found, and though a sample of _arcania_, finely grown, from a warm italian wood, presents a striking contrast with _darwiniana_ from an alpine pasture, one certainly may get samples which fill all the gradations. generally the sample from a given locality is fairly homogeneous. of _satyrion_ i have little personal experience. i only twice found it, namely at zinal, and at hallstatt in austria, but it occurs at zermatt, arolla, and in several swiss localities above , feet, and i understand that it is the typical alpine form in the engadine. with its darkened colour and reduced size it might well be expected to be a still further stunted form of _darwiniana_. yet i have never found the one succeed to the other at the higher levels. if _darwiniana_ appears when alpine conditions are reached in a valley it will be met with up to the highest level at which such butterflies live. tutt was of opinion that _satyrion_ is a distinct species.[ ] i once, at the top of the vorderrheinthal caught a sample of _darwiniana_ a few of which (males) were so dark and had the eye spots so poorly developed that they looked like transitions to _satyrion_. otherwise i never found any such transitional forms and they are certainly exceptional. there is further a record[ ] of _satyrion_ having been taken flying with _arcania_. this was near susa, at about , feet i infer. mr. h. e. page has similar specimens from caud and from st. anton (arlberg). the females, however, both of mine and of mr. page's samples are a pale brown, quite unlike the females both of _arcania_ and of the dark zinal _satyrion_. the difficulty thus raised has not i think yet been considered by the authorities, and it is possible that the alpine forms of _arcania_ are in reality three, not two. the evidence taken together suggests, i think, that _darwiniana_ is related to _arcania_ much as so many of the alpine varieties of plants are to the well-developed individuals of the lower levels. i do not anticipate that factorial differences will be found in these insects, and it is by no means impossible that the distinctions between them are the direct consequences of altered conditions. the relations of _arcania_ to _satyrion_ are more doubtful, and in that case a factorial difference may at least be suspected. the species of the genus _setina_ have alpine forms which agree in possessing a characteristic extension of the black pigment to form radiating junctions between the spots on the wings. speyer, who discussed the interrelations of these forms in detail,[ ] lays stress on the absence of genuine transitional forms between _aurita_ and the variety _ramosa_. both are mountain insects but _ramosa_ extends to levels higher than that at which _aurita_ ceases, which is about , feet. the two forms are often found flying together. speyer says that his brother searched diligently for transitional forms at the level of overlapping, but found none, so that at least they may be regarded as rare. the variety _ramosa_ is not infrequent at much lower levels (_e. g._, chiavenna, , feet; reussthal, , feet) and extends as high as the permanent snows. in the british museum collection, however, i have seen several that i should regard as transitional. speyer perhaps would have classed as _ramosa_ all in which the spots of the central field were united, and it is by no means unlikely that breeding would prove such individuals to be heterozygous.[ ] there can scarcely be a doubt that the distinction between _aurita_ and _ramosa_ is factorial, the radiate _ramosa_ probably having the factor for striping. in support of this view may be mentioned the observation of boisduval,[ ] respecting a gynandromorphous individual, which was _aurita_ male on one side, and _ramosa_ female on the other. speyer makes another excellent comment. he points out that the simple notion that the radiation is a mere extension of pigmentation consequent on the climate of the higher levels, will not fit the facts very easily, because the size of the spots varies greatly in _aurita_ itself at any level, and lowland specimens may actually have more black confined to the spots alone than some _ramosa_ possess on spots and lines combined.[ ] the two salamanders, _s. maculosa_ and its alpine form _atra_, might not improbably furnish evidence bearing on the same problem. the two are of course very distinct, not merely in colour (_maculosa_ being spotted with yellow or orange while _atra_ is entirely black) but also in the mode of reproduction, a feature to which reference will be made in the next chapter. i cannot, however, find any evidence as to the overlapping of the two forms. _s. atra_ occurs from about , feet or somewhat less, and reaches great elevations in the eastern alps, but i do not know if the two forms ever occur in the same localities. leydig,[ ] boulenger,[ ] and most modern authorities regard the two types as distinct species, but they are in any case closely allied, and it would be of interest to have exact knowledge of their geographical delimitations. the reader who has considered the cases adduced will appreciate the difficulties which must be faced in any attempt to account for the facts in a rational way. as always in a problem of evolution, two separate questions have to be answered. first how did the form under consideration come into existence, and secondly, how did it succeed in maintaining itself so as to become a race? the evidence from the local forms, though very far from giving complete answers to either of these questions definitely refutes the popular notion that a new race comes into existence by transformation of an older race. if a gradual mass-transformation of this kind took place we should certainly expect that when two types, nearly allied and capable of interbreeding, overlap each other in their geographical distribution, a normally intermediate population would exist. if each type can maintain itself, and if each came into existence by gradual transformation, then there must have been an intermediate capable of existing and maintaining itself as a population; and if this had ever been, surely in the region of overlapping, that intermediate population should continue. especially should such a population be found when the two extreme types are adaptational forms and the region of overlap is a region of intermediate conditions. but of the examples we have examined there is only one, that of _pararge egeria_ and _egerides_, which can at all be so interpreted, and even in that case it is not impossible that more minute observation would reveal discontinuity between the extremes and the admittedly normal intermediate population. granting provisionally however that this example, as it stands, is consistent with the conventional theory of evolution, i know not where we should look for another case equally good. when the distinctions are produced by direct influence of conditions operating during the lifetime of the individuals, examples of intermediate populations occupying the areas of intermediate conditions can no doubt be produced. many turf-like alpine plants, for instance, if protected from exposure and properly nourished can grow as large as those of the same species found in the valleys, and in the case of such quantitative effects, intermediate conditions can doubtless produce intermediate characters. even these examples however are not very abundant, and often the intermediate locality has not a form intermediate between those of the two extreme localities, but some third form distinct from either. this is the case for instance in the fauna of brackish waters. we are taught to believe that the fresh water fauna was evolved from the marine fauna, which it well may have been; but as students of crustacea and mollusca know familiarly, the brackish water forms are not as a rule intermediates between fresh water species and sea species, but more usually they are special forms belonging to the brackish waters, with the peculiar property that they can tolerate a great range of conditions, and live without ostensible variation in waters of most various compositions and densities, which very few marine or fresh water species are able to do. sometimes the distinction between local races, as in _rhamphocoelus passerinii_ and _icteronotus_ may be regarded with confidence as due to one simple mendelian factor possessed by one race and absent from the other, but i think, more often, as in _colaptes_ or in the varieties of _pieris napi_, the existence of several distinct factors is to be inferred. as we have seen, the races of _colaptes_ show almost beyond doubt that in different areas at least three distinct factorial combinations can be perpetuated as races. in the distribution of variability we find, i think, some hint as to the steps by which the phenomena under consideration have come to their present stage, and i am disposed to regard the facts so well attested in the case of our own melanic moths as a true indication of the process. following this indication we should regard the change in the character of a population as beginning sporadically, by the appearance of varying individuals, possibly only one varying individual, in, it may be, one place only. as to _why_ a variety should increase in numbers we have nothing but mere speculation to offer, and for the present we must simply recognise the fact that it may. that such survival and replacement may reasonably be taken as an indication that the replacing race has some superior power of holding its own i am quite disposed to admit. nevertheless it seems in the highest degree unlikely that the outward and perceptible character or characters which we recognise as differentiating the race should be the actual features which contribute effectively to that result. in discussions of geographical distribution in relation to problems of origin it is generally said that very nearly allied species usually occupy distinct areas, while other competent observers state the exact contrary. lately, for example, dr. r. g. leavitt[ ] has published an important collection of evidence upholding the latter proposition, taken chiefly from the botanical side, showing how in numerous genera two or more closely allied species coexist, frequently without intermediates, in the same localities, and may even be thus found in company throughout their distribution. the difference of opinion evidently arises from a confusion as to the sense in which the term "species" is understood and applied. leavitt, for example, is avowedly following jordan and, among moderns, sargent, in applying a close analysis, and denoting as species all forms which are distinct and breed true. against this use of the term i know no valid objection[ ] but it must be obvious that if others follow a different practice confusion may result when observations are summarised in general statements. we will consider this subject again in another place, but here it may be sufficient to say that there can scarcely now be a doubt that numbers of these associated species, such as jordan discriminated, represent various combinations of the presence and absence of mendelian factors. this does not in any way weaken the argument which leavitt founds upon the facts, namely, that the observed distribution of these forms is consistent with the supposition of an evolution largely discontinuous. on the other hand, those who have come to the opinion that nearly allied species generally occupy distinct ground are presumably more impressed by the characters differentiating the geographically distinct or adaptational races, seeing that genuine intermediates between them are less commonly found. those geographical races may no doubt contain various differentiated forms; but when all live together, occasional intermediates are usually to be found even in the case of characters habitually segregating. these segregating forms jordan would certainly have determined as species, and it must be conceded that no physiological definition has yet been drawn which consistently excludes them. footnotes: [ ] often referred to by older writers as _meone_, esper's name. [ ] there are also two distinct island forms, unlike the european, _xiphia_ of madeira, and a smaller variety, _xiphioides_ of canary. see especially, baker, g. t., _trans. ent. soc. london_, , p. . [ ] speyer, adolf, and august. _verbreitung der schmetterlinge_, , i, p. . [ ] _lepid. comparée_, fsc. iii, p. . [ ] mr. rowland-brown has called my attention to a statement by dr. vaillantin (_petites nouv. ent._, ii, ) that in indre-et-cher the first brood is of the northern type and the second of the southern. my experience is that in captivity these distinctions do not occur, and i have true _egeria_ as first brood from vienne and as the late brood from the landes. i never collected in indre-et-cher. [ ] i have since seen true _egeria_ from ferrol in the extreme northwest, which was in mr. tutt's collection. [ ] mr. g. wheeler kindly showed me a series identical with this type, from guernsey, and others from near laon. [ ] _ent. rec._, v, , p. . [ ] mr. wheeler has some pale but rather worn specimens from the rhone valley at vernayaz. [ ] see fleck, e., die macrolep. rumäniens, _bul. soc. sciinte_, viii, , p. . [ ] my experience agrees with that of mr. h. williams (_ent. rec._, viii, , p. ) that pupae, well-formed, can stand considerable frost; but i used to find that half-grown larvae usually died if unprotected, and i believe that larvae which attempted to pupate in warm autumn weather and then got caught by frosts, always died. small larvae which can creep into shelter at the bottom of the plants survived, and i expect that in the north the winter is usually passed in that state (see also merrifield, f., _ent. rec._, viii, , p. , and carpenter, j. h., _ibid._). [ ] some most unlikely species do this. i once had a larva of _parnassius delius_, found at about , feet, which emerged late in the autumn (in october i believe), a season at which it must have perished in its own country. [ ] see, for examples, barrett, g. c., _lepidoptera of the brit. islands_, i, , p. ; also grover, w., _ent. rec._, ix, , p. ; williams, h., _proc. ent. soc._, , who reared several specimens from the new forest which would pass for bretons, though the rest of the family were true _egerides_. [ ] above the tosa falls. [ ] _bul. soc. sciinte_, viii, , p. . [ ] the fact that weismann by heating pupæ obtained only one autumn specimen seems to me to show rather that a second brood can be produced than that it cannot, which is the inference usually drawn. [ ] schima, k., _verh. zool. bot. ges. wien_, lx, , p. . [ ] _rhopalocera palaearctica_, florence, - , especially pl. xxxii. [ ] see figures in barrett, g. c., _lepidoptera of brit. islands_, i, pt. , p. . [ ] tutt, j. w., _ent. rec._, xviii, , p. . in the same place he states that on the mendel pass _arcania_ "runs into" _darwiniana_ and that in the tyrolean localities the transition is especially evident. wheeler (_ibid._, xiii, , p. ) expresses the contrary opinion, that _satyrion_ does grade to _arcania_. [ ] h. rowland-brown, _ent. rec._, xi, , p. . [ ] speyer, stettiner, _ent. ztg._, xxxi, , p. . [ ] in regard to the closely analogous case of _spilosoma lubricipeda_, standfuss makes a similar statement. he bred the type on a large scale with the radiate form which he calls _intermedia_, and says that in four years of miscellaneous crossing he never obtained really transitional forms. nevertheless after examining large series, especially those of mr. w. h. b. fletcher, i came to the conclusion that several might be so classed, but i am quite prepared to find that such specimens are heterozygous. (see standfuss, _handb. d. gross-schmet._, , p. .) it is by no means unlikely that various dark forms of _lubricipeda_ correspond with a progressive series of factorial additions. many of the stages have been named, and of these the most definite are the _intermedia_ of standfuss (probably = _eboraci_ of tugwell) and the very dark _zatima_ of heligoland, in which only the thorax, the nervures and a small field in the fore-wings remain yellow. a form was bred by deschange from _zatima_ in which even the field in the forewing is obliterated. the exact circumstances in which _zatima_ occurs in heligoland would be worthy of special investigation, for the normal _lubricipeda_ is also found on the island. for references as to the british occurrences see especially, hewett, w., _naturalist_, , p. . as to _zatima_ see especially krancher, _soc. ent._, ii, - , p. . i am indebted to dr. hartlaub for information as to the heligoland types. [ ] boisduval, _bull. soc. ent. fr._, iii, , p. . [ ] the systematics of _setina_ have been much controverted, but no one i believe doubts that _aurita_ and _ramosa_ are forms of one species. see also chapman, a. t., _ent. rec._, xiii, , p. . [ ] _arch. naturg._, , , p. . [ ] _brit. mus. cat., batrachia gradientia_, . [ ] the geographical distribution of nearly related species. _amer. nat._, xli. , p. . [ ] see later, p. . chapter ix the effects of changed conditions: adaptation in the attempt to conceive a process by which evolution may have come about, the first phenomenon to be recognized and accounted for is specific difference. with that recognition the outline of the problem is defined. the second prerogative fact is adaptation. forms of life are _on the whole_ divided into species, and these species _on the whole_ are adapted and fit the places in which they live. to many students of evolution, adaptation has proved so much more interesting and impressive than specific diversity that they have preferred it to the first place in their considerations. whether this is, as i believe, an inversion of the logical order or not, there is one most serious practical objection to such preference, that whereas specific diversity is a subject which can be investigated both by the study of variation and by the analytical apparatus which modern genetic science has developed, we have no very effectual means of directly attacking the problems of adaptation. the absence of any definite progress in genetics in the last century was in great measure due to the exclusive prominence given to the problem of adaptation. almost all debates on heredity centered in that part of the subject. no one disputes that the adaptation of organisms to their surroundings is one of the great problems of nature, but it is not the primary problem of descent. moreover, until the normal and undisturbed course of descent under uniform conditions is ascertained with some exactness, it is useless to attempt a survey of the consequences of external interference; nor as a rule can it be even possible to decide with much confidence whether such interferences have or have not definite consequences. those, for example, who debated with enthusiasm whether acquired characters are or are not transmitted were constantly engaged in discussing occurrences which we now know to be ordinary features of descent under uniform conditions, and the origin of variations which were certainly not caused directly by circumstances at all. in the absence of any factorial analysis, or of any conception of what factorial composition means and implies, no one knew what varieties might be expected from given parents. the appearance of any recessive variety was claimed as a consequence of some treatment which might have been applied to the parents. there was no possible standard of evidence or means of controlling it, and thus the discussion was singularly unfruitful. before we can tell how the course of descent has departed from the normal, we must know what the normal would have been if we had let alone. we are still far from having such knowledge in adequate measure, but it does now exist in some degree, and we are steadily approaching a position from which we shall be able to form fairly sound estimates of the true significance of evidence for or against the proposition that environmental treatment can produce positive disturbances in the physiological course of descent. thus described, the field for consideration is very wide. though the effects of changed conditions were especially studied in the hope of solving the problem of adaptation by direct observation, that, as all are now agreed, is but a part of a more general question. we must ask not only do changed conditions produce an _adaptative_ response on the part of the offspring, but whether they produce any response on the part of the offspring at all. it is not in doubt that by violent means, such as starvation or poisoning of the reproductive cells, effects of a kind, stunting and deformity for instance, can be made evident, just as similar effects may follow similar treatment during embryonic or larval life. apart from interferences of this class, are there any that may be reasonably invoked as modifying the course of inheritance? no epitome of the older evidence for the inheritance of adaptative changes is here required. that has often been collected, especially by weismann, who exposed its weaknesses so thoroughly as to carry conviction to most minds, and showed that whether the phenomenon occurs or not, no one can yet prove that it does. belief in these transmissions, after being almost universally held, was with singular unanimity abandoned. this change in opinion, though doing credit to the faith of the scientific community in evidential reasoning, is the more remarkable inasmuch as the strength of the idea was not derived from the minute amounts of supposed facts now demolished. on the contrary, it was really an instinctive deduction from a wide superficial acquaintance with the properties of animals and plants. they _can_ accommodate themselves to circumstances. they _do_ make responses sometimes marvellously appropriate to demands for which they can scarcely have been prepared. what more natural than to suppose that the permanent adaptations have been achieved by inherited summation of such responses? no one had actually been driven to believe in the inheritance of adaptative changes because bitches which had been docked had been known to give birth to tailless puppies, or because certain wheat in norway was alleged to have become acclimatized in a few generations. evidence of this kind was collected and produced rather as an ornamental appendix to a proposition already accepted, and held to be plainly demonstrated by the facts of nature. looked at indeed in that preliminary and uncritical way, the case is simply overwhelming. those who desire to see how strong it is should turn to samuel butler's _life and habit_, and even if in reading they reiterate to themselves that no experimental evidence exists in support of the propositions advanced, the misgiving that none the less they may be true is likely to remain. making every deduction for the fact that the wonders of adaptation have been grossly exaggerated, and that marvels of fitness and correspondence between means and ends have grown out of mere anthropomorphic speculations, there is much more left to be accounted for than can at all comfortably be accepted as the product of happy accidents. so oppressive are these difficulties that we can scarcely blame those who imagine that the study of heredity is primarily directed to the problem of the transmission of acquired characters, a preconception still almost universal among the laity. but since the belief in transmission of acquired adaptations arose from preconception rather than from evidence, it is worth observing that, rightly considered, the probability should surely be the other way. for the adaptations relate to every variety of exigency. to supply themselves with food, to find it, to seize and digest it, to protect themselves from predatory enemies whether by offence or defence, to counter-balance the changes of temperature, or pressure, to provide for mechanical strains, to obtain immunity from poison and from invading organisms, to bring the sexual elements into contact, to ensure the distribution of the type; all these and many more are accomplished by organisms in a thousand most diverse and alternative methods. those are the things that are hard to imagine as produced by any concatenation of natural events; but the suggestions that organisms had had from the beginning innate in them a power of modifying themselves, their organs and their instincts so as to meet these multifarious requirements does not materially differ from the more overt appeals to supernatural intervention. the conception, originally introduced by hering and independently by s. butler, that adaptation is a consequence or product of accumulated _memory_ was of late revived by semon and has been received with some approval, especially by f. darwin. i see nothing fantastic in the notion that memory may be unconsciously preserved with the same continuity that the protoplasmic basis of life possesses. that idea, though purely speculative and, as yet, incapable of proof or disproof contains nothing which our experience of matter or of life at all refutes. on the contrary, we probably do well to retain the suggestion as a clue that may some day be of service. but if adaptation is to be the product of these accumulated experiences, _they must in some way be translated into terms of physiological and structural change_, a process frankly inconceivable. to attempt any representation of heredity as a product of memory is, moreover, to substitute the more obscure for the less. both are now inscrutable; but while we may not unreasonably aspire to analyse heredity into simpler components by ordinary methods of research, the case of memory is altogether different. memory is a mystery as deep as any that even psychology can propound. philosophers might perhaps encourage themselves to attack the problem of the nature of memory by reflecting that after all the process may in some of its aspects be comparable with that of inheritance, but the student of genetics, as long as he can keep in close touch with a profitable basis of material fact, will scarcely be tempted to look for inspiration in psychical analogies. for a summary of the recent evidence i may refer the reader to semon's paper[ ] where he will find a collection of these observations described from the standpoint of a convinced believer. at the outset one cannot help being struck by the fact that of the instances alleged, very few, even if authentic, show the transmission of acquired modifications which can in any sense be regarded as adaptative, and many are examples not so much of a transmission of characters produced in the parents as of variation induced in the offspring as a consequence of treatment to which the parents were submitted, the parents themselves remaining apparently unmodified. no one questions the great importance of evidence of this latter class as touching the problem of the causes of variation, but it is not obvious why it is introduced in support of the thesis that acquired characters are inherited. it is most difficult to form a clear judgment of the value of the evidence as a whole. to doubt the validity of testimony put forward by reputable authors is to incur a charge of obstinacy or caprice; nevertheless in matters of this kind, where the alleged phenomena are, if genuine, of such exceptional significance, belief should only be extended to evidence after every possible source of doubt has been excluded. we believe such things when we must, but not before. at the very least we are entitled to require that confirmatory evidence should be forthcoming from independent witnesses. so far as i have seen, this requirement is satisfied in scarcely any of the examples that have been lately published, and until it is, judgment may reasonably be suspended. in some cases, however, the facts are not doubtful. standfuss, by subjecting pupae of _vanessa urticae_ to cold, produced the now well-known temperature-aberrations in which the dark pigment is greatly extended. he put together in a breeding-cage males and females showing this modification in various degrees. two of these females died without leaving young. seven produced exclusively normal offspring. from the eighth female butterflies were bred, and of these there were four (all males) which to a greater or less extent exhibited the aberrational form.[ ] the mother of this family was the most abnormal of the females originally put in. fischer's experiment with _aretia caja_ was on similar lines. from pupae which had been frozen almost all the moths which emerged showed aberrational markings. a pair of these mated and produced young which pupated. those which emerged early were all normal, but of those which emerged late, had in various degrees abnormal markings like those of the parents.[ ] in neither of these examples is there any question as to the facts. both observers have great experience and give full details of their work. as regards _vanessa urticae_, however, it must be recalled that fischer himself showed that in nymphalids somewhat similar aberrations could be produced both by heat and by cold, and even by centrifuging the pupae. frl. von linden produced a transitional form of the same aberration in _v. urticae_ by the action of carbonic acid gas.[ ] it is highly probable that the appearance is due to a morbid change, perhaps an arrest of development, which may be brought about by a great diversity of causes. in the experiments the cause probably was a diseased condition of the tissues of the mother herself. she had been subjected to freezing sufficiently severe to prevent the proper development of the pigments and some of the ovarian cells presumably suffered also. it will be observed that the only specimens which were affected were the offspring of the most abnormal female, and of them only four out of forty-three showed any change. the same interpretation probably applies to the cases in _arctia caja_. in this species the markings are well known to be liable to great variation. as barrett says, even in nature individuals are rarely quite alike, and an immense number of strange forms occur in collections.[ ] these are greatly sought after by some collectors, especially in england, where they fetch high prices at auctions, and it is notorious that most of them come from lancashire and the west riding of yorkshire. it is commonly supposed that the breeders of that district subject them to abnormal conditions, and especially to unnatural feeding, but i know no clear evidence that this is true. from whatever cause it is certain that the natural pattern is, in some strains at all events, very easily disturbed. the elaborate experiments of schröder with _abraxas grossulariata_ are difficult to follow and are complicated by the fact that the series which was submitted to abnormal temperatures was derived from an abnormal original pair. from the evidence given it is not clear to me whether the temperature had a distinct effect. this insect, like _arctia caja_, produces an immense number of variations (especially in the amount of the black pigment) and as most of these are, i believe, reared in domestication for sale, it is highly probable that the species is easily influenced by cultural conditions. schröder describes two other experiments which have been accepted by semon and other supporters of the view that acquired characters are transmitted. in the first, _phratora vitellinae_, a phytophagous beetle living on the undersides of leaves, was used. it naturally feeds on _salix fragilis_, a species without a felt, or tomentum, on the underside of the leaves. larvae were transferred to another willow (near _s. viminalis_) which has the undersides of the leaves felted. the larvae took readily to the new food, pushing the tomentum before them as they gnawed the leaves. they came to maturity and when they were about to lay their eggs they were given a free choice between _s. fragilis_ and the tomentose species. the greater number of ovipositions, , took place on _fragilis_, and there were on the tomentose bush, which we are told was six times as large as the _fragilis_. the larvae from _fragilis_ were next put on the tomentose species and reared on it. when they became imagines they were similarly given their choice, with the result that there were ovipositions on the tomentose species and only on _fragilis_. in the next generations there were ovipositions on the tomentose and on _fragilis_. finally the fourth generation made ovipositions on the tomentose and none on _fragilis_. the difficulty about such experiments is obviously that one has no assurance that the change of instinct, in so far as there is any, may not be a mere consequence of the captivity. it must, besides, be extremely difficult to arrange the experiment so that there is really an equal choice between the two bushes, when one stands beside the other. przibram, in quoting this case, considers that as the tomentose bush was about six times as large as the _fragilis_, some indication of the relative attractiveness of the two may be obtained by dividing the ovipositions on the larger bush by six, but i imagine the matter must be much more complex. schröder's second example is not more convincing, in my opinion, though semon regards it as one of the most important pieces of evidence. it concerns a leaf-rolling moth, _gracilaria stigmatella_, the larva of which is said normally to make its house by bending over the _tips_ of the sallow leaves on which it feeds. schröder placed larvae on leaves from which the tips had been cut, and these larvae made their houses by rolling over the _sides_ of the leaves. their offspring were again fed on leaves without tips, and as before, they rolled in the leaf-margins either on one side or both. the offspring of this second generation were then fed on entire leaves. there were houses made by these (? ) larvae, and of them were normal, made by folding down the tips of the leaves, while were abnormal, made by rolling in the leaf-margins. schröder says that in nature he has only twice seen abnormal houses; but it is clearly essential not only that the frequency of such variability in nature should be thoroughly examined, but also that we should know whether when the species is bred in captivity these irregularities of behaviour do or do not occur when the larvae are fed on uninjured leaves. the famous case of schübeler's wheat is revived by semon. the story will be familiar to most readers of the literature of the subject. briefly it is that annuals, especially wheat and maize, raised from seed in central europe take more time in coming to maturity and ripening than similar plants raised in norway, where the summer days are much longer. the received account is that he imported seed especially of maize and of wheat from central europe to norway and found that in successive years the period of growth and ripening was increasingly reduced. after two generations seed of the accelerated wheat was sent back to breslau where it was grown, and was found to ripen rather more slowly than in norway, but much more quickly than the original stock had done. the facts recorded by schübeler[ ] are that he received seed from eldena, which is on the baltic near greifswald. the variety is described as "_ tägiger sommer weizen_," but no more exact record of its behaviour in germany is given. this wheat, grown at christiania in , took days to harvest. its seed was again grown in christiania in , and took days, and sown again in it took only days, days less than in the first year of cultivation in norway. seed of the crop was sent to breslau, and grown there by roedelius in ; it took days. evidently before such a record can be used as proving an inheritance of acquired characters numbers of particulars should be forthcoming. the view that johannsen has taken is that the result was probably due to unconscious selection of the earlier individuals among a population consisting of many types of various compositions. some effect may no doubt be ascribed to that cause, but i cannot think that alone it would account for the results. my impression is rather that they were produced by differences in the cultivation and especially in the seasons. research of an elaborate character would be necessary in order to eliminate the various sources of error, and nothing of the kind has been done; nor does semon allude to these difficulties in prominently adducing schübeler's evidence. a difference of even three weeks in time of harvesting may easily be due to variation in the season. it would in any case be difficult to analyse the meteorological conditions, and to decide how much effect in postponing or accelerating the harvest might be due to cold days, to cloudy days, to wet weather, to fluctuations in average temperature, to hot days, and other such incidents occurring at the different periods of growth, even if they were specially watched while the experiments were in progress, and at this distance of time such analysis is practically impossible. without careful simultaneous control-experiments this evidence is almost worthless. the director of the meteorological office[ ] has, however, kindly sent me some details of the weather at breslau from to , and i notice that as a matter of fact july, , was an exceptionally hot month, _having an average of . ° c. above the mean_ for the twenty years - . june in that year was slightly ( . ° c.) below the mean and may slightly above it ( . ° c.). august was also abnormally hot, . ° c. above the average. the breslau wheat was sown on _may _ and harvested on august . there was a cold spell from may to , which this wheat escaped, as it was sown on may . in the other years the cold spell came much later. these elements of the weather may possibly have done something to hurry the ripening in . it unfortunate that we are not told how long similar wheat from breslau seed took to ripen in that year. as regards the norway cultivations we have the average monthly temperatures recorded by schübeler, though he does not discuss them in connection with this special problem. it is quite clear that , in which the period was days, was an exceptionally cold summer, especially as regards the months of june and july, but though there was, so far as the temperature records go, no great difference between and , the year , in which the period of ripening was the shortest, was somewhat colder in norway than . but we have the further difficulty that there were ten days difference in sowing, for in the sowing was made on may , and in on may . with all these possibilities uncontrolled, and indeed unconsidered, i am surprised that semon should claim these experiments as one of the chief supports for his views. schübeler's other allegations respecting the influence of climate on plants grown in various places and especially at different elevations in norway have been destructively criticised by wille[ ] to whose paper readers interested in the subject should refer. before the appearance of wille's criticisms wettstein[ ] made a favourable reference to schübeler's work, accepting his conclusion. he states also that he has himself made analogous experiments with flax, finding that the length of the period of development and a series of morphological characters show an adaptation to local conditions, and that on transference of seed to other conditions the previous effects are maintained. no details, however, are given, and i do not know if anything more on the subject has appeared since. the other examples cited by wettstein, such as the observations of cieslar on forest-trees and those of jakowatz on gentians seem to me open to all the usual objections applicable to evidence of this kind. such work, to be of any value for the purpose to which it is applied, must be preceded by a study of the normal heredity and of the variations of the species. most of the recent writers (semon, przibram, etc.) on the inheritance of acquired characters accept the story of brown-séquard's guinea pigs, which are said to have inherited a liability to peculiar epileptiform attacks induced in their parents by various nervous lesions. the question has been often debated and several observers have repeated the experiments with varying results, some failing to confirm brown-séquard, others finding evidence which in various degrees supported his conclusions. recently a new and especially valuable paper has been published by mr. t. graham brown[ ] which goes far towards settling this outstanding question. he states that "the brown-séquard phenomenon is nothing more or less than a specific instance of the scratch-reflex," and it is due to a raised excitability of the mechanism of this reflex. this raised excitability is the character acquired as a consequence, for instance, of the removal of part of one great sciatic nerve. the nature of this raised excitability and its causation are discussed and elucidated, but this part of the work is not essential to the present consideration. mr. graham brown in his summary of conclusions remarks that it is very difficult to see how this condition of raised excitability can be transmitted to the offspring, and this comment which might be made in reference to any of the alleged cases certainly applies with special cogency to the present example. he then calls special attention to three observations: . that guinea pigs which had a "trophic" change in the foot, as a result of division of the great sciatic nerve, have repeatedly been seen to nibble the feet of other guinea pigs which had this change in the foot from the same causes. . that accidental injury to the toes may be followed by the brown-séquard phenomenon in an otherwise normal animal. . that in several instances the young of guinea pigs which exhibited the phenomenon have been noticed to have one or more toes eaten off by the mother. brown-séquard noticed that almost all his animals in which the great sciatic was divided acquired the "epilepsy" and nibbled those parts of their feet in which sensation had been lost. of the offspring of such animals he found that a very small proportion exhibited a malformation of the feet, and of these some showed the "epilepsy." the proportion which showed the "epilepsy" was one to two per cent. of the offspring. morgan[ ] is quoted by graham brown as having suggested that the loss of toes in the offspring may have been due to mutilation by the mother, following his experience in a case in which the tails of mice in succeeding litters were thus devoured, and there can be little doubt that in this suggestion lies the clue to the explanation of the whole mystery. graham brown concludes that it may be supposed with every degree of probability that the "transmission" was due to injuries inflicted upon the young by their parents. with this conclusion most people will now be disposed to agree, and we may hope that we shall hear the last of this curious myth--to the elucidation of which a vast quantity of research has been devoted. the series of experiments made by kammerer with various amphibia have attracted much attention and have been acclaimed by semon and other believers in the transmission of acquired characters as giving proof of the truth of their views. with respect to these observations the chief comment to be made is that they are as yet unconfirmed. many of the results that are described, it is scarcely necessary to say, will strike most readers as very improbable; but coming from a man of dr. kammerer's wide experience, and accepted as they are by dr. przibram, under whose auspices the work was done in the biologische vesuchsanstalt at vienna, the published accounts are worthy of the most respectful attention. the evidence relates chiefly to three distinct groups of occurrences: . modification in _alytes obstetricans_, the midwife toad, affecting both the structure and the mode of reproduction, induced by compulsory change of habits. . modification in the mode of reproduction of _salamandra atra_ and _maculosa_ induced by compulsory change of habits. . modification in the colour of _salamandra maculosa_ induced by change in the colour of the soil on which the animals were kept. . i will take first the case of _alytes_,[ ] because it is the most definite example, and because it is the case which most readily admits of repetition and verification. the habits of _alytes obstetricans_ are well known. the animals copulate on land. as the strings of eggs leave the female they are entangled by the hind legs of the male, and being adhesive they stick to him and undergo their development attached to his back and legs. the number of eggs varies from to , a number much smaller than is usual in toads and frogs which lay their eggs in water. the eggs are large and full of yolk. there are two breeding seasons, one about april and the other about september, and a winter hibernation. not only animals brought in from outside, but their offspring reared in domestication maintain these normal habits in confinement, if the temperature does not exceed ° c. (pp. and ). if, however, the temperature be artificially raised and kept at - ° c., the males do not attach the eggs to themselves when spawning occurs on land but let them lie. the adhesion of the eggs is said to be hindered by the comparatively rapid drying of their surfaces. more usually in the high temperatures the animals _take to the water_ and copulate there. the eggs are ejected into the water, and as their gelatinous coverings immediately swell up, they do not stick to the males. the offspring thus derived from the parents subjected to heat for one breeding-period only, whether they were laid in water or on land, did not show departures from the normal type. kammerer states next, however, that in subsequent breeding-periods the same parents frequently take to the water to breed, though they have become quite accustomed to the heated chamber; and furthermore that if such animals, having thus lost their instinct to brood their young, be transferred to ordinary temperatures they do not readily reassume their normal habits, but for several breeding seasons--at least four--will take to the water. these parents lay from to eggs, which are small and contain little yolk, and the larvae, on hatching, breathe with their embryonic gills until they are absorbed instead of being broken off as normally. the offspring thus abnormally developed when they mature are said never to brood their eggs. if they are derived from the earlier spawnings of their parents, before, that is to say, the parents had been submitted to the changed conditions long enough to transmit their effects, they lay on land; but if they are derived from the later spawnings, they lay in the water. these changes of habit are manifested without the continued application of the abnormal experimental conditions, and, as i understand the account, in normal conditions of temperature. if the abnormal experimental conditions are continued, the toads always lay in water, and their eggs become progressively smaller and more numerous. the larvae in the fourth generation acquire three pairs of gills instead of one pair, and are in other respects also different from the normal form. respecting the _alytes_ bred in this way kammerer makes the very striking statement that _the males in the third generation_ (p. ) _have roughened swellings on their thumbs and that in the fourth generation_ (pp. and ) _these swellings develop black pigment_. together with the appearance of this secondary sexual character there is hypertrophy of the muscles of the fore-arm. to my mind this is the critical observation. if it can be substantiated it would go far towards proving kammerer's case. _alytes_, among toads and frogs, is peculiar in that the males do not develop these lumps in the breeding season, and the fact may no doubt be taken to be correlated with the breeding habits, copulation occurring on land and not in water as is usual with batrachians. it is to be expressly noticed that these lumps on the thumbs or arms of male toads and frogs are not merely pigmented swellings, but are pads bearing numerous minute horny black spines, which are used in holding the females in the water. the figures which kammerer gives (taf. xvi, figs. and a) are quite inadequate, and as they merely indicate a dark patch on the thumbs it is not possible to form any opinion as to the nature of the structure they represent. the systematists who have made a special study of batrachia appear to be agreed that _alytes_ in nature does not have these structures; and when individuals possessing them can be produced for inspection it will, i think be time to examine the evidence for the inheritance of acquired characters more seriously. i wrote to dr. kammerer in july, , asking him for the loan of such a specimen[ ] and on visiting the biologische versuchsanstalt in september of the same year i made the same request, but hitherto none has been produced. in matters of this kind much generally depends on interpretations made at the time of observation; here, however, is an example which could readily be attested by preserved material. i notice with some surprise that in a later publication on the same subject no reference to the development of these structures is made (see below). the statements here given represent but a small part of kammerer's papers on the subject. he gives much further information as to the course of the experiments, especially in regard to the fate of the eggs laid on land and the aberrations induced in them by treatment. the ramifications of the experiments are, however, very difficult to follow, and as i am not sure that i have always understood them i must refer the reader to the original. more recently kammerer has published[ ] a most curious account of experiments in crossing his modified and abnormal _alytes_, derived from the water-eggs, with normal individuals. in the first case the cross was made between a _normal female_ and an _abnormal male_. the offspring were normal in their habits. in the next generation bred from these almost exactly a quarter showed the abnormal instinct. the reciprocal cross was made between an _abnormal female_ and a _normal male_. in this case the offspring were abnormal in their behaviour; but the second generation bred from them showed three quarters abnormal and one quarter normal. certain details as to numbers and sexes of the various families bred in the course of this amazing experiment are given in a subsequent publication.[ ] this later paper goes somewhat fully into the question of the difference in behaviour between the normal and modified individuals, describing the ways in which the males and females possessing the acquired character could be recognised from the males and females which were normal, but in this account i find no reference to the development of the "_brunftschwielen_"--the horny pads on the hands of the males. as these structures would be of special value in such a diagnosis the omission of any allusion to them calls for explanation. kammerer claims the evidence as proof of mendelian segregation in regard to an acquired character, the first example recorded. pending a repetition of the experiments there is no more to be said. . _the mode of reproduction of salamandra atra and maculosa._[ ]--_salamandra maculosa_, the common lowland form, with yellow bands or spots, deposits its young in water, generally as gill-bearing tadpoles, with a wide, swimming tail, though occasionally they are born still enclosed in the egg-capsule out of which they soon hatch. spawning extends over a considerable period, often many weeks, and during the season one female may bear more than young. _s. atra_, the black alpine form, produces its young on land. they are born without gills, ready to breathe air, and with the rounded tail of the adult. these differences may, as kammerer says, naturally be regarded as adaptations to the alpine conditions. moreover, the female bears _only two_ young in a season, and this reduction in the number must be taken to be a consequence or condition of viviparity. there are many eggs in the ovary, but all except the two which are destined to develop degenerate and form a yolk-material on which these two survivors feed. kammerer gives a long account of the various conditions to which he subjected both species. the treatment was complicated in many ways, but the essential statements are, as regards _s. maculosa_, that when no water was provided in which the young might be born, they were dropped on land, larger and in a later stage of development and of a darker colour than is normal; that the larvae so born gradually diminished in number until only two were deposited in each breeding-period; that dissection showed that the other ova degenerated to form a yolk-material. the larvae so produced reached maturity. the summary of results describes their behaviour, stating that they produced: (_a_) _in water_, either ( ) _very_ advanced, large-headed larvae mm. long (instead of - mm.) with gills already reduced, which had awkward, embryo-like movements, and in some few days metamorphosed into small perfect salamanders; or ( ) moderately advanced, properly proportioned larvae, - mm. long, provided with large gills of (at first) intrauterine character, which were reduced during aquatic life. (_b_) _on land_, small ( mm. long) larvae with rudimentary gills, having the body rounded instead of being flattened from above downwards, and an elongated narrow head, which were unable to live in deep water. these larvae changed to the salamander colour in - days, and after four weeks metamorphosed into salamanders mm. long. (_c_) in the foregoing cases the experimental conditions were not continued, or in other words, basins of water were provided in which they could spawn. but if the experimental conditions are continued, these _salamandra maculosa_ which were born newt-like (viz., not in a larval condition), are themselves newt-bearing from the first time they give birth, using the dry land, and bringing forth only two young, the normal number for the births of _s. atra_. these young are - mm. long, and are dark-coloured, resembling greatly the normal new-born _s. atra_. this epitome of the observations illustrating the inheritance of acquired characters has been very widely quoted, and may not unnaturally be taken to summarize a wide experience of the modified animals. reference to the details given in the same paper shows that, as alleged, each of the four types of behaviour enumerated was witnessed _once_ only in the case of each of four females, no two agreeing with each other. as to the number of the males or their habits nothing is said. the first female, _a_ ( ), bore five young; the second, _a_ ( ), bore two, of which one was a partial albino; the third, _b_, produced four young; and the fourth, _c_, two as already stated. in the case of _c_ the details show that the female gave birth immediately after being transferred from the open-air terrarium to one indoors, which contained no basin of water. this is the example of the consequences which follow on a continuance of the experimental conditions.[ ] as regards _s. atra_ the converse is reported. various means were used to induce them to eject their young prematurely in water, such as massaging the sides of the mothers, or raising the temperature to ° or ° c., with various degrees of success. but afterwards it was found that specimens collected wild at an elevation of about , metres responded to much simpler treatment, and gave birth prematurely in water when they were kept in a large shallow basin of water not so deep but that they could everywhere touch the bottom with their feet and keep their heads above the surface. with specimens collected at higher elevations this treatment was inoperative, and the suggestion is made that _s. atra_ at the lower confines of its habitat partakes more of the nature of _maculosa_ than do the individuals from greater heights; for kammerer argues that pools suitable for breeding must be more uncommon at those elevations than they are lower down. in the earlier paper[ ] kammerer states that newly caught females of _s. atra_ often give birth in the water, and show an undoubted preference for doing so. he describes also how he once saw several females, wild in their natural habitat, lay their young in a rain-puddle at , metres elevation, but the larvae thus born were fully formed. when the deposition of the young as larvae has become "habitual"[ ] with _s. atra_, three to nine larvae may be produced at one spawning period, from to mm. long, with gills at most mm. long, and a tail-fin - mm. broad. such larvae are generally coffee-brown, or grey (instead of black), and show other minor differences. the summary states that when grown to maturity they become in their turn larva-bearing, and go into the water to bring forth. their young are more than two ( to being the numbers observed) with a length of - mm. or of - mm. at birth. they are light grey, spotted (mottled with lighter and darker colour), have relatively short gills ( to mm. at most) and a broad tail-fin ( mm. wide). at metamorphosis they are relatively long ( mm.) and one of them had some yellow pigment. here again this summary is, as a matter of fact, describing the behaviour of two mothers, of which one produced three, and the other five young. to my mind these experiments suggest that the reproductive habits of both species, if closely observed, will be found to be subject to considerable variation, and i think it not impossible that each species is, especially in confinement, capable of being a good deal deflected from its normal behaviour. moreover, there seems to me no great improbability in the idea that there is an interdependence between the number of young and the stage of maturity in which they are born. but, at the same time, the case as told by kammerer strikes me as proving too much. if each species is so sensitive to conditions that the normal procedure is gravely modified in one generation, and if that modification can reappear in a pronounced form in the next generation without a renewal of the disturbing conditions, it becomes extremely difficult to understand how the regularity which each species is believed to display in nature can be maintained. surely both species might be expected to be in confusion. from a passage in kammerer's earlier paper ( , p. ) on the subject, i infer that he also would expect considerable irregularity in the natural behaviour, but that he has not investigated the point.[ ] . _modification of the colour of salamandra maculosa induced by change in the colour of the soil on which the animals were kept._--kammerer speaks of this as the most convincing of all his experiments on the transmission of acquired characters. so far, however, no full account of them has been published.[ ] the statement is that when salamanders are kept in yellow surroundings the yellow markings gradually in the course of years increase in amount relatively to the black ground colour. conversely by keeping the animals on black garden soil, the yellow may be greatly diminished in quantity until it largely disappears. (the account in _natur_ adds that very moist conditions also favour the increase of yellow, and that with less moist conditions the yellow diminishes.) from each kind, the (induced) yellower and the (induced) blacker, a second generation was raised, on soil of neutral colour, and each family was later divided into two parts, half being put on black and half on yellow ground. as regards the offspring of those which had lived on _black_ soil no positive result had been reached up to the date of publication, but it is stated that these young resembled their parents in having the yellow distributed in _irregular spots_. as regards the offspring of those which had lived on yellow soil the account follows up the story of that part of the offspring which were put on yellow soil again. it is stated that these, though derived from parents with irregular spots, _developed the yellow as longitudinal bands_. this account is given with slight differences of expression in the three places to which i have referred. on returning from vienna in i consulted mr. g. a. boulenger in reference to the subject, and he very kindly showed me the fine series from many localities in the british museum, and pointed out that in nature the colour-varieties can be grouped into two distinct types, one in which the yellow of the body is irregularly distributed in spots and one in which this yellow is arranged for the most part in two longitudinal bands which may be continuous or interrupted. _the spotted form is, as he showed me, an eastern variety, and the striped form belongs to western europe._ mr. e. g. boulenger[ ] has since published a careful account of the distribution of the two forms. the spotted he regards as the typical form, var. _typica_, and for the striped he uses the name var. _taeniata_. the typical form occupies eastern europe in general, including austria and italy, extending as far west as parts of eastern france. the var. _taeniata_ is found all over france, excepting parts of the eastern border, belgium and western germany, spain and portugal. of the very large series examined there was only one specimen (lausanne) which could not with confidence be referred to one or other of the two varieties. mr. e. g. boulenger points out that both varieties inhabit very large areas, and live on soils of most different colours and compositions. both are liable to variations in the amount and the shade of the yellow, but that any suggestion that _taeniata_ belongs especially to yellow soils and _typica_ to black soils is altogether inadmissible. he expresses surprise that kammerer should not allude to these peculiarities in the geographical distribution of the two forms. he suggests further that it is more likely that some mistake occurred in kammerer's observations than that the east european _typica_ should, in the course of a generation, have been transformed into the west european _taeniata_ by the influence of yellow clay soil. in his last paper on the subject kammerer states incidentally[ ] that he has found the _striped form recessive to the spotted_. no evidence for this statement is given, and i have not found any other reference to crosses effected between the two natural types. if, however, this representation is correct, it is conceivable that the production of _taeniata_ from _typica_ was in fact the re-appearance of a recessive form. the plate which kammerer gives in illustration of his modified parent figures a single animal at four stages, and though it is certainly more like the spotted than the striped form, it has a certain suggestion of the striped arrangement, such as i can well imagine being produced in the heterozygote.[ ] in continuation[ ] of the experiments on the colour of _s. maculosa_ kammerer publishes an account of elaborate experiments in grafting ovaries of the various forms, modified and unmodified, into each other, and describes the offspring which followed. before pursuing this part of the inquiry i am disposed to wait until the earlier steps have been made much more secure than they yet are. more recently kammerer has published similar statements in regard to the inheritance of characters induced in various lizards by keeping them in abnormal temperatures, high and low. the changes induced affected in some species the colours, in others the reproductive habits. respecting these examples i feel the same scepticism that i have indicated in regard to the others, somewhat heightened by the fact that insufficient evidence is given both regarding the behaviour of these various species in captivity when not subjected to abnormal temperatures, and in the wild state. respecting this part of the evidence mr. g. a. boulenger has lately published a criticism[ ] from which i extract the following passages. referring to a previous note[ ] on the question of the melanism of the various insular forms of _lacerta muralis_ he writes: "i also alluded (_l. c._) to the theories that have been propounded to explain the melanism of various insular forms. this is a subject which has been lately taken up by dr. kammerer at the biologische versuchsanstalt in vienna, and he claims to have produced nigrinos artificially by a very strong elevation of the temperature, accompanied by extreme dryness. dr. werner[ ] has already opposed his own experiments to those of kammerer, artificial melanism having been produced by him in _lacerta oxycephala_ by keeping two very light specimens from ragusa for a whole summer in very damp conditions. neither is kammerer's theory in accordance with the distribution of the black lizards, as pointed out by werner. kammerer also finds that those forms which are known to produce melanic races in a state of nature, lend themselves more readily than the others to the success of his experiments. but he shows himself misinformed when he states that the variety called _lacerta fiumana_ belongs to the category of those of which black forms are not known. he overlooks the fact, first pointed out by scherer in , and which i can confirm, that the black lizard from melisello near lissa in the adriatic is unquestionably derived from the lizard from lissa, which he correctly regards as not separable from _l. fiumana_...." "another colour modification which dr. kammerer states that he obtained by raising the temperature is the assumption by the female of the typical _lacerta muralis_ of the bright red colour of the lower parts which often distinguishes the male from the female, and which was not shown by the individuals of the latter sex kept by him under normal conditions. he quotes various authorities to show that the lower parts are never red in the females, but he has omitted to consult others who say the contrary. thus bedriaga ( and ) remarks that a so-called var. _rubriventris_ of the typical wall lizard has the lower parts red in both sexes."[ ] in reading such papers as those of semon or kammerer the thought uppermost in my mind is that to multiply illustrations of supposed transmission of acquired characters is of little use until some one example has been thoroughly investigated. if we had certain assurance that even a single unimpeachable case could be repeated at will, the whole matter would assume a more serious aspect. if, for instance, kammerer were able to show us _alytes_ males with horny pads on their hands, it would be something tangible; still more, if the experiment were repeated by others until no doubt remained that the offspring of _alytes_ which had bred in water for some three generations did acquire these pads and that they could transmit these novelties to descendants raised in normal conditions. till evidence of this kind is published by at least two independent observers investigating similar material, i find it easier to believe that mistakes of observation or of interpretation have been made than that any genuine transmission of acquired characters has been witnessed. meanwhile there is no denying that the origin of adaptational features is a very grave difficulty. with the lapse of time since evolutionary conceptions have become a universal subject of study that difficulty has, so far as i see, been in nowise diminished. but i find nothing in the evidence recently put forward which justifies departure from the agnostic position which most of us have felt obliged to assume.[ ] appendix to chapter ix. professor g. klebs, as is well known to students of evolutionary phenomena, has for several years been engaged in investigations relating to the inheritance of acquired characters. in his many publications on the subject the issue has always been represented as more or less uncertain. desiring to know how the matter now stands according to professor klebs' present judgment i wrote to him asking him to favour me with a brief general statement. this he most kindly sent in a letter dated th july, . as such a statement will be read with the greatest interest by all who are watching the progress of these studies i obtained permission to publish it as follows: . juli ihre liebenswurdige anfrage will ich sehr gern beantworten, obwohl ich sie nicht so beantworten kann wie ich erwünschte. ihr skepticismus in der frage der uebertragung erworbener charactere auf die nachkommen ist nur zu berechtigt. meine versuche mit veronica sind _nicht_ beweisend, da es mir bisher nicht gelungen ist eine einigermasse konstante varietät mit verlaubten inflorescenze zu erzeugen. in bezug auf mein semper vivum bin ich allerdings noch heute der meinung dass die starke künstliche veränderung der blüte einen einfluss auf einzelnen nachkommen gehabt hat. ich habe seither nichts darüber veröffentlicht: die mehrzahl der anormalen gefüllten blüten war leider steril. von einem weniger veränderten exemplar erhielt ich einige sämlinge, aber sie haben noch nicht geblüht. es kann sich in diesem falle nur um eine _nachwirkung in der ersten generation_ handeln, vergleichbar jenen fällen in denen samen von bäumen aus den hohen alpen in der ebene gewisse nachwirkungen zeigen. aber es ist bisher kein sicherer. fall bekannt in den der kunstliche herbeigeführte charakter _mehrere generationen hindurch unter der gewöhnlichen "normalen" bedingungen_ übertragen worden ist. auf der andere seite sind diese negativen resultaten nicht entscheidend. denn wie wenig ist in dieser beziehung überhaupt ernstlich versucht worden! und zweifellos geht die sache nicht so einfach. ich versuche es mit anderen pflanzen weil ich der meinung bin dass es möglich sein müsse wenigstens solche neuen varietäten zu erzeugen, wie sie die gartenvarietäten entsprechen. aber bis jetzt leider sind die versuche nicht gelungen, weder mir noch irgend einem anderen. footnotes: [ ] semon, r., der stand der frage nach der vererbung erworbener eigenschaften, published in _fortschr. der naturw. forschung._, bd. , . [ ] standfuss, m., _denks. schweiz. naturf. ges._, xxxvi, , p. . [ ] fischer, e., _allg. ztschr. f. entomologie_, bd. vi, . [ ] out of pupae treated died and of the survivors, one only was affected. see m. v. linden, _archiv. rassen. u. gesells._, , i. [ ] for illustrations see _oberthur's Ã�tudes d'entom._, , where many of these curious aberrations are represented; also barrett, _lepid. brit. islands_, ii, pp. and . [ ] schübeler, f. c., _die culturpflanzen norwegens_, , especially pp. and . [ ] i am obliged to him and to dr. e. gold for much trouble taken to answer my questions. some idea of the kind of weather indicated by an average of . ° c. above the mean may be got from a comparison with the year , which most people will remember as one of the hottest summers they have known. the july of that year was in east and southeast england about ° f. above the mean but . c. means about . ° f. above the mean. at greenwich july, , was about . ° f. above the average. [ ] wille, n., _biol. cbltt._, xxv, , p. . [ ] wettstein, r. von. _der neo-marckismus u. seine beziehungen zum darwinismus_, jena, . [ ] t. graham brown, _proc. roy. soc._, , vol. , b, p. . this paper gives full reference to the previous literature of the subject. [ ] morgan, t. h., _evolution and adaptation_, new york, . [ ] kammerer's chief paper on this subject is in _arch. f. entwm._, , xxviii, p. , and it is to this that the paginal references in the present text relate. his previous paper appeared, _ibid._, , xxii, p. . an account of his further experiments with _alytes_ is given in _natur_, - , heft , p. . [ ] in reply to my letter dr. kammerer who was then away from home very kindly replied that he was not quite sure whether he had killed specimens of _alytes_ with "_brunftschwielen_" or whether he only had living males of the fourth generation, but that he would send illustrative material. [ ] kammerer, p., _ natur_, december, , heft , p. , repeated in _ flugschrift d. deutsch ges. f. züchtungskunde_, berlin, . [ ] _festschrift zum andenken an gregor mendel_, being vol. xlix of the _verh. naturf. ver. in brünn_, , p. . [ ] kammerer's chief papers on this subject are _archiv fur entwm._, xvii, , and _ibid._, xxv, . an epitome of results is also given by him in _ flugschrift d. deutsch. ges. f. züchtungskunde_, berlin, . [ ] "_bei fortdauer der versuchsbedingungen sind als vollmolche geborene salamandra maculosa_ gleich bei der ersten geburt _abermals voll molchgebärend_, benutzen zum geburtsakt das trockene land, und zwar unter erreichung der (bei _salamandra atra_ normalen) _embryonen-zweizahl_," kammerer, , p. . [ ] , p. . [ ] throughout kammerer's papers this is used almost as a technical term. it means, i presume, that the feature was manifested more than once. [ ] it should be stated that the papers contain a quantity of detail, especially descriptive of the state of the larvae, which i have not attempted to represent, but the account here given contains all that seemed essential to an understanding of the more important features of the account. [ ] the first appeared in _natur_, - , heft , p. ; and the second, which contains coloured plates of the animals, in the lecture already referred to, _ flugschr. d. deut. ges. f. züchtungkunde_, berlin, , p. . in the paper in _mendel festschrift_, , the subject is continued, but no more is added as to this part of the experiment. [ ] e. g. boulenger, _proc. zool. soc._, , p. . [ ] _mendel festschrift_, , p. . [ ] _ flugschrift. deut. ges. züchtungskunde_, , fig. , _p. reihe_. [ ] _mendel festschrift_, , p. . [ ] field, , march. [ ] _ibid._, , p. . [ ] _mitth. naturw. ver. a. d. univ. wien_, , p. . [ ] as to the variations of _lacerta muralis_ in western europe and north africa see boulenger, g. a., _trans. zool. soc._, , vol. xvii, p. . [ ] as to the experiments of klebs relating to the transmission of acquired characters, see appendix. chapter x effects of changed conditions continued the causes of genetic variation in the last chapter we examined some of the evidence offered in support of the belief that adaptation in highly organised forms is a consequence of the inheritance of adaptative changes induced by the influence of external conditions. the state of knowledge of this whole subject is, as i have said, most unsatisfactory, chiefly for the reason that in none of the cases which are alleged to show a positive result have two observers been over the same ground, or as yet confirmed each other. in the wider consideration respecting the causes of variation at large we find ourselves still in the same difficulty. the study has thus far proved sadly unfruitful. in spite of the considerable efforts lately made by many observers to induce genetic variation in highly organised plants or animals, and though successes have occasionally been announced, i do not know a single case which has been established and confirmed in such a way that we could with confidence expect to witness the alleged phenomena if we were to repeat the experiment. abundant illustrations are available in which individuals exposed to novel conditions manifest considerable changes in characters or properties, but as yet there is no certain means of determining that germ-cells of a new type shall be formed. of the direct effect of conditions the lower organisms, especially bacteria, offer the best examples, the alterations of virulence which can be produced in so many distinct ways being the most striking and familiar. that attenuation of virulence can be produced by high temperatures or by exposure to chemical agents, and that this diminution in virulence may remain permanent is, from our point of view, not surprising; but the fact that in many cases the full virulence can by suitable cultivation be restored is difficult to understand. similar variations have been observed in power of pigment production and other properties. these phenomena naturally raise the question whether any cases of apparent loss of factors in higher forms may be comparable. the subject of variations in the lower organisms and their dependence on conditions is a highly special one, and i have no knowledge which can justify me in offering any discussion of them, but i understand that hitherto little beyond empirical recognition of the phenomena has been attempted. a useful summary of observations made by many investigators was lately published by hans pringsheim,[ ] who enumerates the different agencies which have been observed to produce modifications, and the various ways in which these changes are manifested. one of the most comprehensive studies of the subject from the genetic point of view is that made by f. wolf.[ ] in his extensive cultivations of _bacillus prodigiosus_, _staphylococcus pyogenes_ and _myxococcus_ he succeeded in producing many strains with modified properties. in most of these the modifications arose in consequence of the application of high or low temperatures or of the addition of various chemical substances to the culture-media. some of the variations, which are for the most part in the powers of pigment-formation, persisted when the strains were returned to normal conditions, and others did not. in reference especially to the variations witnessed in the cocci the reader should consult the critical account of variation in that group published by the winslows,[ ] where much information on the subject is to be found. the authors attempted to determine the systematic relationships of the several forms, as far as possible, by the application of statistical methods. the result is interesting as showing that the problem of species in its main features is presented by these organisms in a form identical with that which we know so well in the higher animals and plants, whatever properties be selected as the diagnostic characters. there are many types perfectly distinct and others which intergrade. some of the types change greatly with conditions while others do not. this is exactly what we encounter whenever we study the problem of species on an extended scale among the higher forms of life. there is now practically complete agreement among bacteriologists that the observations made first by massini on the change in color of _bacterium coli mutabile_ grown in endo's medium, associated with the acquisition of the power to ferment lactose, are perfectly reliable and free from possibilities of mistake. the work has been extended and confirmed by many workers, especially r. müller, who finds that this bacterium can similarly acquire and maintain the power to ferment other sugars. a careful account of the whole subject written by müller for the information of biologists will be found in _zts. für abstammungsl._, viii, . after discussing the biological significance of the facts, he concludes with a caution to the effect that bacteria are so different from all other living things that generalizations from their behavior must not be indiscriminately applied to animals and plants. in all work with this class of material there is obviously danger of error through foreign infection of the cultures, but there can be no doubt that though some of the "mutations" recorded may be due to this cause, the majority of the instances observed under stringent conditions are genuine. another and equally serious difficulty besetting work with bacteria and fungi cultivated from spores is that the appearance of variation may in reality be due to the selection of a special strain previously living masked among other strains. this possibility must be remembered especially in those instances which are claimed as exemplifying the effects of acclimatisation. manifestly this consideration can be urged with most force when the strain which gave rise to the novelty was not raised from a single individual spore. moreover, when once the possibility of spontaneous variation is admitted, it must be difficult to be quite confident that any given variation observed is in reality due to the novel conditions applied, and as i understand the evidence, the appearance of the mutational forms does not with any regularity follow upon the application of the changed conditions. researches into the variation of these lower forms will, no doubt, be continued on a comprehensive scale. so long as the instances recorded are each isolated examples it is impossible to know what value they possess. if they could be coordinated in such a way as to provide some general conception of the types of variation in properties to which bacteria, or any considerable group of them, are habitually liable, the knowledge might greatly advance the elucidation of genetic problems. of mutational changes directly produced with regularity in micro-organisms by treatment, the experiments with trypanosomes provide some of the clearest examples. a summary of the evidence was lately published by dobell,[ ] from which the present account is taken. the most definite fact of this kind established is that certain dyes introduced into the blood of the host have the effect of destroying the small organ known as the "kinetonucleus" in the trypanosomes. the trypanosomes thus altered continue to breed, and give rise to races destitute of kinetonuclei. this observation was originally made by werbitzki and has been confirmed by several observers. the exact way in which this alteration is effected in the trypanosomes is not quite definitely made out, but there is good reason for supposing that the dyes have a direct and specific action upon the kinetonucleus itself, and circumstances make it improbable that in some division a daughter-organism without that body is produced, or that any selection of a pre-existing defective variety occurs. ehrlich has suggested with great probability that the dyes which possess this action owe it to the fact that they have the particular chemical linkage which he calls "ortho-quinoid." in outward respects, such as motility and general appearance, the modified organisms are unchanged, but their virulence is diminished. as regards the possibility of the defective strain reacquiring the kinetonucleus, werbitzki states that in one case passage through animals and treatment with dyes left the strain unaltered; but that in another case at the sixteenth passage per cent. of the trypanosomes were found to have re-acquired the organ, and in subsequent passages the percentage increased, until at the twenty-seventh passage practically all had re-acquired it. kudicke, however, in similar experiments did not succeed in causing re-acquisition by transplantation. by the action of various drugs and anti-bodies races of trypanosomes resistant to those substances have been obtained. these breed true, at least when kept in the same species of animal in which the resistance was acquired. as to whether change of virulence is produced by passage through certain animals or not, there is as yet no general agreement. other changes, especially in size and some points of structure, are said to occur when certain trypanosomes proper to mammals are passed through cold-blooded vertebrates (wendelstadt and fellmer), and it is stated that these changes persist, but the observations have not yet been confirmed. experiments lately conducted by woltereck with _daphnia_ are interesting as having given a definite positive result, in so far, at least, as the ova were affected by conditions before leaving the bodies of the parent individuals. the observations relate to the offspring resulting from _parthenogenetic_ eggs. females bearing ephippia (fertilised eggs) were isolated until the ephippia were dropped, and in this way the offspring of fertilisation were excluded. males, of course, appeared from time to time in the cultures, but as fertilised eggs were rejected, their presence did not disturb the result. the most remarkable observations related to _daphnia longispina_. this species as found in the lower lake at lunz had the front end of the body blunt and nearly round in profile; but on being cultivated in a warm temperature and with abundant nourishment the front end of the body became produced into an elongated "helmet," as woltereck calls it. experiment showed that the change was primarily due to the abundance of food, and owing to temperature in a subordinate degree. this distinction arose as soon as the species was taken into the hothouse, but when the modified individuals were put back into the original conditions, a lower temperature and scanty food-supply, the next generation returned to their original form. after being cultivated for two years and about generations in the more favourable conditions, when similarly put back into the lower temperature with scanty food the _first generation_ born in these conditions was helmeted like the modified parents. woltereck is of opinion that the ova were still unformed at the time the parents were put back, and the influence of the favourable conditions upon the unformed ova he speaks of as a "prae-induction." the effect never extended beyond the one generation, after which the strain returned to its original state. the fact that the influence on the offspring was not manifested at first led woltereck to expect that by more prolonged cultivation in the favourable conditions a further extension of this influence would be produced, but this expectation was never fulfilled, though the attempt was made again and again. similar experiments were made with _hyalodaphnia cucullata_, which is far more sensitive to cultural influences, and in nature manifests a considerable elongation of the helmet as a seasonal modification, but the results were essentially the same as in the preceding case, no modification extending beyond the first generation born after the restoration to _normal conditions_.[ ] the only criticism of these extremely interesting results which suggests itself is that perhaps the original appearance of the modification was not in reality due to an _accumulated_ effect of the conditions, but to some change in the conditions themselves which was not noticed. it is difficult to see how length of time or even the lapse of several generations could have so specific an effect on the race. it is no doubt often vaguely supposed by many that a long period of time may be necessary for the effect of climate or of other environmental conditions to be produced in an organism which does not thus respond at first. i have never been able to see any reason for this opinion nor how it is to be translated into terms of physiological fact, and i imagine that in those cases in which the lapse of time is really required for the production of an effect, the influence of the prolongation is rather on the conditions than on the organisms. the response of the organisms thus probably indicates not that the creature is at length feeling the effects because of their accumulated action on itself, but that the conditions have at length ripened. as this sheet is passing through the press agar has published[ ] an abstract of evidence as to another comparable case in a parthenogenetic strain in the daphnid, _simocephalus vetulus_. when fed on certain abnormal foods the shape of the body is changed, the edges of the carapace being rolled backwards so as to expose the appendages. the offspring of animals thus modified showed similar modification in the first, and to a very slight degree, in the second generation, though the original mothers were removed to normal conditions before their eggs were laid. in the third generation there was "a very pronounced reaction in the opposite direction." agar suggests that the change may be due to some toxin-like substances, carried on passively by the egg into the next generation, against which the protoplasm eventually produces an anti-body. the experiments which have been in recent years regarded by evolutionary writers as the most conclusive proof that direct environmental action may produce germinal variation are those of professor w. l. tower, of chicago, on _leptinotarsa_, the potato beetles. this work has attained considerable celebrity and has been generally accepted as making a definite extension of knowledge. after frequently reading tower's papers and after having been privileged to see some of the experiments in progress (in ) i am still in doubt as to the weight which should be assigned to this contribution. the work is described in two chief publications, the first of which appeared in .[ ] this treatise contains a vast amount of information about numerous species and varieties of these beetles which the author has observed and bred in many parts of their distribution throughout the united states, mexico and central america. the part of the book which has naturally excited the greatest interest is that in which tower states that by subjecting the beetles to change in temperature and moisture, he caused them to produce offspring quite unlike themselves, which in several cases bred true. it is much to be regretted that the author did not happen to become acquainted with mendelian analysis at an earlier stage in the investigation. the evidence might then have been handled in a much more orderly and comprehensive way, and a watch would have been kept for several possibilities of error. the headquarters of the genus is evidently as tower states, in mexico and the adjoining countries. in this region there is a great profusion of forms, some very local, some as for instance the well-known _decemlineata_,[ ] more widely spread. the distinctions are almost all found in peculiarities of colour and pattern, and the limits of species are even more indefinable than is usual in multiform animals. tower arranges the various types into seven groups of which the one most studied is that which he calls the _lineata_ group. to this group belong all the forms to which reference is here made, and, as i understand, they differ among themselves entirely in size, colour and pattern. there is no suggestion of infertility in the crosses made between the several forms of the _lineata_ group; in fact they present, like many chrysomelidae, a good example of what most of us would now call a polymorphic species, consisting of many types, some found existing in the same locality, others being geographically isolated. a series of experiments was devoted to the attempt to fix strains corresponding to the extremes of continuous variations. for example, those with most black pigment and those with least black taken from a population continuously varying in this respect, were separately bred; but almost always the selection led to no sensible change in the position of the mean of the population. the variations in these cases were evidently fluctuational. in some instances, however, real genetic differences were met with, and strains exhibiting them were, as usual, rapidly fixed. tower points out that several of the varieties (or species, as he prefers to call them) were obviously recessive to _decemlineata_. this is most clearly demonstrated in the case of the form called _pallida_, which is a pale depauperated-looking creature, with the orange of the thorax almost white and the eyes devoid of pigment.[ ] this form behaved as an ordinary mendelian recessive, breeding true whenever it appeared in the cultures, or when individuals found wild were studied in captivity. a black form which tower names _melanicum_ was similarly shown to be a mendelian recessive. wild specimens of this variety of opposite sexes were not found simultaneously in nature, and there was thus no opportunity of breeding them together, but the hereditary behaviour was seen in the f_{ } generation from a _melanicum_ found coupled with _decemlineata_. experiments also occurred giving indication that a variety with the stripes anastomosing in pairs (_tortuosa_), was another recessive, and that a variety--called "_rubri-vittata_"--gave an intermediate f_{ } with subsequent segregation. all these are forms of _decemlineata_ stÃ¥l. similar observations were made regarding forms recessive to _multitaeniata_ stÃ¥l. of these two were thrown by _multitaeniata_ itself, namely a form named by stÃ¥l _melanothorax_, and regarded by him as a species, and one which tower names _rubicunda_ n. sp. the facts proving the recessive behaviour of their several forms will be found in the following places in tower's book: _pallida_, pp. - . _melanicum_, p. . _tortuosa_, p. . _rubrivittata_, pp. - . _melanothorax_ and _rubicunda_, pp. - . following this evidence of recessive nature of the six forms enumerated, tower describes experiments showing, as he believes, that some of them may be caused to appear by applying special treatment to the parents during the "growth and fertilisation" (p. ) of the eggs. the most striking example is that in which males and females of _decemlineata_ were kept very hot (average ° c.) and dry, and at low atmospheric pressure ( - inches). the eggs laid were restored to natural conditions. these gave larvae, from which emerged normal, _pallida_ and "_immaculothorax_," viz., without pigment on the pronotum. the account of the rest of the experiment is somewhat involved, but i understand that the _pallida_, of which two only survived, behaved as normal recessives when bred to the type: also that the parents, after having laid the eggs whose history has been given, were restored to normal conditions and laid eggs which gave normals. in another case normal parents laid eggs in the hot and dry conditions, and on restoration to normal conditions, the same parents laid eggs. then eggs gave adults as follows: _males_ _females_ _decemlineata_ _pallida_ _immaculothorax_ _albida_ --- --- the eggs laid in normal conditions gave normal _decemlineata_. similar experiments were made with _multitaeniata_ and gave comparable results, the two recessives (_melanothorax_, _rubicunda_) being produced in large numbers when the parents were subjected to heat, but in this case the atmosphere was kept _saturated_ with moisture, instead of dry, as in the previous instance. the same parents transferred to normal conditions gave normals only. lastly the form _undecimlineata_ was exposed "to an extreme stimulus of high temperature, ° c. above the average," and a dry atmosphere, with the result that from eggs there emerged beetles, all of the form _angustovittata_ jacoby, which subsequently bred true to that type (see p. ). in the results of these experiments, as described, there is one feature which i regard as quite unaccountable. tower makes no comment upon it. indeed, from the general tenour of the paper, i infer, not only that he does not perceive that he is recounting anything contrary to usual experience, but rather that he regards the result as conforming to expectations previously formed. the point in question is the genetic behaviour of the dominant normals produced under the abnormal conditions. these normals were the result of the breeding of parents declared to be at the same time giving off many recessive gametes. some of these normals must be expected therefore to be heterozygous unless some selective fertilisation occurs. nevertheless in every case they and their offspring are reported to have continually bred true. i allude especially to the tables given on pp. , , , and . tower does not mention any misgiving about this result, and i think he regards himself as recounting phenomena in general harmony with the ideas of mutation expressed by de vries. this they may be; but to anyone familiar with analytical breeding the course of these experiments must seem so surprising as to call for most careful, independent confirmation. in [ ] tower published an account of further experiments with _leptinotarsa_. the work described related to two subjects. crosses were made between three forms, _undecimlineata_ stÃ¥l, _signaticollis_ stÃ¥l and "_diversa_" named by tower as a new species. the distinctions between these three depend partly on characters of the adults and partly on those of the larvae. the adults of _undecimlineata_ and _diversa_ have the elytra striped, but the elytra of _signaticollis_ are unstriped. the larvae of _signaticollis_ and of _diversa_ are yellow, but those of _undecimlineata_ are white.[ ] moreover, in _signaticollis_ and _diversa_ the black increases in the third stage of the larvae to form transverse bands which are absent in _undecimlineata_. the general course of the experiments shows that these differences may be approximately represented as due to the action of three factors, any of which may be independently present or absent. the stripings of the elytra and of the larvae are each due to a separate factor. as regards the distinction between the yellow and the white larvae the evidence does not prove that there is decided dominance of either colour and i infer that the heterozygotes are often intermediate. the chief contribution which this new paper claims to make relates to differences in the results which ensue from crosses effected between these three types at different average temperatures. we are first concerned with four experiments which i number ( ), ( ), ( ), ( ): . _signaticollis_ [f] Ã� _diversa_ [m] bred at an average temperature of º f. by day and ° f. by night, gave two groups in about equal numbers. the first ( ) was pure _signaticollis_ and bred true. the second ( ) was of an intermediate type, which on being bred together gave the typical mendelian result-- _sig._: _intermediate_: _div_. . next, as the account originally stood in the published paper, we are told that _sig_ [f] Ã� _div_ [m] bred together at a day-temp. average ° f. and night average ° f. gave an _intermediate_ only, which subsequently produced a normal : : ratio. the two crosses were repeated eleven times with identical results. in a further experiment ( ) _signaticollis_ [f] Ã� _diversa_ [m] were bred under the same conditions as those used in expt. ( ). they again gave _sig._ and intermediates as before in fairly equal numbers. the _sig._ as before bred true, and the intermediate gave : : , all exactly as in expt. ( ). in expt. ( ) _the same parents used_ in ( ) were again mated under conditions of expt. ( ) at the lower temperature, and this time gave _signaticollis_ exclusively, which bred true for four generations. this experiment was repeated seven times with uniform results. diagrams are given representing all these histories in graphic fashion. from these observations, tower concludes that the determination of dominance, and the ensuing type of behaviour, is clearly a function of the conditions incident upon the combining germ plasms. it will be observed that expts. ( ) and ( ) gave identical results but ( ) and ( ), though much the same conditions were applied, are at variance, for ( ) gave all intermediates, while ( ) gave all _signaticollis_. in _amer. nat._, xliv, , p. , professor t. d. a. cockerell commented on this paper of tower's and pointed out that there must be an error somewhere, for when he discusses these experiments tower speaks of ( ) and ( ) as confirming each other. to this tower replied[ ] that there had been a mistake. he states that in preparing the paper "certain minor experiments were taken from a larger series and combined to illustrate a general point in the behaviour of alternative characters in inheritance," and that expt. ( ) was introduced inadvertently in place of another which he desires to substitute. in this, which i number ( ), _signaticollis_ [f] Ã� _diversa_ [m] from exactly the same stocks as those used in ( ), were mated at the lower temperatures specified for ( ), day average ° f., night average ° f. these gave all of the _signaticollis_ type with a narrow range of variability, which bred true, in some cases to f_{ }. tower says he has repeated this experiment six times with identical results. nevertheless he proceeds to say that the description of expt. ( ), which was repeated eleven times with identical results, was correct "as far as given." that experiment was "from a second series of cultures parallel to the one given, but in which there are other factors involved, which in h. [my ( )] are productive of a typical mendelian behaviour." he adds he does "not care at this time to make any statement of what these factors are, nor of their relations to the behaviours given in the h. , h. , h. / series [my ( ), ( ) and ( )--( )] which are the simplest and most easily presented series obtained in the crossing of _signaticollis_ and _diversa_." professor cockerell's intervention has thus elicited the fact that we have as yet only a small selected part of the evidence before us, even as concerning the effect of temperature on the cross between _signaticollis_ [f] Ã� _diversa_ [m]. we learn that at the lower temperatures the result was eleven times the expected one, and six times an unexpected one; further, that we owe it to the author's inadvertence that we have come to hear of the expected result at all, and that though he knows the factors which determine the discrepancy, he declines for the present to name them. in these circumstances we can scarcely venture as yet to estimate the significance of these records. the paper goes on to recount somewhat comparable, but more complex instances in which the descent of the colour of adults and of larvae was affected by temperature in crosses between _undecimlineata_ and _signaticollis_. as they stand the results are very striking and unexpected, but i think, in view of what has been admitted respecting the former part of the paper, full discussion may be postponed till confirmation is forthcoming. one feature, however, calls for remark. this second paper is written apparently without any reference to the discoveries related by tower in his previous book, to which no allusion is made. this is most noticeable in the case of an experiment in which (p. , h. a) _undecimlineata_ [f] (the dominant) was mated to _signaticollis_ [m] with the result that all the offspring were _undecimlineata_ and bred true to that type (parthenogenesis was tested for, but never found to occur). this experiment was made at a temperature averaging ° f. ± . ° by day and ° f. ± . ° by night, and in a humidity given as per cent. by day and per cent. by night; but in the previous book (p. ) we are told that pure _undecimlineata_ bred together "under an extreme stimulus of high temperature, ° c. above the average" and a relative humidity of per cent. gave beetles only, all _angustovittata_. but reference to the plate , fig. , shows that _angustovittata_ must be exceedingly like _signaticollis_, having, like it, the elytral stripes obsolete, and if there is any marked difference at all, it can only be in the larvae. it seems strange that if _undecimlineata_ really gives off ova of this recessive type at high temperatures, the fact should not be alluded to in connection with expt. h. a, where, as the father was _signaticollis_, having the same recessive character, their appearance might have been expected not to pass unobserved. the temperature in the older experiment is, of course, not given with the great accuracy used in the second, and it may have been higher still. the humidity also was widely different. still, in discussing the phenomena we should expect some reference to the very remarkable and closely cognate discovery which tower himself had previously reported in regard to the same species.[ ] the hesitation which i had come to feel respecting these two publications of tower's has been, i confess, increased by the appearance of a destructive criticism by gortner[ ] who has examined the parts of chapter iii of tower's book, in which he discusses at some length the chemistry of the pigments in _leptinotarsa_ and other animals. as gortner has shown, this discussion, though offered with every show of confidence, exhibits such elementary ignorance, both of the special subject and of chemistry in general, that it cannot be taken into serious consideration. some observations made by dr. w. t. macdougal[ ] have also been interpreted as showing the actual causation of genetic variation by chemical treatment. of these perhaps the least open to objection were the experiments with _raimannia odorata_, a patagonian plant closely allied to _oenothera_. the ovaries were injected with various substances and from some of the seeds which subsequently formed in them a remarkable new variety was raised. this varying or mutational form was strikingly different from the parental type, with which it was not connected by any intergradational forms, and it bred true. it made no rosette, growing to a much smaller size than the parent, and was totally glabrous instead of being very hairy as the parental type is. i was shown specimens of these plants by the kindness of dr. britton in the bronx park botanic garden in and can testify to their very remarkable peculiarities. they had a somewhat weakly look, and might at first sight be thought to be a pathological product, but they had bred true for several generations. from the evidence, however, i am by no means satisfied that their original appearance was a consequence of the treatment applied. this treatment was of a most miscellaneous description. two of the mutants came from an ovary which had been treated with a ten per cent. sugar solution. ten came from one into which a . per cent. solution of calcium nitrate had been injected. one was from a capsule which "had been exposed to the action of a radium pencil." macdougal speaks of these results as decisive, but clearly before such evidence can be admitted even for consideration it must be shown by control experiments that the individual plants which threw the mutant were themselves breeding true in ordinary circumstances. nothing is more likely than that the mutant was an ordinary recessive. i may add that mr. r. h. compton made a number of experiments with _raimannia odorata_, raised from seeds kindly given me by dr. britton, injecting the ovaries with a variety of substances, including those named by macdougal; but though a numerous progeny was raised from the ovaries treated, all were normal. macdougal relates also that some mutational forms came from ovaries of _oenothera lamarckiana_ exposed to radium pencils, and also from _oenothera biennis_ injected with zinc sulphate a peculiar mutant was raised, but taking into account the frequency of these occurrences in those species, he very properly regarded this evidence as of doubtful application. in a later paper,[ ] however, he has returned to the subject and affirms his conviction that the appearance of a mutant among seedlings raised from an ovary of _oenothera biennis_ treated with zinc sulphate was really a consequence of the injection, saying that the variation previously observed in the species was afterwards shown to be due to fungoid disease. the circumstances to which he mainly points in support of his view is that the mutation bred true, but this is only evidence of its genetic distinctness, which may, of course, be admitted by those who remain unconvinced as to the original cause of its appearance. he adds that he is making similar experiments with some twenty genera; but what is more urgently needed is repeated confirmation of the original observation. when it has been shown that this mutation can be produced with any regularity from a plant which does not otherwise produce it on normal self-fertilisation, the enquiry may be profitably extended to other plants. a curious and novel experiment, which however, led ultimately to a negative result, was made by f. payne. many discussions have been held respecting the blindness of cave animals. the phenomenon is one of the well-known difficulties, and most of us would admit that the theory of evolution by the natural selection of small differences does not offer a really satisfying account of it. those who believe in the causation of such modifications by environmental influences and in their hereditary transmission make, of course, the simple suggestion that the darkness is the cause of the loss of sight, and that disuse has led to the reduction of the visual organs. payne bred _drosophila ampelophila_, the pomace-fly (which is easy to keep in confinement, fed on fermenting bananas), for sixty-nine generations in darkness. at the end of that period there was no perceptible change in the structure of the eyes, or in any other respect. the number of generations may possibly be regarded as insufficient to prove anything, but comparing them, as he does, with the generations of mankind, we see that they correspond with a period of about two thousand years, an interval far longer than those which many writers in particular cases have deemed sufficient. in his first paper payne states that, though no structural difference could be perceived, the flies which had been bred in the dark reacted less readily to light than those which had been reared under normal conditions, and he inclined to think that the treatment had thus produced a definite effect. after more careful tests, however, he withdrew this opinion. it proved that both individual flies and individual groups of flies, both of those bred in the light and of those bred in the dark, differed greatly in their reactions, which were measured by counting the time that it took for a fly to travel to the light end of a covered tube, various sources of error being eliminated. he found further that these differences of behaviour were not inherited in any simple way, but he is disposed to attribute them to accidental differences in the nature of the food, an account which seems probable enough.[ ] in several recent publications blaringhem[ ] has described the origin of many abnormal forms of plants, especially of maize, which he attributes to various mutilations practised upon the parents. respecting these the same difficulty which has been expressed in other cases reappears, that before drawing any conclusion as to the value of such evidence we require to know that the plants treated belong to a really pure line, which if left to nature in the ordinary circumstances of its life in that locality would have had normal offspring. abnormalities abound in the experience of everyone who examines pans of seedlings of almost any species of plant, and in maize they are well known to be exceptionally common. some of those which we meet with when we attempt to ripen maize in this country are very similar to those which blaringhem describes, consisting in irregularities in the distribution of the sexes, in the shapes of the panicles, etc. many of these are doubtless imperfections of development, due to the dullness of our climate, but others are presumably genetic and would recur in the offspring however treated. if some one working in a climate where maize could be raised in perfection would repeat these experiments, and show that a strain which was thoroughly reliable and normal in its genetic behaviour did, after mutilation, throw the miscellaneous types observed by blaringhem, that would be evidence at least that the development of the seed could be so influenced by injury to the parental tissues that its properties were changed. such evidence could be used for what it is worth; but pending an inquiry of this kind i am disposed to regard these observations of variation following on parental injury as suggestive rather than convincing. some evidence of a remarkably interesting kind has been collected by j. h. powers[ ] respecting the structure and habits of _amblystoma tigrinum_, which led him to the conclusion that striking differences in the form, anatomy, and developmental processes could be effected directly by change in the conditions of life. it is well known that a profusion of forms, distinct in various degrees, is grouped round _amblystoma tigrinum_. some of these are believed to be geographically isolated, others occur together in the same waters, and, as usual, authorities have differed greatly as to the number of names to be given. these forms were studied in detail by cope who described them in the _batrachia of north america_. the view which he inclined to take was that the individual variations of _amblystoma tigrinum_ resulted from variations in the time and completeness of the metamorphosis, and these were regarded as due to external causes, such as differences in season, temperature, and geographical conditions. powers, however, states that collecting within a radius of six or eight miles he found almost if not quite the whole "gamut of recorded variation in this species." some, however, as he states, occurred rarely except under experimental conditions, but considerable differences in temperature were not found necessary in producing them. every year, he says, he has been able to add to the number of peculiar types found in the same small area in nature, until the amount of natural variation at least equals that seen by cope in the collections of the national museum and those of the philadelphia academy. powers states that his observations by no means confirm cope's view that these differences are in the main referable to variation in the completeness of metamorphosis, and on the contrary, he regards metamorphosis as on the whole a levelling process, tending to obliterate diversity. the enormous differences in size and proportions which he describes can only be appreciated by reference to his figures. they affect almost all features of bodily organisation. these striking differences he looks upon as brought about by differences in nutrition, "diversities in habitual locomotion," and diversity in the age at which metamorphosis occurs, and to sexual difference. apart from sexual difference he regards the chief distinctions, in brief, as "acquired variations of the larva." as an example he gives the great elongation of some of the forms as "due first to slow growth, second to the free-swimming habit, third to the prolongation of larval life, and finally to the assumption of sexual maturity as males," either in the branchiate or non-branchiate condition. he describes the rapid growth of some and the slow growth of others. a larva of intermediate type may grow about a centimeter a month, but a rapidly growing specimen may grow more than four times as much. the slower rate of growth may, he says, be induced by winter feeding, and other treatment.[ ] when, however, he goes on to describe the influences which he regards as exerted by the habit of freely swimming, i am led to wonder whether after all in most of these illustrations, the primary distinctions are not in reality genetic. "specimens raised in the same aquarium or in similar aquaria, side by side with all conditions as uniform as it is possible to make them, seldom fail to furnish striking examples of broad-headed, short-bodied, and short-tailed types which are habitually found at the bottom, while others, slender and elongated, are free swimmers, and maintain themselves in almost as continual suspension and motion as does a gold fish." later, again, he writes, "yet despite the uniformity of these favourable conditions, the larvae soon began to split up into two noticeably distinct groups, the one of unusually compact proportions, the other of uniform intermediate build, such is most commonly met with." it is to my mind scarcely possible to resist the inference that, though there may be definite responses to certain conditions, yet the chief distinctions are genetic, and that it is these distinctions which confer the power to respond. the parts respectively played by cause and effect are always difficult to assign; but when it is stated that "a weak-limbed, long-bodied and long-tailed animal becomes well nigh perforce an undulatory swimmer, while the strong-limbed, short-tailed, heavy-bodied specimen, when these characteristics are rapidly forced upon it, is, under certain circumstances, just as forcibly induced to become a crawler," we feel how erroneous any estimates of causation are likely to be. one of the most remarkable and interesting sections of powers' paper is that in which he describes the differences in bodily structure and habits which he attributes to cannibalism, and the whole account of the phenomena should be read in the original. it appears that there are two extremely distinct types of larvae, those with narrow heads and slender bodies which live for the most part on small crustacea such as _daphnias_, and those with huge mouths and very wide heads, which disregard such small animals altogether and live on amphibian larvae, whether of their own or other species. as the illustrations show, the differences between these two types are very great, and the differences in instinct and behaviour are no less. the cannibals take no heed of the pelagic crustacea, lying sluggishly at the bottom, rousing themselves immediately to a violent attack on the larger living things which approach them. nothing but the most incontrovertible evidence based on abundant control experiments should convince us that such differences are not primarily genetic, and in the present state of knowledge i incline to think that the families really consist of individuals which are ready to assume the cannibal habit if opportunity offers, and others which are congenitally incapable of it. it may readily be that if all chance of cannibal diet be excluded, the full development of the wide head and mouth, or the other peculiarities, would never become pronounced, but i doubt whether such change could be induced in any individual taken at random. footnotes: [ ] pringsheim, h., _die variabilität niederer organismen_, berlin, . [ ] f. wolf, modifikationen u. mutationen von bakterien, _zts. f. indukt. abstam. u. vererbungslehre_, ii, , p. . [ ] winslow, c. e. a. and a. r., _systematic relationships of the coccaceae_. new york. . [ ] c. c. dobell, _jour. genetics_, , ii, p. , where full references are given. still more recently the same author has contributed an excellent summary of the evidence relating to bacteria (_ibid._, ii. , p. ). [ ] see woltereck, _verh. d. deut. zool. ges._, , p. ; and , p. . this is a subject which can only be properly appreciated on reference to the original papers. several complications are involved to which i have not here alluded. [ ] _proc. roy. soc._, b, vol. , , p. . [ ] _an investigation of evolution in chrysomelid beetles of the genus leptinotarsa_, carnegie publications, , no. . [ ] this is the famous colorado beetle or potato-bug, which has caused such serious destruction in potato crops. there seems to be no doubt that this insect, formerly unknown in the eastern states, made its way east along the mining trails when the west was opened up. [ ] this is indicated in the coloured plate, but i have not found any explicit statement to this effect in the text, and am not sure if the absence of pigment was regarded as complete. [ ] _biol. bull._, xviii, , p. . [ ] this description does not quite agree with the representation of the larvae in pl. of the book _evolution in the genus leptinotarsa_ for there the larva of _undecimlineata_ is shown as white in the second stage, but yellowish in the third stage; perhaps there is an error in printing. [ ] _biol. bull._, xx, , p. . [ ] as to the interrelations of these three forms, tower states ( , p. ) that _angustovittata_, which he reared from _undecimlineata_, is intermediate between it and _signaticollis_. compare stÃ¥l, "_monogr. des chrysomélides_," , p. ; and jacoby, _biol. centr. amer. celeopt._, vi, pt. , p. , pl. xiii, fig. ; tab. , fig. ; _ibid._, suppl., p. . all these forms are evidently very closely related, and the delimitation of species is quite arbitrary. jacoby indeed suggests that _undecimlineata_ may be a variety of _decemlineata_. [ ] gortner, _amer. nat._, dec., , xlv, p. . [ ] _mutations, variations, and relationships of the oenotheras_, carnegie institution publication no. , , pp. - . [ ] macdougal, d. t., "alterations in heredity induced by ovarial treatments", _bot. gaz._, vol. , , p. . [ ] payne, fernandus, _biol. bull._, xviii, , p. , and _ibid._, xxi, , p. . [ ] see especially, _mutation et traumatismes_, paris, felix alcan, . [ ] j. h. powers, "morphological variation and its causes in _amblystoma tigrinum_." _studies from the zoological laboratory. _ the university of nebraska, no. , . [ ] in connexion with this case i would refer the reader to some remarkable observations of dr. t. a. chapman on various types of larvae which he reared from the moth _arctia caja_ (_ent. rec._, iv, , p. , and following parts). from a single mother he raised a great diversity of forms, some which fed up rapidly and passed through their development without assuming certain stages, and others which were, as he called them, "laggards," moulting more times than their brethren and developing at a much slower rate. it is greatly to be hoped that such a case may be critically investigated by analytical breeding. chapter xi. sterility of hybrids. concluding remarks. when we consider the bearing of recent discoveries on those comprehensive schemes of evolution with which we were formerly satisfied, we find that certain details of the process are more easy to imagine. we readily now understand how varieties once formed, can persist, but at the same time difficulties hitherto faced with complacency become formidable in the light of the new knowledge. so generally is this admitted by those familiar with modern genetic research that most are rightly inclined to postpone the discussion. the premisses, indeed, on which such a discussion must be based are almost wholly wanting. the difficulties to which i chiefly refer are not those created by the phenomena of adaptation, though they are serious enough. in treating of that subject i have felt obliged to express scepticism as to the validity of nearly all the new evidence for the transmission of acquired characters. at the present time the utmost we are bound to accept is the proof that ( ) in some parthenogenetic forms variations, or perhaps we may say malformations, produced in response to special conditions, recur in one or perhaps two generations asexually produced after removal to other conditions. ( ) that violent maltreatment may in rare instances so affect the germ-cells contained in the parents as to cause the individuals resulting from the fertilisation of those cells to exhibit an arrest of development similar to that which their parents underwent. i do not doubt that evidence of this type will be greatly extended. as a contribution to genetic physiology these facts are very important and interesting, but i cannot think that any one, on reflexion, will feel encouraged by such indications to revive old beliefs in the direct origin of adaptations. in these respects we are simply left where we were. the force of objections based upon the existence of adaptative mechanisms is no greater than it has always been. on the contrary the fact that variations can now so generally be recognized as definite is some alleviation of the difficulty. we can moreover disabuse ourselves of the notion that for all characters which are definite or fixed, some utilitarian rationale may be presumed. upon that point the study of variation has provided a perfectly clear answer. in frankly recognizing that the fixity of characters in general need not connote usefulness to their possessors we deliver ourselves of a distracting pre-occupation and prepare our minds for an investigation of the properties of living organisms in the same spirit as that in which the chemist and the physicist examine the properties of unorganized materials. the creature persists not merely by virtue of its characteristics but in spite of them, and the fact of its persistence proves no more than that on the whole the balance of its properties leaves something in its favour. it may be noted by the way that the fact that the structures of living things are on the whole adaptative was not always obvious. though to naturalists of this generation it is a truism, we have only to turn to buffon to find that in his philosophy of nature it played no essential part. the passage in which buffon describes what he regards as the forlorn and degraded condition of the woodpecker is well known. we have come to think of the woodpecker as a capital example of adaptation to the mode of life; but buffon after enumerating the hard features of the bird's existence, forced to earn its living by piercing the bark of trees in an attitude of perpetual constraint, remarks[ ] "tel est l'instinct étroit et grossier d'un oiseau borné a une vie triste et chétive. il a reçu de la nature des organes et des instrumens appropriés a cette destinée _ou plutôt il tient cette destinée même des organes avec lesquels il est né_" (my italics). his reflexions on the stilt (_himantopus_) read even more strangely to us, accustomed as we are to see in the prodigious length and thinness of the shanks and in the other features of its organisation palpable adaptations to a wading life. for buffon, however, this curious bird seemed a poor, neglected production, extravagant in its disproportions, one of the misfits of creation, left as a shadow in the picture composed of nature's more successful efforts.[ ] this theme he develops at some length, being evidently well pleased with the idea. our way of regarding these things is doubtless sounder and more fruitful than buffon's, but it is well to remember that what seems so obvious to us looked quite differently to other excellent observers; and stupid as it may have been to have overlooked plain examples of adaptation, it is a far worse mistake to see adaptation everywhere. i do not seek to minimise the real and permanent difficulty which the existence of adaptations creates, but by the suggestion that all normal specific differences are adaptational that difficulty was quite gratuitously increased. in these respects it may be claimed that progress has been made, even if that progress seem outwardly of small account. but all constructive theories of evolution have been built on the understanding that what we know of the relation of varieties to species justifies the assumption that the one phenomenon is a phase of the other, and that each species arises or has arisen from another species either by one or several genetic steps. in the varieties we have accustomed ourselves to think that we see those steps. we still know little enough of the mode of occurrence of variation, but we do begin to know something, and if we ask ourselves whether our knowledge, such as it is, conforms at all readily with our former expectations, we cannot with any confidence assert that it does. among the plants and animals genetically investigated are many illustrations of very striking and distinct varieties. many of these might readily enough be accepted as species by even the most exacting systematists, and not a few have been so treated in classification; but when we have examined their relationship to each other we feel not merely that they are not species in any strict sense but that the distinctions they present cannot be regarded as stages in the direction of specific difference. complete fertility of the results of inter-crossing is and i think must rightly be regarded as inconsistent with actual specific difference; and of variations leading to that consequence no clear indication has yet been found. as an example of possible exceptions mention should perhaps be made of the case of a giant form of _primula sinensis_ investigated by keeble.[ ] it arose from a "star" primula of normal size, and though fertile with its own pollen all attempts to fertilise it with the pollen of other forms failed. miss pellew, who did these fertilisations, tells me that very extensive trials were made, and repeated in several seasons. ultimately two plants were raised from it fertilised with a plant of the strain from which it sprang, and these proved sterile. in the light of modern experience the significance of such isolated instances is doubtful. all the strains known as "giants" are, as messrs. sutton have always found, more or less sterile, and their sterility is presumably due to some negative defect. in regard to the fertility of primula species there are several paradoxes. for example the long-styled varieties, apart from giants, are fertile with their own pollen, and for many years short-styled plants have not been used in most strains. auriculas and polyanthuses, on the contrary, are generally if not always bred from short-styled plants, as the florists have decided that the long-styled are inadmissible. mr. r. p. gregory tells me that, though most strains of _p. sinensis_ give seed enough when only long-styled plants are used, he finds nevertheless that when a "legitimate" union is made the amount of seed usually increases much as darwin observed. darwin's statement that plants of "illegitimate" origin are less fertile than the "legitimately" raised plants is also in general confirmed by his experience. to this rule there were some marked exceptions in strains derived from _long_-styled plants, which though illegitimate showed a high degree of fertility, but illegitimate unions between _short_-styled plants always produced comparatively sterile offspring. i have no records of the behavior of auriculas and polyanthuses. it would be interesting to know whether among them pure strains of short-styled plants (dominants) have appeared, and, if so, how their fertility is affected. without much more critical data i suppose no one would nowadays be inclined to follow darwin in instituting a comparison between the sterility of hybrids and that of illegitimately raised plants of heterostyle species.[ ] it is even difficult to imagine any essential resemblance between these two phenomena, nor has evidence ever been produced to show that illegitimately raised plants have bad pollen grains, which is the usual symptom of sterility in hybrid plants and the consequence, as we believe, of failure of some essential division in the process of maturation. the difficulty that we have no knowledge of the contemporary origin of forms, from a common stock, which when crossed together give a sterile product, is one of the objections constantly and prominently adduced from the time of the first promulgation of evolutionary ideas. in the light of recent work the objection has gathered strength. why, if we are able to produce instances of variation colourably simulating specific difference in almost all other respects, do we never find an original appearance of this most widely spread of all specific characteristics? no doubt all breeders know that sterile animals and plants occasionally appear in their cultures, but it is more in accordance with probability that the sterility in these sporadic instances should be regarded as due to defect than that it should be thought comparable with that of the sterile hybrids. for their sterility must, by all analogy with results elsewhere seen, be attributed not to the absence of something, but to the presence and operation of complementary factors leading to the production of inhibition of division; and consistently with that interpretation, we find that when from a partially sterile hybrid comparatively fertile offspring can be raised, their comparative fertility continues in the posterity generally if not always without diminution. the distinction between these several kinds of sterility was of course not understood in darwin's time. the comparison, for example, which he instituted[ ] between the sterility of "contabescent" anthers and that of hybrids no longer holds, for at least in those cases in which the nature of contabescent anthers have been genetically investigated (sweet pea, _tropaeolum_) they proved to be a simple recessive character. nor can we now easily suppose that the attempt there made by darwin to suggest resemblance between the sterility produced by unnatural conditions and that of hybrids has any physiological justification. in regarding the power to produce a sterile or partially sterile hybrid as a distinction in kind, of a nature other than those which we perceive among our varieties, i am aware that i am laying stress on an impression which may hereafter prove false. the distinction nevertheless is so striking and so continually before the eyes of a practical breeder that he can scarcely avoid the inference that when he meets a considerable degree of sterility in a cross-bred he is dealing with something belonging to a distinct category, and not merely a varietal feature of an exceptional kind. besides the sterility of hybrids appeal has often been made to the phenomenon of incompatibility, in its several stages of completeness, as distinguishing species. no one doubts that incompatibility may arise from a variety of causes of most diverse degrees of importance, but though sometimes referred to as an extreme case of interspecific sterility, it is really a very different matter. in regard to one phase of this incompatibility, that associated with self-sterility, some progress has been made, and we are not wholly without experimental evidence of its being within the range of contemporary variation. given the outline of mendelian teaching as to gametic differentiation and the classification of individuals in a mixed population, it seemed highly probable that what we call self-sterility must mean that the species really consisted of _classes_, some of which are capable of interbreeding with others while others are not. according to the received account every individual, though incapable of fertilising itself, was supposed to be able both to fertilise and to be fertilised by any other individual. this notion has always seemed to me a self-evident absurdity, for it would imply that there can be as many categories as individuals. such experiments, however, as i made did certainly give results consistent with that belief. i first tried cinerarias, which are usually self-sterile, but i found no incompatible pairs of plants. whether i was deceived by the consequences of apogamy, or whether the pollen of certain plants may belong to more than one class i do not know. the results were confused in various ways. usually the self-fertilised plants set little or nothing, and cross-fertilised they set fully with such uniformity that the few failures could plausibly be attributed to mistakes in manipulation or to other extraneous causes. later de vries announced[ ] (without giving particulars) that he had proved the existence of such classes in _linaria vulgaris_; but on making experiments with that species i again got no positive results, and i came to the conclusion that in spite of inherent improbability the conventional belief must be substantially true. at last, however, the work of correns, lately published,[ ] does definitely show that in one species, _cardamine pratensis_, classes of individuals exist such that individuals of the same class are incapable of fertilising themselves or each other, but fertilisation made between the classes is usually completely effective. many complications were encountered and some contradictory evidence is recorded, but the general bearing of the results was positive and indubitable. we know far too little of this phenomenon as yet to be able to understand its significance, but i suppose we may anticipate with some confidence that it will be found to be a manifestation of dissimilarity between the male and female gametes of the same individual, comparable with that first seen in the stocks (_matthiola_) which throw doubles--a state of things in all likelihood to be found widely spread among hermaphrodite organisms. whether the incompatibility between species is to be associated with that of the self-steriles also cannot be positively asserted, though it seems not unreasonable to expect that such an association will be discovered. the case of the apple and the pear is an impressive illustration of this possibility. the two species are of course exceedingly alike in all outward respects, but nevertheless the pollen of each is entirely without effect on the other. presumably we should interpret this fact as meaning not so much that the apple and the pear are in reality very wide apart, but rather that either, each is lacking in one of two complementary elements, or that each possesses a factor with an inhibitory effect. their incompatibility may well be of the same nature as that of the classes in _cardamine pratensis_. returning now to the problem of inter-specific sterility; we note, as i have said, the absence of contemporary evidence that variation can confer on a variety the power to form a sterile hybrid with the parent species. the considerations based on this want of evidence have for a long while been familiar to all who have discussed evolutionary theories, and it is worth observing the exact reason why the difficulty strikes us now with a new and special force. in pre-mendelian times all that was known was that some forms could freely interbreed without diminution of fertility in the product, while others could not. but now we find that, by virtue of segregation, from one and the same pair of parents, or even, in the case of hermaphrodites, from one and the same individual, offspring commonly arises showing among themselves exactly such differences as distinguish species--and very good species too. this we see happening again and again. but to forms capable of arising as brethren in one family the title species has never been meant to apply, and if we are going to use the term in application to fraternal groups we must definitely recognise that by "specific" difference is to be understood simply _difference_, without any immediate or even ulterior physiological limitation whatever. naturally, therefore, we begin to think of the appearance of sterility in crosses as something apart, and as a manifestation which distinguishes certain kinds of unions in a very special way. i am perfectly aware that there are gradations in the sterility of hybrids as in every other characteristic upon which it has been proposed to base specific definitions; but, as also so often happens in the matter of defining intergrading categories, the difficulty in practice is not often such as to lead to actual ambiguity. i am speaking of course of those examples which are amenable to genetic experiment. as to the rest there is complete and permanent uncertainty. but the experience of the practical breeder does, i think, on the whole, support the contention to which systematists have so steadily clung under all the assaults of evolutionary philosophers, that, though we cannot strictly define species, they yet have properties which varieties have not, and that the distinction is not merely a matter of degree. the first step is to discover the nature of the factors which by their complementary action inhibit the critical divisions and so cause the sterility of the hybrid. thus expressed, we see the problem of inter-specific sterility in its right place; and the question why we do not now find contemporary instances of varieties lately arisen in domestication, which when crossed back with their parents, or with their coderivatives, can produce sterile products, is perceived to be only a special case of a problem which in its more general form is that of the origin of new and additional factors. for the requisite evidence no comprehensive search has been made, but perhaps it will yet be found. all that we can say at the present time is that the incidence both of hybrid sterility, and of incompatibility also, is most capricious; and provided that two forms have such features in common that a cross between them seems not altogether out of the question, no one can predict without experiment whether such a cross is feasible, and if feasible whether the product will be fertile, or sterile more or less completely. for instance, though probably all the british and some foreign finches (fringillidae) have been crossed together, and some of these crosses, as for instance, the various canary-mules have been made in thousands, i believe no quite clear example of a fertile hybrid can be produced. many species of anatidae cross readily and produce fertile hybrids: others give results uniformly sterile. though most of the equidae can be crossed and some of the hybrids are among the commonest of domesticated animals there is no certain record of a fertile mule. among the canidae the dogs, wolves and jackals all give fertile hybrids, but there is no clearly authenticated instance of a cross between any of these forms and the european fox. in spite of their close anatomical resemblance it is doubtful if the rabbit and the hare have ever interbred. many of the wild species of _bos_ have been crossed and recrossed both with each other and with many domesticated races, but i understand that no cross with the indian buffalo (_bos bubalus_) has yet been successful even in producing a live calf.[ ] in the genus _primula_ many hybrids are known and several of them occur in nature, but hitherto no certain hybrid between _p. sinensis_ and any other species has been made, in spite of repeated attempts. in _nicotiana_ many--doubtless all--the various forms of _n. tabacum_ can be crossed together without diminution of fertility, though some are very distinct in appearance, but crosses between _tabacum_ and _sylvestris_ are highly sterile (in my experience totally sterile[ ]), though the distinctions between them are not to outward observation nearly so great as those which can be found between the various races of _primula sinensis_. recently some remarkable experiments bearing closely on these questions have been published by f. rosen.[ ] they concern the forms of _erophila (draba) verna_, celebrated in the history of evolutionary theory as the plants especially chosen by alexis jordan for the exposition of his views on these subjects. the "species" contains a profusion of forms dissimilar in many structural characters, such as the size and shape of leaves, flowers, fruits, etc. of these forms many grow in association. jordan found, on experiment, that each, to the number of some two hundred, bred true, and that therefore, the conventional assumption that polymorphism of this kind must mean great contemporary variability had no foundation in fact. so far indeed is the evidence from favouring the belief that such forms are in any way transitional or indeterminate, that, as is well known, jordan used it with every plausibility to support the doctrine of the fixity of species. to certain aspects of jordan's work we will return later in this chapter, but the matter is in the present connection of especial interest for the reason that rosen has lately found by experiment that some of these presumably very closely allied forms, crossed together, gave hybrids more or less sterile. in the case of the offspring of one pair of forms only (_e. cochleata_ and _stricta_) was the fertility undiminished, and the various degrees of sterility found in the other crosses ranged up to the extreme infertility of the hybrids between _e. stricta_ Ã� _elata_. from this cross ten plants were bred. of these the four strongest were chosen to breed from, but two of the four proved totally sterile; one had only bad seeds; and from the fourth a single seedling was raised which in its turn proved to be sterile. from the less sterile hybrids f_{ } families were raised, with the usual experience that in this and subsequent generations the sterility diminished among extracted forms, new and true-breeding types with complete fertility being thus derived from the original cross.[ ] the production of sterility as a consequence of crossing plants so nearly approaching each other as these _erophila_ "species" do is not a little interesting, and the fact well exemplifies the futility of the various attempts to frame general expressions as to specific properties or behaviour. commenting on his results rosen argues that the polymorphic group commonly called by systematists _erophila (draba) verna_ may now be regarded as having arisen by crossing, as did his own types mentioned above. the question, however, _what_ species were the original progenitors of the group cannot be answered. rosen considers that no form which he knows satisfies the requirements, and that it or they must be supposed to be lost. this conclusion will recall the similar problem raised by the _oenothera_ mutants (chap. v); and unsatisfactory as it may be to have recourse to such hypotheses we must remember the possibility that as a consequence of hybridisation, subsequent segregation and recombination of factors, species may have thus actually, as we may say, exploded, and left nothing but a polymorphic group of miscellaneous types to represent them in posterity. if this way of regarding the phenomena be a true one, the sterility now seen when some of the group are re-crossed, becomes analogous to that "reversion or crossing" which we now so well understand to be a consequence of the recombination of characters separated at some previous point in the history of descent. in the partial sterility of the contemporary hybrid we see this character reappearing, formed now as it was on the occasion of the original cross, by the meeting of complementary factors. another case that may be mentioned in this connection is that of the crosses between various culinary peas (_pisum sativum_) and a peculiar form found by mr. arthur sutton growing ostensibly in a wild state in palestine. this palestine pea is low growing, rarely reaching inches. it is in general appearance like a small and poorly grown field pea. the stems are thin and rather hard. the most obvious differences which distinguish this from other field peas are the marked serration of the stipules, and the development of pith in the pods. such pith is often present in the pods of peas more or less, but in the palestines it is so strongly developed as almost to form a lomentum. curiously enough, though the flowers are purple much as those of ordinary field peas, there is no coloured spot in the axils. on the other hand, the stems have coloured stripes running up from the axils. though this plant differs so little from domesticated peas, all crosses with them either failed, or produced hybrids quite or almost quite sterile. this was mr. sutton's experience, and on repeating the experiments with material kindly given by him i found the same result.[ ] in a large series of crosses some seeds died or gave rise to feeble plants. of the plants which lived, few gave any seed. the seed, however, that was obtained from f_{ } plants grew well enough, and the f_{ } plants proved, as often in such cases, fertile. in these, indeed, no sign of sterility was noticeable. the experiment is being repeated in various ways, for, as the genetic behaviour of peas is comparatively well known, the subject is an exceptionally favourable one for these investigations. such an example shows the confusion produced the moment we attempt to harmonize conceptions of specific difference with results attained by experimental methods. it has been usual to regard the field pea (_p. arvense_) as a species distinct from the edible pea (_p. sativum_). de candolle and others regard the field pea as derived from a form wild in italy, but the origin of the edible pea is considered to be unknown. from breeding experiments we find no sterility whatever in the crosses between the various _arvense_ and _sativum_ types, nor in the crosses made between them and several other peculiar types from various countries; whereas this palestine pea, which only differs from a small _arvense_ in what might have been thought trivial characters,[ ] either fails to cross altogether or gives a sterile product, whatever type be chosen as the other parent. examples of this kind have at least the merit that they lead to more precise delimitations of the problem. we are confronted with two distinct alternatives. . we may apply the term species promiscuously to all distinct forms. if we do so it must be clearly understood that we cannot even rule out the several combinations of "presences and absences" represented by the various types whether wild or domesticated. for we may feel perfectly assured that at least all the _arvense_ and all the _sativum_ types yet subjected to experimental tests are on precisely the same level in this respect. there is no distinction, logical or physiological, to be drawn between them. some contain more factors, and others contain fewer. in some the re-combinations have been brought about by natural variation or crossing, while the same consequences in the others have resulted from man's interference. . we may follow the conventions of systematists and distinguish the outstanding or conspicuous forms such as _arvense_, _quadratum_, _sativum_ and perhaps a few more as species, and leave the rest unheeded. if this course is followed it must be clearly understood and permitted as a piece of pure pragmatism, deliberately adopted for the convenience of cataloguers and collectors, without regard to any natural fact or system whatsoever. but while following either the one plan or the other we shall be still awaiting the answer, which only genetic experiment can provide, to the question whether among the various types there are some which differ from the rest in a peculiar way: whether by having groups of characters linked together in especially durable combinations, or by possessing ingredients which cause greater or less disturbance in the processes of cell-division, and especially in the processes of gametic maturation, when they are united by fertilisation with complementary ingredients. before any but the vaguest ideas regarding the nature and significance of inter-specific sterility can be formed, a vast amount of detailed work must be done. sterility as a result of crossing, as well as that which is alleged sometimes to arise in consequence of changed conditions, is at best a negative characteristic, and there are endless opportunities for mistake and misinterpretation in studying features of this kind. no one, i suppose, would now feel any great confidence in most of the data which from time to time are resuscitated for the purpose of such discussions. even the best collections of evidence, such as those given by darwin in _forms of flowers_, cannot be regarded as critical when judged by present-day standards. nothing short of the most familiar acquaintance with the habitual behaviour of individuals, and of strains kept under constant scrutiny for several years would enable the experimenter to form reliable judgments as to the value to be attached to observations of this class. the admission must, however, be faced that nothing in recent work materially tends to diminish the surprise which has always been felt at the absence of sterility in the crosses between co-derivatives. we should expect such groups of forms to behave like the _erophila_ types, and frequently to produce sterile products on crossing. whatever be the explanation, the fact remains that such evidence is wanting almost completely. in spite of all that we know of variability nothing readily comparable with the power to produce a sterile hybrid on crossing with a near ally, has yet been observed spontaneously arising, though that characteristic of specificity is one of the most widely distributed in nature. it may be that the lacuna in our evidence is due merely to want of attention to this special aspect of genetic inquiry, and on the whole that is the most acceptable view which can be proposed. but seeing that naturalists are more and more driven to believe the domesticated animals and plants to be poly-phyletic in origin--the descendants, that is to say, of several wild forms--the difficulty is proportionately greater than it was formerly, when variation spontaneously occurring was regarded as a sufficient account of their diversity. concluding remarks. the many converging lines of evidence point so clearly to the central fact of the origin of the forms of life by an evolutionary process that we are compelled to accept this deduction, but as to almost all the essential features, whether of cause or mode, by which specific diversity has become what we perceive it to be, we have to confess an ignorance nearly total. the transformation of masses of population by imperceptible steps guided by selection, is, as most of us now see, so inapplicable to the facts, whether of variation or of specificity, that we can only marvel both at the want of penetration displayed by the advocates of such a proposition, and at the forensic skill by which it was made to appear acceptable even for a time. in place of this doctrine we have little teaching of a positive kind to offer. we have direct perception that new forms of life may arise sporadically, and that they differ from their progenitors quite sufficiently to pass for species. by the success and maintenance of such sporadically arising forms, moreover, there is no reasonable doubt that innumerable strains, whether in isolation or in community with their co-derivatives, have as a fact arisen, which now pass in the lists of systematists as species. for an excellent account of typical illustrations i would refer the reader to the book lately published by r. e. lloyd[ ] on the rat-population of india. the observations there recorded are typical of the state of things disclosed whenever the variations of large numbers of individuals are closely investigated, whether in domestication or in natural conditions. guided by such clues we may get a good way into the problem. we see the origin of colourable species in abundance. then, however, doubt arises whether though these new forms are as good species as many which are accepted as such by even cautious systematists, there may not be a stricter physiological sense in which the term species can be consistently used, which would exclude the whole mass of these _petites espèces_. if further we find that we have, with certain somewhat doubtful exceptions, never seen the contemporary origin of a dominant factor, or of inter-racial sterility between indubitable co-derivatives, it needs no elaboration of argument to show that the root of the matter has not been reached. examination of the inter-relations of unquestionably distinct species nearly allied, such as the two common species of _lychnis_, leads to the same disquieting conclusion, and the best suggestion we can make as to their origin is that _conceivably_ they may have arisen as two re-combinations of factors brought together by the crossing of parent species, one or both of which must be supposed to be lost. all this is, as need hardly be said, an unsatisfying conclusion. to those permanently engaged in systematics it may well bring despair. the best course for them is once for all to recognise that whether or no specific distinction may prove hereafter to have any actual physiological meaning, it is impossible for the systematist with the means at his disposal to form a judgment of value in any given case. their business is purely that of the cataloguer, and beyond that they cannot go. they will serve science best by giving names freely and by describing everything to which their successors may possibly want to refer, and generally by subdividing their material into as many species as they can induce any responsible society or journal to publish. between jordan with his odd species for _erophila_, and grenier and godron with one, there is no hesitation possible. jordan's view, as he again and again declares with vehemence, is at least a view of natural facts, whereas the collective species is a mere abstraction, convenient indeed for librarians and beginners, but an insidious misrepresentation of natural truth, perhaps more than any other the source of the plausible fallacies regarding evolution that have so long obstructed progress. nevertheless though we have been compelled to retreat from the speculative position to which scientific opinion had rashly advanced, the prospect of permanent progress is greatly better than it was. with the development of genetic research clear conceptions have at length been formed of the kind of knowledge required and of the methods by which it is to be attained. if we no longer see how varieties give rise to species, we may feel confident that a minute study of genetic physiology of varieties and species is the necessary beginning of any critical perception of their inter-relations. it is little more than a century since no valid distinction between a mechanical mixture and a chemical combination could be perceived, and in regard to the forms of life we may well be in a somewhat similar confusion. as yet the genetic behaviour of animals and plants has only been sampled. when the work has been done on a scale so large as to provide generalisations, we may be in a position to declare whether specific difference is or is not a physiological reality. footnotes: [ ] buffon, _hist. nat._, oiseaux, , vii, p. . [ ] ibid., viii, p. . [ ] keeble, _jour. gen._, , ii, p. . [ ] _animals and plants_, ed. , , ii, pp. - . [ ] _animals and plants_, ed. , , ii, p. . [ ] _species and varieties_, , p. . [ ] correns, _festschr. med.-nat. ges. zur versamml. deutsch. naturf. u. aertze. münster i. w._, . [ ] this is a case of a somewhat different order and i mention it partly for that reason as an illustration of the complexity which such negative instances may present. the difficulty is that though the buffalo and the zebu can breed together, the foetus is too large to be born alive. (see ackermann _ber. d. ver. f. naturk._, kassel, , p. . prof. s. nathusius, of halle, who has great experience in crossing bovidae, tells me that he has always failed to cross the buffalo with other species.) [ ] in a paper to be published in the report of the genetic conference, paris, , bellair states that he obtained some partially fertile hybrids in the cross _n. sylvestris_ Ã� _tabacum_. as to the various degrees of sterility in hybrids between _nicotiana_ species see lock, r. h., _ann. roy. bot. gardens_. peradeniya, iv, , p. . [ ] _beitrage zur biol. der pflanzen._, x, , p. . [ ] one very peculiar feature was observed, namely, that all the new forms in f_{ } which were bred from came true. as i understand, this statement applied to five such new types, and they were represented by individuals in f_{ }, but further details on this point are desirable. another curious fact was observed, namely that one of the f_{ } forms (_cochleata_ Ã� _radiata_) when fertilised by _cochleata_ gave a highly polymorphic family, but fertilised by _radiata_ the resulting offspring were almost uniform. [ ] i also had a few f_{ } seeds given me by mr. r. h. lock. [ ] in a paper about to appear in _jour. linn. soc._ mr. a. w. sutton identifies this palestine pea as _pisum humile_ of boissier and noé. [ ] lloyd, r. e., _the growth of groups in the animal kingdom_, london, . index of subjects page abraxa grossulariata, , aceras hircina, local variability, achatinellidae, local forms of, acquired characters, inheritance of, et seq., , acronycta psi, melanic, adaptation, problem of, , agelaius, local forms, agrotis, fixed and variable species, alkaptonuria, alpine plants, growing larger, if protected, alpine varieties, alytes obstetricans, kammerer's experiments on, , amblystoma, races of, amphidasys betularia, melanic form, , dimorphic larvae, anodonta, polymorphism of, antirrhinum, striped, species-hybrids, albinos, apple, will not cross with pear, arctia caja, effects of temperature, larval variation in, arctic varieties, argynnis paphia and valesina in italy, armadillo, polyembryony, artistic faculty, arum, rights and lefts, auriculas, short-styled selected, axis of symmetry in hand and foot, axolotl, alleged effect of conditions, azalea, bud-sports, bacillus anthracis, unsegmented form, bacillus prodigiosus, variation in, bacteria, variation in, bacterium coli, variation in, baeolophus, geographical races of, barley, right and left-handed, basilarchia, geographical races of, begonia phyllomaniaca, hybrids, bizarre carnation, genetics of, black, as a variation from red, blackbird, varying, black cock, fixity of, boarmia repandata, melanic form, rhomboidaria, , botrytis susceptibility to, bovidae, hybrid, brachydactyly, , bradypus, vertebral variation, bud-sports geometrically irregular, - buffalo, attempts to hybridize, bullfinch, gynandromorph, bulimus detritus, local variation of, canary, asymmetrical markings in, canidae, hybrid, capsella, cardamine pratensis, cat, polydactylism, carnation, picotees and bizarres compared, , cataract, hereditary, certhiola, melanic, chladni figures, choloepus, vertebral variation in, local variation in, cinerarias, self-sterility in, cistudo, local variation in, climatic varieties, coccaceae, variation in, coenonympha arcania, climatic forms of, satyrion, coereba, melanic, colaptes, geographical races, et seq. chrysoides, colloids, growth in, colorado beetles, experiments on, colour blindness in twins, continuous variation, possible example of, coracias, geographical races of, cotton, genetics of, , coupling, crab, extra claws, crustacean appendages and serial homology, crystals, analogy with, cyclopian monsters, artificial, daphnia, changed by environment, dasypus, polyembryony, dianthoecia, fixed and variable species, disease-resistance, division, power of, a fundamental attribute of living things, genetics of, , dogger bank, large varieties on, dogs, hybrid, dominance, nature of, dominants, origin of new, , , double monsters, draba, experiments with, drosophila, payne's experiments on, earthworm, regeneration, elephant, tusk segmented, entelechy, environmental treatment, effects of, et seq. enzymes and genetic factors, epilepsy, inheritance of traumatic, equidae, sterility of hybrid, erophila, experiments with, species, exacum, right and left, euphonia elegantissima, local forms, eupithecia rectangulata, melanic form, factors, new, loss of, factorial representation of varieties, , falcons, geographical races, fasciation, ferments, boyle on, finger-prints of twins, fixity and variability in species, flax, climatic experiments, fowl, silky, leghorn, , dominant white, wyandotte, rumpless, foxes, incompatibility with dogs, free-martin, fringillidae, sterility of hybrid, fundulus, cyclopian, gallus, invariability of wild species, and origin of poultry, , genitalia, a basis for classification in insects, gentians, climatic experiments, geometrical structure and differentiation, , geometrical distinction between germ-cells and somatic cells, gladiolus, right and left, gnophus obscurata, protective colouring, goldfinch, geographical races, gonioctena variabilis, variation in sexes of, gouldian finch, polymorphism, , gracilaria stigmatella, experiments on, grantia, large varieties of, ground-squirrels, local forms of, grouse, red, variation, guillemot, ringed, guinea-pig, brown-séquard's experiments on, gynandromorphs, heliconius erato, forms of, , helix lapicida, local variation of, striata, heripensis, caespitum, trochoides, nemoralis and hortensis, helminthophila, geographical races of, hemerophila abruptaria, melanic, hepialus humuli, in shetland, heterostyle plants, hieracium, himantopus, homoeosis, hybernia progemmaria, hybrids, sterility of, et seq. incompatibility between certain allied species, individual, geometrical independence of, inhibiting factors, intermediates, nature of, , isolation, consequences of, lacerta muralis, kammerer's experiments on, fiumana, leptinotarsa, power's experiments on, limbs, extra, in pairs, limnaea, sinistral, linaria vulgaris, self-sterility, loasa fruits, right and left, lobster, extra claws, locality, variation connected with, , , et seq., lumbricus, regeneration, lychnis dioica and vespertina, inter-relations of, macrocarpa, possibly a common parent of, machetes pugnax, polymorphism of male, maize, blaringhem's experiments on, maize, cumulative factors in, malformations, dominants, arising de novo, manx cat, heredity, matthiola, , , melanic varieties, et seq. memory, analogy with heredity, meristic variation, , , mirabilis, striped, models of segmentation, , "modes," coutagne's conception of, mödling, peculiar race of _pieris napi_ at, mole, albino, , mule, linnaeus on, mutation, matthioli on, in mercurialis, in kales, alleged in bulbs, theory, periods of, in bacteria, mutilation, consequences of, alleged effect of, on offspring, myxococcus, variation in, narwhal, asymmetry of tusks, nemesia strumosa, neuration, a basis for classification, nicotiana, sterility of hybrid, nightjars, varying, noctuidae, fixity and variability, noctua, polymorphic and fixed species, noctua castanea, local forms of, nomenclature, future of, , notonecta, variations of, odontoptera bidentata, melanic form, oedipodidae, protectively coloured, oenothera, new dominant in, rubricalyx and rubrinervis, , lamarckiana, , origin of, , has bad pollen-grains, factorial analysis of, pollen and egg-cells genetically dissimilar, oenothera, "twin hybrids", laeta and velutina, reciprocal crosses in, et seq. possible coupling in, dwarfs, , "triple hybrids", alleged variation due to treatment, ophrys, local variability, orange, polyembryony, osmotic growth, overlapping forms, , papilio, geographical races of, papilio turnus, variation of, pararge egeria, geographical forms, et seq. parthenogenesis, partula, local forms of, passer domesticus and montanus, distinctions, pea, round and wrinkled, pear, will not cross with apple, pelargonium, variegated, bud-sports, periodic phenomena in structure, peronea, fixed and variable species, "petites espèces", petunia, double, phalanger maculatus, local variation, pheasant, fixity of, phigalia pilosaria, melanic, , phratora vitellinae, experiments on, phyllotaxis, pied varieties common in passer domesticus unknown in montanus, pieris napi and bryoniae, et seq. pig, mule-footed, pigeon, web-footed, , indian rock, a recessive form, pigments, nature of, pisum humile, hybrids with culinary peas, species, planarian, regeneration of, , plotheia frontalis, polymorphic, , plusia, fixity and variation in, poephila gouldiae, variation of, , polarity of individual, polia chi, melanic, polyanthus, short-styled selected, polydactylism in cat, , polyembryony, potato, variation in, poultry, evolution of, primula obconica, primula sinensis, flaked, leaf-shapes, new dominant in, sterility in, "giants", primula, species-hybrids, protective coloration, pyrrhulagra, local forms, python, twin-vertebrae, quiscalus, geographical races of, rabbit, angora, colours of, incompatibility with hare, raimannia odorata, macdougal's experiments on, rats, variation in, recessives, origin of, reciprocal crosses, giving distinct results, et seq. regeneration, repulsion, reversal on regeneration, rhamphocoelus, geographical forms, , rhinoptera, variation in jaws of, rhythm in repetition, ribs, variation of, rights and lefts, - ripples, analogous to segments, , , regeneration of, rollers, geographical races of, ruff, polymorphism of male, salamandra, maculosa and atra, , , spotted and striped, geographical variation of, segmentation, nature of, simulated mechanically, compared with rippling, analogies with, segmentation of normally unsegmented structures, selection, natural, an insufficient cause of definiteness of types, , , sempervivum, serial homology, the true nature of, , setina, alpine varieties, sex of twins, sex-factors, possible coupling of, sexual characters, variation in, et seq. siamese twins, silky fowl, , simocephalus, changed by environment, sinistral forms, - situs transversus, skate's jaws, variation in, sloths, vertebral variation, species, conceptions of, , , , , allied, distribution of, alternative uses of the term, specific difference, universality of, of organisms compared with those of inorganic materials, failure of theory of selection to explain, , , sphyropicus varius, , spilosoma lubricipeda, varieties of, zatima, heligoland form, spinal nerves, segmentation of, sporadic variation, , , squashes, polymorphism of, staphylococcus pyogenes, variation in, sterility of hybrids, in general, in lychnis hybrids, et seq. in crossing forms of draba, significance of, self, stilt, stocks, , , striped varieties, substantive variation, subtraction-stages, supernumerary limbs, - sweet pea, variation of, sterile anthers in, symmetry compared with heredity, symmetry of body approximate, syndactyly, in foot, synthetic formulae, in nomenclature, taeniocampa, fixed and variable species, tamias, local forms of, tanagers, geographical races of, teeth, variation in, , tephrosia consortaria and consonaria, , , tephrosia species, separated by season, terminal members, variation of, thais rumina, local variation in, tolerance, persistence of diversity due to, , tomato, number of cells in fruit, transitional populations, rarity of, an example, tropaeolum, sterile anthers in, trypanosomes, variation in, tusk, of elephant, segmented, of narwhal, twinning, , , heredity of, in organs, uria troile, variety of, vanessa urticae, effects of temperature, variation, a medley of phenomena, , sporadic, , and locality, causes of genetic, , , , substantive and meristic, veronica, specific difference in, intermediates between species, vertebrae, division in, , homologies of, vespa, specific difference in, vortex, living organism compared with, wave-motion compared with repetition of parts, , , wheat, cumulative factors in, climatic experiments on, woodpecker, zebra, pattern of stripes compared with ripples, index of persons page ackermann, agar, allen, j. a., , , annandale, arrigoni degli oddi, backhouse, baker, g. t., bangs, outram, , , barrett, , , , , , baur, e., , baur, g., beneden, van, bentham, on species of veronica, lychnis, primula, bernadin, bishop, l. b., , blaringhem, bobart, boisduval, boissier, borradaile, , boulenger, e. g., boulenger, g. a., , , boyle, , brewster, w., , britton, brown, t. graham, brown-séquard, et seq. bruant, p., buffon, butler, s., , buysson, r. du, candolle, de, carpenter, j. h., chapman, f. m., , , , chapman, t. a., , , , church, a. h., cieslar, clark, austin, , cockayne, e. a., cockerell, t. d. a., compton, r. h., , , cope, cory, correns, coutagne, et seq. darwin, on variation, , systematics, selection, , heterostyle plants, , darwin, f., darwin, sir g., davenport, davis, h. m., delcourt, deschange, dobell, doncaster, , , driesch, , duchartre, east, , edwards, w. h., ehrlich, fellmer, field, w. l. w., fischer, e., fleck, , fletcher, w. h. b., , foster, sir n., gallé, garrod, gates, , , gayner, f., godron, gold, e., goldschmidt, goodwin, e., gortner, greene, e. l., gregory, r. p., , , grenier, grover, gruber, gulick, , hamling, hampson, sir g., harris, hartlaub, herbst, heribert-nilsson, hewett, honing, hunter, john, jakowatz, janet, jeans, jenkinson, jentink, johannsen, jordan, , , kammere, et seq. keeble, klebs, krancher, küchenmeister, kudicke, lamarck, lang, a., lawrence, w. n., , leake, h. martin, , leavitt, lecoq, lederer, leduc, , , leydig, linden, m. von, linnaeus, , , lloyd, r. e., locard, lock, r. h., , loeb, , , , , lotsy, lowe, p. r., macdougal, w. t., , marchant, mathew, matthioli, mayer, a. g., mendel, rediscovery of, on fasciation, merrifield, , miller, w. d., , morgan, , , , moggridge, nathusius, s., nettleship., newman, h. h., newsholme, nilsson-ehle, , norman, a. m., , ober, oberthür, , , oliver, j., page, h. e., , patterson, j. t., payne, f., pellew, poll, porritt, poulton, powers, j. h., pringsheim, h., przibram, , , , , , punnett, ray, , raynor, ridgway, , roedelius, rolfe, rosen, f., rosner, rowland-brown, h., , sargent, saunders, e. r., , , schima, schröder, , schübeler, semon, r., et seq. sharrock, shull, speyer, a., , , spillman, standfuss, , , staples-browne, , staudinger, , stockard, , sutton, , tornier, tower, w. l., - trechmann, tugwell, tutt, j. w. on definiteness of species, on plusia interrogationis, on tephrosia, on n. castanea, on pararge egeria, et seq. verity, r., , vries, h. de, - , , walker, g, weir, jenner, weismann, , wendelstadt, werbitzki, werner, wettstein, wheeler, g., , wheldale, wilder, wille, williams, h., , windle, b. c. a., winslow, wolf, f., woodforde, woltereck, zeijlstra, transcriber's note: a few typographical errors have been corrected: they are listed at the end of the text. the errata on page viii, which were in the original book, have been applied to this e-text. page numbers within curly brackets (such as {iii} and { } have been included so that the reader might use the index. the variation of animals and plants under domestication. by charles darwin, m.a., f.r.s., &c. in two volumes.--vol. ii. with illustrations. london: john murray, albemarle street. . the right of translation is reserved. london: printed by william clowes and sons, stamford street, and charing cross. {iii} contents of volume ii. chapter xii. inheritance. wonderful nature of inheritance--pedigrees of our domesticated animals--inheritance not due to chance--trifling characters inherited--diseases inherited--peculiarities in the eye inherited--diseases in the horse--longevity and vigour--asymmetrical deviations of structure--polydactylism and regrowth of supernumerary digits after amputation--cases of several children similarly affected from non-affected parents--weak and fluctuating inheritance: in weeping trees, in dwarfness, colour of fruit and flowers, colour of horses--non-inheritance in certain cases--inheritance of structure and habits overborne by hostile conditions of life, by incessantly recurring variability, and by reversion--conclusion ... page chapter xiii. inheritance _continued_--reversion or atavism. different forms of reversion--in pure or uncrossed breeds, as in pigeons, fowls, hornless cattle and sheep, in cultivated plants--reversion in feral animals and plants--reversion in crossed varieties and species--reversion through bud-propagation, and by segments in the same flower or fruit--in different parts of the body in the same animal--the act of crossing a direct cause of reversion, various cases of, with instincts--other proximate causes of reversion--latent characters--secondary sexual characters--unequal development of the two sides of the body--appearance with advancing age of characters derived from a cross--the germ with all its latent characters a wonderful object--monstrosities--peloric flowers due in some cases to reversion ... page chapter xiv. inheritance _continued_--fixedness of character--prepotency--sexual limitation--correspondence of age. fixedness of character apparently not due to antiquity of inheritance--prepotency of transmission in individuals of the same family, in crossed breeds and species; often stronger in one sex than the other; sometimes due to the same character being present and visible in one breed and latent in the other--inheritance as limited by sex--newly-acquired characters in our domesticated animals often transmitted by one sex alone, sometimes lost by one sex alone--inheritance at corresponding periods of life--the importance of the principle with respect to embryology; as exhibited in domesticated animals; as exhibited in the appearance and disappearance of inherited diseases; sometimes supervening earlier in the child than in the parent--summary of the three preceding chapters ... page {iv} chapter xv. on crossing. free intercrossing obliterates the differences between allied breeds--when the numbers of two commingling breeds are unequal, one absorbs the other--the rate of absorption determined by prepotency of transmission, by the conditions of life, and by natural selection--all organic beings occasionally intercross; apparent exceptions--on certain characters incapable of fusion; chiefly or exclusively those which have suddenly appeared in the individual--on the modification of old races, and the formation of new races, by crossing--some crossed races have bred true from their first production--on the crossing of distinct species in relation to the formation of domestic races ... page chapter xvi. causes which interfere with the free crossing of varieties--influence of domestication on fertility. difficulties in judging of the fertility of varieties when crossed--various causes which keep varieties distinct, as the period of breeding and sexual preference--varieties of wheat said to be sterile when crossed--varieties of maize, verbascum, hollyhock, gourds, melons, and tobacco, rendered in some degree mutually sterile--domestication eliminates the tendency to sterility natural to species when crossed--on the increased fertility of uncrossed animals and plants from domestication and cultivation ... page chapter xvii. on the good effects of crossing, and on the evil effects of close interbreeding. definition of close interbreeding--augmentation of morbid tendencies--general evidence on the good effects derived from crossing, and on the evil effects from close interbreeding--cattle, closely interbred; half-wild cattle long kept in the same parks--sheep--fallow-deer--dogs--rabbits--pigs--man, origin of his abhorrence of incestuous marriages--fowls--pigeons--hive-bees--plants, general considerations on the benefits derived from crossing--melons, fruit-trees, peas, cabbages, wheat, and forest-trees--on the increased size of hybrid plants, not exclusively due to their sterility--on certain plants which either normally or abnormally are self-impotent, but are fertile, both on the male and female side, when crossed with distinct individuals either of the same or another species--conclusion ... page {v} chapter xviii. on the advantages and disadvantages of changed conditions of life: sterility from various causes. on the good derived from slight changes in the conditions of life--sterility from changed conditions, in animals, in their native country and in menageries--mammals, birds, and insects--loss of secondary sexual characters and of instincts--causes of sterility--sterility of domesticated animals from changed conditions--sexual incompatibility of individual animals--sterility of plants from changed conditions of life--contabescence of the anthers--monstrosities as a cause of sterility--double flowers--seedless fruit--sterility from the excessive development of the organs of vegetation--from long-continued propagation by buds--incipient sterility the primary cause of double flowers and seedless fruit ... page chapter xix. summary of the four last chapters, with remarks on hybridism. on the effects of crossing--the influence of domestication on fertility--close interbreeding--good and evil results from changed conditions of life--varieties when crossed not invariably fertile--on the difference in fertility between crossed species and varieties--conclusions with respect to hybridism--light thrown on hybridism by the illegitimate progeny of dimorphic and trimorphic plants--sterility of crossed species due to differences confined to the reproductive system--not accumulated through natural selection--reasons why domestic varieties are not mutually sterile--too much stress has been laid on the difference in fertility between crossed species and crossed varieties--conclusion ... page chapter xx. selection by man. selection a difficult art--methodical, unconscious, and natural selection--results of methodical selection--care taken in selection--selection with plants--selection carried on by the ancients, and by semi-civilised people--unimportant characters often attended to--unconscious selection--as circumstances slowly change, so have our domesticated animals changed through the action of unconscious selection--influence of different breeders on the same sub-variety--plants as affected by unconscious selection--effects of selection as shown by the great amount of difference in the parts most valued by man ... page {vi} chapter xxi. selection--_continued._ natural selection as affecting domestic productions--characters which appear of trifling value often of real importance--circumstances favourable to selection by man--facility in preventing crosses, and the nature of the conditions--close attention and perseverance indispensable--the production of a large number of individuals especially favourable--when no selection is applied, distinct races are not formed--highly-bred animals liable to degeneration--tendency in man to carry the selection of each character to an extreme point, leading to divergence of character, rarely to convergence--characters continuing to vary in the same direction in which they have already varied--divergence of character, with the extinction of intermediate varieties, leads to distinctness in our domestic races--limit to the power of selection--lapse of time important--manner in which domestic races have originated--summary ... page chapter xxii. causes of variability. variability does not necessarily accompany reproduction--causes assigned by various authors--individual differences--variability of every kind due to changed conditions of life--on the nature of such changes--climate, food, excess of nutriment--slight changes sufficient--effects of grafting on the variability of seedling-trees--domestic productions become habituated to changed conditions--on the accumulative action of changed conditions--close interbreeding and the imagination of the mother supposed to cause variability--crossing as a cause of the appearance of new characters--variability from the commingling of characters and from reversion--on the manner and period of action of the causes which either directly, or indirectly through the reproductive system, induce variability ... page chapter xxiii. direct and definite action of the external conditions of life. slight modifications in plants from the definite action of changed conditions, in size, colour, chemical properties, and in the state of the tissues--local diseases--conspicuous modifications from changed climate or food, etc.--plumage of birds affected by peculiar nutriment, and by the inoculation of poison--land-shells--modifications of organic beings in a state of nature through the definite action of external conditions--comparison of american and european trees--galls--effects of parasitic fungi--considerations opposed to the belief in the potent influence of changed external conditions--parallel series of varieties--amount of variation does not correspond with the degree of change in the conditions--bud-variation--monstrosities produced by unnatural treatment--summary ... page {vii} chapter xxiv. laws of variation--use and disuse, etc. nisus formativus, or the co-ordinating power of the organisation--on the effects of the increased use and disuse of organs--changed habits of life--acclimatisation with animals and plants--various methods by which this can be effected--arrests of development--rudimentary organs ... page chapter xxv. laws of variation, _continued_--correlated variability. explanation of term--correlation as connected with development--modifications correlated with the increased or decreased size of parts--correlated variation of homologous parts--feathered feet in birds assuming the structure of the wings--correlation between the head and the extremities--between the skin and dermal appendages--between the organs of sight and hearing--correlated modifications in the organs of plants--correlated monstrosities--correlation between the skull and ears--skull and crest of feathers--skull and horns--correlation of growth complicated by the accumulated effects of natural selection--colour as correlated with constitutional peculiarities ... page chapter xxvi. laws of variation, _continued_--summary. on the affinity and cohesion of homologous parts--on the variability of multiple and homologous parts--compensation of growth--mechanical pressure--relative position of flowers with respect to the axis of the plant, and of seeds in the capsule, as inducing variation--analogous or parallel varieties--summary of the three last chapters ... page chapter xxvii. provisional hypothesis of pangenesis. preliminary remarks--first part:--the facts to be connected under a single point of view, namely, the various kinds of reproduction--the direct action of the male element on the female--development--the functional independence of the elements or units of the body--variability--inheritance--reversion. second part:--statement of the hypothesis--how far the necessary assumptions are improbable--explanation by aid of the hypothesis of the several classes of facts specified in the first part--conclusion ... page {viii} chapter xxviii. concluding remarks. domestication--nature and causes of variability--selection--divergence and distinctness of character--extinction of races--circumstances favourable to selection by man--antiquity of certain races--the question whether each particular variation has been specially preordained ... page index ... page * * * * * errata. vol. ii., pp. , , , for cratægus oxycantha, read oxyacantha. ,, p. , lines from top, for dianthus armoria read armeria. ,, ,, , lines from bottom, for casuarinus read casuarius. ,, ,, ,, lines from bottom, for grus cineria read cinerea. ,, ,, , lines from top, for oesculus read Æsculus. ,, ,, , lines from top, for anastomising read anastomosing. ,, ,, ,, foot-note, for birckell read brickell. * * * * * { } the variation of animals and plants under domestication. * * * * * chapter xii. inheritance. wonderful nature of inheritance--pedigrees of our domesticated animals--inheritance not due to chance--trifling characters inherited--diseases inherited--peculiarities in the eye inherited--diseases in the horse--longevity and vigour--asymmetrical deviations of structure--polydactylism and regrowth of supernumerary digits after amputation--cases of several children similarly affected from non-affected parents--weak and fluctuating inheritance: in weeping trees, in dwarfness, colour of fruit and flowers, colour of horses--non-inheritance in certain cases--inheritance of structure and habits overborne by hostile conditions of life, by incessantly recurring variability, and by reversion--conclusion. the subject of inheritance is an immense one, and has been treated by many authors. one work alone, 'de l'hérédité naturelle,' by dr. prosper lucas, runs to the length of pages. we must confine ourselves to certain points which have an important bearing on the general subject of variation, both with domestic and natural productions. it is obvious that a variation which is not inherited throws no light on the derivation of species, nor is of any service to man, except in the case of perennial plants, which can be propagated by buds. if animals and plants had never been domesticated, and wild ones alone had been observed, we should probably never have heard the saying, that "like begets like." the proposition would have been as self-evident, as that all the buds on the same tree are alike, though neither proposition is strictly true. for, as has often been remarked, probably no two individuals are { } identically the same. all wild animals recognise each other, which shows that there is some difference between them; and when the eye is well practised, the shepherd knows each sheep, and man can distinguish a fellow-man out of millions on millions of other men. some authors have gone so far as to maintain that the production of slight differences is as much a necessary function of the powers of generation, as the production of offspring like their parents. this view, as we shall see in a future chapter, is not theoretically probable, though practically it holds good. the saying that "like begets like" has in fact arisen from the perfect confidence felt by breeders, that a superior or inferior animal will generally reproduce its kind; but this very superiority or inferiority shows that the individual in question has departed slightly from its type. the whole subject of inheritance is wonderful. when a new character arises, whatever its nature may be, it generally tends to be inherited, at least in a temporary and sometimes in a most persistent manner. what can be more wonderful than that some trifling peculiarity, not primordially attached to the species, should be transmitted through the male or female sexual cells, which are so minute as not to be visible to the naked eye, and afterwards through the incessant changes of a long course of development, undergone either in the womb or in the egg, and ultimately appear in the offspring when mature, or even when quite old, as in the case of certain diseases? or again, what can be more wonderful than the well-ascertained fact that the minute ovule of a good milking cow will produce a male, from whom a cell, in union with an ovule, will produce a female, and she, when mature, will have large mammary glands, yielding an abundant supply of milk, and even milk of a particular quality? nevertheless, the real subject of surprise is, as sir h. holland has well remarked,[ ] not that a character should be inherited, but that any should ever fail to be inherited. in a future chapter, devoted to an hypothesis which i have termed pangenesis, an attempt will be made to show the means by which characters of all kinds are transmitted from generation to generation. { } some writers,[ ] who have not attended to natural history, have attempted to show that the force of inheritance has been much exaggerated. the breeders of animals would smile at such simplicity; and if they condescended to make any answer, might ask what would be the chance of winning a prize if two inferior animals were paired together? they might ask whether the half-wild arabs were led by theoretical notions to keep pedigrees of their horses? why have pedigrees been scrupulously kept and published of the shorthorn cattle, and more recently of the hereford breed? is it an illusion that these recently improved animals safely transmit their excellent qualities even when crossed with other breeds? have the shorthorns, without good reason, been purchased at immense prices and exported to almost every quarter of the globe, a thousand guineas having been given for a bull? with greyhounds pedigrees have likewise been kept, and the names of such dogs, as snowball, major, &c., are as well known to coursers as those of eclipse and herod on the turf. even with the gamecock pedigrees of famous strains were formerly kept, and extended back for a century. with pigs, the yorkshire and cumberland breeders "preserve and print pedigrees;" and to show how such highly-bred animals are valued, i may mention that mr. brown, who won all the first prizes for small breeds at birmingham in , sold a young sow and boar of his breed to lord ducie for guineas; the sow alone was afterwards sold to the rev. f. thursby for guineas; who writes, "she paid me very well, having sold her produce for _l_., and having now four breeding sows from her."[ ] hard cash paid down, over and over again, is an excellent test of inherited superiority. in fact, the whole art of breeding, from which such great results have been attained during the present century, depends on the inheritance of each small { } detail of structure. but inheritance is not certain; for if it were, the breeder's art[ ] would be reduced to a certainty, and there would be little scope left for all that skill and perseverance shown by the men who have left an enduring monument of their success in the present state of our domesticated animals. it is hardly possible, within a moderate compass, to impress on the mind of those who have not attended to the subject, the full conviction of the force of inheritance which is slowly acquired by rearing animals, by studying the many treatises which have been published on the various domestic animals, and by conversing with breeders. i will select a few facts of the kind, which, as far as i can judge, have most influenced my own mind. with man and the domestic animals, certain peculiarities have appeared in an individual, at rare intervals, or only once or twice in the history of the world, but have reappeared in several of the children and grandchildren. thus lambert, "the porcupine-man," whose skin was thickly covered with warty projections, which were periodically moulted, had all his six children and two grandsons similarly affected.[ ] the face and body being covered with long hair, accompanied by deficient teeth (to which i shall hereafter refer), occurred in three successive generations in a siamese family; but this case is not unique, as a woman[ ] with a completely hairy face was exhibited in london in , and another instance has recently occurred. colonel hallam[ ] has described a race of two-legged pigs, "the hinder extremities being entirely wanting;" and this deficiency was transmitted through three generations. in fact, all races presenting any remarkable peculiarity, such as solid-hoofed swine, mauchamp sheep, niata cattle, &c., are instances of the long-continued inheritance of rare deviations of structure. when we reflect that certain extraordinary peculiarities have { } thus appeared in a single individual out of many millions, all exposed in the same country to the same general conditions of life, and, again, that the same extraordinary peculiarity has sometimes appeared in individuals living under widely different conditions of life, we are driven to conclude that such peculiarities are not directly due to the action of the surrounding conditions, but to unknown laws acting on the organisation or constitution of the individual;--that their production stands in hardly closer relation to the conditions than does life itself. if this be so, and the occurrence of the same unusual character in the child and parent cannot be attributed to both having been exposed to the same unusual conditions, then the following problem is worth consideration, as showing that the result cannot be due, as some authors have supposed, to mere coincidence, but must be consequent on the members of the same family inheriting something in common in their constitution. let it be assumed that, in a large population, a particular affection occurs on an average in one out of a million, so that the _à priori_ chance that an individual taken at random will be so affected is only one in a million. let the population consist of sixty millions, composed, we will assume, of ten million families, each containing six members. on these data, professor stokes has calculated for me that the odds will be no less than millions to that in the ten million families there will not be even a single family in which one parent and two children will be affected by the peculiarity in question. but numerous cases could be given, in which several children have been affected by the same rare peculiarity with one of their parents; and in this case, more especially if the grandchildren be included in the calculation, the odds against mere coincidence become something prodigious, almost beyond enumeration. in some respects the evidence of inheritance is more striking when we consider the reappearance of trifling peculiarities. dr. hodgkin formerly told me of an english family in which, for many generations, some members had a single lock differently coloured from the rest of the hair. i knew an irish gentleman, who, on the right side of his head, had a small white lock in the midst of his dark hair: he assured me that his grandmother had { } a similar lock on the same side, and his mother on the opposite side. but it is superfluous to give instances; every shade of expression, which may often be seen alike in parents and children, tells the same story. on what a curious combination of corporeal structure, mental character, and training, must handwriting depend! yet every one must have noted the occasional close similarity of the handwriting in father and son, although the father had not taught his son. a great collector of franks assured me that in his collection there were several franks of father and son hardly distinguishable except by their dates. hofacker, in germany, remarks on the inheritance of handwriting; and it has even been asserted that english boys when taught to write in france naturally cling to their english manner of writing.[ ] gait, gestures, voice, and general bearing are all inherited, as the illustrious hunter and sir a. carlisle have insisted.[ ] my father communicated to me two or three striking instances, in one of which a man died during the early infancy of his son, and my father, who did not see this son until grown up and out of health, declared that it seemed to him as if his old friend had risen from the grave, with all his highly peculiar habits and manners. peculiar manners pass into tricks, and several instances could be given of their inheritance; as in the case, often quoted, of the father who generally slept on his back, with his right leg crossed over the left, and whose daughter, whilst an infant in the cradle, followed exactly the same habit, though an attempt was made to cure her.[ ] i will give one instance which has fallen under my own observation, and which is curious from being a trick associated with a peculiar state of mind, namely, pleasurable emotion. a boy had the singular habit, when pleased, of rapidly moving his fingers parallel to each other, and, when much excited, of raising both hands, with the fingers still moving, to the sides of his face on a level with the eyes; this boy, when almost an old man, could still hardly resist this trick when much pleased, but from its absurdity concealed it. he had eight children. of these, a girl, when { } pleased, at the age of four and a half years, moved her fingers in exactly the same way, and what is still odder, when much excited, the raised both her hands, with her fingers still moving, to the sides of her face, in exactly the same manner as her father had done, and sometimes even still continued to do when alone. i never heard of any one excepting this one man and his little daughter who had this strange habit; and certainly imitation was in this instance out of the question. some writers have doubted whether those complex mental attributes, on which genius and talent depend, are inherited, even when both parents are thus endowed. but he who will read mr. galton's able paper[ ] on hereditary talent will have his doubts allayed. unfortunately it matters not, as far as inheritance is concerned, how injurious a quality or structure may be if compatible with life. no one can read the many treatises[ ] on hereditary disease and doubt this. the ancients were strongly of this opinion, or, as ranchin expresses it, _omnes græci, arabes, et latini in eo consentiunt_. a long catalogue could be given of all sorts of inherited malformations and of predisposition to various diseases. with gout, fifty per cent. of the cases observed in hospital practice are, according to dr. garrod, inherited, and a greater percentage in private practice. every one knows how often insanity runs in families, and some of the cases given by mr. sedgwick are awful,--as of a surgeon, whose brother, father, and four paternal uncles were all insane, the latter dying by suicide; of a jew, whose father, mother, and six brothers and sisters were all mad; and in some other cases several members of the same family, during three or four successive generations, have committed suicide. striking instances { } have been recorded of epilepsy, consumption, asthma, stone in the bladder, cancer, profuse bleeding from the slightest injuries, of the mother not giving milk, and of bad parturition being inherited. in this latter respect i may mention an odd case given by a good observer,[ ] in which the fault lay in the offspring, and not in the mother: in a part of yorkshire the farmers continued to select cattle with large hind-quarters, until they made a strain called "dutch-buttocked," and "the monstrous size of the buttocks of the calf was frequently fatal to the cow, and numbers of cows were annually lost in calving." instead of giving numerous details on various inherited malformations and diseases, i will confine myself to one organ, that which is the most complex, delicate, and probably best-known in the human frame, namely, the eye, with its accessory parts. to begin with the latter: i have heard of a family in which parents and children were affected by drooping eyelids, in so peculiar a manner, that they could not see without throwing their heads backwards; and sir a. carlisle[ ] specifies a pendulous fold to the eyelids as inherited. "in a family," says sir h. holland,[ ] "where the father had a singular elongation of the upper eyelid, seven or eight children were born with the same deformity; two or three other children having it not." many persons, as i year from mr. paget, have two or three of the hairs in their eyebrows (apparently corresponding with the vibrissæ of the lower animals) much longer than the others; and even so trifling a peculiarity as this certainly runs in families. with respect to the eye itself, the highest authority in england, mr. bowman, has been so kind as to give me the following remarks on certain inherited imperfections. first, hypermetropia, or morbidly long sight: in this affection, the organ, instead of being spherical, is too flat from front to back, and is often altogether too small, so that the retina is brought too forward for the focus of the humours; consequently a convex glass is required for clear vision of near objects, and frequently even of distant ones. this state occurs congenitally, or at a very early age, often in several children of the same family, where one of the parents has presented it.[ ] secondly, myopia, or short-sight, in which the eye is egg-shaped, and too long from front to back; the retina in this case lies behind the focus, and is therefore fitted to see distinctly only very near objects. this condition is not commonly congenital, but comes on in youth, the liability to it being well known to be transmissible from parent to child. the change from the spherical to the ovoidal shape seems the immediate { } consequence of something like inflammation of the coats, under which they yield, and there is ground for believing that it may often originate in causes acting directly on the individual affected, and may thenceforward become transmissible. when both parents are myopic mr. bowman has observed the hereditary tendency in this direction to be heightened, and some of the children to be myopic at an earlier age or in a higher degree than their parents. thirdly, squinting is a familiar example of hereditary transmission: it is frequently a result of such optical defects as have been above mentioned; but the more primary and uncomplicated forms of it are also sometimes in a marked degree transmitted in a family. fourthly, _cataract_, or opacity of the crystalline lens, is commonly observed in persons whose parents have been similarly affected, and often at an earlier age in the children than in the parents. occasionally more than one child in a family is thus afflicted, one of whose parents or other relation presents the senile form of the complaint. when cataract affects several members of a family in the same generation, it is often seen to commence at about the same age in each; _e.g._, in one family several infants or young persons may suffer from it; in another, several persons of middle age. mr. bowman also informs me that he has occasionally seen, in several members of the same family, various defects in either the right or left eye; and mr. white cooper has often seen peculiarities of vision confined to one eye reappearing in the same eye in the offspring.[ ] the following cases are taken from an able paper by mr. w. sedgwick, and from dr. prosper lucas.[ ] amaurosis, either congenital or coming on late in life, and causing total blindness, is often inherited; it has been observed in three successive generations. congenital absence of the iris has likewise been transmitted for three generations, a cleft-iris for four generations, being limited in this latter case to the males of the family. opacity of the cornea and congenital smallness of the eyes have been inherited. portal records a curious case, in which a father and two sons were rendered blind, whenever the head was bent downwards, apparently owing to the crystalline lens, with its capsule, slipping through an unusually large pupil into the anterior chamber of the eye. day-blindness, or imperfect vision under a bright light, is inherited, as is night-blindness, or an incapacity to see except under a strong light: a case has been recorded, by m. cunier, of this latter defect having affected eighty-five members of the same family during six generations. the singular incapacity of distinguishing colours, which has been called _daltonism_, is notoriously hereditary, and has been traced through five generations, in which it was confined to the female sex. with respect to the colour of the iris: deficiency of colouring matter is well known to be hereditary in albinoes. the iris of one eye being of a different colour from that of the other, and the iris being spotted, are cases which have been inherited. mr. sedgwick gives, in addition, on the { } authority of dr. osborne,[ ] the following curious instance of strong inheritance: a family of sixteen sons and five daughters all had eyes "resembling in miniature the markings on the back of a tortoiseshell cat." the mother of this large family had three sisters and a brother all similarly marked, and they derived this peculiarity from their mother, who belonged to a family notorious for transmitting it to their posterity. finally, dr. lucas emphatically remarks that there is not one single faculty of the eye which is not subject to anomalies; and not one which is not subjected to the principle of inheritance. mr. bowman agrees with the general truth of this proposition; which of course does not imply that all malformations are necessarily inherited; this would not even follow if both parents were affected by an anomaly which in most cases was transmissible. even if no single fact had been known with respect to the inheritance of disease and malformations by man, the evidence would have been ample in the case of the horse. and this might have been expected, as horses breed much quicker than man, are matched with care, and are highly valued. i have consulted many works, and the unanimity of the belief by veterinaries of all nations in the transmission of various morbid tendencies is surprising. authors, who have had wide experience, give in detail many singular cases, and assert that contracted feet, with the numerous contingent evils, of ring-bones, curbs, splints, spavin, founder and weakness of the front legs, roaring or broken and thick wind, melanosis, specific ophthalmia, and blindness (the great french veterinary hazard going so far as to say that a blind race could soon be formed), crib-biting, jibbing, and ill-temper, are all plainly hereditary. youatt sums up by saying "there is scarcely a malady to which the horse is subject which is not hereditary;" and m. bernard adds that the doctrine "that there is scarcely a disease which does not run in the stock, is gaining new advocates every day."[ ] so it { } is in regard to cattle, with consumption, good and bad teeth, fine skin, &c. &c. but enough, and more than enough, has been said on disease. andrew knight, from his own experience, asserts that disease is hereditary with plants; and this assertion is endorsed by lindley.[ ] seeing how hereditary evil qualities are, it is fortunate that good health, vigour, and longevity are equally inherited. it was formerly a well-known practice, when annuities were purchased to be received during the lifetime of a nominee, to search out a person belonging to a family of which many members had lived to extreme old age. as to the inheritance of vigour and endurance, the english race-horse offers an excellent instance. eclipse begot , and king herod winners. a "cock-tail" is a horse not purely bred, but with only one-eighth or one-sixteenth impure blood in his veins, yet very few instances have ever occurred of such horses having won a great race. they are sometimes as fleet for short distances as thoroughbreds, but as mr. robson, the great trainer, asserts, they are deficient in wind, and cannot keep up the pace. mr. lawrence also remarks, "perhaps no instance has ever occurred of a three-part-bred horse saving his '_distance_' in running two miles with thoroughbred racers." it has been stated by cecil, that when unknown horses, whose parents were not celebrated, have unexpectedly won great races, as in the case of priam, they can always be proved to be descended on both sides, through many generations, from first-rate ancestors. on the continent, baron cameronn challenges, in a german veterinary periodical, the opponents of the english race-horse, to name one good horse on the continent which has not some english race-blood in his veins.[ ] with respect to the transmission of the many slight, but { } infinitely diversified characters, by which the domestic races of animals and plants are distinguished, nothing need be said; for the very existence of persistent races proclaims the power of inheritance. a few special cases, however, deserve some consideration. it might have been anticipated, that deviations from the law of symmetry would not have been inherited. but anderson[ ] states that a rabbit produced in a litter a young animal having only one ear; and from this animal a breed was formed which steadily produced one-eared rabbits. he also mentions a bitch, with a single leg deficient, and she produced several puppies with the same deficiency. from hofacker's account[ ] it appears that a one-horned stag was seen in in a forest in germany, in two, and afterwards, from year to year, many were observed with only one horn on the right side of the head. a cow lost a horn by suppuration,[ ] and she produced three calves which had on the same side of the head, instead of a horn, a small bony lump attached merely to the skin; but we here approach the doubtful subject of inherited mutilations. a man who is left-handed, and a shell in which the spire turns in the wrong direction, are departures from the normal though a symmetrical condition, and they are well known to be inherited. _polydactylism._--supernumerary fingers and toes are eminently liable, as various authors have insisted, to transmission, but they are noticed here chiefly on account of their occasional regrowth after amputation. polydactylism graduates[ ] by multifarious steps from a mere cutaneous appendage, not including any bone, to a double hand. but an additional digit, supported on a metacarpal bone, and furnished with all the proper muscles, nerves, and vessels, is sometimes so perfect, that it escapes detection, unless the fingers are actually counted. occasionally there are several supernumerary digits; but usually only one, making the total number six. this one may represent either a thumb or finger, being attached to the inner or outer margin of the hand. generally, through the law of correlation, both hands and feet are similarly affected. i have tabulated the cases recorded in various works or privately communicated { } to me, of forty-six persons with extra digits on one or both hands and feet; if in each case all four extremities had been similarly affected, the table would have shown a total of ninety-two hands and ninety-two feet each with six digits. as it is, seventy-three hands and seventy-five feet were thus affected. this proves, in contradiction to the result arrived at by dr. struthers,[ ] that the hands are not more frequently affected than the feet. the presence of more than five digits is a great anomaly, for this number is not normally exceeded by any mammal, bird, or existing reptile.[ ] nevertheless, supernumerary digits are strongly inherited; they have been transmitted through five generations; and in some cases, after disappearing for one, two, or even three generations, have reappeared through reversion. these facts are rendered, as professor huxley has observed, more remarkable from its being known in most cases that the affected person had not married one similarly affected. in such cases the child of the fifth generation would have only - nd part of the blood of his first sedigitated ancestor. other cases are rendered remarkable by the affection gathering force, as dr. struthers has shown, in each generation, though in each the affected person had married one not affected; moreover such additional digits are often amputated soon after birth, and can seldom have been strengthened by use. dr. struthers gives the following instance: in the first generation an additional digit appeared on one hand; in the second, on both hands; in the third, three brothers had both hands, and one of the brothers a foot affected; and in the fourth generation all four limbs were affected. yet we must not over-estimate the force of inheritance. dr. struthers asserts that cases of non-inheritance and of the first appearance of additional digits in unaffected families are much more frequent than cases of inheritance. many other deviations of structure, of a nature almost as anomalous as supernumerary digits, such as deficient phalanges, thickened joints, crooked fingers, &c., are in like manner strongly inherited, and are equally subject to intermission with reversion, though in such cases there is no reason to suppose that both parents had been similarly affected.[ ] { } additional digits have been observed in negroes as well as in other races of man, and in several of the lower animals. six toes have been described on the hind feet of the newt (_salamandra cristata_), and, as it is said, of the frog. it deserves notice from what follows, that the six-toed newt, though adult, had preserved some of its larval characters; for part of the hyoidal apparatus, which is properly absorbed during the act of metamorphosis, was retained. in the dog, six toes on the hinder feet have been transmitted through three generations; and i have heard of a race of six-toed cats. in several breeds of the fowl the hinder toe is double, and is generally transmitted truly, as is well shown when dorkings are crossed with common four-toed breeds.[ ] with animals which have properly less than five digits, the number is sometimes increased to five, especially in the front legs, though rarely carried beyond that number; but this is due to the development of a digit already existing in a more or less rudimentary state. thus the dog has properly four toes behind, but in the larger breeds a fifth toe is commonly, though not perfectly, developed. horses, which properly have one toe alone fully developed with rudiments of the others, have been described with each foot bearing two or three small separate hoofs: analogous facts have been noticed with sheep, goats, and pigs.[ ] the most interesting point with respect to supernumerary digits is their occasional regrowth after amputation. mr. white[ ] describes a child, three years old, with a thumb double from the first joint. he removed the lesser thumb, which was furnished with a nail; but to his astonishment it grew again, and reproduced a nail. the child was then taken to an eminent london surgeon, and the newly-grown thumb was wholly removed by its socket-joint, but again it grew and reproduced a nail. dr. struthers mentions a case of partial regrowth of an additional thumb, amputated when the child was three months old; and the late dr. falconer communicated to me an analogous case which had fallen under his own observation. a gentleman, who first called my attention to this subject, has given me the following facts which occurred in his own family. he himself, two brothers, and a sister were born with an extra digit to each extremity. his parents were not affected, and there was no tradition in the family, or in the village in which the family had long resided, of any member having been thus affected. whilst a child, both additional toes, which were attached by bones, were rudely cut off; but the stump of one grew again, and a second operation was performed in his thirty-third year. { } he has had fourteen children, of whom three have inherited additional digits; and one of them, when about six weeks old, was operated on by an eminent surgeon. the additional finger, which was attached by bone to the outer side of the hand, was removed at the joint; the wound healed, but immediately the digit began growing; and in about three months' time the stump was removed for the second time by the root. but it has since grown again, and is now fully a third of an inch in length, including a bone; so that it will for the third time have to be operated on. now the normal digits in adult man and other mammals, in birds, and, as i believe, in true reptiles, have no power of regrowth. the nearest approach to this power is exhibited by the occasional reappearance in man of imperfect nails on the stumps of his fingers after amputation.[ ] but man in his embryonic condition has a considerable power of reproduction, for sir j. simpson[ ] has several times observed arms which had been cut off in the womb by bands of false membrane, and which had grown again to a certain extent. in one instance, the extremity was "divided into three minute nodules, on two of which small points of nails could be detected;" so that these nodules clearly represented fingers in process of regrowth. when, however, we descend to the lower vertebrate classes, which are generally looked at as representing the higher classes in their embryonic condition, we find ample powers of regrowth. spallanzani[ ] cut off the legs and tail of a salamander six times, and bonnet eight times, successively, and they were reproduced. an additional digit beyond the proper number was occasionally formed after bonnet had cut off or had divided longitudinally the hand or foot, and in one instance three additional digits were thus formed.[ ] these latter cases appear at first sight quite distinct from the congenital production of additional digits in the higher animals; but theoretically, as we shall see in a future chapter, they probably present no real difference. the larvæ or tadpoles of the tailless batrachians, but not the adults,[ ] are capable of reproducing lost members.[ ] lastly, as i have been informed by mr. j. j. briggs and mr. f. buckland, when portions of the pectoral and tail fins of various { } fresh-water fish are cut off, they are perfectly reproduced in about six weeks' time. from these several facts we may infer that supernumerary digits in man retain to a certain extent an embryonic condition, and that they resemble in this respect the normal digits and limbs in the lower vertebrate classes. they also resemble the digits of some of the lower animals in the number exceeding five; for no mammal, bird, existing reptile, or amphibian (unless the tubercle on the hind feet of the toad and other tailless batrachians be viewed as a digit) has more than five; whilst fishes sometimes have in their pectoral fins as many as twenty metacarpal and phalangeal bones, which, together with the bony filaments, apparently represent our digits with their nails. so, again, in certain extinct reptiles, namely, the ichthyopterygia, "the digits may be seven, eight, or nine in number, a significant mark," says professor owen, "of piscine affinity."[ ] we encounter much difficulty in attempting to reduce these various facts to any rule or law. the inconstant number of the additional digits--their irregular attachment to either the inner or outer margin of the hand--the gradation which can be traced from a mere loose rudiment of a single digit to a completely double hand--the occasional appearance of additional digits in the salamander after a limb has been amputated--these various facts appear to indicate mere fluctuating monstrosity; and this perhaps is all that can be safely said. nevertheless, as supernumerary digits in the higher animals, from their power of regrowth and from the number thus acquired exceeding five, partake of the nature of the digits in the lower vertebrate animals;--as they occur by no means rarely, and are transmitted with remarkable strength, though perhaps not more strongly than some other anomalies;--and as with animals which have fewer than five digits, when an additional one appears it is generally due to the development of a visible rudiment;--we are led in all cases to suspect, that, although no actual rudiment can be detected, yet that a latent tendency to the formation of an additional digit exists in all mammals, including man. on this view, as we shall more plainly see in the { } next chapter when discussing latent tendencies, we should have to look at the whole case as one of reversion to an enormously remote, lowly-organised, and multidigitate progenitor. * * * * * i may here allude to a class of facts closely allied to, but somewhat different from, ordinary cases of inheritance. sir h. holland[ ] states that brothers and sisters of the same family are frequently affected, often at about the same age, by the same peculiar disease, not known to have previously occurred in the family. he specifies the occurrence of diabetes in three brothers under ten years old; he also remarks that children of the same family often exhibit in common infantile diseases the same peculiar symptoms. my father mentioned to me the case of four brothers who died between the ages of sixty and seventy, in the same highly peculiar comatose state. an instance has been already given of supernumerary digits appearing in four children out of six in a previously unaffected family. dr. devay states[ ] that two brothers married two sisters, their first-cousins, none of the four nor any relation being an albino; but the seven children produced from this double marriage were all perfect albinoes. some of these cases, as mr. sedgwick[ ] has shown, are probably the result of reversion to a remote ancestor, of whom no record had been preserved; and all these cases are so far directly connected with inheritance that no doubt the children inherited a similar constitution from their parents, and, from being exposed to nearly similar conditions of life, it is not surprising that they should be affected in the same manner and at the same period of life. * * * * * most of the facts hitherto given have served to illustrate the force of inheritance, but we must now consider cases, grouped as well as the subject allows into classes, showing how feeble, capricious, or deficient the power of inheritance sometimes is. when a new peculiarity first appears, we can never predict whether it will be inherited. if both parents from their birth present { } the same peculiarity, the probability is strong that it will be transmitted to at least some of their offspring. we have seen that variegation is transmitted much more feebly by seed from a branch which had become variegated through bud-variation, than from plants which were variegated as seedlings. with most plants the power of transmission notoriously depends on some innate capacity in the individual: thus vilmorin[ ] raised from a peculiarly coloured balsam some seedlings, which all resembled their parent; but of these seedlings some failed to transmit the new character, whilst others transmitted it to all their descendants during several successive generations. so again with a variety of the rose, two plants alone out of six were found by vilmorin to be capable of transmitting the desired character. the weeping or pendulous growth of trees is strongly inherited in some cases, and, without any assignable reason, feebly in other cases. i have selected this character as an instance of capricious inheritance, because it is certainly not proper to the parent-species, and because, both sexes being borne on the same tree, both tend to transmit the same character. even supposing that there may have been in some instances crossing with adjoining trees of the same species, it is not probable that all the seedlings would have been thus affected. at moccas court there is a famous weeping oak; many of its branches "are feet long, and no thicker in any part of this length than a common rope:" this tree transmits its weeping character, in a greater or less degree, to all its seedlings; some of the young oaks being so flexible that they have to be supported by props; others not showing the weeping tendency till about twenty years old.[ ] mr. rivers fertilized, as he informs me, the flowers of a new belgian weeping thorn (_cratægus oxyacantha_) with pollen from a crimson not-weeping variety, and three young trees, "now six or seven years old, show a decided tendency to be pendulous, but as yet are not so much so as the mother-plant." according to mr. macnab,[ ] seedlings from a magnificent weeping birch (_betula alba_), in the botanic garden at edinburgh, grew for the first ten or fifteen years upright, but then all became weepers like their parent. a peach with pendulous branches, like those of the weeping willow, has been found capable of propagation by seed.[ ] lastly, a weeping and almost prostrate yew (_taxus baccata_) was found in a hedge in shropshire; it was a male, but one branch bore female flowers, and produced berries; these, { } being sown, produced seventeen trees, all of which had exactly the same peculiar habit with the parent-tree.[ ] these facts, it might have been thought, would have been sufficient to render it probable that a pendulous habit would in all cases be strictly inherited. but let us look to the other side. mr. macnab[ ] sowed seeds of the weeping beech (_fagus sylvanica_), but succeeded in raising only common beeches. mr. rivers, at my request, raised a number of seedlings from three distinct varieties of weeping elm; and at least one of the parent-trees was so situated that it could not have been crossed by any other elm; but none of the young trees, now about a foot or two in height, show the least signs of weeping. mr. rivers formerly sowed above twenty thousand seeds of the weeping ash (_fraxinus excelsior_), and not a single seedling was in the least degree pendulous: in germany, m. borchmeyer raised a thousand seedlings, with the same result. nevertheless, mr. anderson, of the chelsea botanic garden, by sowing seed from a weeping ash, which was found before the year , in cambridgeshire, raised several pendulous trees.[ ] professor henslow also informs me that some seedlings from a female weeping ash in the botanic garden at cambridge were at first a little pendulous, but afterwards became quite upright: it is probable that this latter tree, which transmits to a certain extent its pendulous habit, was derived by a bud from the same original cambridgeshire stock; whilst other weeping ashes may have had a distinct origin. but the crowning case, communicated to me by mr. rivers, which shows how capricious is the inheritance of a pendulous habit, is that a variety of another species of ash (_f. lentiscifolia_) which was formerly pendulous, "now about twenty years old has long lost this habit, every shoot being remarkably erect; but seedlings formerly raised from it were perfectly prostrate, the stems not rising more than two inches above the ground." thus the weeping variety of the common ash, which has been extensively propagated by buds during a long period, did not, with mr. rivers, transmit its character to one seedling out of above twenty thousand; whereas the weeping variety of a second species of ash, which could not, whilst grown in the same garden, retain its own weeping character, transmitted to its seedlings the pendulous habit in excess! many analogous facts could be given, showing how apparently capricious is the principle of inheritance. all the seedlings from a variety of the barberry (_b. vulgaris_) with red leaves inherited the same character; only about one-third of the seedlings of the copper beech (_fagus sylvatica_) had purple leaves. not one out of a hundred seedlings of a variety of the _cerasus padus_, with yellow fruit, bore yellow fruit: one-twelfth of the seedlings of the variety of _cornus mascula_, with yellow fruit, came true:[ ] and lastly, all the trees raised by my father from a yellow-berried holly (_ilex aquifolium_), { } found wild, produced yellow berries. vilmorin[ ] observed in a bed of _saponaria calabrica_ an extremely dwarf variety, and raised from it a large number of seedlings; some of these partially resembled their parent, and he selected their seed; but the grandchildren were not in the least dwarfed: on the other hand, he observed a stunted and bushy variety of _tagetes signata_ growing in the midst of the common varieties by which it was probably crossed; for most of the seedlings raised from this plant were intermediate in character, only two perfectly resembling their parent; but seed saved from these two plants reproduced the new variety so truly, that hardly any selection has since been necessary. flowers transmit their colour truly, or most capriciously. many annuals come true: thus i purchased german seeds of thirty-four named sub-varieties of one _race_ of ten-week stocks (_matthiola annua_), and raised a hundred and forty plants, all of which, with the exception of a single plant, came true. in saying this, however, it must be understood that i could distinguish only twenty kinds out of the thirty-four named sub-varieties; nor did the colour of the flower always correspond with the name affixed to the packet; but i say that they came true, because in each of the thirty-six short rows every plant was absolutely alike, with the one single exception. again, i procured packets of german seed of twenty-five named varieties of common and quilled asters, and raised a hundred and twenty-four plants; of these, all except ten were true in the above limited sense; and i considered even a wrong shade of colour as false. it is a singular circumstance that white varieties generally transmit their colour much more truly than any other variety. this fact probably stands in close relation with one observed by verlot,[ ] namely, that flowers which are normally white rarely vary into any other colour. i have found that the white varieties of _delphinium consolida_ and of the stock are the truest. it is, indeed, sufficient to look through a nurseryman's seed-list, to see the large number of white varieties which can be propagated by seed. the several coloured varieties of the sweet-pea (_lathyrus odoratus_) are very true; but i hear from mr. masters, of canterbury, who has particularly attended to this plant, that the white variety is the truest. the hyacinth, when propagated by seed, is extremely inconstant in colour, but "white hyacinths almost always give by seed white-flowered plants;"[ ] and mr. masters informs me that the yellow varieties also reproduce their colour, but of different shades. on the other hand, pink and blue varieties, the latter being the natural colour, are not nearly so true: hence, as mr. masters has remarked to me, "we see that a garden variety may acquire a more permanent habit than a natural species;" but it should have been added, that this occurs under cultivation, and therefore under changed conditions. with many flowers, especially perennials, nothing can be more fluctuating than the colour of the seedlings, as is notoriously the case with verbenas, carnations, dahlias, cinerarias, and others.[ ] i sowed seed of twelve { } named varieties of snapdragon (_antirrhinum majus_), and utter confusion was the result. in most cases the extremely fluctuating colour of seedling plants is probably in chief part due to crosses between differently-coloured varieties during previous generations. it is almost certain that this is the case with the polyanthus and coloured primrose (_primula veris_ and _vulgaris_), from their reciprocally dimorphic structure;[ ] and these are plants which florists speak of as never come true by seed: but if care be taken to prevent crossing, neither species is by any means very inconstant in colour; thus i raised twenty-three plants from a purple primrose, fertilised by mr. j. scott with its own pollen, and eighteen came up purple of different shades, and only five reverted to the ordinary yellow colour: again, i raised twenty plants from a bright-red cowslip, similarly treated by mr. scott, and every one perfectly resembled its parent in colour, as likewise did, with the exception of a single plant, grandchildren. even with the most variable flowers, it is probable that each delicate shade of colour might be permanently fixed so as to be transmitted by seed, by cultivation in the same soil, by long-continued selection, and especially by the prevention of crosses. i infer this from certain annual larkspurs (_delphinium consolida_ and _ajacis_), of which common seedlings present a greater diversity of colour than any other plant known to me; yet on procuring seed of five named german varieties of _d. consolida_, only nine plants out of ninety-four were false; and the seedlings of six varieties of _d. ajacis_ were true in the same manner and degree as with the stocks above described. a distinguished botanist maintains that the annual species of delphinium are always self-fertilised; therefore i may mention that thirty-two flowers on a branch of _d. consolida_, enclosed in a net, yielded twenty-seven capsules, with an average of . seed in each; whilst five flowers, under the same net, which were artificially fertilised, in the same manner as must be effected by bees during their incessant visits, yielded five capsules with an average of . fine seed; and this shows that the agency of insects is necessary for the full fertility of this plant. analogous facts could be given with respect to the crossing of many other flowers, such as carnations, &c., of which the varieties fluctuate much in colour. as with flowers, so with our domesticated animals, no character is more variable than colour, and probably in no animal more so than with the horse. yet with a little care in breeding, it appears that races of any colour might soon be formed. hofacker gives the result of matching two hundred and sixteen mares of four different colours with like-coloured stallions, without regard to the colour of their ancestors; and of the two hundred and sixteen colts born, eleven alone failed to inherit the colour of their parents: autenrieth and ammon assert that, after two generations, colts of a uniform colour are produced with certainty.[ ] in a few rare cases peculiarities fail to be inherited, apparently from the force of inheritance being too strong. i have been assured by breeders of the canary-bird that to get a good { } jonquil-coloured bird it does not answer to pair two jonquils, as the colour then comes out too strong, or is even brown. so again, if two crested canaries are paired, the young birds rarely inherit this character:[ ] for in crested birds a narrow space of bare skin is left on the back of the head, where the feathers are up-turned to form the crest, and, when both parents are thus characterised, the bareness becomes excessive, and the crest itself fails to be developed. mr. hewitt, speaking of laced sebright bantams, says[ ] that, "why this should be so, i know not, but i am confident that those that are best laced frequently produce offspring very far from perfect in their markings, whilst those exhibited by myself, which have so often proved successful, were bred from the union of heavily-laced birds with those that were scarcely sufficiently laced." it is a singular fact that, although several deaf-mutes often occur in the same family, and though their cousins and other relations are often in the same condition, yet their parents are very rarely deaf-mutes. to give a single instance: not one scholar out of , who were at the same time in the london institution, was the child of parents similarly afflicted. so again, when a male or a female deaf-mute marries a sound person, their children are most rarely affected: in ireland out of children thus produced one alone was mute. even when both parents have been deaf-mutes, as in the case of forty-one marriages in the united states and of six in ireland, only two deaf and dumb children were produced. mr. sedgwick,[ ] in commenting on this remarkable and fortunate failure in the power of transmission in the direct line, remarks that it may possibly be owing to "excess having reversed the action of some natural law in development." but it is safer in the present state of our knowledge to look at the whole case as simply unintelligible. * * * * * with respect to the inheritance of structures mutilated by injuries or altered by disease it is difficult to come to any { } definite conclusion. in some cases mutilations have been practised for a vast number of generations without any inherited result. godron has remarked[ ] that different races of man have from time immemorial knocked out their upper incisors, cut off joints of their fingers, made holes of immense size through the lobes of their ears or through their nostrils, made deep gashes in various parts of their bodies, and there is no reason whatever to suppose that these mutilations have ever been inherited. adhesions due to inflammation and pits from the small-pox (and formerly many consecutive generations must have been thus pitted) are not inherited. with respect to jews, i have been assured by three medical men of the jewish faith that circumcision, which has been practised for so many ages, has produced no inherited effect; blumenbach, on the other hand, asserts[ ] that in germany jews are often born in a condition rendering circumcision difficult, so that a name is here applied to them signifying "born circumcised." the oak and other trees must have borne galls from primeval times, yet they do not produce inherited excrescences; many other such facts could be adduced. on the other hand, various cases have been recorded of cats, dogs, and horses, which have had their tails, legs, &c., amputated or injured, producing offspring with the same parts ill-formed; but as it is not at all rare for similar malformations to appear spontaneously, all such cases may be due to mere coincidence. nevertheless, dr. prosper lucas has given, on good authorities, such a long list of inherited injuries, that it is difficult not to believe in them. thus, a cow that had lost a horn from an accident with consequent suppuration, produced three calves which were hornless on the same side of the head. with the horse, there seems hardly a doubt that bony exostoses on the legs, caused by too much travelling on hard roads, are inherited. blumenbach records the case of a man who had his little finger on the right hand almost cut off, and which in consequence grew crooked, and his sons had the same finger on the same hand similarly crooked. a soldier, fifteen years before his marriage, lost his left eye from purulent ophthalmia, and his { } two sons were microphthalmic on the same side.[ ] in all such cases, if truthfully reported, in which the parent has had an organ injured on one side, and more than one child has been born with the same organ affected on the same side, the chances against mere coincidence are enormous. but perhaps the most remarkable and trustworthy fact is that given by dr. brown-séquard,[ ] namely, that many young guinea-pigs inherited an epileptic tendency from parents which had been subjected to a particular operation, inducing in the course of a few weeks a convulsive disease like epilepsy: and it should be especially noted that this eminent physiologist bred a large number of guinea-pigs from animals which had not been operated on, and not one of these manifested the epileptic tendency. on the whole, we can hardly avoid admitting, that injuries and mutilations, especially when followed by disease, or perhaps exclusively when thus followed, are occasionally inherited. although many congenital monstrosities are inherited, of which examples have already been given, and to which may be added the lately recorded case of the transmission during a century of hare-lip with a cleft-palate in the writer's own family,[ ] yet other malformations are rarely or never inherited. of these later cases, many are probably due to injuries in the womb or egg, and would come under the head of non-inherited injuries or mutilations. with plants, a long catalogue of inherited monstrosities of the most serious and diversified nature could easily be given; and with plants, there is no reason to suppose that monstrosities are caused by direct injuries to the seed or embryo. _causes of non-inheritance._ a large number of cases of non-inheritance are intelligible on the principle, that a strong tendency to inheritance does exist, but { } that it is overborne by hostile or unfavourable conditions of life. no one would expect that our improved pigs, if forced during several generations to travel about and root in the ground for their own subsistence, would transmit, as truly as they now do, their tendency to fatten, and their short muzzles and legs. dray-horses assuredly would not long transmit their great size and massive limbs, if compelled to live on a cold, damp mountainous region; we have indeed evidence of such deterioration in the horses which have run wild on the falkland islands. european dogs in india often fail to transmit their true character. our sheep in tropical countries lose their wool in a few generations. there seems also to be a close relation between certain peculiar pastures and the inheritance of an enlarged tail in fat-tailed sheep, which form one of the most ancient breeds in the world. with plants, we have seen that the american varieties of maize lose their proper character in the course of two or three generations, when cultivated in europe. our cabbages, which here come so true by seed, cannot form heads in hot countries. under changed circumstances, periodical habits of life soon fail to be transmitted, as the period of maturity in summer and winter wheat, barley, and vetches. so it is with animals; for instance, a person whose statement i can trust, procured eggs of aylesbury ducks from that town, where they are kept in houses and are reared as early as possible for the london market; the ducks bred from these eggs in a distant part of england, hatched their first brood on january th, whilst common ducks, kept in the same yard and treated in the same manner, did not hatch till the end of march; and this shows that the period of hatching was inherited. but the grandchildren of these aylesbury ducks completely lost their early habit of incubation, and hatched their eggs at the same time with the common ducks of the same place. many cases of non-inheritance apparently result from the conditions of life continually inducing fresh variability. we have seen that when the seeds of pears, plums, apples, &c., are sown, the seedlings generally inherit some degree of family likeness from the parent-variety. mingled with these seedlings, a few, and sometimes many, worthless, wild-looking plants commonly appear; and their appearance may be attributed to the principle of reversion. but scarcely a single seedling will be found { } perfectly to resemble the parent-form; and this, i believe, may be accounted for by constantly recurring variability induced by the conditions of life. i believe in this, because it has been observed that certain fruit-trees truly propagate their kind whilst growing on their own roots, but when grafted on other stocks, and by this process their natural state is manifestly affected, they produce seedlings which vary greatly, departing from the parental type in many characters.[ ] metzger, as stated in the ninth chapter, found that certain kinds of wheat brought from spain and cultivated in germany, failed during many years to reproduce themselves truly; but that at last, when accustomed to their new conditions, they ceased to be variable,--that is, they became amenable to the power of inheritance. nearly all the plants which cannot be propagated with any approach to certainty by seed, are kinds which have long been propagated by buds, cuttings, offsets, tubers, &c., and have in consequence been frequently exposed during their individual lives to widely diversified conditions of life. plants thus propagated become so variable, that they are subject, as we have seen in the last chapter, even to bud-variation. our domesticated animals, on the other hand, are not exposed during their individual lives to such extremely diversified conditions, and are not liable to such extreme variability; therefore they do not lose the power of transmitting most of their characteristic features. in the foregoing remarks on non-inheritance, crossed breeds are of course excluded, as their diversity mainly depends on the unequal development of characters derived from either parent, modified by the principles of reversion and prepotency. _conclusion._ it has, i think, been shown in the early part of this chapter how strongly new characters of the most diversified nature, whether normal or abnormal, injurious or beneficial, whether affecting organs of the highest or most trifling importance, are inherited. contrary to the common opinion, it is often sufficient for the inheritance of some peculiar character, that one parent alone should possess it, as in most cases in which the rarer { } anomalies have been transmitted. but the power of transmission is extremely variable: in a number of individuals descended from the same parents, and treated in the same manner, some display this power in a perfect manner, and in some it is quite deficient; and for this difference no reason can be assigned. in some cases the effects of injuries or mutilations apparently are inherited; and we shall see in a future chapter that the effects of the long-continued use and disuse of parts are certainly inherited. even those characters which are considered the most fluctuating, such as colour, are with rare exceptions transmitted much more forcibly than is generally supposed. the wonder, indeed, in all cases is not that any character should be transmitted, but that the power of inheritance should ever fail. the checks to inheritance, as far as we know them, are, firstly, circumstances hostile to the particular character in question; secondly, conditions of life incessantly inducing fresh variability; and lastly, the crossing of distinct varieties during some previous generation, together with reversion or atavism--that is, the tendency in the child to resemble its grand-parents or more remote ancestors instead of its immediate parents. this latter subject will be fully discussed in the following chapter. * * * * * { } chapter xiii. inheritance _continued_--reversion or atavism. different forms of reversion--in pure or uncrossed breeds, as in pigeons, fowls, hornless cattle and sheep, in cultivated plants--reversion in feral animals and plants--reversion in crossed varieties and species--reversion through bud-propagation, and by segments in the same flower or fruit--in different parts of the body in the same animal--the act of crossing a direct cause of reversion, various cases of, with instincts--other proximate causes of reversion--latent characters--secondary sexual characters--unequal development of the two sides of the body--appearance with advancing age of characters derived from a cross--the germ with all its latent characters a wonderful object--monstrosities--peloric flowers due in some cases to reversion. the great principle of inheritance to be discussed in this chapter has been recognised by agriculturists and authors of various nations, as shown by the scientific term _atavism_, derived from atavus, an ancestor; by the english terms of _reversion_, or _throwing back_; by the french _pas-en-arrière_; and by the german _rück-schlag_, or _rück-schritt_. when the child resembles either grandparent more closely than its immediate parents, our attention is not much arrested, though in truth the fact is highly remarkable; but when the child resembles some remote ancestor, or some distant member in a collateral line,--and we must attribute the latter case to the descent of all the members from a common progenitor,--we feel a just degree of astonishment. when one parent alone displays some newly-acquired and generally inheritable character, and the offspring do not inherit it, the cause may lie in the other parent having the power of prepotent transmission. but when both parents are similarly characterised, and the child does not, whatever the cause may be, inherit the character in question, but resembles its grandparents, we have one of the simplest cases of reversion. we continually see another and even more simple case of atavism, though not generally included under this head, namely, when { } the son more closely resembles his maternal than his paternal grandsire in some male attribute, as in any peculiarity in the beard of man, the horns of the bull, the hackles or comb of the cock, or, as in certain diseases necessarily confined to the male sex; for the mother cannot possess or exhibit such male attributes, yet the child has inherited them, through her blood, from his maternal grandsire. the cases of reversion may be divided into two main classes, which, however, in some instances, blend into each other; namely, first, those occurring in a variety or race which has not been crossed, but has lost by variation some character that it formerly possessed, and which afterwards reappears. the second class includes all cases in which a distinguishable individual, sub-variety, race, or species, has at some former period been crossed with a distinct form, and a character derived from this cross, after having disappeared during one or several generations, suddenly reappears. a third class, differing only in the manner of reproduction, might be formed to include all cases of reversion effected by means of buds, and therefore independent of true or seminal generation. perhaps even a fourth class might be instituted, to include reversions by segments in the same individual flower or fruit, and in different parts of the body in the same individual animal as it grows old. but the two first main classes will be sufficient for our purpose. * * * * * _reversion to lost characters by pure or uncrossed forms._--striking instances of this first class of cases were given in the sixth chapter, namely, of the occasional reappearance, in variously-coloured pure breeds of the pigeon, of blue birds with all the marks which characterise the wild _columba livia_. similar cases were given in the case of the fowl. with the common ass, as we now know that the legs of the wild progenitor are striped, we may feel assured that the occasional appearance of such stripes in the domestic animal is a case of simple reversion. but i shall be compelled to refer again to these cases, and therefore will here pass them over. the aboriginal species from which our domesticated cattle and sheep are descended, no doubt possessed horns; but several hornless breeds are now well established. yet in these--for instance, { } in southdown sheep--"it is not unusual to find among the male lambs some with small horns." the horns, which thus occasionally reappear in other polled breeds, either "grow to the full size, or are curiously attached to the skin alone and hang loosely down, or drop off."[ ] the galloways and suffolk cattle have been hornless for the last or years, but a horned calf, with the horn often loosely attached, is occasionally born.[ ] there is reason to believe that sheep in their early domesticated condition were "brown or dingy black;" but even in the time of david certain flocks were spoken of as white as snow. during the classical period the sheep of spain are described by several ancient authors as being black, red, or tawny.[ ] at the present day, notwithstanding the great care which is taken to prevent it, particoloured lambs and some entirely black are occasionally dropped by our most highly improved and valued breeds, such as the southdowns. since the time of the famous bakewell, during the last century, the leicester sheep have been bred with the most scrupulous care; yet occasionally grey-faced, or black-spotted, or wholly black lambs appear.[ ] this occurs still more frequently with the less improved breeds, such as the norfolks.[ ] as bearing on this tendency in sheep to revert to dark colours, i may state (though in doing so i trench on the reversion of crossed breeds, and likewise on the subject of prepotency) that the rev. w. d. fox was informed that seven white southdown ewes were put to a so-called spanish ram, which had two small black spots on his sides, and they produced thirteen lambs, all perfectly black. mr. fox believes that this ram belonged to a breed which he has himself kept, and which is always spotted with black and white; and he finds that leicester sheep crossed by rams of this breed always produce black lambs: he has gone on recrossing these crossed sheep with pure white leicesters during three successive { } generations, but always with the same result. mr. fox was also told by the friend from whom the spotted breed was procured, that he likewise had gone on for six or seven generations crossing with white sheep, but still black lambs were invariably produced. similar facts could be given with respect to tailless breeds of various animals. for instance, mr. hewitt[ ] states that chickens bred from some rumpless fowls, which were reckoned so good that they won a prize at an exhibition, "in a considerable number of instances were furnished with fully developed tail-feathers." on inquiry, the original breeder of these fowls stated that, from the time when he had first kept them, they had often produced fowls furnished with tails; but that these latter would again reproduce rumpless chickens. analogous cases of reversion occur in the vegetable kingdom; thus "from seeds gathered from the finest cultivated varieties of heartsease (_viola tricolor_), plants perfectly wild both in their foliage and their flowers are frequently produced;"[ ] but the reversion in this instance is not to a very ancient period, for the best existing varieties of the heartsease are of comparatively modern origin. with most of our cultivated vegetables there is some tendency to reversion to what is known to be, or may be presumed to be, their aboriginal state; and this would be more evident if gardeners did not generally look over their beds of seedlings, and pull up the false plants or "rogues" as they are called. it has already been remarked, that some few seedling apples and pears generally resemble, but apparently are not identical with, the wild trees from which they are descended. in our turnip[ ] and carrot-beds a few plants often "break"--that is, flower too soon; and their roots are generally found to be hard and stringy, as in the parent-species. by the aid of a little selection, carried on during a few generations, most of our cultivated plants could probably be brought back, without any great change in their conditions of life, to a wild or nearly wild condition: mr. buckman has effected this with the parsnip;[ ] { } and mr. hewett c. watson, as he informs me, selected, during three generations, "the most diverging plants of scotch kail, perhaps one of the least modified varieties of the cabbage; and in the third generation some of the plants came very close to the forms now established in england about old castle-walls, and called indigenous." * * * * * _reversion in animals and plants which have run wild._--in the cases hitherto considered, the reverting animals and plants have not been exposed to any great or abrupt change in their conditions of life which could have induced this tendency; but it is very different with animals and plants which have become feral or run wild. it has been repeatedly asserted in the most positive manner by various authors, that feral animals and plants invariably return to their primitive specific type. it is curious on what little evidence this belief rests. many of our domesticated animals could not subsist in a wild state; thus, the more highly improved breeds of the pigeon will not "field" or search for their own food. sheep have never become feral, and would be destroyed by almost every beast of prey. in several cases we do not know the aboriginal parent-species, and cannot possibly tell whether or not there has been any close degree of reversion. it is not known in any instance what variety was first turned out; several varieties have probably in some cases run wild, and their crossing alone would tend to obliterate their proper character. our domesticated animals and plants, when they run wild, must always be exposed to new conditions of life, for, as mr. wallace[ ] has well remarked, they have to obtain their own food, and are exposed to competition with the native productions. under these circumstances, if our domesticated animals did not undergo change of some kind, the result would be quite opposed to the conclusions arrived at in this work. nevertheless, i do not doubt that the simple fact of animals and plants becoming feral, does cause some tendency to reversion to the primitive state; though this tendency has been much exaggerated by some authors. { } i will briefly run through the recorded cases. with neither horses nor cattle is the primitive stock known; and it has been shown in former chapters that they have assumed different colours in different countries. thus the horses which have run wild in south america are generally brownish-bay, and in the east dun-coloured; their heads have become larger and coarser, and this may be due to reversion. no careful description has been given of the feral goat. dogs which have run wild in various countries have hardly anywhere assumed a uniform character; but they are probably descended from several domestic races, and aboriginally from several distinct species. feral cats, both in europe and la plata, are regularly striped; in some cases they have grown to an unusually large size, but do not differ from the domestic animal in any other character. when variously-coloured tame rabbits are turned out in europe, they generally reacquire the colouring of the wild animal; there can be no doubt that this does really occur, but we should remember that oddly-coloured and conspicuous animals would suffer much from beasts of prey and from being easily shot; this at least was the opinion of a gentleman who tried to stock his woods with a nearly white variety; and when thus destroyed, they would in truth be supplanted by, instead of being transformed into, the common rabbit. we have seen that the feral rabbits of jamaica, and especially of porto santo, have assumed new colours and other new characters. the best known case of reversion, and that on which the widely-spread belief in its universality apparently rests, is that of pigs. these animals have run wild in the west indies, south america, and the falkland islands, and have everywhere acquired the dark colour, the thick bristles, and great tusks of the wild boar; and the young have reacquired longitudinal stripes. but even in the case of the pig, roulin describes the half-wild animals in different parts of south america as differing in several respects. in louisiana the pig[ ] has run wild, and is said to differ a little in form, and much in colour, from the domestic animal, yet does not closely resemble the wild boar of europe. with pigeons and fowls,[ ] it is not known what variety was first turned out, nor what character the feral birds have assumed. the guinea-fowl in the west indies, when feral, seems to vary more than in the domesticated state. with respect to plants run wild, dr. hooker[ ] has strongly insisted on what slight evidence the common belief in their power of reversion rests. godron[ ] describes wild turnips, carrots, and celery; but these plants in their cultivated state hardly differ from their wild prototypes, except in the { } succulency and enlargement of certain parts,--characters which would be surely lost by plants growing in a poor soil and struggling with other plants. no cultivated plant has run wild on so enormous a scale as the cardoon (_cynara cardunculus_) in la plata. every botanist who has seen it growing there, in vast beds, as high as a horse's back, has been struck with its peculiar appearance; but whether it differs in any important point from the cultivated spanish form, which is said not to be prickly like its american descendant, or whether it differs from he wild mediterranean species, which is said not to be social, i do not know. * * * * * _reversion to characters derived from a cross, in the case of sub-varieties, races, and species._--when an individual having some recognizable peculiarity unites with another of the same sub-variety, not having the peculiarity in question, it often reappears in the descendants after an interval of several generations. every one must have noticed, or heard from old people of children closely resembling in appearance or mental disposition, or in so small and complex a character as expression, one of their grandparents, or some more distant collateral relation. very many anomalies of structure and diseases,[ ] of which instances have been given in the last chapter, have come into a family from one parent, and have reappeared in the progeny after passing over two or three generations. the following case has been communicated to me on good authority, and may, i believe, be fully trusted: a pointer-bitch produced seven puppies; four were marked with blue and white, which is so unusual a colour with pointers that she was thought to have played false with one of the greyhounds, and the whole litter was condemned; but the gamekeeper was permitted to save one as a curiosity. two years afterwards a friend of the owner saw the young dog, and declared that he was the image of his old pointer-bitch sappho, the only blue and white pointer of pure descent which he had ever seen. this led to close inquiry, and it was proved that he was the great-great-grandson of sappho; so that, according to the common expression, he had only - th of her blood in his veins. here it can hardly be doubted that a character derived from a cross with an individual of the same variety reappeared after passing over three generations. { } when two distinct races are crossed, it is notorious that the tendency in the offspring to revert to one or both parent-forms is strong, and endures for many generations. i have myself seen the clearest evidence of this in crossed pigeons and with various plants. mr. sidney[ ] states that, in a litter of essex pigs, two young ones appeared which were the image of the berkshire boar that had been used twenty-eight years before in giving size and constitution to the breed. i observed in the farmyard at betley hall some fowls showing a strong likeness to the malay breed, and was told by mr. tollet that he had forty years before crossed his birds with malays; and that, though he had at first attempted to get rid of this strain, he had subsequently given up the attempt in despair, as the malay character would reappear. this strong tendency in crossed breeds to revert has given rise to endless discussions in how many generations after a single cross, either with a distinct breed or merely with an inferior animal, the breed may be considered as pure, and free from all danger of reversion. no one supposes that less than three generations suffices, and most breeders think that six, seven, or eight are necessary, and some go to still greater lengths.[ ] but neither in the case of a breed which has been contaminated by a single cross, nor when, in the attempt to form an intermediate breed, half-bred animals have been matched together during many generations, can any rule be laid down how soon the tendency to reversion will be obliterated. it depends on the difference in the strength or prepotency of transmission in the two parent-forms, on their actual amount of difference, and on the nature of the conditions of life to which the crossed offspring are exposed. but we must be careful not to confound these cases of reversion to characters gained from a cross, with those given under the first class, in which characters originally common to _both_ parents, but lost at some former period, reappear; for such characters may recur after an almost indefinite number of generations. { } the law of reversion is equally powerful with hybrids, when they are sufficiently fertile to breed together, or when they are repeatedly crossed with either pure parent-form, as with mongrels. it is not necessary to give instances, for in the case of plants almost every one who has worked on this subject from the time of kölreuter to the present day has insisted on this tendency. gärtner has recorded some good instances; but no one has given more striking cases than naudin.[ ] the tendency differs in degree or strength in different groups, and partly depends, as we shall presently see, on the fact of the parent-plants having been long cultivated. although the tendency to reversion is extremely general with nearly all mongrels and hybrids, it cannot be considered as invariably characteristic of them; there is, also, reason to believe that it may be mastered by long-continued selection; but these subjects will more properly be discussed in a future chapter on crossing. from what we see of the power and scope of reversion, both in pure races and when varieties or species are crossed, we may infer that characters of almost every kind are capable of reappearance after having been lost for a great length of time. but it does not follow from this that in each particular case certain characters will reappear: for instance, this will not occur when a race is crossed with another endowed with prepotency of transmission. in some few cases the power of reversion wholly fails, without our being able to assign any cause for the failure: thus it has been stated that in a french family in which out of above members, during six generations, had been subject to night-blindness, "there has not been a single example of this affection in the children of parents who were themselves free from it."[ ] * * * * * _reversion through bud-propagation--partial reversion, by segments in the same flower or fruit, or in different parts of the { } body in the same individual animal._--in the eleventh chapter, many cases of reversion by buds, independently of seminal generation, were given--as when a leaf-bud on a variegated, curled, or laciniated variety suddenly reassumes its proper character; or as when a provence-rose appears on a moss-rose, or a peach on a nectarine-tree. in some of these cases only half the flower or fruit, or a smaller segment, or mere stripes, reassumed their former character; and here we have with buds reversion by segments. vilmorin[ ] has also recorded several cases with plants derived from seed, of flowers reverting by stripes or blotches to their primitive colours: he states that in all such cases a white or pale-coloured variety must first be formed, and, when this is propagated for a length of time by seed, striped seedlings occasionally make their appearance; and these can afterwards by care be multiplied by seed. the stripes and segments just referred to are not due, as far as is known, to reversion to characters derived from a cross, but to characters lost by variation. these cases, however, as naudin[ ] insists in his discussion on disjunction of character, are closely analogous with those given in the eleventh chapter, in which crossed plants are known to have produced half-and-half or striped flowers and fruit, or distinct kinds of flowers on the same root resembling the two parent-forms. many piebald animals probably come under this same head. such cases, as we shall see in the chapter on crossing, apparently result from certain characters not readily blending together, and, as a consequence of this incapacity for fusion, the offspring either perfectly resemble one of their two parents, or resemble one parent in one part and the other parent in another part; or whilst young are intermediate in character, but with advancing age revert wholly or by segments to either parent-form, or to both. thus young trees of the _cytisus adami_ are intermediate in foliage and flowers between the two parent-forms; but when older the buds continually revert either partially or wholly to both forms. the cases given in the eleventh chapter on the changes which occurred during growth { } in crossed plants of tropæolum, cereus, datura, and lathyrus are all analogous. as however these plants are hybrids of the first generation, and as their buds after a time come to resemble their parents and not their grandparents, these cases do not at first appear to come under the law of reversion in the ordinary sense of the word; nevertheless, as the change is effected through a succession of bud-generations on the same plant, they may be thus included. analogous facts have been observed in the animal kingdom, and are more remarkable, as they occur strictly in the same individual, and not as with plants through a succession of bud-generations. with animals the act of reversion, if it can be so designated, does not pass over a true generation, but merely over the early stages of growth in the same individual. for instance, i crossed several white hens with a black cock, and many of the chickens were during the first year perfectly white, but acquired during the second year black feathers; on the other hand, some of the chickens which were at first black became during the second year piebald with white. a great breeder[ ] says, that a pencilled brahma hen which has any of the blood of the light brahma in her, will "occasionally produce a pullet well pencilled during the first year, but she will most likely moult brown on the shoulders and become quite unlike her original colours in the second year." the same thing occurs with light brahmas if of impure blood. i have observed exactly similar cases with the crossed offspring from differently coloured pigeons. but here is a more remarkable fact: i crossed a turbit, which has a frill formed by the feathers being reversed on its breast, with a trumpeter; and one of the young pigeons thus raised showed at first not a trace of the frill, but, after moulting thrice, a small yet unmistakably distinct frill appeared on its breast. according to girou,[ ] calves produced from a red cow by a black bull, or from a black cow by a red bull, are not rarely born red, and subsequently become black. in the foregoing cases, the characters which appear with advancing age are the result of a cross in the previous or some { } former generation; but in the following cases, the characters which thus reappear formerly appertained to the species, and were lost at a more or less remote epoch. thus, according to azara,[ ] the calves of a hornless race of cattle which originated in corrientes, though at first quite hornless, as they become adult sometimes acquire small, crooked, and loose horns; and these in succeeding years occasionally become attached to the skull. white and black bantams, both of which generally breed true, sometimes assume as they grow old a saffron or red plumage. for instance, a first-rate black bantam has been described, which during three seasons was perfectly black, but then annually became more and more red; and it deserves notice that this tendency to change, whenever it occurs in a bantam, "is almost certain to prove hereditary."[ ] the cuckoo or blue-mottled dorking cock, when old, is liable to acquire yellow or orange hackles in place of his proper bluish-grey hackles.[ ] now, as _gallus bankiva_ is coloured red and orange, and as dorking fowls and both kinds of bantams are descended from this species, we can hardly doubt that the change which occasionally occurs in the plumage of these birds as their age advances, results from a tendency in the individual to revert to the primitive type. * * * * * _crossing as a direct cause of reversion._--it has long been notorious that hybrids and mongrels often revert to both or to one of their parent-forms, after an interval of from two to seven or eight, or according to some authorities even a greater number of generations. but that the act of crossing in itself gives an impulse towards reversion, as shown by the reappearance of long-lost characters, has never, i believe, been hitherto proved. the proof lies in certain peculiarities, which do not characterise the immediate parents, and therefore cannot have been derived from them, frequently appearing in the offspring of two breeds when crossed, which peculiarities never appear, or appear with extreme rarity, in these same breeds, as long as they are { } precluded from crossing. as this conclusion seems to me highly curious and novel, i will give the evidence in detail. my attention was first called to this subject, and i was led to make numerous experiments, by mm. boitard and corbié having stated that, when they crossed certain breeds, pigeons coloured like the wild _c. livia_, or the common dovecot, namely, slaty-blue, with double black wing-bars, sometimes chequered with black, white loins, the tail barred with black, with the outer feathers edged with white, were almost invariably produced. the breeds which i crossed, and the remarkable results attained, have been fully described in the sixth chapter. i selected pigeons, belonging to true and ancient breeds, which had not a trace of blue or any of the above specified marks; but when crossed, and their mongrels recrossed, young birds were continually produced, more or less plainly coloured slaty-blue, with some or all of the proper characteristic marks. i may recall to the reader's memory one case, namely, that of a pigeon, hardly distinguishable from the wild shetland species, the grandchild of a red-spot, white fantail, and two black barbs, from any of which, when purely-bred, the production of a pigeon coloured like the wild _c. livia_ would have been almost a prodigy. i was thus led to make the experiments, recorded in the seventh chapter, on fowls. i selected long-established, pure breeds, in which there was not a trace of red, yet in several of the mongrels feathers of this colour appeared; and one magnificent bird, the offspring of a black spanish cock and white silk hen, was coloured almost exactly like the wild _gallus bankiva_. all who know anything of the breeding of poultry will admit that tens of thousands of pure spanish and of pure white silk fowls might have been reared without the appearance of a red feather. the fact, given on the authority of mr. tegetmeier, of the frequent appearance, in mongrel fowls, of pencilled or transversely-barred feathers, like those common to many gallinaceous birds, is likewise apparently a case of reversion to a character formerly possessed by some ancient progenitor of the family. i owe to the kindness of this same excellent observer the inspection of some neck-hackles and tail-feathers from a hybrid between the common fowl and a very distinct species, the _gallus varius_; and these feathers are transversely striped in a conspicuous manner with dark metallic blue and grey, a character which could not have been derived from either immediate parent. i have been informed by mr. b. p. brent, that he crossed a white aylesbury drake and a black so-called labrador duck, both of which are true breeds, and he obtained a young drake closely like the mallard (_a. boschas_). of the musk-duck (_a. moschata_, linn.) there are two sub-breeds, namely, white and slate-coloured; and these i am informed breed true, or nearly true. but the rev. w. d. fox tells me that, by putting a white drake to a slate-coloured duck, black birds, pied with white, like the wild musk-duck, were always produced. we have seen in the fourth chapter, that the so-called himalayan rabbit, with its snow-white body, black ears, nose, tail, and feet, breeds { } perfectly true. this race is known to have been formed by the union of two varieties of silver-grey rabbits. now, when a himalayan doe was crossed by a sandy-coloured buck, a silver-grey rabbit was produced; and this is evidently a case of reversion to one of the parent varieties. the young of the himalayan rabbit are born snow-white, and the dark marks do not appear until some time subsequently; but occasionally young himalayan rabbits are born of a light silver-grey, which colour soon disappears; so that here we have a trace of reversion, during an early period of life, to the parent-varieties, independently of any recent cross. in the third chapter is was shown that at an ancient period some breeds of cattle in the wilder parts of britain were white with dark ears, and that the cattle now kept half wild in certain parks, and those which have run quite wild in two distant parts of the world, are likewise thus coloured. now, an experienced breeder, mr. j. beasley, of northamptonshire,[ ] crossed some carefully selected west highland cows with purely-bred shorthorn bulls. the bulls were red, red and white, or dark roan; and the highland cows were all of a red colour, inclining to a light or yellow shade. but a considerable number of the offspring--and mr. beasley calls attention to this as a remarkable fact--were white, or white with red ears. bearing in mind that none of the parents were white, and that they were purely-bred animals, it is highly probable that here the offspring reverted, in consequence of the cross, to the colour either of the aboriginal parent-species or of some ancient and half-wild parent-breed. the following case, perhaps, comes under the same head: cows in their natural state have their udders but little developed, and do not yield nearly so much milk as our domesticated animals. now there is some reason to believe[ ] that cross-bred animals between two kinds, both of which are good milkers, such as alderneys and shorthorns, often turn out worthless in this respect. in the chapter on the horse reasons were assigned for believing that the primitive stock was striped and dun-coloured; and details were given, showing that in all parts of the world stripes of a dark colour frequently appear along the spine, across the legs, and on the shoulders, where they are occasionally double or treble, and even sometimes on the face and body of horses of all breeds and of all colours. but the stripes appear most frequently on the various kinds of duns. they may sometimes plainly be seen on foals, and subsequently disappear. the dun-colour and the stripes are strongly transmitted when a horse thus characterised is crossed with any other; but i was not able to prove that striped duns are generally produced from the crossing of two distinct breeds, neither of which are duns, though this does sometimes occur. the legs of the ass are often striped, and this may be considered as a reversion to the wild parent-form, the _asinus tæniopus_ of abyssinia,[ ] which is thus striped. in the domestic animal the stripes on the shoulder are occasionally double, or forked at the extremity, as in certain zebrine { } species. there is reason to believe that the foal is frequently more plainly striped on the legs than the adult animal. as with the horse, i have not acquired any distinct evidence that the crossing of differently-coloured varieties of the ass brings out the stripes. but now let us turn to the result of crossing the horse and ass. although mules are not nearly so numerous in england as asses, i have seen a much greater number with striped legs, and with the stripes far more conspicuous than in either parent-form. such mules are generally light-coloured, and might be called fallow-duns. the shoulder-stripe in one instance was deeply forked at the extremity, and in another instance was double, though united in the middle. mr. martin gives a figure of a spanish mule with strong zebra-like marks on its legs,[ ] and remarks, that mules are particularly liable to be thus striped on their legs. in south america, according to roulin,[ ] such stripes are more frequent and conspicuous in the mule than in the ass. in the united states, mr. gosse,[ ] speaking of these animals, says, "that in a great number, perhaps in nine out of every ten, the legs are banded with transverse dark stripes." many years ago i saw in the zoological gardens a curious triple hybrid, from a bay mare, by a hybrid from a male ass and female zebra. this animal when old had hardly any stripes; but i was assured by the superintendent, that when young it had shoulder-stripes, and faint stripes on its flanks and legs. i mention this case more especially as an instance of the stripes being much plainer during youth than in old age. as the zebra has such conspicuously striped legs, it might have been expected that the hybrids from this animal and the common ass would have had their legs in some degree striped; but it appears from the figures given in dr. gray's 'knowsley gleanings,' and still more plainly from that given by geoffroy and f. cuvier,[ ] that the legs are much more conspicuously striped than the rest of the body; and this fact is intelligible only on the belief that the ass aids in giving, through the power of reversion, this character to its hybrid offspring. the quagga is banded over the whole front part of its body like a zebra, but has no stripes on its legs, or mere traces of them. but in the famous hybrid bred by lord morton,[ ] from a chesnut, nearly purely-bred, arabian mare, by a male quagga, the stripes were "more strongly defined and darker than those on the legs of the quagga." the mare was subsequently put to a black arabian horse, and bore two colts, both of which, as formerly stated, were plainly striped on the legs, and one of them likewise had stripes on the neck and body. the _asinus indicus_[ ] is characterised by a spinal stripe, without shoulder { } or leg stripes; but traces of these latter stripes may occasionally be seen even in the adult;[ ] and colonel s. poole, who has had ample opportunities for observation, informs me that in the foal, when first born, the head and legs are often striped, but the shoulder-stripe is not so distinct as in the domestic ass; all these stripes, excepting that along the spine, soon disappear. now a hybrid, raised at knowsley[ ] from a female of this species by a male domestic ass, had all four legs transversely and conspicuously striped, had three short stripes on each shoulder, and had even some zebra-like stripes on its face! dr. gray informs me that he has seen a second hybrid of the same parentage similarly striped. from these facts we see that the crossing of the several equine species tends in a marked manner to cause stripes to appear on various parts of the body, especially on the legs. as we do not know whether the primordial parent of the genus was striped, the appearance of the stripes can only hypothetically be attributed to reversion. but most persons, after considering the many undoubted cases of variously coloured marks reappearing by reversion in crossed pigeons, fowls, ducks, &c., will come to the same conclusion with respect to the horse-genus; and in this case we must admit that the progenitor of the group was striped on the legs, shoulders, face, and probably over the whole body, like a zebra. if we reject this view, the frequent and almost regular appearance of stripes in the several foregoing hybrids is left without any explanation. * * * * * it would appear that with crossed animals a similar tendency to the recovery of lost characters holds good even with instincts. there are some breeds of fowls which are called "everlasting layers," because they have lost the instinct of incubation; and so rare is it for them to incubate that i have seen notices published in works on poultry, when hens of such breeds have taken to sit.[ ] yet the aboriginal species was of course a good incubator; for with birds in a state of nature hardly any { } instinct is so strong as this. now, so many cases have been recorded of the crossed offspring from two races, neither of which are incubators, becoming first-rate sitters, that the reappearance of this instinct must be attributed to reversion from crossing. one author goes so far as to say, "that a cross between two non-sitting varieties almost invariably produces a mongrel that becomes broody, and sits with remarkable steadiness."[ ] another author, after giving a striking example, remarks that the fact can be explained only on the principle that "two negatives make a positive." it cannot, however, be maintained that hens produced from a cross between two non-sitting breeds invariably recover their lost instinct, any more than that crossed fowls or pigeons invariably recover the red or blue plumage of their prototypes. i raised several chickens from a polish hen by a spanish cock,--breeds which do not incubate,--and none of the young hens at first recovered their instinct, and this appeared to afford a well-marked exception to the foregoing rule; but one of these hens, the only one which was preserved, in the third year sat well on her eggs and reared a brood of chickens. so that here we have the appearance with advancing age of a primitive instinct, in the same manner as we have seen that the red plumage of the _gallus bankiva_ is sometimes reacquired by crossed and purely-bred fowls of various kinds as they grow old. the parents of all our domesticated animals were of course aboriginally wild in disposition; and when a domesticated species is crossed with a distinct species, whether this is a domesticated or only tamed animal, the hybrids are often wild { } to such a degree, that the fact is intelligible only on the principle that the cross has caused a partial return to the primitive disposition. the earl of powis formerly imported some thoroughly domesticated humped cattle from india, and crossed them with english breeds, which belong to a distinct species; and his agent remarked to me, without any question having been asked, how oddly wild the cross-bred animals were. the european wild boar and the chinese domesticated pig are almost certainly specifically distinct: sir f. darwin crossed a sow of the latter breed with a wild alpine boar which had become extremely tame, but the young, though having half-domesticated blood in their veins, were "extremely wild in confinement, and would not eat swill like common english pigs." mr. hewitt, who has had great experience in crossing tame cock-pheasants with fowls belonging to five breeds, gives as the character of all "extraordinary wildness;"[ ] but i have myself seen one exception to this rule. mr. s. j. salter,[ ] who raised a large number of hybrids from a bantam-hen by _gallus sonneratii_, states that "all were exceedingly wild." mr. waterton[ ] bred some wild ducks from eggs hatched under a common duck, and the young were allowed to cross freely both amongst themselves and with the tame ducks; they were "half wild and half tame; they came to the windows to be fed, but still they had a wariness about them quite remarkable." on the other hand, mules from the horse and ass are certainly not in the least wild, yet they are notorious for obstinacy and vice. mr. brent, who has crossed canary-birds with many kinds of finches, has not observed, as he informs me, that the hybrids were in any way remarkably wild. hybrids are often raised between the common and musk duck, and i have been assured by three persons, who have kept these crossed birds, that they were not wild; but mr. garnett[ ] observed that his female hybrids exhibited "migratory propensities," of which there is not a vestige in the common or musk duck. no case is { } known of this latter bird having escaped and become wild in europe or asia, except, according to pallas, on the caspian sea; and the common domestic duck only occasionally becomes wild in districts where large lakes and fens abound. nevertheless, a large number of cases have been recorded[ ] of hybrids from these two ducks, although so few are reared in comparison with purely-bred birds of either species, having been shot in a completely wild state. it is improbable that any of these hybrids could have acquired their wildness from the musk-duck having paired with a truly wild duck; and this is known not to be the case in north america; hence we must infer that they have reacquired, through reversion, their wildness, as well as renewed powers of flight. these latter facts remind us of the statements, so frequently made by travellers in all parts of the world, on the degraded state and savage disposition of crossed races of man. that many excellent and kind-hearted mulattos have existed no one will dispute; and a more mild and gentle set of men could hardly be found than the inhabitants of the island of chiloe, who consist of indians commingled with spaniards in various proportions. on the other hand, many years ago, long before i had thought of the present subject, i was struck with the fact that, in south america, men of complicated descent between negroes, indians, and spaniards, seldom had, whatever the cause might be, a good expression.[ ] livingstone,--and a more unimpeachable authority cannot be quoted,--after speaking of a half-caste man on the zambesi, described by the portuguese as a rare monster of inhumanity, remarks, "it is unaccountable why half-castes, such as he, are so much more cruel than the portuguese, but such is undoubtedly the case." an inhabitant remarked to livingstone, "god made white men, and god made black men, but the devil made half-castes."[ ] when two races, both { } low in the scale, are crossed, the progeny seems to be eminently bad. thus the noble-hearted humboldt, who felt none of that prejudice against the inferior races now so current in england, speaks in strong terms of the bad and savage disposition of zambos, or half-castes between indians and negroes; and this conclusion has been arrived at by various observers.[ ] from these facts we may perhaps infer that the degraded state of so many half-castes is in part due to reversion to a primitive and savage condition, induced by the act of crossing, as well as to the unfavourable moral conditions under which they generally exist. * * * * * _summary on the proximate causes leading to reversion._--when purely-bred animals or plants reassume long-lost characters,--when the common ass, for instance, is born with striped legs, when a pure race of black or white pigeons throws a slaty-blue bird, or when a cultivated heartsease with large and rounded flowers produces a seedling with small and elongated flowers,--we are quite unable to assign any proximate cause. when animals run wild, the tendency to reversion, which, though it has been greatly exaggerated, no doubt exists, is sometimes to a certain extent intelligible. thus, with feral pigs, exposure to the weather will probably favour the growth of the bristles, as is known to be the case with the hair of other domesticated animals, and through correlation the tusks will tend to be redeveloped. but the reappearance of coloured longitudinal stripes on young feral pigs cannot be attributed to the direct action of external conditions. in this case, and in many others, we can only say that changed habits of life apparently have favoured a tendency, inherent or latent in the species, to return to the primitive state. it will be shown in a future chapter that the position of flowers on the summit of the axis, and the position of seeds within the capsule, sometimes determine a tendency towards reversion; and this apparently depends on the amount of sap or nutriment which the flower-buds and seeds receive. the position, also, of buds, either on branches or on roots, sometimes determines, as was formerly shown, the transmission of the { } proper character of the variety, or its reversion to a former state. we have seen in the last section that when two races or species are crossed there is the strongest tendency to the reappearance in the offspring of long-lost characters, possessed by neither parent nor immediate progenitor. when two white, or red, or black pigeons, of well-established breeds, are united, the offspring are almost sure to inherit the same colours; but when differently-coloured birds are crossed, the opposed forces of inheritance apparently counteract each other, and the tendency which is inherent in both parents to produce slaty-blue offspring becomes predominant. so it is in several other cases. but when, for instance, the ass is crossed with _a. indicus_ or with the horse,--animals which have not striped legs,--and the hybrids have conspicuous stripes on their legs and even on their faces, all that can be said is, that an inherent tendency to reversion is evolved through some disturbance in the organisation caused by the act of crossing. another form of reversion is far commoner, indeed is almost universal with the offspring from a cross, namely, to the characters proper to either pure parent-form. as a general rule, crossed offspring in the first generation are nearly intermediate between their parents, but the grandchildren and succeeding generations continually revert, in a greater or lesser degree, to one or both of their progenitors. several authors have maintained that hybrids and mongrels include all the characters of both parents, not fused together, but merely mingled in different proportions in different parts of the body; or, as naudin[ ] has expressed it, a hybrid is a living mosaic-work, in which the eye cannot distinguish the discordant elements, so completely are they intermingled. we can hardly doubt that, in a certain sense, this is true, as when we behold in a hybrid the elements of both species segregating themselves into segments in the same flower or fruit, by a process of self-attraction or self-affinity; this segregation taking place either by seminal or by bud-propagation. naudin further believes that the segregation of the two specific elements or essences is eminently liable to occur in the male and female reproductive matter; and he thus explains the almost { } universal tendency to reversion in successive hybrid generations. for this would be the natural result of the union of pollen and ovules, in both of which the elements of the same species had been segregated by self-affinity. if, on the other hand, pollen which included the elements of one species happened to unite with ovules including the elements of the other species, the intermediate or hybrid state would still be retained, and there would be no reversion. but it would, as i suspect, be more correct to say that the elements of both parent-species exist in every hybrid in a double state, namely, blended together and completely separate. how this is possible, and what the term specific essence or element may be supposed to express, i shall attempt to show in the hypothetical chapter on pangenesis. but naudin's view, as propounded by him, is not applicable to the reappearance of characters lost long ago by variation; and it is hardly applicable to races or species which, after having been crossed at some former period with a distinct form, and having since lost all traces of the cross, nevertheless occasionally yield an individual which reverts (as in the case of the great-great-grandchild of the pointer sappho) to the crossing form. the most simple case of reversion, namely, of a hybrid or mongrel to its grandparents, is connected by an almost perfect series with the extreme case of a purely-bred race recovering characters which had been lost during many ages; and we are thus led to infer that all the cases must be related by some common bond. gärtner believed that only those hybrid plants which are highly sterile exhibit any tendency to reversion to their parent-forms. it is rash to doubt so good an observer, but this conclusion must i think be an error; and it may perhaps be accounted for by the nature of the genera observed by him, for he admits that the tendency differs in different genera. the statement is also directly contradicted by naudin's observations, and by the notorious fact that perfectly fertile mongrels exhibit the tendency in a high degree,--even in a higher degree, according to gärtner himself, than hybrids.[ ] gärtner further states that reversions rarely occur with { } hybrid plants raised from species which have not been cultivated, whilst, with those which have been long cultivated, they are of frequent occurrence. this conclusion explains a curious discrepancy: max wichura,[ ] who worked exclusively on willows, which had not been subjected to culture, never saw an instance of reversion; and he goes so far as to suspect that the careful gärtner had not sufficiently protected his hybrids from the pollen of the parent-species: naudin, on the other hand, who chiefly experimented on cucurbitaceous and other cultivated plants, insists more strenuously than any other author on the tendency to reversion in all hybrids. the conclusion that the condition of the parent-species, as affected by culture, is one of the proximate causes leading to reversion, agrees fairly well with the converse case of domesticated animals and cultivated plants being liable to reversion when they become feral; for in both cases the organisation or constitution must be disturbed, though in a very different way. finally, we have seen that characters often reappear in purely-bred races without our being able to assign any proximate cause; but when they become feral this is either indirectly or directly induced by the change in their conditions of life. with crossed breeds, the act of crossing in itself certainly leads to the recovery of long-lost characters, as well as of those derived from either parent-form. changed conditions, consequent on cultivation, and the relative position of buds, flowers, and seeds on the plant, all apparently aid in giving this same tendency. reversion may occur either through seminal or bud generation, generally at birth, but sometimes only with an advance of age. segments or portions of the individual may alone be thus affected. that a being should be born resembling in certain characters an ancestor removed by two or three, and in some cases by hundreds or even thousands of generations, is assuredly a wonderful fact. in these cases the child is commonly said to inherit such characters directly from its grandparents or more remote ancestors. but this view is hardly conceivable. if, however, we suppose that every character is derived { } exclusively from the father or mother, but that many characters lie latent in both parents during a long succession of generations, the foregoing facts are intelligible. in what manner characters may be conceived to lie latent, will be considered in a future chapter to which i have lately alluded. * * * * * _latent characters._--but i must explain what is meant by characters lying latent. the most obvious illustration is afforded by secondary sexual characters. in every female all the secondary male characters, and in every male all the secondary female characters, apparently exist in a latent state, ready to be evolved under certain conditions. it is well known that a large number of female birds, such as fowls, various pheasants, partridges, peahens, ducks, &c., when old or diseased, or when operated on, partly assume the secondary male characters of their species. in the case of the hen-pheasant this has been observed to occur far more frequently during certain seasons than during others.[ ] a duck ten years old has been known to assume both the perfect winter and summer plumage of the drake.[ ] waterton[ ] gives a curious case of a hen which had ceased laying, and had assumed the plumage, voice, spurs, and warlike disposition of the cock; when opposed to an enemy she would erect her hackles and show fight. thus every character, even to the instinct and manner of fighting, must have lain dormant in this hen as long as her ovaria continued to act. the females of two kinds of deer, when old, have been known to acquire horns; and, as hunter has remarked, we see something of an analogous nature in the human species. on the other hand, with male animals, it is notorious that the secondary sexual characters are more or less completely lost when they are subjected to castration. thus, if the operation be performed on a young cock, he never, as yarrell states, crows { } again; the comb, wattles, and spurs do not grow to their full size, and the hackles assume an intermediate appearance between true hackles and the feathers of the hen. cases are recorded of confinement alone causing analogous results. but characters properly confined to the female are likewise acquired; the capon takes to sitting on eggs, and will bring up chickens; and what is more curious, the utterly sterile male hybrids from the pheasant and the fowl act in the same manner, "their delight being to watch when the hens leave their nests, and to take on themselves the office of a sitter."[ ] that admirable observer réaumur[ ] asserts that a cock, by being long confined in solitude and darkness, can be taught to take charge of young chickens; he then utters a peculiar cry, and retains during his whole life this newly acquired maternal instinct. the many well-ascertained cases of various male mammals giving milk, show that their rudimentary mammary glands retain this capacity in a latent condition. we thus see that in many, probably in all cases, the secondary characters of each sex lie dormant or latent in the opposite sex, ready to be evolved under peculiar circumstances. we can thus understand how, for instance, it is possible for a good milking cow to transmit her good qualities through her male offspring to future generations; for we may confidently believe that these qualities are present, though latent, in the males of each generation. so it is with the game-cock, who can transmit his superiority in courage and vigour through his female to his male offspring; and with man it is known [ ] that diseases, such as hydrocele, necessarily confined to the male sex, can be transmitted through the female to the grandson. such cases as these offer, as was remarked at the commencement of this chapter, the simplest possible examples of reversion; and they are intelligible on the belief that characters common to the grandparent and grandchild of the same sex are present, though latent, in the intermediate parent of the opposite sex. the subject of latent characters is so important, as we shall see in a future chapter, that i will give another illustration. { } many animals have the right and left sides of their body unequally developed: this is well known to be the case with flat-fish, in which the one side differs in thickness and colour, and in the shape of the fins, from the other; and during the growth of the young fish one eye actually travels, as shown by steenstrup, from the lower to the upper surface.[ ] in most flat-fishes the left is the blind side, but in some it is the right; though in both cases "wrong fishes," which are developed in a reversed manner to what is usual, occasionally occur, and in _platessa flesus_ the right or left side is indifferently developed, the one as often as the other. with gasteropods or shell-fish, the right and left sides are extremely unequal; the far greater number of species are dextral, with rare and occasional reversals of development, and some few are normally sinistral; but certain species of bulimus, and, many achatinellæ,[ ] are as often sinistral as dextral. i will give an analogous case in the great articulate kingdom: the two sides of verruca[ ] are so wonderfully unlike, that without careful dissection it is extremely difficult to recognise the corresponding parts on the opposite sides of the body; yet it is apparently a mere matter of chance whether it be the right or the left side that undergoes so singular an amount of change. one plant is known to me[ ] in which the flower, according as it stands on the one or other side of the spike, is unequally developed. in all the foregoing cases the two sides of the animal are perfectly symmetrical at an early period of growth. now, whenever a species is as liable to be unequally developed on the one as on the other side, we may infer that the capacity for such development is present, though latent, in the undeveloped side. and as a reversal of development occasionally occurs in animals of many kinds, this latent capacity is probably very common. the best yet simplest instances of characters lying dormant are, perhaps, those previously given, in which chickens and { } young pigeons, raised from a cross between differently coloured birds, are at first of one colour, but in a year or two acquire feathers of the colour of the other parent; for in this case the tendency to a change of plumage is clearly latent in the young bird. so it is with hornless breeds of cattle, some of which acquire, as they grow old, small horns. purely bred black and white bantams, and some other fowls, occasionally assume, with advancing years, the red feathers of the parent-species. i will here add a somewhat different case, as it connects in a striking manner latent characters of two classes. mr. hewitt[ ] possessed an excellent sebright gold-laced hen bantam, which, as she became old, grew diseased in her ovaria, and assumed male characters. in this breed the males resemble the females in all respects except in their combs, wattles, spurs, and instincts; hence it might have been expected that the diseased hen would have assumed only those masculine characters which are proper to the breed, but she acquired, in addition, well-arched tail sickle-feathers quite a foot in length, saddle-feathers on the loins, and hackles on the neck,--ornaments which, as mr. hewitt remarks, "would be held as abominable in this breed." the sebright bantam is known[ ] to have originated about the year from a cross between a common bantam and a polish fowl, recrossed by a hen-tailed bantam, and carefully selected; hence there can hardly be a doubt that the sickle-feathers and hackles which appeared in the old hen were derived from the polish fowl or common bantam; and we thus see that not only certain masculine characters proper to the sebright bantam, but other masculine characters derived from the first progenitors of the breed, removed by a period of above sixty years, were lying latent in this hen-bird, ready to be evolved as soon as her ovaria became diseased. from these several facts it must be admitted that certain characters, capacities, and instincts may lie latent in an individual, and even in a succession of individuals, without our being able to detect the least signs of their presence. we have { } already seen that the transmission of a character from the grandparent to the grandchild, with its apparent omission in the intermediate parent of the opposite sex, becomes simple on this view. when fowls, pigeons, or cattle of different colours are crossed, and their offspring change colour as they grow old, or when the crossed turbit acquired the characteristic frill after its third moult, or when purely-bred bantams partially assume the red plumage of their prototype, we cannot doubt that these qualities were from the first present, though latent, in the individual animal, like the characters of a moth in the caterpillar. now, if these animals had produced offspring before they had acquired with advancing age their new characters, nothing is more probable than that they would have transmitted them to some of their offspring, which in this case would in appearance have received such characters from their grandparents or more distant progenitors. we should then have had a case of reversion, that is, of the reappearance in the child of an ancestral character, actually present, though during youth completely latent, in the parent; and this we may safely conclude is what occurs with reversions of all kinds to progenitors however remote. this view of the latency in each generation of all the characters which appear through reversion, is also supported by their actual presence in some cases during early youth alone, or by their more frequent appearance and greater distinctness at this age than during maturity. we have seen that this is often the case with the stripes on the legs and faces of the several species of the horse-genus. the himalayan rabbit, when crossed, sometimes produces offspring which revert to the parent silver-grey breed, and we have seen that in purely bred animals pale-grey fur occasionally reappears during early youth. black cats, we may feel assured, would occasionally produce by reversion tabbies; and on young black kittens, with a pedigree[ ] known to have been long pure, faint traces of stripes may almost always be seen which afterwards disappear. hornless suffolk cattle occasionally produce by reversion horned animals; and youatt[ ] asserts that even in hornless individuals { } "the rudiment of a horn may be often felt at an early age." no doubt it appears at first sight in the highest degree improbable that in every horse of every generation there should be a latent capacity and tendency to produce stripes, though these may not appear once in a thousand generations; that in every white, black, or other coloured pigeon, which may have transmitted its proper colour during centuries, there should be a latent capacity in the plumage to become blue and to be marked with certain characteristic bars; that in every child in a six-fingered family there should be the capacity for the production of an additional digit; and so in other cases. nevertheless there is no more inherent improbability in this being the case than in a useless and rudimentary organ, or even in only a tendency to the production of a rudimentary organ, being inherited during millions of generations, as is well known to occur with a multitude of organic beings. there is no more inherent improbability in each domestic pig, during a thousand generations, retaining the capacity and tendency to develop great tusks under fitting conditions, than in the young calf having retained for an indefinite number of generations rudimentary incisor teeth, which never protrude through the gums. i shall give at the end of the next chapter a summary of the three preceding chapters; but as isolated and striking cases of reversion have here been chiefly insisted on, i wish to guard the reader against supposing that reversion is due to some rare or accidental combination of circumstances. when a character, lost during hundreds of generations, suddenly reappears, no doubt some such combination must occur; but reversions may be constantly observed, at least to the immediately preceding generations, in the offspring of most unions. this has been universally recognised in the case of hybrids and mongrels, but it has been recognised simply from the difference between the united forms rendering the resemblance of the offspring to their grandparents or more remote progenitors of easy detection. reversion is likewise almost invariably the rule, as mr. sedgwick has shown, with certain diseases. hence we must conclude that a tendency to this peculiar form of transmission is an integral part of the general law of inheritance. { } * * * * * _monstrosities._--a large number of monstrous growths and of lesser anomalies are admitted by every one to be due to an arrest of development, that is to the persistence of an embryonic condition. if every horse or ass had striped legs whilst young, the stripes which occasionally appear on these animals when adult would have to be considered as due to the anomalous retention of an early character, and not as due to reversion. now, the leg-stripes in the horse-genus, and some other characters in analogous cases, are apt to occur during early youth and then to disappear; thus the persistence of early characters and reversion are brought into close connexion. but many monstrosities can hardly be considered as the result of an arrest of development; for parts of which no trace can be detected in the embryo, but which occur in other members of the same class of animals or plants, occasionally appear, and these may probably with truth be attributed to reversion. for instance: supernumerary mammæ, capable of secreting milk, are not extremely rare in women; and as many as five have been observed. when four are developed, they are generally arranged symmetrically on each side of the chest; and in one instance a woman (the daughter of another with supernumerary mammæ) had one mamma, which yielded milk, developed in the inguinal region. this latter case, when we remember the position of the mammæ in some of the lower animals on both the chest and inguinal region, is highly remarkable, and leads to the belief that in all cases the additional mammæ in woman are due to reversion. the facts given in the last chapter on the tendency in supernumerary digits to regrowth after amputation, indicate their relation to the digits of the lower vertebrate animals, and lead to the suspicion that their appearance may in some manner be connected with reversion. but i shall have to recur, in the chapter on pangenesis, to the abnormal multiplication of organs, and likewise to their occasional transposition. the occasional development in man of the coccygeal vertebræ into a short and free tail, though it thus becomes in one sense more perfectly developed, may at the same time be considered as an arrest of development, and as a case of reversion. the greater frequency of a monstrous kind of proboscis in the pig than in any other mammal, considering the position of the pig { } in the mammalian series, has likewise been attributed, perhaps truly, to reversion.[ ] when flowers which are properly irregular in structure become regular or peloric, the change is generally looked at by botanists as a return to the primitive state. but dr. maxwell masters,[ ] who has ably discussed this subject, remarks that when, for instance, all the sepals of a tropæolum become green and of the same shape, instead of being coloured with one alone prolonged into a spur, or when all the petals of a linaria become simple and regular, such cases may be due merely to an arrest of development; for in these flowers all the organs during their earliest condition are symmetrical, and, if arrested at this stage of growth, they would not become irregular. if, moreover, the arrest were to take place at a still earlier period of development, the result would be a simple tuft of green leaves; and no one probably would call this a case of reversion. dr. masters designates the cases first alluded to as regular peloria; and others, in which all the corresponding parts assume a similar form of irregularity, as when all the petals in a linaria become spurred, as irregular peloria. we have no right to attribute these latter cases to reversion, until it can be shown to be probable that the parent-form, for instance, of the genus linaria had had all its petals spurred; for a change of this nature might result from the spreading of an anomalous structure, in accordance with the law, to be discussed in a future chapter, of homologous parts tending to vary in the same manner. but as both forms of peloria frequently occur on the same individual plant of the linaria,[ ] they probably stand in some close relation to each other. on the doctrine that peloria is simply the result of an arrest of development, it is difficult to understand how an organ arrested at a very early period of growth should acquire its full functional perfection;--how a petal, supposed to be thus arrested, should acquire its brilliant colours, and serve as an envelope to the flower, or a stamen produce efficient pollen; yet this occurs with many peloric flowers. that pelorism is not due to mere chance variability, but either to an arrest of development or to reversion, we may infer from an observation made by ch. morren,[ ] namely, that families which have irregular flowers often "return by these monstrous growths to their regular form; whilst we never see a regular flower realise the structure of an irregular one." some flowers have almost certainly become more or less completely peloric through reversion. _corydalis tuberosa_ properly has one of its two nectaries colourless, destitute of nectar, only half the size of the other, and { } therefore, to a certain extent, in a rudimentary state; the pistil is curved towards the perfect nectary, and the hood, formed of the inner petals, slips off the pistil and stamens in one direction alone, so that, when a bee sucks the perfect nectary, the stigma and stamens are exposed and rubbed against the insect's body. in several closely allied genera, as in dielytra, &c., there are two perfect nectaries, the pistil is straight, and the hood slips off on either side, according as the bee sucks either nectary. now, i have examined several flowers of _corydalis tuberosa_, in which both nectaries were equally developed and contained nectar; in this we see only the redevelopment of a partially aborted organ; but with this redevelopment the pistil becomes straight, and the hood slips off in either direction; so that these flowers have acquired the perfect structure, so well adapted for insect agency, of dielytra and its allies. we cannot attribute these coadapted modifications to chance, or to correlated variability; we must attribute them to reversion to a primordial condition of the species. the peloric flowers of pelargonium have their five petals in all respects alike, and there is no nectary; so that they resemble the symmetrical flowers of the closely allied geranium-genus; but the alternate stamens are also sometimes destitute of anthers, the shortened filaments being left as rudiments, and in this respect they resemble the symmetrical flowers of the closely allied genus, erodium. hence we are led to look at the peloric flowers of pelargonium as having probably reverted to the state of some primordial form, the progenitor of the three closely related genera of pelargonium, geranium, and erodium. in the peloric form of _antirrhinum majus_, appropriately called the "_wonder_," the tubular and elongated flowers differ wonderfully from those of the common snapdragon; the calyx and the mouth of the corolla consist of six equal lobes, and include six equal instead of four unequal stamens. one of the two additional stamens is manifestly formed by the development of a microscopically minute papilla, which may be found at the base of the upper lip of the flower in all common snapdragons, at least in nineteen plants examined by me. that this papilla is a rudiment of a stamen was well shown by its various degrees of development in crossed plants between the common and peloric antirrhinum. again, a peloric _galeobdolon luteum_, growing in my garden, had five equal petals, all striped like the ordinary lower lip, and included five equal instead of four unequal stamens; but mr. r. keeley, who sent me this plant, informs me that the flowers vary greatly, having from four to six lobes to the corolla, and from three to six stamens.[ ] now, as the members of the two great families to which the antirrhinum and galeobdolon belong are properly pentamerous, with some of the parts confluent and others suppressed, we ought not to look at the sixth stamen and the sixth lobe to the corolla in either case as due to reversion, any more than the additional petals in double flowers in these same two families. but the case is different with the fifth stamen in the peloric antirrhinum, which { } is produced by the redevelopment of a rudiment always present, and which probably reveals to us the state of the flower, as far as the stamens are concerned, at some ancient epoch. it is also difficult to believe that the other four stamens and the petals, after an arrest of development at a very early embryonic age, would have come to full perfection in colour, structure, and function, unless these organs had at some former period normally passed through a similar course of growth. hence it appears to me probable that the progenitor of the genus antirrhinum must at some remote epoch have included five stamens and borne flowers in some degree resembling those now produced by the peloric form. lastly, i may add that many instances have been recorded of flowers, not generally ranked as peloric, in which certain organs, normally few in number, have been abnormally augmented. as such an increase of parts cannot be looked at as an arrest of development, nor as due to the redevelopment of rudiments, for no rudiments are present, and as these additional parts bring the plant into closer relationship with its natural allies, they ought probably to be viewed as reversions to a primordial condition. these several facts show us in an interesting manner how intimately certain abnormal states are connected together; namely, arrests of development causing parts to become rudimentary or to be wholly suppressed,--the redevelopment of parts at present in a more or less rudimentary condition,--the reappearance of organs of which not a vestige can now be detected,--and to these may be added, in the case of animals, the presence during youth, and subsequent disappearance, of certain characters which occasionally are retained throughout life. some naturalists look at all such abnormal structures as a return to the ideal state of the group to which the affected being belongs; but it is difficult to conceive what is meant to be conveyed by this expression. other naturalists maintain, with greater probability and distinctness of view, that the common bond of connection between the several foregoing cases is an actual, though partial, return to the structure of the ancient progenitor of the group. if this view be correct, we must believe that a vast number of characters, capable of evolution, lie hidden in every organic being. but it would be a mistake to suppose that the number is equally great in all beings. we know, for instance, that plants of many orders occasionally become peloric; but many more cases have been observed in the labiatæ and scrophulariaceæ than in any other order; and in one genus of the scrophulariaceæ, namely linaria, no less { } than thirteen species have been described in a peloric condition.[ ] on this view of the nature of peloric flowers, and bearing in mind what has been said with respect to certain monstrosities in the animal kingdom, we must conclude that the progenitors of most plants and animals, though widely different in structure, have left an impression capable of redevelopment on the germs of their descendants. the fertilised germ of one of the higher animals, subjected as it is to so vast a series of changes from the germinal cell to old age,--incessantly agitated by what quatrefages well calls the _tourbillon vital_,--is perhaps the most wonderful object in nature. it is probable that hardly a change of any kind affects either parent, without some mark being left on the germ. but on the doctrine of reversion, as given in this chapter, the germ becomes a far more marvellous object, for, besides the visible changes to which it is subjected, we must believe that it is crowded with invisible characters, proper to both sexes, to both the right and left side of the body, and to a long line of male and female ancestors separated by hundreds or even thousands of generations from the present time; and these characters, like those written on paper with invisible ink, all lie ready to be evolved under certain known or unknown conditions. * * * * * { } chapter xiv. inheritance _continued_--fixedness of character--prepotency--sexual limitation--correspondence of age. fixedness of character apparently not due to antiquity of inheritance--prepotency of transmission in individuals of the same family, in crossed breeds and species; often stronger in one sex than the other; sometimes due to the same character being present and visible in one breed and latent in the other--inheritance as limited by sex--newly-acquired characters in our domesticated animals often transmitted by one sex alone, sometimes lost by one sex alone--inheritance at corresponding periods of life--the importance of the principle with respect to embryology; as exhibited in domesticated animals; as exhibited in the appearance and disappearance of inherited diseases; sometimes supervening earlier in the child than in the parent--summary of the three preceding chapters. in the two last chapters the nature and force of inheritance, the circumstances which interfere with its power, and the tendency to reversion, with its many remarkable contingencies, were discussed. in the present chapter some other related phenomena will be treated of, as fully as my materials permit. _fixedness of character._ it is a general belief amongst breeders that the longer any character has been transmitted by a breed, the more firmly it will continue to be transmitted. i do not wish to dispute the truth of the proposition, that inheritance gains strength simply through long continuance, but i doubt whether it can be proved. in one sense the proposition is little better than a truism; if any character has remained constant during many generations, it will obviously be little likely, the conditions of life remaining the same, to vary during the next generation. so, again, in improving a breed, if care be taken for a length of time to exclude all inferior individuals, the breed will obviously tend to become truer, as it will not have been crossed during many generations by an inferior animal. we have previously seen, { } but without being able to assign any cause, that, when a new character appears, it is occasionally from the first well fixed, or fluctuates much, or wholly fails to be transmitted. so it is with the aggregate of slight differences which characterise a new variety, for some propagate their kind from the first much truer than others. even with plants multiplied by bulbs, layers, &c., which may in one sense be said to form parts of the same individual, it is well known that certain varieties retain and transmit through successive bud-generations their newly-acquired characters more truly than others. in none of these, nor in the following cases, does there appear to be any relation between the force with which a character is transmissible and the length of time during which it has already been transmitted. some varieties, such as white and yellow hyacinths and white sweet-peas, transmit their colours more faithfully than do the varieties which have retained their natural colour. in the irish family, mentioned in the twelfth chapter, the peculiar tortoiseshell-like colouring of the eyes was transmitted far more faithfully than any ordinary colour. ancon and mauchamp sheep and niata cattle, which are all comparatively modern breeds, exhibit remarkably strong powers of inheritance. many similar cases could be adduced. as all domesticated animals and cultivated plants have varied, and yet are descended from aboriginally wild forms, which no doubt had retained the same character from an immensely remote epoch, we see that scarcely any degree of antiquity ensures a character being transmitted perfectly true. in this case, however, it may be said that changed conditions of life induce certain modifications, and not that the power of inheritance fails; but in every case of failure, some cause, either internal or external, must interfere. it will generally be found that the parts in our domesticated productions which have varied, or which still continue to vary,--that is, which fail to retain their primordial state,--are the same with the parts which differ in the natural species of the same genus. as, on the theory of descent with modification, the species of the same genus have been modified since they branched off from a common progenitor, it follows that the characters by which they differ from each other have varied whilst other parts of the organisation have remained unchanged; and it might be argued that { } these same characters now vary under domestication, or fail to be inherited, owing to their lesser antiquity. but we must believe structures, which have already varied, would be more liable to go on varying, rather than structures which during an immense lapse of time have remained unaltered; and this variation is probably the result of certain relations between the conditions of life and the organisation, quite independently of the greater or less antiquity of each particular character. fixedness of character, or the strength of inheritance, has often been judged of by the preponderance of certain characters in the crossed offspring between distinct races; but prepotency of transmission here comes into play, and this, as we shall immediately see, is a very different consideration from the strength or weakness of inheritance. it has often been observed[ ] that breeds of animals inhabiting wild and mountainous countries cannot be permanently modified by our improved breeds; and as these latter are of modern origin, it has been thought that the greater antiquity of the wilder breeds has been the cause of their resistance to improvement by crossing; but it is more probably due to their structure and constitution being better adapted to the surrounding conditions. when plants are first subjected to culture, it has been found that, during several generations, they transmit their characters truly, that is, do not vary, and this has been attributed to ancient characters being strongly inherited; but it may with equal or greater probability be consequent on changed conditions of life requiring a long time for their accumulative action. notwithstanding these considerations, it would perhaps be rash to deny that characters become more strongly fixed the longer they are transmitted; but i believe that the proposition resolves itself into this,--that all characters of all kinds, whether new or old, tend to be inherited, and that those which have already withstood all counteracting influences and been truly transmitted, will, as a general rule, continue to withstand them, and consequently be faithfully inherited. { } _prepotency in the transmission of character._ when individuals distinct enough to be recognised, but of the same family, or when two well-marked races, or two species, are crossed, the usual result, as stated in the previous chapter, is, that the offspring in the first generation are intermediate between their parents, or resemble one parent in one part and the other parent in another part. but this is by no means the invariable rule; for in many cases it is found that certain individuals, races, and species are prepotent in transmitting their likeness. this subject has been ably discussed by prosper lucas,[ ] but is rendered extremely complicated by the prepotency sometimes running equally in both sexes, and sometimes more strongly in one sex than in the other; it is likewise complicated by the presence of secondary sexual characters, which render the comparison of mongrels with their parent-breeds difficult. it would appear that in certain families some one ancestor, and after him others in the same family, must have had great power in transmitting their likeness through the male line; for we cannot otherwise understand how the same features should so often be transmitted after marriages with various females, as has been the case with the austrian emperors, and as, according to niebuhr, formerly occurred in certain roman families with their mental qualities.[ ] the famous bull favourite is believed[ ] to have had a prepotent influence on the shorthorn race. it has also been observed[ ] with english race-horses that certain mares have generally transmitted their own character, whilst other mares of equally pure blood have allowed the character of the sire to prevail. the truth of the principle of prepotency comes out more clearly when certain races are crossed. the improved shorthorns, notwithstanding that the breed is comparatively modern, are generally acknowledged to possess great power in impressing their likeness on all other breeds; and it is chiefly in consequence of this power that they are so highly valued { } for exportation.[ ] godine has given a curious case of a ram of a goat-like breed of sheep from the cape of good hope, which produced offspring hardly to be distinguished from himself, when crossed with ewes of twelve other breeds. but two of these half-bred ewes, when put to a merino ram, produced lambs closely resembling the merino breed. girou de buzareingues[ ] found that of two races of french sheep the ewes of one, when crossed during successive generations with merino rams, yielded up their character far sooner than the ewes of the other race. sturm and girou have given analogous cases with other breeds of sheep and with cattle, the prepotency running in these cases through the male side; but i was assured on good authority in south america, that when niata cattle are crossed with common cattle, though the niata breed is prepotent whether males or females are used, yet that the prepotency is strongest through the female line. the manx cat is tailless and has long hind legs; dr. wilson crossed a male manx with common cats, and, out of twenty-three kittens, seventeen were destitute of tails; but when the female manx was crossed by common male cats all the kittens had tails, though they were generally short and imperfect.[ ] in making reciprocal crosses between pouter and fantail pigeons, the pouter-race seemed to be prepotent through both sexes over the fantail. but this is probably due to weak power in the fantail rather than to any unusually strong power in the pouter, for i have observed that barbs also preponderated over fantails. this weakness of transmission in the fantail, though the breed is an ancient one, is said[ ] to be general; but i have observed one exception to the rule, namely, in a cross between a fantail and laugher. the most curious instance known to me of weak power in both sexes is in the trumpeter pigeon. this breed has been well known for at least years: it breeds perfectly true, as i have been assured by those who have long kept many birds: it is characterised by a peculiar tuft of feathers over the beak, by a crest on the head, by a most peculiar coo quite unlike that of any other breed, and by much-feathered feet. i have crossed both sexes with turbits of two sub-breeds, with almond tumblers, spots, and runts, and reared many mongrels and recrossed them; and though the crest on the head and feathered feet were inherited (as is generally the case with most breeds), i have never seen a vestige of the tuft over the beak or heard the peculiar coo. boitard and corbié[ ] assert that this is the invariable result of crossing trumpeters with any other breed: neumeister,[ ] however, states that in germany mongrels have been obtained, though very rarely, which were furnished with the tuft and would trumpet: but a pair of these mongrels with a tuft, which i imported, never trumpeted. mr. brent states[ ] that the crossed offspring of a trumpeter were crossed { } with trumpeters for three generations, by which time the mongrels had - ths of this blood in their veins, yet the tuft over the beak did not appear. at the fourth generation the tuft appeared, but the birds, though now having - ths trumpeter's blood, still did not trumpet. this case well shows the wide difference between inheritance and prepotency; for here we have a well-established old race which transmits it characters faithfully, but which, when crossed with any other race, has the feeblest power of transmitting its two chief characteristic qualities. i will give one other instance with fowls and pigeons of weakness and strength in the transmission of the same character to their crossed offspring. the silk-fowl breeds true, and there is reason to believe is a very ancient race; but when i reared a large number of mongrels from a silk-hen by a spanish cock, not one exhibited even a trace of the so-called silkiness. mr. hewitt also asserts that in no instance are the silky feathers transmitted by this breed when crossed with any other variety. but three birds out of many raised by mr. orton from a cross between a silk-cock and a bantam-hen, had silky feathers.[ ] so that it is certain that this breed very seldom has the power of transmitting its peculiar plumage to its crossed progeny. on the other hand, there is a silk sub-variety of the fantail pigeon, which has its feathers in nearly the same state as in the silk-fowl: now we have already seen that fantails, when crossed, possess singularly weak power in transmitting their general qualities; but the silk sub-variety when crossed with any other small-sized race invariably transmits its silky feathers![ ] the law of prepotency comes into action when species are crossed, as with races and individuals. gärtner has unequivocally shown[ ] that this is the case with plants. to give one instance: when _nicotiana paniculata_ and _vincæflora_ are crossed, the character of _n. paniculata_ is almost completely lost in the hybrid; but if _n. quadrivalvis_ be crossed with _n. vincæflora_, this later species, which was before so prepotent, now in its turn almost disappears under the power of _n. quadrivalvis_. it is remarkable that the prepotency of one species over another in transmission is quite independent, as shown by gärtner, of the greater or less facility with which the one fertilises the other. with animals, the jackal is prepotent over the dog, as is stated by flourens who made many crosses between these animals; and this was likewise the case with a hybrid which i once saw between a jackal and terrier. i cannot doubt, from the observations of colin and others, that the ass is prepotent over the horse; the prepotency in this instance running more strongly through the male than through the female ass; so that the mule resembles the ass more closely than does the hinny.[ ] the { } male pheasant, judging from mr. hewitt's descriptions,[ ] and from the hybrids which i have seen, preponderates over the domestic fowl; but the latter, as far as colour is concerned, has considerable power of transmission, for hybrids raised from five differently coloured hens differed greatly in plumage. i formerly examined some curious hybrids in the zoological gardens, between the penguin variety of the common duck and the egyptian goose (_tadorna Ægyptiaca_); and although i will not assert that the domesticated variety preponderated over the natural species, yet it had strongly impressed its unnatural upright figure on these hybrids. i am aware that such cases as the foregoing have been ascribed by various authors, not to one species, race, or individual being prepotent over the other in impressing it character on its crossed offspring, but to such rules as that the father influences the external characters and the mother the internal or vital organs. but the great diversity of the rules given by various authors almost proves their falseness. dr. prosper lucas has fully discussed this point, and has shown[ ] that none of the rules (and i could add others to those quoted by him) apply to all animals. similar rules have been enounced for plants, and have been proved by gärtner[ ] to be all erroneous. if we confine our view to the domesticated races of a single species, or perhaps even to the species of the same genus, some such rules may hold good; for instance, it seems that in reciprocally crossing various breeds of fowls the male generally gives colour;[ ] but conspicuous exceptions have passed under my own eyes. in sheep it seems that the ram usually gives its peculiar horns and fleece to its crossed offspring, and the bull the presence or absence of horns. in the following chapter on crossing i shall have occasion to show that certain characters are rarely or never blended by crossing, but are { } transmitted in an unmodified state from either parent-form; i refer to this fact here because it is sometimes accompanied on the one side by prepotency, which thus acquires the false appearance of unusual strength. in the same chapter i shall show that the rate at which a species or breed absorbs and obliterates another by repeated crosses, depends in chief part on prepotency in transmission. in conclusion, some of the cases above given,--for instance, that of the trumpeter pigeon,--prove that there is a wide difference between mere inheritance and prepotency. this latter power seems to us, in our ignorance, to act in most cases quite capriciously. the very same character, even though it be an abnormal or monstrous one, such as silky feathers, may be transmitted by different species, when crossed, either with prepotent force or singular feebleness. it is obvious, that a purely-bred form of either sex, in all cases in which prepotency does not run more strongly in one sex than the other, will transmit its character with prepotent force over a mongrelized and already variable form.[ ] from several of the above-given cases we may conclude that mere antiquity of character does not by any means necessarily make it prepotent. in some cases prepotency apparently depends on the same character being present and visible in one of the two breeds which are crossed, and latent or invisible in the other breed; and in this case it is natural that the character which is potentially present in both should be prepotent. thus, we have reason to believe that there is a latent tendency in all horses to be dun-coloured and striped; and when a horse of this kind is crossed with one of any other colour, it is said that the offspring are almost sure to be striped. sheep have a similar latent tendency to become dark-coloured, and we have seen with what prepotent force a ram with a few black spots, when crossed with sheep of various breeds, coloured its offspring. all pigeons have a latent tendency to become slaty-blue, with certain characteristic marks, and it is known that, when a bird thus coloured is crossed with one of any other colour, it is most difficult afterwards to eradicate the blue tint. a nearly parallel case is offered by those black bantams which, as they grow { } old, develop a latent tendency to acquire red feathers. but there are exceptions to the rule: hornless breeds of cattle possess a latent capacity to reproduce horns, yet when crossed with horned breeds they do not invariably produce offspring bearing horns. we meet with analogous cases with plants. striped flowers, though they can be propagated truly by seed, have a latent tendency to become uniformly coloured, but when once crossed by a uniformly coloured variety, they ever afterwards fail to produce striped seedlings.[ ] another case is in some respects more curious: plants bearing peloric or regular flowers have so strong a latent tendency to reproduce their normally irregular flowers, that this often occurs by buds when a plant is transplanted into poorer or richer soil.[ ] now i crossed the peloric snapdragon (_antirrhinum majus_), described in the last chapter, with pollen of the common form; and the latter, reciprocally, with peloric pollen. i thus raised two great beds of seedlings, and not one was peloric. naudin[ ] obtained the same result from crossing a peloric linaria with the common form. i carefully examined the flowers of ninety plants of the crossed antirrhinum in the two beds, and their structure had not been in the least affected by the cross, except that in a few instances the minute rudiment of the fifth stamen, which is always present, was more fully or even completely developed. it must not be supposed that this entire obliteration of the peloric structure in the crossed plants can be accounted for by any incapacity of transmission; for i raised a large bed of plants from the peloric antirrhinum, artificially fertilised by its own pollen, and sixteen plants, which alone survived the winter, were all as perfectly peloric as the parent-plant. here we have a good instance of the wide difference between the inheritance of a character and the power of transmitting it to crossed offspring. the crossed plants, which perfectly resembled the common snapdragon, were allowed to sow themselves, and, out of a hundred and twenty-seven seedlings, eighty-eight proved to be common snapdragons, two were in an intermediate condition between the peloric and normal state, { } and thirty-seven were perfectly peloric, having reverted to the structure of their one grandparent. this case seems at first sight to offer an exception to the rule formerly given, namely, that a character which is present in one form and latent in the other is generally transmitted with prepotent force when the two forms are crossed. for in all the scrophulariaceæ, and especially in the genera antirrhinum and linaria, there is, as was shown in the last chapter, a strong latent tendency to become peloric; and there is also, as we have just seen, a still stronger tendency in all peloric plants to reacquire their normal irregular structure. so that we have two opposed latent tendencies in the same plants. now, with the crossed antirrhinums the tendency to produce normal or irregular flowers, like those of the common snapdragon, prevailed in the first generation; whilst the tendency to pelorism, appearing to gain strength by the intermission of a generation, prevailed to a large extent in the second set of seedlings. how it is possible for a character to gain strength by the intermission of a generation, will be considered in the chapter on pangenesis. on the whole, the subject of prepotency is extremely intricate,--from its varying so much in strength, even in regard to the same character, in different animals,--from its running either equally in both sexes, or, as frequently is the case with animals, but not with plants, much stronger in the one sex than the other,--from the existence of secondary sexual characters,--from the transmission of certain characters being limited, as we shall immediately see, by sex,--from certain characters not blending together,--and, perhaps, occasionally from the effects of a previous fertilisation on the mother. it is therefore not surprising that every one hitherto has been baffled in drawing up general rules on the subject of prepotency. _inheritance as limited by sex._ new characters often appear in one sex, and are afterwards transmitted to the same sex, either exclusively or in a much greater degree than to the other. this subject is important, because with animals of many kinds in a state of nature, both high and low in the scale, secondary sexual characters, not in { } any way directly connected with the organs of reproduction, are often conspicuously present. with our domesticated animals, also, these same secondary characters are often found to differ greatly from the state in which they exist in the parent-species. and the principle of inheritance as limited by sex shows how such characters might have been first acquired and subsequently modified. dr. p. lucas, who has collected many facts on this subject, shows[ ] that when a peculiarity, in no manner connected with the reproductive organs, appears in either parent, it is often transmitted exclusively to the offspring of the same sex, or to a much greater number of them than of the opposite sex. thus, in the family of lambert, the horn-like projections on the skin were transmitted from the father to his sons and grandsons alone; so it has been with other cases of ichthyosis, with supernumerary digits, with a deficiency of digits and phalanges, and in a lesser degree with various diseases, especially with colour-blindness, and a hæmorrhagic diathesis, that is, an extreme liability to profuse and uncontrollable bleeding from trifling wounds. on the other hand, mothers have transmitted, during several generations, to their daughters alone, supernumerary and deficient digits, colour-blindness, and other peculiarities. so that we see that the very same peculiarity may become attached to either sex, and be long inherited by that sex alone; but the attachment in certain cases is much more frequent to one than the other sex. the same peculiarities also may be promiscuously transmitted to either sex. dr. lucas gives other cases, showing that the male occasionally transmits his peculiarities to his daughters alone, and the mother to her sons alone; but even in this case we see that inheritance is to a certain extent, though inversely, regulated by sex. dr. lucas, after weighing the whole evidence, comes to the conclusion that every peculiarity, according to the sex in which it first appears, tends to be transmitted in a greater or lesser degree to that sex. a few details from the many cases collected by mr. sedgwick,[ ] may be here given. colour-blindness, from some unknown cause, shows itself much oftener in males than in females; in upwards of two hundred cases collected by mr. sedgwick, nine-tenths related to men; but it is eminently liable to be transmitted through women. in the case given by dr. earle, members of eight related families were affected during five generations: these families consisted of sixty-one individuals, namely, of thirty-two males, of whom nine-sixteenths were incapable of distinguishing colour, and of twenty-nine females, of whom only one-fifteenth were thus affected. { } although colour-blindness thus generally clings to the male sex, nevertheless, in one instance in which it first appeared in a female, it was transmitted during five generations to thirteen individuals, all of whom were females. a hæmorrhagic diathesis, often accompanied by rheumatism, has been known to affect the males alone during five generations, being transmitted, however, through the females. it is said that deficient phalanges in the fingers have been inherited by the females alone during ten generations. in another case, a man thus deficient in both hands and feet, transmitted the peculiarity to his two sons and one daughter; but in the third generation, out of nineteen grandchildren, twelve sons had the family defect, whilst the seven daughters were free. in ordinary cases of sexual limitation, the sons or daughters inherit the peculiarity, whatever it may be, from their father or mother, and transmit it to their children of the same sex; but generally with the hæmorrhagic diathesis, and often with colour-blindness, and in some other cases, the sons never inherit the peculiarity directly from their fathers, but the daughters, and the daughters alone, transmit the latent tendency, so that the sons of the daughters alone exhibit it. thus, the father, grandson, and great-great-grandson will exhibit a peculiarity,--the grandmother, daughter, and great-granddaughter having transmitted it in a latent state. hence we have, as mr. sedgwick remarks, a double kind of atavism or reversion; each grandson apparently receiving and developing the peculiarity from his grandfather, and each daughter apparently receiving the latent tendency from her grandmother. from the various facts recorded by dr. prosper lucas, mr. sedgwick, and others, there can be no doubt that peculiarities first appearing in either sex, though not in any way necessarily or invariably connected with that sex, strongly tend to be inherited by the offspring of the same sex, but are often transmitted in a latent state through the opposite sex. turning now to domesticated animals, we find that certain characters not proper to the parent-species are often confined to, and inherited by, one sex alone; but we do not know the history of the first appearance of such characters. in the chapter on sheep, we have seen that the males of certain races differ greatly from the females in the shape of their horns, these being absent in the ewes of some breeds, in the development of fat in the tail in certain fat-tailed breeds, and in the outline of the forehead. these differences, judging from the character of the allied wild species, cannot be accounted for by supposing that they have been derived from distinct parent-forms. there is, also, a great difference between the horns of the two sexes in one indian breed of goats. the bull zebu is said to have a larger hump than the cow. in the scotch deer-hound the two sexes differ in size more than in any other variety of the dog,[ ] and, judging from analogy, more than in the aboriginal parent-species. the peculiar colour called tortoise-shell is very rarely seen in a male cat; the males of this variety being of a rusty tint. a tendency to baldness in man before the advent of old age is certainly inherited; and in the european, or at least in the { } englishman, is an attribute of the male sex, and may almost be ranked as an incipient secondary sexual character. in various breeds of the fowl the males and females often differ greatly; and these differences are far from being the same with those which distinguish the two sexes in the parent-species, the _gallus bankiva_; and consequently have originated under domestication. in certain sub-varieties of the game race we have the unusual case of the hens differing from each other more than the cocks. in an indian breed of a white colour stained with soot, the hens invariably have black skins, and their bones are covered by a black periosteum, whilst the cocks are never or most rarely thus characterised. pigeons offer a more interesting case; for the two sexes rarely differ throughout the whole great family, and the males and females of the parent-form, the _c. livia_, are undistinguishable; yet we have seen that with pouters the male has the characteristic quality of pouting more strongly developed than the female; and in certain sub-varieties[ ] the males alone are spotted or striated with black. when male and female english carrier-pigeons are exhibited in separate pens, the difference in the development of the wattle over the beak and round the eyes is conspicuous. so that here we have instances of the appearance of secondary sexual characters in the domesticated races of a species in which such differences are naturally quite absent. on the other hand, secondary sexual characters which properly belong to the species are sometimes quite lost, or greatly diminished, under domestication. we see this in the small size of the tusks in our improved breeds of the pig, in comparison with those of the wild boar. there are sub-breeds of fowls in which the males have lost the fine flowing tail-feathers and hackles; and others in which there is no difference in colour between the two sexes. in some cases the barred plumage, which in gallinaceous birds is commonly the attribute of the hen, has been transferred to the cock, as in the cuckoo sub-breeds. in other cases masculine characters have been partly transferred to the female, as with the splendid plumage of the golden-spangled hamburgh hen, the enlarged comb of the spanish hen, the pugnacious disposition of the game hen, and as in the well-developed spurs which occasionally appear in the hens of various breeds. in polish fowls both sexes are ornamented with a topknot, that of the male being formed of hackle-like feathers, and this is a new male character in the genus gallus. on the whole, as far as i can judge, new characters are more apt { } to appear in the males of our domesticated animals than in the females, and afterwards to be either exclusively or more strongly inherited by the males. finally, in accordance with the principle of inheritance as limited by sex, the appearance of secondary sexual characters in natural species offers no especial difficulty, and their subsequent increase and modification, if of any service to the species, would follow through that form of selection which in my 'origin of species' i have called sexual selection. _inheritance at corresponding periods of life._ this is an important subject. since the publication of my 'origin of species,' i have seen no reason to doubt the truth of the explanation there given of perhaps the most remarkable of all the facts in biology, namely, the difference between the embryo and the adult animal. the explanation is, that variations do not necessarily or generally occur at a very early period of embryonic growth, and that such variations are inherited at a corresponding age. as a consequence of this the embryo, even when the parent-form undergoes a great amount of modification, is left only slightly modified; and the embryos of widely-different animals which are descended from a common progenitor remain in many important respects like each other and their common progenitor. we can thus understand why embryology should throw a flood of light on the natural system of classification, for this ought to be as far as possible genealogical. when the embryo leads an independent life, that is, becomes a larva, it has to be adapted to the surrounding conditions in its structure and instincts, independently of those of its parents; and the principle of inheritance at corresponding periods of life renders this possible. this principle is, indeed, in one way so obvious that it escapes attention. we possess a number of races of animals and plants, which, when compared with each other and with their parent-forms, present conspicuous differences, both in the immature and mature states. look at the seeds of the several kinds of peas, beans, maize, which can be propagated truly, and see how they differ in size, colour, and shape, whilst the { } full-grown plants differ but little. cabbages on the other hand differ greatly in foliage and manner of growth, but hardly at all in their seeds; and generally it will be found that the differences between cultivated plants at different periods of growth are not necessarily closely connected together, for plants may differ much in their seeds and little when full-grown, and conversely may yield seeds hardly distinguishable, yet differ much when full-grown. in the several breeds of poultry, descended from a single species, differences in the eggs and chickens, in the plumage at the first and subsequent moults, in the comb and wattles during maturity, are all inherited. with man peculiarities in the milk and second teeth, of which i have received the details, are inheritable, and with man longevity is often transmitted. so again with our improved breeds of cattle and sheep, early maturity, including the early development of the teeth, and with certain breeds of fowl the early appearance of secondary sexual characters, all come under the same head of inheritance at corresponding periods. numerous analogous facts could be given. the silk-moth, perhaps, offers the best instance; for in the breeds which transmit their characters truly, the eggs differ in size, colour, and shape;--the caterpillars differ, in moulting three or four times, in colour, even in having a dark-coloured mark like an eyebrow, and in the loss of certain instincts;--the cocoons differ in size, shape, and in the colour and quality of the silk; these several differences being followed by slight or barely distinguishable differences in the mature moth. but it may be said that, if in the above cases a new peculiarity is inherited, it must be at the corresponding stage of development; for an egg or seed can resemble only an egg or seed, and the horn in a full-grown ox can resemble only a horn. the following cases show inheritance at corresponding periods more plainly, because they refer to peculiarities which might have supervened, as far as we can see, earlier or later in life, yet are inherited at the same period at which they first appeared. in the lambert family the porcupine-like excrescences appeared in the father and sons at the same age, namely, about nine weeks after { } birth.[ ] in the extraordinary hairy family described by mr. crawfurd,[ ] children were produced during three generations with hairy ears; in the father the hair began to grow over his body at six years old; in his daughter somewhat earlier, namely, at one year; and in both generations the milk teeth appeared late in life, the permanent teeth being afterwards singularly deficient. greyness of hair at an unusually early age has been transmitted in some families. these cases border on diseases inherited at corresponding periods of life, to which i shall immediately refer. it is a well-known peculiarity with almond-tumbler pigeons, that the full beauty and peculiar character of the plumage does not appear until the bird has moulted two or three times. neumeister describes and figures a breed of pigeons in which the whole body is white except the breast, neck, and head; but before the first moult all the white feathers acquire coloured edges. another breed is more remarkable: its first plumage is black, with rusty-red wing-bars and a crescent-shaped mark on the breast; these marks then became white, and remain so during three or four moults; but after this period the white spreads over the body, and the bird loses its beauty.[ ] prize canary-birds have their wings and tail black: "this colour, however, is only retained until the first moult, so that they must be exhibited ere the change takes place. once moulted, the peculiarity has ceased. of course all the birds emanating from this stock have black wings and tails the first year."[ ] a curious and somewhat analogous account has been given[ ] of a family of wild pied rooks which were first observed in , near chalfont, and which every year from that date up to the period of the published notice, viz. , "have several of their brood particoloured, black and white. this variegation of the plumage, however, disappears with the first moult; but among the next young families there are always a few pied ones." these changes of plumage, which appear and are inherited at various corresponding periods of life in the pigeon, canary-bird, and rook, are remarkable, because the parent-species undergo no such change. inherited diseases afford evidence in some respects of less value than the foregoing cases, because diseases are not necessarily connected with any change in structure; but in other respects of more value, because the periods have been more carefully observed. certain diseases are communicated to the child apparently by a process like inoculation, and the child is from the first affected; such cases may be here passed over. large classes of diseases usually appear at certain ages, such as st. vitus's dance in youth, consumption in early mid-life, gout later, and apoplexy still later; and these are naturally inherited at the same period. but even in diseases of this class, instances have been recorded, as with st. vitus's { } dance, showing that an unusually early or late tendency to the disease is inheritable.[ ] in most cases the appearance of any inherited disease is largely determined by certain critical periods in each person's life, as well as by unfavourable conditions. there are many other diseases, which are not attached to any particular period, but which certainly tend to appear in the child at about the same age at which the parent was first attacked. an array of high authorities, ancient and modern, could be given in support of this proposition. the illustrious hunter believed in it; and piorry[ ] cautions the physician to look closely to the child at the period when any grave inheritable disease attacked the parent. dr. prosper lucas,[ ] after collecting facts from every source, asserts that affections of all kinds, though not related to any particular period of life, tend to reappear in the offspring at whatever period of life they first appeared in the progenitor. as the subject is important, it may be well to give a few instances, simply as illustrations, not as proof; for proof, recourse must be had to the authorities above quoted. some of the following cases have been selected for the sake of showing that, when a slight departure from the rule occurs, the child is affected somewhat earlier in life than the parent. in the family of le compte blindness was inherited during three generations, and no less than thirty-seven children and grandchildren were all affected at about the same age, namely seventeen or eighteen.[ ] in another case a father and his four children all became blind at twenty-one years old; in another, a grandmother grew blind at thirty-five, her daughter at nineteen, and three grandchildren at the ages of thirteen and eleven.[ ] so with deafness, two brothers, their father and paternal grandfather, all became deaf at the age of forty.[ ] esquirol gives several striking instances of insanity coming on at the same age, as that of a grandfather, father, and son, who all committed suicide near their fiftieth year. many other cases could be given, as of a whole family who became insane at the age of forty.[ ] other cerebral affections sometimes follow the same rule,--for instance, epilepsy and apoplexy. a woman died of the latter disease when sixty-three years old; one of her daughters at forty-three, and the other at sixty-seven: the latter had twelve children, who all died from tubercular meningitis.[ ] i mention this latter case because it illustrates a frequent occurrence, namely, a change in the precise nature of an inherited disease, though still affecting the same organ. { } asthma has attacked several members of the same family when forty years old, and other families during infancy. the most different diseases, as angina pectoris, stone in the bladder, and various affections of the skin, have appeared in successive generations at nearly the same age. the little finger of a man began from some unknown cause to grow inwards, and the same finger in his two sons began at the same age to bend inwards in a similar manner. strange and inexplicable neuralgic affections have caused parents and children to suffer agonies at about the same period of life.[ ] i will give only two other cases, which are interesting as illustrating the disappearance as well as the appearance of disease at the same age. two brothers, their father, their paternal uncles, seven cousins, and their paternal grandfather, were all similarly affected by a skin-disease, called pityriasis versicolor; "the disease, strictly limited to the males of the family (though transmitted through the females), usually appeared at puberty, and disappeared at about the age of forty or forty-five years." the second case is that of four brothers, who when about twelve years old suffered almost every week from severe headaches, which were relieved only by a recumbent position in a dark room. their father, paternal uncles, paternal grandfather, and paternal granduncles all suffered in the same way from headaches, which ceased at the age of fifty-four or fifty-five in all those who lived so long. none of the females of the family were affected.[ ] it is impossible to read the foregoing accounts, and the many others which have been recorded, of diseases coming on during three or even more generations, at the same age in several members of the same family, especially in the case of rare affections in which the coincidence cannot be attributed to chance, and doubt that there is a strong tendency to inheritance in disease at corresponding periods of life. when the rule fails, the disease is apt to come on earlier in the child than in the parent; the exceptions in the other direction being vey much rarer. dr. lucas[ ] alludes to several cases of inherited diseases coming on at an earlier period. i have already given one striking instance with blindness during three generations; and mr. bowman remarks that this frequently occurs with cataract. with cancer there seems to be a peculiar liability to earlier inheritance: mr. paget, who has particularly { } attended to this subject, and tabulated a large number of cases, informs me that he believes that in nine cases out of ten the later generation suffers from the disease at an earlier period than the previous generation. he adds, "in the instances in which the opposite relation holds, and the members of later generations have cancer at a later age than their predecessors, i think it will be found that the non-cancerous parents have lived to extreme old ages." so that the longevity of a non-affected parent seems to have the power of determining in the offspring the fatal period; and we thus apparently get another element of complexity in inheritance. the facts, showing that with certain diseases the period of inheritance occasionally or even frequently advances, are important with respect to the general descent-theory, for they render it in some degree probable that the same thing would occur with ordinary modifications of structure. the final result of a long series of such advances would be the gradual obliteration of characters proper to the embryo and larva, which would thus come to resemble more and more closely the mature parent-form. but any structure which was of service to the embryo or larva would be preserved by the destruction at this stage of growth of each individual which manifested any tendency to lose at too early an age its own proper character. finally, from the numerous races of cultivated plants and domestic animals, in which the seed or eggs, the young or old, differ from each other and from their parent-species;--from the cases in which new characters have appeared at a particular period, and afterwards have been inherited at the same period;--and from what we know with respect to disease, we must believe in the truth of the great principle of inheritance at corresponding periods of life. * * * * * _summary of the three preceding chapters._--strong as is the force of inheritance, it allows the incessant appearance of new characters. these, whether beneficial or injurious, of the most trifling importance, such as a shade of colour in a flower, a coloured lock of hair, or a mere gesture; or of the highest importance, as when affecting the brain or an organ so perfect { } and complex as the eye; or of so grave a nature as to deserve to be called a monstrosity, or so peculiar as not to occur normally in any member of the same natural class, are all sometimes strongly inherited by man, the lower animals, and plants. in numberless cases it suffices for the inheritance of a peculiarity that one parent alone should be thus characterised. inequalities in the two sides of the body, though opposed to the law of symmetry, may be transmitted. there is a considerable body of evidence showing that even mutilations, and the effects of accidents, especially or perhaps exclusively when followed by disease, are occasionally inherited. there can be no doubt that the evil effects of long-continued exposure in the parent to injurious conditions are sometimes transmitted to the offspring. so it is, as we shall see in a future chapter, with the effects of the use and disuse of parts, and of mental habits. periodical habits are likewise transmitted, but generally, as it would appear, with little force. hence we are led to look at inheritance as the rule, and non-inheritance as the anomaly. but this power often appears to us in our ignorance to act capriciously, transmitting a character with inexplicable strength or feebleness. the very same peculiarity, as the weeping habit of trees, silky-feathers, &c., may be inherited either firmly or not at all by different members of the same group, and even by different individuals of the same species, though treated in the same manner. in this latter case we see that the power of transmission is a quality which is merely individual in its attachment. as with single characters, so it is with the several concurrent slight differences which distinguish sub-varieties or races; for of these, some can be propagated almost as truly as species, whilst others cannot be relied on. the same rule holds good with plants, when propagated by bulbs, offsets, &c., which in one sense still form parts of the same individual, for some varieties retain or inherit through successive bud-generations their character far more truly than others. some characters not proper to the parent-species have certainly been inherited from an extremely remote epoch, and may therefore be considered as firmly fixed. but it is doubtful whether length of inheritance in itself gives fixedness of character; { } though the chances are obviously in favour of any character which has long been transmitted true or unaltered, still being transmitted true as long as the conditions of life remain the same. we know that many species, after having retained the same character for countless ages, whilst living under their natural conditions, when domesticated have varied in the most diversified manner, that is, have failed to transmit their original form; so that no character appears to be absolutely fixed. we can sometimes account for the failure of inheritance by the conditions of life being opposed to the development of certain characters; and still oftener, as with plants cultivated by grafts and buds, by the conditions causing new and slight modifications incessantly to appear. in this latter case it is not that inheritance wholly fails, but that new characters are continually superadded. in some few cases, in which both parents are similarly characterised, inheritance seems to gain so much force by the combined action of the two parents, that it counteracts its own power, and a new modification is the result. in many cases the failure of the parents to transmit their likeness is due to the breed having been at some former period crossed; and the child takes after his grandparent or more remote ancestor of foreign blood. in other cases, in which the breed has not been crossed, but some ancient character has been lost through variation, it occasionally reappears through reversion, so that the parents apparently fail to transmit their own likeness. in all cases, however, we may safely conclude that the child inherits all its characters from its parents, in whom certain characters are latent, like the secondary sexual characters of one sex in the other. when, after a long succession of bud-generations, a flower or fruit becomes separated into distinct segments, having the colours or other attributes of both parent-forms, we cannot doubt that these characters were latent in the earlier buds, though they could not then be detected, or could be detected only in an intimately commingled state. so it is with animals of crossed parentage, which with advancing years occasionally exhibit characters derived from one of their two parents, of which not a trace could at first be perceived. certain monstrosities, which resemble what naturalists call the typical form of the group in question, { } apparently come under the same law of reversion. it is assuredly an astonishing fact that the male and female sexual elements, that buds, and even full-grown animals, should retain characters, during several generations in the case of crossed breeds, and during thousands of generations in the case of pure breeds, written as it were in invisible ink, yet ready at any time to be evolved under the requisite conditions. what these conditions are, we do not in many cases at all know. but the act of crossing in itself, apparently from causing some disturbance in the organisation, certainly gives a strong tendency to the reappearance of long-lost characters, both corporeal and mental, independently of those derived from the cross. a return of any species to its natural conditions of life, as with feral animals and plants, favours reversion; though it is certain that this tendency exists, we do not know how far it prevails, and it has been much exaggerated. on the other hand, the crossed offspring of plants which have had their organisation disturbed by cultivation, are more liable to reversion than the crossed offspring of species which have always lived under their natural conditions. when distinguishable individuals of the same family, or races, or species are crossed, we see that the one is often prepotent over the other in transmitting its own character. a race may possess a strong power of inheritance, and yet when crossed, as we have seen with trumpeter-pigeons, yield to the prepotency of every other race. prepotentcy of transmission may be equal in the two sexes of the same species, but often runs more strongly in one sex. it plays an important part in determining the rate at which one race can be modified or wholly absorbed by repeated crosses with another. we can seldom tell what makes one race or species prepotent over another; but it sometimes depends on the same character being present and visible in one parent, and latent or potentially present in the other. characters may first appear in either sex, but oftener in the male than in the female, and afterwards be transmitted to the offspring of the same sex. in this case we may feel confident that the peculiarity in question is really present though latent in the opposite sex; hence the father may transmit through his daughter any character to his grandson; and the mother { } conversely to her granddaughter. we thus learn, and the fact is an important one, that transmission and development are distinct powers. occasionally these two powers seem to be antagonistic, or incapable of combination in the same individual; for several cases have been recorded in which the son has not directly inherited a character from his father, or directly transmitted it to his son, but has received it by transmission through his non-affected mother, and transmitted it through his non-affected daughter. owing to inheritance being limited by sex, we can see how secondary sexual characters may first have arisen under nature; their preservation and accumulation being dependent on their service to either sex. at whatever period of life a new character first appears, it generally remains latent in the offspring until a corresponding age is attained, and then it is developed. when this rule fails, the child generally exhibits the character at an earlier period than the parent. on this principle of inheritance at corresponding periods, we can understand how it is that most animals display from the germ to maturity such a marvellous succession of characters. finally, though much remains obscure with respect to inheritance, we may look at the following laws as fairly well established. firstly, a tendency in every character, new and old, to be transmitted by seminal and bud generation, though often counteracted by various known and unknown causes. secondly, reversion or atavism, which depends on transmission and development being distinct powers: it acts in various degrees and manners through both seminal and bud generation. thirdly, prepotency of transmission, which may be confined to one sex, or be common to both sexes of the prepotent form. fourthly, transmission, limited by sex, generally to the same sex in which the inherited character first appeared. fifthly, inheritance at corresponding periods of life, with some tendency to the earlier development of the inherited character. in these laws of inheritance, as displayed under domestication, we see an ample provision for the production, through variability and natural selection, of new specific forms. * * * * * { } chapter xv. on crossing. free intercrossing obliterates the differences between allied breeds--when the numbers of two commingling breeds are unequal, one absorbs the other--the rate of absorption determined by prepotency of transmission, by the conditions of life, and by natural selection--all organic beings occasionally intercross; apparent exceptions--on certain characters incapable of fusion; chiefly or exclusively those which have suddenly appeared in the individual--on the modification of old races, and the formation of new races, by crossing--some crossed races have bred true from their first production--on the crossing of distinct species in relation to the formation of domestic races. in the two previous chapters, when discussing reversion and prepotency, i was necessarily led to give many facts on crossing. in the present chapter i shall consider the part which crossing plays in two opposed directions,--firstly, in obliterating characters, and consequently in preventing the formation of new races; and secondly, in the modification of old races, or in the formation of new and intermediate races, by a combination of characters. i shall also show that certain characters are incapable of fusion. the effects of free or uncontrolled breeding between the members of the same variety or of closely allied varieties are important; but are so obvious that they need not be discussed at much length. it is free intercrossing which chiefly gives uniformity, both under nature and under domestication, to the individuals of the same species or variety, when they live mingled together and are not exposed to any cause inducing excessive variability. the prevention of free crossing, and the intentional matching of individual animals, are the corner-stones of the breeder's art. no man in his senses would expect to improve or modify a breed in any particular manner, or keep an old breed true and distinct, unless he separated his animals. the killing of inferior animals in each generation comes to the { } same thing as their separation. in savage and semi-civilised countries, where the inhabitants have not the means of separating their animals, more than a single breed of the same species rarely or never exists. in former times, even in a country so civilised as north america, there were no distinct races of sheep, for all had been mingled together.[ ] the celebrated agriculturist marshall[ ] remarks that "sheep that are kept within fences, as well as shepherded flocks in open countries, have generally a similarity, if not a uniformity, of character in the individuals of each flock;" for they breed freely together, and are prevented from crossing with other kinds; whereas in the unenclosed parts of england the unshepherded sheep, even of the same flock, are far from true or uniform, owing to various breeds having mingled and crossed. we have seen that the half-wild cattle in the several british parks are uniform in character in each; but in the different parks, from not having mingled and crossed during many generations, they differ in a slight degree. we cannot doubt that the extraordinary number of varieties and sub-varieties of the pigeon, amounting to at least one hundred and fifty, is partly due to their remaining, differently from other domesticated birds, paired for life when once matched. on the other hand, breeds of cats imported into this country soon disappear, for their nocturnal and rambling habits render it hardly possible to prevent free crossing. rengger[ ] gives an interesting case with respect to the cat in paraguay: in all the distant parts of the kingdom it has assumed, apparently from the effects of the climate, a peculiar character, but near the capital this change has been prevented, owing, as he asserts, to the native animal frequently crossing with cats imported from europe. in all cases like the foregoing, the effects of an occasional cross will be augmented by the increased vigour and fertility of the crossed offspring, of which fact evidence will hereafter be given; for this will lead to the mongrels increasing more rapidly than the pure parent-breeds. { } when distinct breeds are allowed to cross freely, the result will be a heterogenous body; for instance, the dogs in paraguay are far from uniform, and can no longer be affiliated to their parent-races.[ ] the character which a crossed body of animals will ultimately assume must depend on several contingencies,--namely, on the relative numbers of the individuals belonging to the two or more races which are allowed to mingle; on the prepotency of one race over the other in the transmission of character; and on the conditions of life to which they are exposed. when two commingled breeds exist at first in nearly equal numbers, the whole will sooner or later become intimately blended, but not so soon, both breeds being equally favoured in all respects, as might have been expected. the following calculation[ ] shows that this is the case: if a colony with an equal number of black and white men were founded, and we assume that they marry indiscriminately, are equally prolific, and that one in thirty annually dies and is born; then "in years the number of blacks, whites, and mulattoes would be equal. in years the whites would be - th, the blacks - th, and the mulattoes, or people of intermediate degrees of colour, - ths of the whole number. in three centuries not - th part of the whites would exist." when one of two mingled races exceeds the other greatly in number, the latter will soon be wholly, or almost wholly, absorbed and lost.[ ] thus european pigs and dogs have been largely introduced into the islands of the pacific ocean, and the native races have been absorbed and lost in the course of about fifty or sixty years;[ ] but the imported races no doubt were favoured. rats may be considered as semi-domesticated animals. some snake-rats (_mus alexandrinus_) escaped in the zoological gardens of london, "and for a long time afterwards the keepers frequently caught cross-bred rats, at first half-breds, afterwards with less and less of the character of the snake-rat, till at length all traces of it disappeared."[ ] on the other hand, { } in some parts of london, especially near the docks, where fresh rats are frequently imported, an endless variety of intermediate forms may be found between the brown, black, and snake rat, which are all three usually ranked as distinct species. how many generations are necessary for one species or race to absorb another by repeated crosses has often been discussed;[ ] and the requisite number has probably been much exaggerated. some writers have maintained that a dozen, or score, or even more generations, are necessary; but this in itself is improbable, for in the tenth generation there will be only - th part of foreign blood in the offspring. gärtner found,[ ] that with plants one species could be made to absorb another in from three to five generations, and he believes that this could always be effected in from six to seventh generations. in one instance, however, kölreuter[ ] speaks of the offspring of _mirabilis vulgaris_, crossed during eight successive generations by _m. longiflora_, as resembling this latter species so closely, that the most scrupulous observer could detect "vix aliquam notabilem differentiam;"--he succeeded, as he says, "ad plenariam fere transmutationem." but this expression shows that the act of absorption was not even then absolutely complete, though these crossed plants contained only the - th part of _m. vulgaris_. the conclusions of such accurate observers as gärtner and kölreuter are of far higher worth than those made without scientific aim by breeders. the most remarkable statement which i have met with of the persistent endurance of the effects of a single cross is given by fleischmann,[ ] who, in reference to german sheep, says "that the original coarse sheep have fibres of wool on a square inch; grades of the third or fourth merino cross produced about , the twentieth cross , , the perfect pure merino blood , to , ." so that in this case common german sheep crossed twenty times successively with merinos have not by any means acquired wool as fine as that of the pure breed. in all cases, the rate of absorption will { } depend largely on the conditions of life being favourable to any particular character; and we may suspect that there would be under the climate of germany a constant tendency to degeneration in the wool of merinos, unless prevented by careful selection; and thus perhaps the foregoing remarkable case may be explained. the rate of absorption must also depend on the amount of distinguishable difference between the two forms which are crossed, and especially, as gärtner insists, on prepotency of transmission in the one form over the other. we have seen in the last chapter that one of two french breeds of sheep yielded up its character, when crossed with merinos, very much slower than the other; and the common german sheep referred to by fleischmann may present an analogous case. but in all cases there will be during many subsequent generations more or less liability to reversion, and it is this fact which has probably led authors to maintain that a score or more of generations are requisite for one race to absorb another. in considering the final result of the commingling of two or more breeds, we must not forget that the act of crossing in itself tends to bring back long-lost characters not proper to the immediate parent-forms. with respect to the influence of the conditions of life on any two breeds which are allowed to cross freely, unless both are indigenous and have long been accustomed to the country where they live, they will, in all probability, be unequally affected by the conditions, and this will modify the result. even with indigenous breeds, it will rarely or never occur that both are equally well adapted to the surrounding circumstances; more especially when permitted to roam freely, and not carefully tended, as will generally be the case with breeds allowed to cross. as a consequence of this, natural selection will to a certain extent come into action, and the best fitted will survive, and this will aid in determining the ultimate character of the commingled body. how long a time it would require before such a crossed body of animals would assume within a limited area a uniform character no one can say; that they would ultimately become uniform from free intercrossing, and from the survival of the fittest, we may feel assured; but the character thus acquired would rarely or never, as we may infer from the several previous { } considerations, be exactly intermediate between that of the two parent-breeds. with respect to the very slight differences by which the individuals of the same sub-variety, or even of allied varieties, are characterised, it is obvious that free crossing would soon obliterate such small distinctions. the formation of new varieties, independently of selection, would also thus be prevented; except when the same variation continually recurred from the action of some strongly predisposing cause. hence we may conclude that free crossing has in all cases played an important part in giving to all the members of the same domestic race, and of the same natural species, uniformity of character, though largely modified by natural selection and by the direct action of the surrounding conditions. _on the possibility of all organic beings occasionally intercrossing._--but it may be asked, can free crossing occur with hermaphrodite animals and plants? all the higher animals, and the few insects which have been domesticated, have separated sexes, and must inevitably unite for each birth. with respect to the crossing of hermaphrodites, the subject is too large for the present volume, and will be more properly treated in a succeeding work. in my 'origin of species,' however, i have given a short abstract of the reasons which induce me to believe that all organic beings occasionally cross, though perhaps in some cases only at long intervals of time.[ ] i will here just recall the fact that many plants, though hermaphrodite in structure, are unisexual in function;--such as those called by c. k. sprengel _dichogamous_, in which the pollen and stigma of the same flower are matured at different periods; or those called by me _reciprocally dimorphic_, in which the flower's own pollen is not fitted to fertilise its own stigma; or again, the many kinds in which curious mechanical contrivances exist, effectually preventing self-fertilisation. there are, however, many hermaphrodite plants which are not in any way specially constructed to favour intercrossing, but which nevertheless commingle almost as freely as animals with separated sexes. this is the case with cabbages, radishes, and onions, as i know from { } having experimented on them: even the peasants of liguria say that cabbages must be prevented "from falling in love" with each other. in the orange tribe, gallesio[ ] remarks that the amelioration of the various kinds is checked by their continual and almost regular crossing. so it is with numerous other plants. nevertheless some cultivated plants can be named which rarely intercross, as the common pea, or which never intercross, as i have reason to believe is the case with the sweet-pea (_lathyrus odoratus_); yet the structure of these flowers certainly favours an occasional cross. the varieties of the tomato and aubergine (_solanum_) and pimenta (_pimenta vulgaris?_) are said[ ] never to cross, even when growing alongside each other. but it should be observed that these are all exotic plants, and we do not know how they would behave in their native country when visited by the proper insects. it must also be admitted that some few natural species appear under our present state of knowledge to be perpetually self-fertilised, as in the case of the bee ophrys (_o. apifera_), though adapted in its structure to be occasionally crossed. the _leersia oryzoides_ produces minute enclosed flowers which cannot possibly be crossed, and these alone, to the exclusion of the ordinary flowers, have as yet been known to yield seed.[ ] a few additional and analogous cases could be advanced. but these facts do not make me doubt that it is a general law of nature that the individuals of the same species occasionally intercross, and that some great advantage is derived from this act. it is well known (and i shall hereafter have to give instances) that some plants, both indigenous and naturalised, rarely or never produce flowers; or, if they flower, never produce seeds. but no one is thus led to doubt that it is a general law of nature that phanerogamic plants should produce flowers, and that these flowers should produce seed. when they fail, we believe that such plants would perform their proper functions under different conditions, or that they formerly did so and will do so again. on analogous grounds, i believe that the few flowers { } which do not now intercross, either would do so under different conditions, or that they formerly fertilised each other at intervals--the means for effecting this being generally still retained--and they will do so again at some future period, unless indeed they become extinct. on this view alone, many points in the structure and action of the reproductive organs in hermaphrodite plants and animals are intelligible,--for instance, the male and female organs never being so completely enclosed as to render access from without impossible. hence we may conclude that the most important of all the means for giving uniformity to the individuals of the same species, namely, the capacity of occasionally intercrossing, is present, or has been formerly present, with all organic beings. _on certain characters not blending._--when two breeds are crossed their characters usually become intimately fused together; but some characters refuse to blend, and are transmitted in an unmodified state either from both parents or from one. when grey and white mice are paired, the young are not piebald nor of an intermediate tint, but are pure white or of the ordinary grey colour: so it is when white and common collared turtle-doves are paired. in breeding game fowls, a great authority, mr. j. douglas, remarks, "i may here state a strange fact: if you cross a black with a white game, you get birds of both breeds of the clearest colour." sir r. heron crossed during many years white, black, brown, and fawn-coloured angora rabbits, and never once got these colours mingled in the same animal, but often all four colours in the same litter.[ ] additional cases could be given, but this form of inheritance is very far from universal even with respect to the most distinct colours. when turnspit dogs and ancon sheep, both of which have dwarfed limbs, are crossed with common breeds, the offspring are not intermediate in structure, but take after either parent. when tailless or hornless animals are crossed with perfect animals, it frequently, but by no means invariably, happens that the offspring are { } either perfectly furnished with these organs or are quite destitute of them. according to rengger, the hairless condition of the paraguay dog is either perfectly or not at all transmitted to its mongrel offspring; but i have seen one partial exception in a dog of this parentage which had part of its skin hairy, and part naked; the parts being distinctly separated as in a piebald animal. when dorking fowls with five toes are crossed with other breeds, the chickens often have five toes on one foot and four on the other. some crossed pigs raised by sir r. heron between the solid-hoofed and common pig had not all four feet in an intermediate condition, but two feet were furnished with properly divided, and two with united hoofs. analogous facts have been observed with plants: major trevor clarke crossed the little, glabrous-leaved, annual stock (_matthiola_), with pollen of a large, red-flowered, rough-leaved, biennial stock, called _cocardeau_ by the french, and the result was that half the seedlings had glabrous and the other half rough leaves, but none had leaves in an intermediate state. that the glabrous seedlings were the product of the rough-leaved variety, and not accidentally of the mother-plant's own pollen, was shown by their tall and strong habit of growth.[ ] in the succeeding generations raised from the rough-leaved crossed seedlings, some glabrous plants appeared, showing that the glabrous character, though incapable of blending with and modifying the rough leaves, was all the time latent in this family of plants. the numerous plants formerly referred to, which i raised from reciprocal crosses between the peloric and common antirrhinum, offer a nearly parallel case; for in the first generation all the plants resembled the common form, and in the next generation, out of one hundred and thirty-seven plants, two alone were in an intermediate condition, the others perfectly resembling either the peloric or common form. major trevor clarke also fertilised the above-mentioned red-flowered stock with pollen from the purple queen stock, and about half the seedlings scarcely differed in habit, and not at all in the red colour of the flower, from the mother-plant, the other half bearing blossoms of a rich purple, closely like those of the paternal plant. gärtner crossed many white and yellow-flowered species and varieties of verbascum; and these colours were never blended, but the offspring bore either pure white or pure yellow blossoms; the former in the larger proportion.[ ] dr. herbert raised many seedlings, as he informed me, from swedish turnips crossed by two other varieties, and these never produced flowers of an intermediate tint, but always like one of their parents. i fertilised the purple sweet-pea (_lathyrus odoratus_), which has a dark reddish-purple standard-petal and violet-coloured wings and keel, with pollen of the painted-lady sweet-pea, which has a pale cherry-coloured standard, and almost white wings and keel; and from the same pod i twice raised plants perfectly resembling both sorts; the greater number resembling the father. so perfect was the resemblance, that i should have thought there had { } been some mistake, if the plants which were at first identical with the paternal variety, namely, the painted-lady, had not later in the season produced, as mentioned in a former chapter, flowers blotched and streaked with dark purple. i raised grandchildren and great-grandchildren from these crossed plants, and they continued to resemble the painted-lady, but during the later generations became rather more blotched with purple, yet none reverted completely to the original mother-plant, the purple sweet-pea. the following case is slightly different, but still shows the same principle: naudin[ ] raised numerous hybrids between the yellow _linaria vulgaris_ and the purple _l. purpurea_, and during three successive generations the colours kept distinct in different parts of the same flower. from such cases as the foregoing, in which the offspring of the first generation perfectly resemble either parent, we come by a small step to those cases in which differently coloured flowers borne on the same root resemble both parents, and by another step to those in which the same flower or fruit is striped or blotched with the two parental colours, or bears a single stripe of the colour or other characteristic quality of one of the parent-forms. with hybrids and mongrels it frequently or even generally happens that one part of the body resembles more or less closely one parent and another part the other parent; and here again some resistance to fusion, or, what comes to the same thing, some mutual affinity between the organic atoms of the same nature, apparently comes into play, for otherwise all parts of the body would be equally intermediate in character. so again, when the offspring of hybrids or mongrels, which are themselves nearly intermediate in character, revert either wholly or by segments to their ancestors, the principle of the affinity of similar, or the repulsion of dissimilar atoms, must come into action. to this principle, which seems to be extremely general, we shall recur in the chapter on pangenesis. it is remarkable, as has been strongly insisted upon by isidore geoffroy st. hilaire in regard to animals, that the transmission of characters without fusion occurs most rarely when species are crossed; i know of one exception alone, namely, with the hybrids naturally produced between the common and hooded crow (_corvus corone_ and _cornix_), which, however, are closely allied species, differing in nothing except colour. nor have i met with any well-ascertained cases of transmission of this kind, even when one form is strongly prepotent over another, when two races are crossed which have been slowly formed by man's selection, and therefore resemble to a certain extent natural species. such cases as puppies in the same litter closely resembling two distinct breeds, are probably due to super-foetation,--that is, to the influence of two fathers. all the characters above enumerated, which are transmitted in a perfect state to some of the offspring and not to others,--such as distinct colours, nakedness of skin, smoothness of leaves, absence of horns or tail, additional toes, pelorism, dwarfed structure, &c.,--have all been known to appear suddenly in individual animals and plants. from this fact, and from the several slight, aggregated differences which distinguish domestic races and species from { } each other, not being liable to this peculiar form of transmission, we may conclude that it is in some way connected with the sudden appearance of the characters in question. _on the modification of old races and the formation of new races by crossing._--we have hitherto chiefly considered the effects of crossing in giving uniformity of character; we must now look to an opposite result. there can be no doubt that crossing, with the aid of rigorous selection during several generations, has been a potent means in modifying old races, and in forming new ones. lord orford crossed his famous stud of greyhounds once with the bulldog, which breed was chosen from being deficient in scenting powers, and from having what was wanted, courage and perseverance. in the course of six or seven generations all traces of the external form of the bulldog were eliminated, but courage and perseverance remained. certain pointers have been crossed, as i hear from the rev. w. d. fox, with the foxhound, to give them dash and speed. certain strains of dorking fowls have had a slight infusion of game blood; and i have known a great fancier who on a single occasion crossed his turbit-pigeons with barbs, for the sake of gaining greater breadth of beak. in the foregoing cases breeds have been crossed once, for the sake of modifying some particular character; but with most of the improved races of the pig, which now breed true, there have been repeated crosses,--for instance, the improved essex owes its excellence to repeated crosses with the neapolitan, together probably with some infusion of chinese blood.[ ] so with our british sheep: almost all the races, except the southdown, have been largely crossed; "this, in fact, has been the history of our principal breeds."[ ] to give an example, the "oxfordshire downs" now rank as an established breed.[ ] they were produced about the year by crossing "hampshire and in some instances southdown ewes with cotswold rams:" now the hampshire ram was itself produced by repeated crosses between the native { } hampshire sheep and southdowns; and the long-woolled cotswold were improved by crosses with the leicester, which latter again is believed to have been a cross between several long-woolled sheep. mr. spooner, after considering the various cases which have been carefully recorded, concludes "that from a judicious pairing of cross-bred animals it is practicable to establish a new breed." on the continent the history of several crossed races of cattle and of other animals has been well ascertained. to give one instance: the king of wurtemberg, after twenty-five years' careful breeding, that is after six or seven generations, made a new breed of cattle from a cross between a dutch and swiss breed, combined with other breeds.[ ] the sebright bantam, which breeds as true as any other kind of fowl, was formed about sixty years ago by a complicated cross.[ ] dark brahmas, which are believed by some fanciers to constitute a distinct species, were undoubtedly formed[ ] in the united states, within a recent period, by a cross between chittagongs and cochins. with plants i believe there is little doubt that some kinds of turnips, now extensively cultivated, are crossed races; and the history of a variety of wheat which was raised from two very distinct varieties, and which after six years' culture presented an even sample, has been recorded on good authority.[ ] until quite lately, cautious and experienced breeders, though not averse to a single infusion of foreign blood, were almost universally convinced that the attempt to establish a new race, intermediate between two widely distinct races, was hopeless: "they clung with superstitious tenacity to the doctrine of purity of blood, believing it to be the ark in which alone true safety could be found."[ ] nor was this conviction unreasonable: when two distinct races are crossed, the offspring of the first generation are generally nearly uniform in character; but even this sometimes fails to be the case, especially with crossed dogs and fowls, the young of which from the first are sometimes much { } diversified. as cross-bred animals are generally of large size and vigorous, they have been raised in great numbers for immediate consumption. but for breeding they are found to be utterly useless; for though they may be themselves uniform in character, when paired together they yield during many generations offspring astonishingly diversified. the breeder is driven to despair, and concludes that he will never form an intermediate race. but from the cases already given, and from others which have been recorded, it appears that patience alone is necessary; as mr. spooner remarks, "nature opposes no barrier to successful admixture; in the course of time, by the aid of selection and careful weeding, it is practicable to establish a new breed." after six or seven generations the hoped-for result will in most cases be obtained; but even then an occasional reversion, or failure to keep true, may be expected. the attempt, however, will assuredly fail if the conditions of life be decidedly unfavourable to the characters of either parent-breed.[ ] although the grandchildren and succeeding generations of cross-bred animals are generally variable in an extreme degree, some curious exceptions to the rule have been observed, both with crossed races and species. thus boitard and corbié[ ] assert that from a pouter and a runt "a cavalier will appear, which we have classed amongst pigeons of pure race, because it transmits all its qualities to its posterity." the editor of the 'poultry chronicle'[ ] bred some bluish fowls from a black spanish cock and a malay hen; and these remained true to colour "generation after generation." the himalayan breed of rabbits was certainly formed by crossing two sub-varieties of the silver-grey rabbit; although it suddenly assumed its present character, which differs much from that of either parent-breed, yet it has ever since been easily and truly propagated. i crossed some labrador and penguin ducks, and recrossed the mongrels with penguins; afterwards, most of the ducks reared during three generations were nearly uniform in character, being brown with a white crescentic mark on the lower part of the breast, { } and with some white spots at the base of the beak; so that by the aid of a little selection a new breed might easily have been formed. in regard to crossed varieties of plants, mr. beaton remarks[ ] that "melville's extraordinary cross between the scotch kale and an early cabbage is as true and genuine as any on record;" but in this case no doubt selection was practised. gärtner[ ] has given five cases of hybrids, in which the progeny kept constant; and hybrids between _dianthus armeria_ and _deltoides_ remained true and uniform to the tenth generation. dr. herbert likewise showed me a hybrid from two species of loasa which from its first production had kept constant during several generations. we have seen in the earlier chapters, that some of our domesticated animals, such as dogs, cattle, pigs, &c., are almost certainly descended from more than one species, or wild race, if any one prefers to apply this latter term to forms which were enabled to keep distinct in a state of nature. hence the crossing of aboriginally distinct species probably came into play at an early period in the formation of our present races. from rütimeyer's observations there can be little doubt that this occurred with cattle; but in most cases some one of the forms which were allowed to cross freely, will, it is probable, have absorbed and obliterated the others. for it is not likely that semi-civilized men would have taken the necessary pains to modify by selection their commingled, crossed, and fluctuating stock. nevertheless, those animals which were best adapted to their conditions of life would have survived through natural selection; and by this means crossing will often have indirectly aided in the formation of primeval domesticated breeds. within recent times, as far as animals are concerned, the crossing of distinct species has done little or nothing in the formation or modification of our races. it is not yet known whether the species of silk-moth which have been recently crossed in france will yield permanent races. in the fourth chapter i alluded with some hesitation to the statement that a new breed, between the hare and rabbit, called leporides, had been formed in france, and was found capable of propagating { } itself; but it is now positively affirmed[ ] that this is an error. with plants which can be multiplied by buds and cuttings, hybridisation has done wonders, as with many kinds of roses, rhododendrons, pelargoniums, calceolarias, and petunias. nearly all these plants can be propagated by seed; most of them freely; but extremely few or none come true by seed. some authors believe that crossing is the chief cause of variability,--that is, of the appearance of absolutely new characters. some have gone so far as to look at it as the sole cause; but this conclusion is disproved by some of the facts given in the chapter on bud-variation. the belief that characters not present in either parent or in their ancestors frequently originate from crossing is doubtful; that they occasionally thus arise is probable; but this subject will be more conveniently discussed in a future chapter on the causes of variability. a condensed summary of this and of the three following chapters, together with some remarks on hybridism, will be given in the nineteenth chapter. * * * * * { } chapter xvi. causes which interfere with the free crossing of varieties--influence of domestication on fertility. difficulties in judging of the fertility of varieties when crossed--various causes which keep varieties distinct, as the period of breeding and sexual preference--varieties of wheat said to be sterile when crossed--varieties of maize, verbascum, hollyhock, gourds, melons, and tobacco, rendered in some degree mutually sterile--domestication eliminates the tendency to sterility natural to species when crossed--on the increased fertility of uncrossed animals and plants from domestication and cultivation. the domesticated races of both animals and plants, when crossed, are with extremely few exceptions quite prolific,--in some cases even more so than the purely bred parent-races. the offspring, also, raised from such crosses are likewise, as we shall see in the following chapter, generally more vigorous and fertile than their parents. on the other hand, species when crossed, and their hybrid offspring, are almost invariability in some degree sterile; and here there seems to exist a broad and insuperable distinction between races and species. the importance of this subject as bearing on the origin of species is obvious; and we shall hereafter recur to it. it is unfortunate how few precise observations have been made on the fertility of mongrel animals and plants during several successive generations. dr. broca[ ] has remarked that no one has observed whether, for instance, mongrel dogs, bred _inter se_, are indefinitely fertile; yet, if a shade of infertility be detected by careful observation in the offspring of natural forms when crossed, it is thought that their specific distinction is proved. but so many breeds of sheep, cattle, pigs, dogs, and poultry, have been crossed and recrossed in various ways, that any sterility, if it had existed, would from being injurious { } almost certainly have been observed. in investigating the fertility of crossed varieties many sources of doubt occur. whenever the least trace of sterility between two plants, however closely allied, was observed by kölreuter, and more especially by gärtner, who counted the exact number of seed in each capsule, the two forms were at once ranked as distinct species; and if this rule be followed, assuredly it will never be proved that varieties when crossed are in any degree sterile. we have formerly seen that certain breeds of dogs do not readily pair together; but no observations have been made whether, when paired, they produce the full number of young, and whether the latter are perfectly fertile _inter se_; but, supposing that some degree of sterility were found to exist, naturalists would simply infer that these breeds were descended from aboriginally distinct species; and it would be scarcely possible to ascertain whether or not this explanation was the true one. the sebright bantam is much less prolific than any other breed of fowls, and is descended from a cross between two very distinct breeds, recrossed by a third sub-variety. but it would be extremely rash to infer that the loss of fertility was in any manner connected with its crossed origin, for it may with more probability be attributed either to long-continued close interbreeding, or to an innate tendency to sterility correlated with the absence of hackles and sickle tail-feathers. before giving the few recorded cases of forms, which must be ranked as varieties, being in some degree sterile when crossed, i may remark that other causes sometimes interfere with varieties freely intercrossing. thus they may differ too greatly in size, as with some kinds of dogs and fowls: for instance, the editor of the 'journal of horticulture, &c.,'[ ] says that he can keep bantams with the larger breeds without much danger of their crossing, but not with the smaller breeds, such as games, hamburgs, &c. with plants a difference in the period of flowering serves to keep varieties distinct, as with the various kinds of maize and wheat: thus colonel le couteur[ ] remarks, "the talavera wheat, from flowering much earlier than any other kind, is sure to continue pure." in different parts of { } the falkland islands the cattle are breaking up into herds of different colours; and those on the higher ground, which are generally white, usually breed, as i am informed by admiral sulivan, three months earlier than those on the lowlands; and this would manifestly tend to keep the herds from blending. certain domestic races seem to prefer breeding with their own kind; and this is a fact of some importance, for it is a step towards that instinctive feeling which helps to keep closely allied species in a state of nature distinct. we have now abundant evidence that, if it were not for this feeling, many more hybrids would be naturally produced than is the case. we have seen in the first chapter that the alco dog of mexico dislikes dogs of other breeds; and the hairless dog of paraguay mixes less readily with the european races, than the latter do with each other. in germany the female spitz-dog is said to receive the fox more readily than will other dogs; a female australian dingo in england attracted the wild male foxes. but these differences in the sexual instinct and attractive power of the various breeds may be wholly due to their descent from distinct species. in paraguay the horses have much freedom, and an excellent observer[ ] believes that the native horses of the same colour and size prefer associating with each other, and that the horses which have been imported from entre rios and banda oriental into paraguay likewise prefer associating together. in circassia six sub-races of the horse are known and have received distinct names; and a native proprietor of rank[ ] asserts that horses of three of these races, whilst living a free life, almost always refuse to mingle and cross, and will even attack each other. it has been observed, in a district stocked with heavy lincolnshire and light norfolk sheep, that both kinds, though bred together, when turned out, "in a short time separate to a sheep;" the lincolnshires drawing off to the rich soil, and the norfolks to their own dry light soil; and as long as there is plenty of grass, "the two breeds keep themselves as distinct as rooks and pigeons." in this case different habits of { } life tend to keep the races distinct. on one of the faroe islands, not more than half a mile in diameter, the half-wild native black sheep are said not to have readily mixed with the imported white sheep. it is a more curious fact that the semi-monstrous ancon sheep of modern origin "have been observed to keep together, separating themselves from the rest of the flock, when put into enclosures with other sheep."[ ] with respect to fallow deer, which live in a semi-domesticated condition, mr. bennett[ ] states that the dark and pale coloured herds, which have long been kept together in the forest of dean, in high meadow woods, and in the new forest, have never been known to mingle: the dark-coloured deer, it may be added, are believed to have been first brought by james i. from norway, on account of their greater hardiness. i imported from the island of porto santo two of the feral rabbits, which differ, as described in the fourth chapter, from common rabbits; both proved to be males, and, though they lived during some years in the zoological gardens, the superintendent, mr. bartlett, in vain endeavoured to make them breed with various tame kinds; but whether this refusal to breed was due to any change in instinct, or simply to their extreme wildness; or whether confinement had rendered them sterile, as often occurs, cannot be told. whilst matching for the sake of experiment many of the most distinct breeds of pigeons, it frequently appeared to me that the birds, though faithful to their marriage vow, retained some desire after their own kind. accordingly i asked mr. wicking, who has kept a larger stock of various breeds together than any man in england, whether he thought that they would prefer pairing with their own kind, supposing that there were males and females enough of each; and he without hesitation answered that he was convinced that this was the case. it has often been noticed that the dovecot pigeon seems to have an actual aversion towards the several fancy breeds;[ ] yet all have { } certainly sprung from a common progenitor. the rev. w. d. fox informs me that his flocks of white and common chinese geese kept distinct. these facts and statements, though some of them are incapable of proof, resting only on the opinion of experienced observers, show that some domestic races are led by different habits of life to keep to a certain extent separate, and that others prefer coupling with their own kind, in the same manner as species in a state of nature, though in a much less degree. with respect to sterility from the crossing of domestic races, i know of no well-ascertained case with animals. this fact, seeing the great difference in structure between some breeds of pigeons, fowls, pigs, dogs, &c., is extraordinary, in contrast with the sterility of many closely allied natural species when crossed; but we shall hereafter attempt to show that it is not so extraordinary as it at first appears. and it may be well here to recall to mind that the amount of external difference between two species will not safely guide us in foretelling whether or not they will breed together,--some closely allied species when crossed being utterly sterile, and others which are extremely unlike being moderately fertile. i have said that no case of sterility in crossed races rests on satisfactory evidence; but here is one which at first seems trustworthy. mr. youatt,[ ] and a better authority cannot be quoted, states, that formerly in lancashire crosses were frequently made between longhorn and shorthorn cattle; the first cross was excellent, but the produce was uncertain; in the third or fourth generation the cows were bad milkers; "in addition to which, there was much uncertainty whether the cows would conceive; and full one-third of the cows among some of these half-breds failed to be in calf." this at first seems a good case; but mr. wilkinson states,[ ] that a breed derived from this same cross was actually established in another part of england; and if it had failed in fertility, the fact would surely have been noticed. moreover, supposing that mr. youatt had proved his case, it might be argued that the sterility was wholly due to the two parent-breeds being descended from primordially distinct species. i will give a case with plants, to show how difficult it is to get sufficient evidence. mr. sheriff, who has been so successful in the formation of new races of wheat, fertilised the hopetoun with the talavera; in the first and second generations the produce was intermediate in character, but in the fourth generation "it was found to consist of many varieties; nine-tenths of the florets proved barren, and many of the seeds seemed shrivelled abortions, void of vitality, and the whole race was evidently verging to extinction."[ ] now, considering how little these { } varieties of wheat differ in any important character, it seems to me very improbable that the sterility resulted, as mr. sheriff thought, from the cross, but from some quite distinct cause. until such experiments are many times repeated, it would be rash to trust them; but unfortunately they have been rarely tried even once with sufficient care. gärtner has recorded a more remarkable and trustworthy case: he fertilised thirteen panicles (and subsequently nine others) on a dwarf maize bearing yellow seed[ ] with pollen of a tall maize having red seed; and one head alone produced good seed, only five in number. though these plants are monoecious, and therefore do not require castration, yet i should have suspected some accident in the manipulation had not gärtner expressly stated that he had during many years grown these two varieties together, and they did not spontaneously cross; and this, considering that the plants are monoecious and abound with pollen, and are well known generally to cross freely, seems explicable only on the belief that these two varieties are in some degree mutually infertile. the hybrid plants raised from the above five seed were intermediate in structure, extremely variable, and perfectly fertile.[ ] no one, i believe, has hitherto suspected that these varieties of maize are distinct species; but had the hybrids been in the least sterile, no doubt gärtner would at once have so classed them. i may here remark, that with undoubted species there is not necessarily any close relation between the sterility of a first cross and that of the hybrid offspring. some species can be crossed with facility, but produce utterly sterile hybrids; others can be crossed with extreme difficulty, but the hybrids when produced are moderately fertile. i am not aware, however, of any instance quite like this of the maize with natural species, namely, of a first cross made with difficulty, but yielding perfectly fertile hybrids. the following case is much more remarkable, and evidently perplexed gärtner, whose strong wish it was to draw a broad line of distinction between species and varieties. in the genus verbascum, he made, during eighteen years, a vast number of experiments, and crossed no less than flowers and counted their seeds. many of these experiments consisted in crossing white and yellow varieties of both _v. lychnitis_ and _v. blattaria_ with nine other species and their hybrids. that the white and yellow flowered plants of these two species are really varieties, no one has doubted; and gärtner actually raised in the case of both species one variety from the seed of the other. now in two of his works[ ] he distinctly asserts that crosses between similarly-coloured flowers yield more seed than between dissimilarly-coloured; so that the yellow-flowered variety of either species (and conversely with the white-flowered variety), when crossed with pollen of its own kind, yields more seed than when crossed with that of the white variety; and so it is when differently coloured species are crossed. the general results may be seen in the table at the { } end of his volume. in one instance he gives[ ] the following details; but i must premise that gärtner, to avoid exaggerating the degree of sterility in his crosses, always compares the _maximum_ number obtained from a cross with the _average_ number naturally given by the pure mother-plant. the white-variety of _v. lychnitis_, naturally fertilised by its own pollen, gave from an _average_ of twelve capsules ninety-six good seeds in each; whilst twenty flowers fertilised with pollen from the yellow variety of this same species, gave as the _maximum_ only eighty-nine good seed; so that we have the proportion of to , according to gärtner's usual scale. i should have thought it possible that so small a difference in fertility might have been accounted for by the evil effects of the necessary castration; but gärtner shows that the white variety of _v. lychnitis_, when fertilised first by the white variety of _v. blattaria_, and then by the yellow variety of this species, yielded seed in the proportion of to ; and in both these cases castration was performed. now the sterility which results from the crossing of the differently coloured varieties of the same species, is fully as great as that which occurs in many cases when distinct species are crossed. unfortunately gärtner compared the results of the first unions alone, and not the sterility of the two sets of hybrids produced from the white variety of _v. lychnitis_ when fertilised by the white and yellow varieties of _v. blattaria_, for it is probable that they would have differed in this respect. mr. j. scott has given me the results of a series of experiments on verbascum, made by him in the botanic gardens of edinburgh. he repeated some of gärtner's experiments on distinct species, but obtained only fluctuating results; some confirmatory, but the greater number contradictory; nevertheless these seem hardly sufficient to overthrow the conclusions arrived at by gärtner from experiments tried on a much larger scale. in the second place mr. scott experimented on the relative fertility of unions between similarly and dissimilarly-coloured varieties of the same species. thus he fertilised six flowers of the yellow variety of _v. lychnitis_ by its own pollen, and obtained six capsules, and calling, for the sake of having a standard of comparison, the average number of good seed in each one hundred, he found that this same yellow variety, when fertilised by the white variety, yielded from seven capsules an average of ninety-four seed. on the same principle, the white variety of _v. lychnitis_ by its own pollen (from six capsules), and by the pollen of the yellow variety (eight capsules), yielded seed in the proportion of to . the yellow variety of _v. thapsus_ by its own pollen (eight capsules), and by that of the white variety (only two capsules), yielded seed in the proportion of to . lastly, the white variety of _v. blattaria_ by its own pollen (eight capsules), and by that of the yellow variety (five capsules), yielded seed in the proportion of to . so that in every case the unions of dissimilarly-coloured varieties of the same species were less fertile than the unions of similarly-coloured varieties; when all the cases are grouped together, the difference of fertility is as to . some additional trials were made, and altogether thirty-six similarly-coloured unions yielded thirty-five good { } capsules; whilst thirty-five dissimilarly-coloured unions yielded only twenty-six good capsules. besides the foregoing experiments, the purple _v. phoeniceum_ was crossed by a rose-coloured and a white variety of the same species; these two varieties were also crossed together, and these several unions yielded less seed than _v. phoeniceum_ by its own pollen. hence it follows from mr. scott's experiments, that in the genus verbascum the similarly and dissimilarly-coloured varieties of the same species behave, when crossed, like closely allied but distinct species.[ ] this remarkable fact of the sexual affinity of similarly-coloured varieties, as observed by gärtner and mr. scott, may not be of very rare occurrence; for the subject has not been attended to by others. the following case is worth giving, partly to show how difficult it is to avoid error. dr. herbert[ ] has remarked that variously-coloured double varieties of the hollyhock (_althæa rosea_) may be raised with certainty by seed from plants growing close together. i have been informed that nurserymen who raise seed for sale do not separate their plants; accordingly i procured seed of eighteen named varieties; of these, eleven varieties produced sixty-two plants all perfectly true to their kind; and seven produced forty-nine plants, half of which were true and half false. mr. masters of canterbury has given me a more striking case; he saved seed from a great bed of twenty-four named varieties planted in closely adjoining rows, and each variety reproduced itself truly with only sometimes a shade of difference in tint. now in the hollyhock the pollen, which is abundant, is matured and nearly all shed before the stigma of the same flower is ready to receive it;[ ] and as bees covered with pollen incessantly fly from plant to plant, it would appear that adjoining varieties could not escape being crossed. as, however, this does not occur, it appeared to me probable that the pollen { } of each variety was prepotent on its own stigma over that of all other varieties. but mr. c. turner of slough, well known for his success in the cultivation of this plant, informs me that it is the doubleness of the flowers which prevents the bees gaining access to the pollen and stigma; and he finds that it is difficult even to cross them artificially. whether this explanation will fully account for varieties in close proximity propagating themselves so truly by seed, i do not know. the following cases are worth giving, as they relate to monoecious forms, which do not require, and consequently have not been injured by, castration. girou de buzareingues crossed what he designates three varieties of gourd,[ ] and asserts that their mutual fertilisation is less easy in proportion to the difference which they present. i am aware how imperfectly the forms in this group were until recently known; but sageret,[ ] who ranked them according to their mutual fertility, considers the three forms above alluded to as varieties, as does a far higher authority, namely, m. naudin.[ ] sageret[ ] has observed that certain melons have a greater tendency, whatever the cause may be, to keep true than others; and m. naudin, who has had such immense experience in this group, informs me that he believes that certain varieties intercross more readily than others of the same species; but he has not proved the truth of this conclusion; the frequent abortion of the pollen near paris being one great difficulty. nevertheless, he has grown close together, during seven years, certain forms of citrullus, which, as they could be artificially crossed with perfect facility and produced fertile offspring, are ranked as varieties; but these forms when not artificially crossed kept true. many other varieties, on the other hand, in the same group cross with such facility, as m. naudin repeatedly insists, that without being grown far apart they cannot be kept in the least true. another case, though somewhat different, may be here given, as it is highly remarkable, and is established on excellent evidence. kölreuter minutely describes five varieties of the common tobacco,[ ] which were reciprocally crossed, and the offspring were intermediate in character and as fertile as their parents: from this fact kölreuter inferred that they are really varieties; and no one, as far as i can discover, seems to have doubted that such is the case. he also crossed reciprocally these five varieties with _n. glutinosa_, and they yielded very sterile hybrids; but those raised from the _var. perennis_, whether used as the father or mother plant, were not so sterile as the hybrids from the four other varieties.[ ] so that the sexual { } capacity of this one variety has certainly been in some degree modified, so as to approach in nature that of _n. glutinosa_.[ ] these facts with respect to plants show that in some few cases certain varieties have had their sexual powers so far modified, that they cross together less readily and yield less seed than other varieties of the same species. we shall presently see that the sexual functions of most animals and plants are eminently liable to be affected by the conditions of life to which they are exposed; and hereafter we shall briefly discuss the conjoint bearing of this and other facts on the difference in fertility between crossed varieties and crossed species. _domestication eliminates the tendency to sterility which is general with species when crossed._ this hypothesis was first propounded by pallas,[ ] and has been adopted by several authors. i can find hardly any direct facts in its support; but unfortunately no one has compared, in the case of either animals or plants, the fertility of anciently domesticated varieties, when crossed with a distinct species, with that of the wild parent-species when similarly crossed. no one has compared, for instance, the fertility of _gallus bankiva_ and of the domesticated fowl, when crossed with a distinct species of gallus or phasianus; and the { } experiment would in all cases be surrounded by many difficulties. dureau de la malle, who has so closely studied classical literature, states[ ] that in the time of the romans the common mule was produced with more difficulty than at the present day; but whether this statement may be trusted i know not. a much more important, though somewhat different, case is given by m. groenland,[ ] namely, that plants, known from their intermediate character and sterility to be hybrids between Ægilops and wheat, have perpetuated themselves under culture since , _with a rapid but varying increase of fertility in each generation_. in the fourth generation the plants, still retaining their intermediate character, had become as fertile as common cultivated wheat. the indirect evidence in favour of the pallasian doctrine appears to me to be extremely strong. in the earlier chapters i have attempted to show that our various breeds of dogs are descended from several wild species; and this probably is the case with sheep. there can no longer be any doubt that the zebu or humped indian ox belongs to a distinct species from european cattle: the latter, moreover, are descended from two or three forms, which may be called either species or wild races, but which co-existed in a state of nature and kept distinct. we have good evidence that our domesticated pigs belong to at least two specific types, _s. scrofa_ and _indica_, which probably lived together in a wild state in south-eastern europe. now, a widely-extended analogy leads to the belief that if these several allied species, in the wild state or when first reclaimed, had been crossed, they would have exhibited, both in their first unions and in their hybrid offspring, some degree of sterility. nevertheless the several domesticated races descended from them are now all, as far as can be ascertained, perfectly fertile together. if this reasoning be trustworthy, and it is apparently sound, we must admit the pallasian doctrine that long-continued domestication tends to eliminate that sterility which is natural to species when crossed in their aboriginal state. { } _on increased fertility from domestication and cultivation._ increased fertility from domestication, without any reference to crossing, may be here briefly considered. this subject bears indirectly on two or three points connected with the modification of organic beings. as buffon long ago remarked,[ ] domestic animals breed oftener in the year and produce more young at a birth than wild animals of the same species; they, also, sometimes breed at an earlier age. the case would hardly have deserved further notice, had not some authors lately attempted to show that fertility increases and decreases in an inverse ratio with the amount of food. this strange doctrine has apparently arisen from individual animals when supplied with an inordinate quantity of food, and from plants of many kinds when grown on excessively rich soil, as on a dunghill, becoming sterile; but to this latter point i shall have occasion presently to return. with hardly an exception, our domesticated animals, which have long been habituated to a regular and copious supply of food, without the labour of searching for it, are more fertile than the corresponding wild animals. it is notorious how frequently cats and dogs breed, and how many young they produce at a birth. the wild rabbit is said generally to breed four times yearly, and to produce from four to eight young; the tame rabbit breeds six or seven times yearly, and produces from four to eleven young. the ferret, though generally so closely confined, is more prolific than its supposed wild prototype. the wild sow is remarkably prolific, for she often breeds twice in the year, and produces from four to eight and sometimes even twelve young at a birth; but the domestic sow regularly breeds twice a year, and would breed oftener if permitted; and a sow that produces less than eight at a birth "is worth little, and the sooner she is fattened for the butcher the better." the amount of food affects the fertility even of the same individual: thus sheep, which on mountains never produce more than one lamb at a birth, when brought { } down to lowland pastures frequently bear twins. this difference apparently is not due to the cold of the higher land, for sheep and other domestic animals are said to be extremely prolific in lapland. hard living, also, retards the period at which animals conceive; for it has been found disadvantageous in the northern islands of scotland to allow cows to bear calves before they are four years old.[ ] birds offer still better evidence of increased fertility from domestication: the hen of the wild _gallus bankiva_ lays from six to ten eggs, a number which would be thought nothing of with the domestic hen. the wild duck lays from five to ten eggs; the tame one in the course of the year from eighty to one hundred. the wild grey-lag goose lays from five to eight eggs; the tame from thirteen to eighteen, and she lays a second time; as mr. dixon has remarked, "high-feeding, care, and moderate warmth induce a habit of prolificacy which becomes in some measure hereditary." whether the semi-domesticated dovecot pigeon is more fertile than the wild rock-pigeon _c. livia_, i know not; but the more thoroughly domesticated breeds are nearly twice as fertile as dovecots: the latter, however, when caged and highly fed, become equally fertile with house pigeons. the peahen alone of domesticated birds is rather more fertile, according to some accounts, when wild in its native indian home, than when domesticated in europe and exposed to our much colder climate.[ ] with respect to plants, no one would expect wheat to tiller more, and each ear to produce more grain, in poor than in rich soil; or to get in poor soil a heavy crop of peas or beans. seeds vary so much in number { } that it is difficult to estimate them; but on comparing beds of carrots saved for seed in a nursery garden with wild plants, the former seemed to produce about twice as much seed. cultivated cabbages yielded thrice as many pods by measure as wild cabbages from the rocks of south wales. the excess of berries produced by the cultivated asparagus in comparison with the wild plant is enormous. no doubt many highly cultivated plants, such as pears, pineapples, bananas, sugar-cane, &c., are nearly or quite sterile; and i am inclined to attribute this sterility to excess of food and to other unnatural conditions; but to this subject i shall presently recur. in some cases, as with the pig, rabbit, &c., and with those plants which are valued for their seed, the direct selection of the more fertile individuals has probably much increased their fertility; and in all cases this may have occurred indirectly, from the better chance of the more numerous offspring produced by the more fertile individuals having survived. but with cats, ferrets, and dogs, and with plants like carrots, cabbages, and asparagus, which are not valued for their prolificacy, selection can have played only a subordinate part; and their increased fertility must be attributed to the more favourable conditions of life under which they have long existed. * * * * * { } chapter xvii. on the good effects of crossing, and on the evil effects of close interbreeding. definition of close interbreeding--augmentation of morbid tendencies--general evidence on the good effects derived from crossing, and on the evil effects from close interbreeding--cattle, closely interbred; half-wild cattle long kept in the same parks--sheep--fallow-deer--dogs--rabbits--pigs--man, origin of his abhorrence of incestuous marriages--fowls--pigeons--hive-bees--plants, general considerations on the benefits derived from crossing--melons, fruit-trees, peas, cabbages, wheat, and forest-trees--on the increased size of hybrid plants, not exclusively due to their sterility--on certain plants which either normally or abnormally are self-impotent, but are fertile, both on the male and female side, when crossed with distinct individuals either of the same or another species--conclusion. the gain in constitutional vigour, derived from an occasional cross between individuals of the same variety, but belonging to distinct families, or between distinct varieties, has not been so largely or so frequently discussed, as have the evil effects of too close interbreeding. but the former point is the more important of the two, inasmuch as the evidence is more decisive. the evil results from close interbreeding are difficult to detect, for they accumulate slowly, and differ much in degree with different species; whilst the good effects which almost invariably follow a cross are from the first manifest. it should, however, be clearly understood that the advantage of close interbreeding, as far as the retention of character is concerned, is indisputable, and often outweighs the evil of a slight loss of constitutional vigour. in relation to the subject of domestication, the whole question is of some importance, as too close interbreeding interferes with the improvement of old races, and especially with the formation of new ones. it is important as indirectly bearing on hybridism; and perhaps on the extinction of species, when any form has become so rare that only a few individuals { } remain within a confined area. it bears in an important manner on the influence of free intercrossing, in obliterating individual differences, and thus giving uniformity of character to the individuals of the same race or species; for if additional vigour and fertility be thus gained, the crossed offspring will multiply and prevail, and the ultimate result will be far greater than otherwise would have occurred. lastly, the question is of high interest, as bearing on mankind. hence i shall discuss this subject at full length. as the facts which prove the evil effects of close interbreeding are more copious, though less decisive, than those on the good effects of crossing, i shall, under each group of beings, begin with the former. there is no difficulty in defining what is meant by a cross; but this is by no means easy in regard to "breeding in and in" or "too close interbreeding," because, as we shall see, different species of animals are differently affected by the same degree of interbreeding. the pairing of a father and daughter, or mother and son, or brothers and sisters, if carried on during several generations, is the closest possible form of interbreeding. but some good judges, for instance sir j. sebright, believe that the pairing of a brother and sister is closer than that of parents and children; for when the father is matched with his daughter he crosses, as is said, with only half his own blood. the consequences of close interbreeding carried on for too long a time, are, as is generally believed, loss of size, constitutional vigour, and fertility, sometimes accompanied by a tendency to malformation. manifest evil does not usually follow from pairing the nearest relations for two, three, or even four generations; but several causes interfere with our detecting the evil--such as the deterioration being very gradual, and the difficulty of distinguishing between such direct evil and the inevitable augmentation of any morbid tendencies which may be latent or apparent in the related parents. on the other hand, the benefit from a cross, even when there has not been any very close interbreeding, is almost invariably at once conspicuous. there is reason to believe, and this was the opinion of that most experienced observer sir j. sebright,[ ] that the evil effects of close interbreeding may be checked by the related individuals { } being separated during a few generations and exposed to different conditions of life. that evil directly follows from any degree of close interbreeding has been denied by many persons; but rarely by any practical breeder; and never, as far as i know, by one who has largely bred animals which propagate their kind quickly. many physiologists attribute the evil exclusively to the combination and consequent increase of morbid tendencies common to both parents: that this is an active source of mischief there can be no doubt. it is unfortunately too notorious that men and various domestic animals endowed with a wretched constitution, and with a strong hereditary disposition to disease, if not actually ill, are fully capable of procreating their kind. close interbreeding, on the other hand, induces sterility; and this indicates something quite distinct from the augmentation of morbid tendencies common to both parents. the evidence immediately to be given convinces me that it is a great law of nature, that all organic beings profit from an occasional cross with individuals not closely related to them in blood; and that, on the other hand, long-continued close interbreeding is injurious. various general considerations have had much influence in leading me to this conclusion; but the reader will probably rely more on special facts and opinions. the authority of experienced observers, even when they do not advance the grounds of their belief, is of some little value. now almost all men who have bred many kinds of animals and have written on the subject, such as sir j. sebright, andrew knight, &c.,[ ] have expressed the strongest conviction on the impossibility of long-continued close interbreeding. those who have compiled works on agriculture, and have associated much with breeders, such as the sagacious youatt, low, &c., have strongly declared their opinion to the same effect. prosper lucas, trusting largely to french authorities, has come to a similar conclusion. the distinguished german agriculturist hermann von nathusius, who has written the most able treatise on this subject which i have met with, concurs; and as i shall have to quote from { } this treatise, i may state that nathusius is not only intimately acquainted with works on agriculture in all languages, and knows the pedigrees of our british breeds better than most englishmen, but has imported many of our improved animals, and is himself an experienced breeder. evidence of the evil effects of close interbreeding can most readily be acquired in the case of animals, such as fowls, pigeons, &c., which propagate quickly, and, from being kept in the same place, are exposed to the same conditions. now i have inquired of very many breeders of these birds, and i have hitherto not met with a single man who was not thoroughly convinced that an occasional cross with another strain of the same sub-variety was absolutely necessary. most breeders of highly-improved or fancy birds value their own strain, and are most unwilling, at the risk, in their opinion, of deterioration, to make a cross. the purchase of a first-rate bird of another strain is expensive, and exchanges are troublesome; yet all breeders, as far as i can hear, excepting those who keep large stocks at different places for the sake of crossing, are driven after a time to take this step. another general consideration which has had great influence on my mind is, that with all hermaphrodite animals and plants, which it might have been thought would have perpetually fertilised themselves, and thus have been subjected for long ages to the closest interbreeding, there is no single species, as far as i can discover, in which the structure ensures self-fertilisation. on the contrary, there are in a multitude of cases, as briefly stated in the fifteenth chapter, manifest adaptations which favour or inevitably lead to an occasional cross between one hermaphrodite and another of the same species; and these adaptive structures are utterly purposeless, as far as we can see, for any other end. with _cattle_ there can be no doubt that extremely close interbreeding may be long carried on, advantageously with respect to external characters and with no manifestly apparent evil as far as constitution is concerned. the same remark is applicable to sheep. whether these animals have gradually been rendered less susceptible than others to this evil, in order to permit them to live in herds,--a habit which leads the old and vigorous males to expel all intruders, and in consequence often to pair with their own daughters, i will not pretend to decide. the case of bakewell's long-horns, which were closely interbred for a long period, has often been { } quoted; yet youatt says[ ] the breed "had acquired a delicacy of constitution inconsistent with common management," and "the propagation of the species was not always certain." but the shorthorns offer the most striking case of close interbreeding; for instance, the famous bull favourite (who was himself the offspring of a half-brother and sister from foljambe) was matched with his own daughter, granddaughter, and great-granddaughter; so that the produce of this last union, or the great-great-granddaughter, had - ths, or . per cent. of the blood of favourite in her veins. this cow was matched with the bull wellington, having . per cent. of favourite blood in his veins, and produced clarissa; clarissa was matched with the bull lancaster, having . of the same blood, and she yielded valuable offspring.[ ] nevertheless collings, who reared these animals, and was a strong advocate for close breeding, once crossed his stock with a galloway, and the cows from this cross realised the highest prices. bates's herd was esteemed the most celebrated in the world. for thirteen years he bred most closely in and in; but during the next seventeen years, though he had the most exalted notion of the value of his own stock, he thrice infused fresh blood into his herd: it is said that he did this, not to improve the form of his animals, but on account of their lessened fertility. mr. bates's own view, as given by a celebrated breeder,[ ] was, that "to breed in and in from a bad stock was ruin and devastation; yet that the practice may be safely followed within certain limits when the parents so related are descended from first-rate animals." we thus see that there has been extremely close interbreeding with shorthorns; but nathusius, after the most careful study of their pedigrees, says that he can find no instance of a breeder who has strictly followed this practice during his whole life. from this study and his own experience, he concludes that close interbreeding is necessary to ennoble the stock; but that in effecting this the greatest care is necessary, on account of the tendency to infertility and weakness. it may be added, that another high authority[ ] asserts that many more calves are born cripples from shorthorns than from other and less closely interbred races of cattle. although by carefully selecting the best animals (as nature effectually does by the law of battle) close interbreeding may be long carried on with cattle, yet the good effects of a cross between almost any two breeds is at once shown by the greater size and vigour of the offspring; as mr. spooner writes to me, "crossing distinct breeds certainly improves cattle for the butcher." such crossed animals are of course of no value to the breeder; but they have been raised during many years in several { } parts of england to be slaughtered;[ ] and their merit is now so fully recognised, that at fat-cattle shows a separate class has been formed for their reception. the best fat ox at the great show at islington in was a crossed animal. the half-wild cattle, which have been kept in british parks probably for or years, or even for a longer period, have been advanced by culley and others as a case of long-continued interbreeding within the limits of the same herd without any consequent injury. with respect to the cattle at chillingham, the late lord tankerville owned that they were bad breeders.[ ] the agent, mr. hardy, estimates (in a letter to me, dated may, ) that in the herd of about fifty the average number annually slaughtered, killed by fighting, and dying, is about ten, or one in five. as the herd is kept up to nearly the same average number, the annual rate of increase must be likewise about one in five. the bulls, i may add, engage in furious battles, of which battles the present lord tankerville has given me a graphic description, so that there will always be rigorous selection of the most vigorous males. i procured in from mr. d. gardner, agent to the duke of hamilton, the following account of the wild cattle kept in the duke's park in lanarkshire, which is about acres in extent. the number of cattle varies from sixty-five to eighty; and the number annually killed (i presume by all causes) is from eight to ten; so that the annual rate of increase can hardly be more than one in six. now in south america, where the herds are half-wild, and therefore offer a nearly fair standard of comparison, according to azara the natural increase of the cattle on an estancia is from one-third to one-fourth of the total number, or one in between three and four; and this, no doubt, applies exclusively to adult animals fit for consumption. hence the half-wild british cattle which have long interbred within the limits of the same herd are relatively far less fertile. although in an unenclosed country like paraguay there must be some crossing between the different herds, yet even there the inhabitants believe that the occasional introduction of animals from distant localities is necessary to prevent "degeneration in size and diminution of fertility."[ ] the decrease in size from ancient times in the chillingham and hamilton cattle must have been prodigious, for professor rütimeyer has shown that they are almost certainly the descendants of the gigantic _bos primigenius_. no doubt this decrease in size may be largely attributed to less favourable conditions of life; yet animals roaming over large parks, and fed during severe winters, can hardly be considered as placed under very unfavourable conditions. with _sheep_ there has often been long-continued interbreeding within the limits of the same flock; but whether the nearest relations have been matched so frequently as in the case of shorthorn cattle, i do not know. the messrs. brown during fifty years have never infused fresh blood into their excellent flock of leicesters. since mr. barford has acted on the same principle with the foscote flock. he asserts that half a century { } of experience has convinced him that when two nearly related animals are quite sound in constitution, in-and-in breeding does not induce degeneracy; but he adds that he "does not pride himself on breeding from the nearest affinities." in france the naz flock has been bred for sixty years without the introduction of a single strange ram.[ ] nevertheless, most great breeders of sheep have protested against close interbreeding prolonged for too great a length of time.[ ] the most celebrated of recent breeders, jonas webb, kept five separate families to work on, thus "retaining the requisite distance of relationship between the sexes."[ ] although by the aid of careful selection the near interbreeding of sheep may be long continued without any manifest evil, yet it has often been the practice with farmers to cross distinct breeds to obtain animals for the butcher, which plainly shows that good is derived from this practice. mr. spooner sums up his excellent essay on crossing by asserting that there is a direct pecuniary advantage in judicious cross-breeding, especially when the male is larger than the female. a former celebrated breeder, lord somerville, distinctly states that his half-breeds from ryelands and spanish sheep were larger animals than either the pure ryelands or pure spanish sheep.[ ] as some of our british parks are ancient, it occurred to me that there must have been long-continued close interbreeding with the fallow deer (_cervus dama_) kept in them; but on inquiry i find that it is a common practice to infuse new blood by procuring bucks from other parks. mr. shirley,[ ] who has carefully studied the management of deer, admits that in some parks there has been no admixture of foreign blood from a time beyond the memory of man. but he concludes "that in the end the constant breeding in-and-in is sure to tell to the disadvantage of the whole herd, though it may take a very long time to prove it; moreover, when we find, as is very constantly the case, that the introduction of fresh blood has been of the very greatest use to deer, both by improving their size and appearance, and particularly by being of service in removing the taint of 'rickback,' if not of other diseases, to which deer are sometimes subject when the blood has not been changed, there can, i think, be no doubt but that a judicious cross with a good stock is of the greatest consequence, and is indeed essential, sooner or later, to the prosperity of every well-ordered park." mr. meynell's famous foxhounds have been adduced, as showing that no ill effects follow from close interbreeding; and sir j. sebright ascertained from him that he frequently bred from father and daughter, mother and { } son, and sometimes even from brothers and sisters. sir j. sebright, however, declares,[ ] that by breeding _in-and-in_, by which he means matching brothers and sisters, he has actually seen strong spaniels become weak and diminutive lapdogs. the rev. w. d. fox has communicated to me the case of a small lot of bloodhounds, long kept in the same family, which had become very bad breeders, and nearly all had a bony enlargement in the tail. a single cross with a distinct strain of bloodhounds restored their fertility, and drove away the tendency to malformation in the tail. i have heard the particulars of another case with bloodhounds, in which the female had to be held to the male. considering how rapid is the natural increase of the dog, it is difficult to understand the high price of most highly improved breeds, which almost implies long-continued close interbreeding, except on the belief that this process lessens fertility and increases liability to distemper and other diseases. a high authority, mr. scrope, attributes the rarity and deterioration in size of the scotch deerhound (the few individuals now existing throughout the country being all related) in large part to close interbreeding. with all highly-bred animals there is more or less difficulty in getting them to procreate quickly, and all suffer much from delicacy of constitution; but i do not pretend that these effects ought to be wholly attributed to close interbreeding. a great judge of rabbits[ ] says, "the long-eared does are often too highly bred or forced in their youth to be of much value as breeders, often turning out barren or bad mothers." again: "very long-eared bucks will also sometimes prove barren." these highly-bred rabbits often desert their young, so that it is necessary to have nurse-rabbits. with _pigs_ there is more unanimity amongst breeders on the evil effects of close interbreeding than, perhaps, with any other large animal. mr. druce, a great and successful breeder of the improved oxfordshires (a crossed race), writes, "without a change of boars of a different tribe, but of the same breed, constitution cannot be preserved." mr. fisher hobbs, the raiser of the celebrated improved essex breed, divided his stock into three separate families, by which means he maintained the breed for more than twenty years, "by judicious selection from the _three distinct families_."[ ] lord western was the first importer of a neapolitan boar and sow. "from this pair he bred in-and-in, until the breed was in danger of becoming extinct, a sure result (as mr. sidney remarks) of in-and-in breeding." lord western then crossed his neapolitan pigs with the old essex, and made the first great step towards the improved essex breed. here is a more interesting case. mr. j. wright, well known as a breeder, crossed[ ] the same boar with the daughter, granddaughter, and great-granddaughter, and so on for seven generations. the result was, that in many instances the offspring failed to breed; in others they produced few that lived; and of the latter many were idiotic, without sense { } even to suck, and when attempting to move could not walk straight. now it deserves especial notice, that the two last sows produced by this long course of interbreeding were sent to other boars, and they bore several litters of healthy pigs. the best sow in external appearance produced during the whole seven generations was one in the last stage of descent; but the litter consisted of this one sow. she would not breed to her sire, yet bred at the first trial to a stranger in blood. so that, in mr. wright's case, long-continued and extremely close interbreeding did not affect the external form or merit of the young; but with many of them the general constitution and mental powers, and especially the reproductive functions, were seriously affected. nathusius gives[ ] an analogous and even more striking case: he imported from england a pregnant sow of the large yorkshire breed, and bred the product closely in-and-in for three generations: the result was unfavourable, as the young were weak in constitution, with impaired fertility. one of the latest sows, which he esteemed a good animal, produced, when paired with her own uncle (who was known to be productive with sows of other breeds), a litter of six, and a second time a litter of only five weak young pigs. he then paired this sow with a boar of a small black breed, which he had likewise imported from england, and which boar, when matched with sows of his own breed, produced from seven to nine young: now, the sow of the large breed, which was so unproductive when paired with her own uncle, yielded to the small black boar, in the first litter twenty-one, and in the second litter eighteen young pigs; so that in one year she produced thirty-nine fine young animals! as in the case of several other animals already mentioned, even when no injury is perceptible from moderately close interbreeding, yet, to quote the words of mr. coate, a most successful breeder (who five times won the annual gold medal of the smithfield club show for the best pen of pigs), "crosses answer well for profit to the farmer, as you get more constitution and quicker growth; but for me, who sell a great number of pigs for breeding purposes, i find it will not do, as it requires many years to get anything like purity of blood again."[ ] before passing on to birds, i ought to refer to man, though i am unwilling to enter on this subject, as it is surrounded by natural prejudices. it has moreover been discussed by various authors under many points of view.[ ] mr. tylor[ ] has shown { } that with widely different races, in the most distant quarters of the world, marriages between relations--even between distant relations--have been strictly prohibited. a few exceptional cases can be specified, especially with royal families; and these have been enlarged on in a learned article[ ] by mr. w. adam, and formerly in by hofacker. mr. tylor is inclined to believe that the almost universal prohibition of closely-related marriages has arisen from their evil effects having been observed, and he ingeniously explains some apparent anomalies in the prohibition not extending equally to the relations on both the male and female side. he admits, however, that other causes, such as the extension of friendly alliances, may have come into play. mr. w. adam, on the other hand, concludes that related marriages are prohibited and viewed with repugnance from the confusion which would thus arise in the descent of property, and from other still more recondite reasons; but i cannot accept this view, seeing that the savages of australia and south america,[ ] who have no property to bequeath or fine moral feelings to confuse, hold the crime of incest in abhorrence. it would be interesting to know, if it could be ascertained, as throwing light on this question with respect to man, what occurs with the higher anthropomorphous apes--whether the young males and females soon wander away from their parents, or whether the old males become jealous of their sons and expel them, or whether any inherited instinctive feeling, from being beneficial, has been generated, leading the young males and females of the same family to prefer pairing with distinct families, and to dislike pairing with each other. a considerable body of evidence has already been advanced, showing that the offspring from parents which are not related are more vigorous and fertile than those from parents which are closely related; hence any slight feeling, arising from the sexual excitement of novelty or other cause, which led to the former rather than to the latter unions, would be augmented through natural selection, and thus might become instinctive; for those individuals which had an innate preference of this kind would increase in number. it seems more probable, that degraded savages should { } thus unconsciously have acquired their dislike and even abhorrence of incestuous marriages, rather than that they should have discovered by reasoning and observation the evil results. the abhorrence occasionally failing is no valid argument against the feeling being instinctive, for any instinct may occasionally fail or become vitiated, as sometimes occurs with parental love and the social sympathies. in the case of man, the question whether evil follows from close interbreeding will probably never be answered by direct evidence, as he propagates his kind so slowly and cannot be subjected to experiment; but the almost universal practice of all races at all times of avoiding closely-related marriages is an argument of considerable weight; and whatever conclusion we arrive at in regard to the higher animals may be safely extended to man. turning now to birds: in the case of the _fowl_ a whole array of authorities could be given against too close interbreeding. sir j. sebright positively asserts that he made many trials, and that his fowls, when thus treated, became long in the legs, small in the body, and bad breeders.[ ] he produced the famous sebright bantams by complicated crosses, and by breeding in-and-in; and since his time there has been much close interbreeding with these bantams; and they are now notoriously bad breeders. i have seen silver bantams, directly descended from his stock, which had become almost as barren as hybrids; for not a single chicken had been that year hatched from two full nests of eggs. mr. hewitt says that with these bantams the sterility of the male stands, with rare exceptions, in the closest relation with their loss of certain secondary male characters: he adds, "i have noticed, as a general rule, that even the slightest deviation from feminine character in the tail of the male sebright--say the elongation by only half an inch of the two principal tail-feathers--brings with it improved probability of increased fertility."[ ] mr. wright states[ ] that mr. clark, "whose fighting-cocks were so notorious, continued to breed from his own kind till they lost their disposition to fight, but stood to be cut up without making any resistance, and were so reduced in size as to be under those weights required for the best prizes; but on obtaining a cross from mr. leighton, they again resumed their former courage and weight." it should be borne in mind that game-cocks before they fought were always weighed, so that nothing was left to the imagination about any reduction or increase of { } weight. mr. clark does not seem to have bred from brothers and sisters, which is the most injurious kind of union; and he found, after repeated trials, that there was a greater reduction in weight in the young from a father paired with his daughter, than from a mother with her son. i may add that mr. eyton, of eyton, the well-known ornithologist, who is a large breeder of grey dorkings, informs me that they certainly diminish in size, and become less prolific, unless a cross with another strain is occasionally obtained. so it is with malays, according to mr. hewitt, as far as size is concerned.[ ] an experienced writer[ ] remarks that the same amateur, as is well known, seldom long maintains the superiority of his birds; and this, he adds, undoubtedly is due to all his stock "being of the same blood;" hence it is indispensable that he should occasionally procure a bird of another strain. but this is not necessary with those who keep a stock of fowls at different stations. thus, mr. ballance, who has bred malays for thirty years, and has won more prizes with these birds than any other fancier in england, says that breeding in-and-in does not necessarily cause deterioration; "but all depends upon how this is managed." "my plan has been to keep about five or six distinct runs, and to rear about two hundred or three hundred chickens each year, and select the best birds from each run for crossing. i thus secure sufficient crossing to prevent deterioration."[ ] we thus see that there is almost complete unanimity with poultry-breeders that, when fowls are kept at the same place, evil quickly follows from interbreeding carried on to an extent which would be disregarded in the case of most quadrupeds. on the other hand, it is a generally received opinion that cross-bred chickens are the hardiest and most easily reared.[ ] mr. tegetmeier, who has carefully attended to poultry of all breeds, says[ ] that dorking hens, allowed to run with houdan or crevecoeur cocks, "produce in the early spring chickens that for size, hardihood, early maturity, and fitness for the market, surpass those of any pure breed that we have ever raised." mr. hewitt gives it as a general rule with fowls, that crossing the breed increases their size. he makes this remark after stating that hybrids from the pheasant and fowl are considerably larger than either progenitor: so again, hybrids from the male golden pheasant and hen common pheasant "are of far larger size than either parent-bird."[ ] to this subject of the increased size of hybrids i shall presently return. with _pigeons_, breeders are unanimous, as previously stated, that it is absolutely indispensable, notwithstanding the trouble and expense thus caused, occasionally to cross their much-prized birds with individuals of another strain, but belonging, of course, to the same variety. it deserves { } notice that, when large size is one of the desired characters, as with pouters,[ ] the evil effects of close interbreeding are much sooner perceived than when small birds, such as short-faced tumblers, are valued. the extreme delicacy of the high fancy breeds, such as these tumblers and improved english carriers, is remarkable; they are liable to many diseases, and often die in the egg or during the first moult; and their eggs have generally to be hatched under foster-mothers. although these highly-prized birds have invariably been subjected to much close interbreeding, yet their extreme delicacy of constitution cannot perhaps be thus fully explained. mr. yarrell informed me that sir j. sebright continued closely interbreeding some owl-pigeons, until from their extreme sterility he as nearly as possible lost the whole family. mr. brent[ ] tried to raise a breed of trumpeters, by crossing a common pigeon, and recrossing the daughter, granddaughter, great-granddaughter, and great-great-granddaughter, with the same male trumpeter, until he obtained a bird with / ths of trumpeter's blood; but then the experiment failed, for "breeding so close stopped reproduction." the experienced neumeister[ ] also asserts that the offspring from dovecotes and various other breeds are "generally very fertile and hardy birds:" so again, mm. boitard and corbié,[ ] after forty-five years' experience, recommend persons to cross their breeds for amusement; for, if they fail to make interesting birds, they will succeed under an economical point of view, "as it is found that mongrels are more fertile than pigeons of pure race." i will refer only to one other animal, namely, the hive-bee, because a distinguished entomologist has advanced this as a case of inevitable close interbreeding. as the hive is tenanted by a single female, it might have been thought that her male and female offspring would always have bred together, more especially as bees of different hives are hostile to each other; a strange worker being almost always attacked when trying to enter another hive. but mr. tegetmeier has shown[ ] that this instinct does not apply to drones, which are permitted to enter any hive; so that there is no _à priori_ improbability of a queen receiving a foreign drone. the fact of the union invariably and necessarily taking place on the wing, during the queen's nuptial flight, seems to be a special provision against continued interbreeding. however this may be, experience has shown, since the introduction of the yellow-banded ligurian race into germany and england, that bees freely cross: mr. woodbury, who introduced ligurian bees into devonshire, found during a single season that three stocks, at distances of from one to two miles from his hives, were crossed by his drones. in one case the ligurian drones must have flown over the city of exeter, and over several intermediate hives. on another occasion several common black queens were crossed by ligurian drones at a distance of from one to three and a half miles.[ ] { } _plants._ when a single plant of a new species is introduced into any country, if propagated by seed, many individuals will soon be raised, so that if the proper insects be present there will be crossing. with newly-introduced trees or other plants not propagated by seed we are not here concerned. with old-established plants it is an almost universal practice occasionally to make exchanges of seed, by which means individuals which have been exposed to different conditions of life,--and this, as we have seen, diminishes the evil from close interbreeding,--will occasionally be introduced into each district. experiments have not been tried on the effects of fertilising flowers with their own pollen during _several_ generations. but we shall presently see that certain plants, either normally or abnormally, are more or less sterile, even in the first generation, when fertilised by their own pollen. although nothing is directly known on the evil effects of long-continued close interbreeding with plants, the converse proposition that great good is derived from crossing is well established. with respect to the crossing of individuals belonging to the same sub-variety, gärtner, whose accuracy and experience exceeded that of all other hybridisers, states[ ] that he has many times observed good effects from this step, especially with exotic genera, of which the fertility is somewhat impaired, such as passiflora, lobelia, and fuchsia. herbert also says,[ ] "i am inclined to think that i have derived advantage from impregnating the flower from which i wished to obtain seed with pollen from another individual of the same variety, or at least from another flower, rather than with its own." again, professor lecoq asserts that he has ascertained that crossed offspring are more vigorous and robust than their parents.[ ] general statements of this kind, however, can seldom be fully trusted; consequently i have begun a series of experiments, which, if they continue to give the same results as hitherto, will for ever settle the question of the good effects of crossing two distinct plants of the same variety, and of the evil effects of self-fertilisation. a clear light will thus also be thrown on the fact that flowers are invariably constructed so as to permit, or favour, or necessitate the union of two individuals. we shall clearly understand why monoecious and dioecious,--why dimorphic and trimorphic plants exist, and many other such cases. the plan which i have followed in my experiments is to grow plants in the same pot, or in pots of the same size, or close together in the open ground; to carefully exclude insects; and then to fertilise some of the flowers with pollen from the same flower, and others on the same plant with pollen from a distinct but adjoining plant. in many, but not all, of these experiments, the crossed plants yielded much more seed than the self-fertilised plants; and i have never seen the { } reversed case. the self-fertilised and crossed seeds thus obtained were allowed to germinate in the same glass vessel on damp sand; and as the seeds successively germinated, they were planted in pairs on opposite sides of the same pot, with a superficial partition between them, and were placed so as to be equally exposed to the light. in other cases the self-fertilised and crossed seeds were simply sown on opposite sides of the same small pot. i have, in short, followed different plans, but in every case have taken all the precautions which i could think of, so that the two lots should be equally favoured. now, i have carefully observed the growth of plants raised from crossed and self-fertilised seed, from their germination to maturity, in species of the following genera, namely, brassica, lathyrus, lupinus, lobelia, lactuca, dianthus, myosotis, petunia, linaria, calceolaria, mimulus, and ipomoea, and the difference in their powers of growth, and of withstanding in certain cases unfavourable conditions, was most manifest and strongly marked. it is of importance that the two lots of seed should be sown or planted on opposite sides of the same pot, so that the seedlings may struggle against each other; for if sown separately in ample and good soil, there is often but little difference in their growth. i will briefly describe the two most striking cases as yet observed by me. six crossed and six self-fertilised seeds of _ipomoea purpurea_, from plants treated in the manner above described, were planted as soon as they had germinated, in pairs on opposite sides of two pots, and rods of equal thickness were given them to twine up. five of the crossed plants grew from the first more quickly than the opposed self-fertilised plants; the sixth, however, was weakly and was for a time beaten, but at last its sounder constitution prevailed and it shot ahead of its antagonist. as soon as each crossed plant reached the top of its seven-foot rod its fellow was measured, and the result was that, when the crossed plants were seven feet high, the self-fertilised had attained the average height of only five feet four and a half inches. the crossed plants flowered a little before, and more profusely than the self-fertilised plants. on opposite sides of another _small_ pot a large number of crossed and self-fertilised seeds were sown, so that they had to struggle for bare existence; a single rod was given to each lot: here again the crossed plants showed from the first their advantage; they never quite reached the summit of the seven-foot rod, but relatively to the self-fertilised plants their average height was as seven feet to five feet two inches. the experiment was repeated in the two following generations with plants raised from the self-fertilised and crossed plants, treated in exactly the same manner, and with nearly the same result. in the second generation, the crossed plants, which were again crossed, produced seed-capsules, whilst the self-fertilised plants, again self-fertilised, produced only capsules. some flowers of the _mimulus luteus_ were fertilised with their own pollen, and others were crossed with pollen from distinct plants growing in the same pot. the seeds after germinating were thickly planted on opposite sides of a pot. the seedlings were at first equal in height; but when the young crossed plants were exactly half an inch, the { } self-fertilised plants were only a quarter of an inch high. but this inequality did not continue, for, when the crossed plants were four and a half inches high, the self-fertilised were three inches; and they retained the same relative difference till their growth was complete. the crossed plants looked far more vigorous than the uncrossed, and flowered before them; they produced also a far greater number of flowers, which yielded capsules (judging, however, from only a few) containing more seeds. as in the former case, the experiment was repeated in the same manner during the next two generations, and with exactly the same result. had i not watched these plants of the mimulus and ipomoea during their whole growth, i could not have believed it possible, that a difference apparently so slight, as that of the pollen being taken from the same flower, and from a distinct plant growing in the same small pot, could have made so wonderful a difference in the growth and vigour of the plants thus produced. this, under a physiological point of view, is a most remarkable phenomenon. * * * * * with respect to the benefit derived from crossing distinct varieties, plenty of evidence has been published. sageret[ ] repeatedly speaks in strong terms of the vigour of melons raised by crossing different varieties, and adds that they are more easily fertilised than common melons, and produce numerous good seed. here follows the evidence of an english gardener:[ ] "i have this summer met with better success in my cultivation of melons, in an unprotected state, from the seeds of hybrids (_i.e._ mongrels) obtained by cross impregnation, than with old varieties. the offspring of three different hybridisations (one more especially, of which the parents were the two most dissimilar varieties i could select) each yielded more ample and finer produce than any one of between twenty and thirty established varieties." andrew knight[ ] believed that his seedlings from crossed varieties of the apple exhibited increased vigour and luxuriance; and m. chevreul[ ] alludes to the extreme vigour of some of the crossed fruit-trees raised by sageret. by crossing reciprocally the tallest and shortest peas, knight[ ] says, "i had in this experiment a striking instance of the stimulative effects of crossing the breeds; for the smallest variety, whose height rarely exceeded two feet, was increased to six feet; whilst the height of the large and luxuriant kind was very little diminished." mr. laxton gave me seed-peas produced from crosses between four distinct kinds; and the plants thus raised were extraordinarily vigorous, being in each case from one to two or three feet taller than the parent-forms growing close alongside them. { } wiegmann[ ] made many crosses between several varieties of cabbage; and he speaks with astonishment of the vigour and height of the mongrels, which excited the amazement of all the gardeners who beheld them. mr. chaundy raised a great number of mongrels by planting together six distinct varieties of cabbage. these mongrels displayed an infinite diversity of character; "but the most remarkable circumstance was, that, while all the other cabbages and borecoles in the nursery were destroyed by a severe winter, these hybrids were little injured, and supplied the kitchen when there was no other cabbage to be had." mr. maund exhibited before the royal agricultural society[ ] specimens of crossed wheat, together with their parent varieties; and the editor states that they were intermediate in character, "united with that greater vigour of growth, which it appears, in the vegetable as in the animal world, is the result of a first cross." knight also crossed several varieties of wheat,[ ] and he says "that in the years and , when almost the whole crop of corn in the island was blighted, the varieties thus obtained, and these only, escaped in this neighbourhood, though sown in several different soils and situations." here is a remarkable case: m. clotzsch[ ] crossed _pinus sylvestris_ and _nigricans_, _quercus robur_ and _pedunculata, alnus glutinosa_ and _incana_, _ulmus campestris_ and _effusa_; and the cross-fertilised seeds, as well as seeds of the pure parent-trees, were all sown at the same time and in the same place. the result was, that after an interval of eight years, the hybrids were one-third taller than the pure trees! * * * * * the facts above given refer to undoubted varieties, excepting the trees crossed by clotzsch, which are ranked by various botanists as strongly-marked races, sub-species, or species. that true hybrids raised from entirely distinct species, though they lose in fertility, often gain in size and constitutional vigour, is certain. it would be superfluous to quote any facts; for all experimenters, kölreuter, gärtner, herbert, sageret, lecoq, and naudin, have been struck with the wonderful vigour, height, size, tenacity of life, precocity, and hardiness of their hybrid productions. gärtner[ ] sums up his conviction on this head in the strongest terms. kölreuter[ ] gives numerous precise measurements of the weight and height of his hybrids in comparison with measurements of both parent-forms; and speaks with astonishment of their "_statura portentosa_," their "_ambitus vastissimus ac altitudo valde conspicua_." some exceptions to the rule in the case of very sterile hybrids have, however, been noticed by gärtner and { } herbert; but the most striking exceptions are given by max wichura,[ ] who found that hybrid willows were generally tender in constitution, dwarf, and short-lived. kölreuter explains the vast increase in the size of the roots, stems, &c., of his hybrids, as the result of a sort of compensation due to their sterility, in the same way as many emasculated animals are larger than the perfect males. this view seems at first sight extremely probable, and has been accepted by various authors;[ ] but gärtner[ ] has well remarked that there is much difficulty in fully admitting it; for with many hybrids there is no parallelism between the degree of their sterility and their increased size and vigour. the most striking instances of luxuriant growth have been observed with hybrids which were not sterile in any extreme degree. in the genus mirabilis, certain hybrids are unusually fertile, and their extraordinary luxuriance of growth, together with their enormous roots,[ ] have been transmitted to their progeny. the increased size of the hybrids produced between the fowl and pheasant, and between the distinct species of pheasants, has been already noticed. the result in all cases is probably in part due to the saving of nutriment and vital force through the sexual organs not acting, or acting imperfectly, but more especially to the general law of good being derived from a cross. for it deserves especial attention that mongrel animals and plants, which are so far from being sterile that their fertility is often actually augmented, have, as previously shown, their size, hardiness, and constitutional vigour generally increased. it is not a little remarkable that an accession of vigour and size should thus arise under the opposite contingencies of increased and diminished fertility. it is a perfectly well ascertained fact[ ] that hybrids will invariably breed more readily with either pure parent, and not rarely with a distinct species, than with each other. herbert is inclined to explain even this fact by the advantage derived from a cross; but gärtner more justly accounts for it by the pollen of the hybrid, and probably its ovules, being in some degree vitiated, whereas the pollen and ovules of both pure parents and of any third species are sound. nevertheless there are some well-ascertained and remarkable facts, which, as we shall immediately see, show that the act of crossing in itself undoubtedly tends to increase or re-establish the fertility of hybrids. _on certain hermaphrodite plants which, either normally or abnormally, require to be fertilised by pollen from a distinct individual or species._ the facts now to be given differ from those hitherto detailed, as the self-sterility does not here result from long-continued, { } close interbreeding. these facts are, however, connected with our present subject, because a cross with a distinct individual is shown to be either necessary or advantageous. dimorphic and trimorphic plants, though they are hermaphrodites, must be reciprocally crossed, one set of forms by the other, in order to be fully fertile, and in some cases to be fertile in any degree. but i should not have noticed these plants, had it not been for the following cases given by dr. hildebrand:[ ]-- _primula sinensis_ is a reciprocally dimorphic species: dr. hildebrand fertilised twenty-eight flowers of both forms, each by pollen of the other form, and obtained the full number of capsules containing on an average . seed per capsule; here we have complete and normal fertility. he then fertilised forty-two flowers of both forms with pollen of the same form, but taken from a distinct plant, and all produced capsules containing on an average only . seed. lastly, and here we come to our more immediate point, he fertilised forty-eight flowers of both forms with pollen of the same form, taken from the same flower, and now he obtained only thirty-two capsules, and these contained on an average . seed, or one less per capsule than in the former case. so that, with these illegitimate unions, the act of impregnation is less assured, and the fertility slightly less, when the pollen and ovules belong to the same flower, than when belonging to two distinct individuals of the same form. dr. hildebrand has recently made analogous experiments on the long-styled form of _oxalis rosea_, with the same result.[ ] it has recently been discovered that certain plants, whilst growing in their native country under natural conditions, cannot be fertilised with pollen from the same plant. they are sometimes so utterly self-impotent, that, though they can readily be fertilised by the pollen of a distinct species or even distinct genus, yet, wonderful as the fact is, they never produce a single seed by their own pollen. in some cases, moreover, the plant's own pollen and stigma mutually act on each other in a deleterious manner. most of the facts to be given relate to orchids, but i will commence with a plant belonging to a widely different family. sixty-three flowers of _corydalis cava_, borne on distinct plants, were fertilised by dr. hildebrand[ ] with pollen from other plants of the same species; and fifty-eight capsules were obtained, including on an average { } . seed in each. he then fertilised sixteen flowers produced by the same raceme, one with another, but obtained only three capsules, one of which alone contained any good seeds, namely, two in number. lastly, he fertilised twenty-seven flowers, each with its own pollen; he left also fifty-seven flowers to be spontaneously fertilised, and this would certainly have ensued if it had been possible, for the anthers not only touch the stigma, but the pollen-tubes were seen by dr. hildebrand to penetrate it; nevertheless these eighty-four flowers did not produce a single seed-capsule! this whole case is highly instructive, as it shows how widely different the action of the same pollen is, according as it is placed on the stigma of the same flower, or on that of another flower on the same raceme, or on that of a distinct plant. with exotic orchids several analogous cases have been observed, chiefly by mr. john scott.[ ] _oncidium sphacelatum_ has effective pollen, for with it mr. scott fertilised two distinct species; its ovules are likewise capable of impregnation, for they were readily fertilised by the pollen of _o. divaricatum_; nevertheless, between one and two hundred flowers fertilised by their own pollen did not produce a single capsule, though the stigmas were penetrated by the pollen-tubes. mr. robinson munro, of the royal botanic gardens of edinburgh, also informs me ( ) that a hundred and twenty flowers of this same species were fertilised by him with their own pollen, and did not produce a capsule, but eight flowers fertilised by the pollen of _o. divaricatum_ produced four fine capsules: again, between two and three hundred flowers of _o. divaricatum_, fertilised by their own pollen, did not set a capsule, but twelve flowers fertilised by _o. flexuosum_ produced eight fine capsules: so that here we have three utterly self-impotent species, with their male and female organs perfect, as shown by their mutual fertilisation. in these cases fertilisation was effected only by the aid of a distinct species. but, as we shall presently see, distinct plants, raised from seed, of _oncidium flexuosum_, and probably of the other species, would have been perfectly capable of fertilising each other, for this is the natural process. again, mr. scott found that the pollen of a plant of _o. microchilum_ was good, for with it he fertilised two distinct species; he found its ovules good, for they could be fertilised by the pollen of one of these species, and by the pollen of a distinct plant of _o. microchilum_; but they could not be fertilised by pollen of the same plant, though the pollen-tubes penetrated the stigma. an analogous case has been recorded by m. rivière,[ ] with two plants of _o. cavendishianum_, which were both self-sterile, but reciprocally fertilised each other. all these cases refer to the genus oncidium, but mr. scott found that _maxillaria atro-rubens_ was "totally insusceptible of fertilisation with its own pollen," but fertilised, and was fertilised by, a widely distinct species, viz. _m. squalens_. as these orchids had grown under unnatural conditions, in { } hot-houses, i concluded without hesitation that their self-sterility was due to this cause. but fritz müller informs me that at desterro, in brazil, he fertilised above one hundred flowers of the above-mentioned _oncidium flexuosum_, which is there endemic, with its own pollen, and with that taken from distinct plants; all the former were sterile, whilst those fertilised by pollen from any _other plant_ of the same species were fertile. during the first three days there was no difference in the action of the two kinds of pollen: that placed on the stigma of the same plant separated in the usual manner into grains, and emitted tubes which penetrated the column, and the stigmatic chamber shut itself; but the flowers alone which had been fertilised by pollen taken from a distinct plant produced seed-capsules. on a subsequent occasion these experiments were repeated on a large scale with the same result. fritz müller found that four other endemic species of oncidium were in like manner utterly sterile with their own pollen, but fertile with that from any other plant: some of them likewise produced seed-capsules when impregnated with pollen of widely distinct genera, such as leptotes, cyrtopodium, and rodriguezia! _oncidium crispum_, however, differs from the foregoing species in varying much in its self-sterility; some plants producing fine pods with their own pollen, others failing to do so; in two or three instances, fritz müller observed that the pods produced by pollen taken from a distinct flower on the same plant, were larger than those produced by the flower's own pollen. in _epidendrum cinnabarinum_, an orchid belonging to another division of the family, fine pods were produced by the plant's own pollen, but they contained by weight only about half as much seed as the capsules which had been fertilized by pollen from a distinct plant, and in one instance from a distinct species; moreover, a very large proportion, and in some cases nearly all the seed produced by the plant's own pollen, was embryonless and worthless. some self-fertilized capsules of a maxillaria were in a similar state. another observation made by fritz müller is highly remarkable, namely, that with various orchids the plant's own pollen not only fails to impregnate the flower, but acts on the stigma, and is acted on, in an injurious or poisonous manner. this is shown by the surface of the stigma in contact with the pollen, and by the pollen itself, becoming in from three to five days dark brown, and then decaying. the discolouration and decay are not caused by parasitic cryptogams, which were observed by fritz müller in only a single instance. these changes are well shown by placing on the same stigma, at the same time, the plant's own pollen and that from a distinct plant of the same species, or of another species, or even of another and widely remote genus. thus, on the stigma of _oncidium flexuosum_, the plant's own pollen and that from a distinct plant were placed side by side, and in five days' time the latter was perfectly fresh, whilst the plant's own pollen was brown. on the other hand, when the pollen of a distinct plant of the _oncidium flexuosum_, and of the _epidendrum zebra_ (_nov. spec.?_), were placed together on the same stigma, they behaved in exactly the same manner, the grains separating, emitting tubes, and penetrating the stigma, so that the two { } pollen-masses, after an interval of eleven days, could not be distinguished except by the difference of their caudicles, which, of course, undergo no change. fritz müller has, moreover, made a large number of crosses between orchids belonging to distinct species and genera, and he finds that in all cases when the flowers are not fertilised their footstalks first begin to wither; and the withering slowly spreads upwards until the germens fall off, after an interval of one or two weeks, and in one instance of between six and seven weeks; but even in this latter case, and in most other cases, the pollen and stigma remained in appearance fresh. occasionally, however, the pollen becomes brownish, generally on the external surface, and not in contact with the stigma, as is invariably the case when the plant's own pollen is applied. fritz müller observed the poisonous action of the plant's own pollen in the above-mentioned _oncidium flexuosum_, _o. unicorne, pubes_ (_?_), and in two other unnamed species. also in two species of rodriguezia, in two of notylia, in one of burlingtonia, and of a fourth genus in the same group. in all these cases, except the last, it was proved that the flowers were, as might have been expected, fertile with pollen from a distinct plant of the same species. numerous flowers of one species of notylia were fertilized with pollen from the same raceme; in two days' time they all withered, the germens began to shrink, the pollen-masses became dark brown, and not one pollen-grain emitted a tube. so that in this orchid the injurious action of the plant's own pollen is more rapid than with _oncidium flexuosum_. eight other flowers on the same raceme were fertilized with pollen from a distinct plant of the same species: two of these were dissected, and their stigmas were found to be penetrated by numberless pollen-tubes; and the germens of the other six flowers became well developed. on a subsequent occasion many other flowers were fertilized with their own pollen, and all fell off dead in a few days; whilst some flowers on the same raceme which had been left simply unfertilised adhered and long remained fresh. we have seen that in cross-unions between extremely distinct orchids the pollen long remains undecayed; but notylia behaved in this respect differently; for when its pollen was placed on the stigma of _oncidium flexuosum_, both the stigma and pollen quickly became dark brown, in the same manner as if the plant's own pollen had been applied. fritz müller suggests that, as in all these cases the plant's own pollen is not only impotent (thus effectually preventing self-fertilization), but likewise prevents, as was ascertained in the case of the notylia and _oncidium flexuosum_, the action of subsequently applied pollen from a distinct individual, it would be an advantage to the plant to have its own pollen rendered more and more deleterious; for the germens would thus quickly be killed, and, dropping off, there would be no further waste in nourishing a part which ultimately could be of no avail. fritz müller's discovery that a plant's own pollen and stigma in some cases act on each other as if mutually poisonous, is certainly most remarkable. we now come to cases closely analogous with those just { } given, but different, inasmuch as individual plants alone of the species are self-impotent. this self-impotence does not depend on the pollen or ovules being in a state unfit for fertilisation, for both have been found effective in union with other plants of the same or of a distinct species. the fact of these plants having spontaneously acquired so peculiar a constitution, that they can be fertilised more readily by the pollen of a distinct species than by their own, is remarkable. these abnormal cases, as well as the foregoing normal cases, in which certain orchids, for instance, can be much more easily fertilised by the pollen of a distinct species than by their own, are exactly the reverse of what occurs with all ordinary species. for in these latter the two sexual elements of the same individual plant are capable of freely acting on each other; but are so constituted that they are more or less impotent when brought into union with the sexual elements of a distinct species, and produce more or less sterile hybrids. it would appear that the pollen or ovules, or both, of the individual plants which are in this abnormal state, have been affected in some strange manner by the conditions to which they themselves or their parents have been exposed; but whilst thus rendered self-sterile, they have retained the capacity common to most species of partially fertilizing and being partially fertilized by allied forms. however this may be, the subject, to a certain extent, is related to our general conclusion that good is derived from the act of crossing. gärtner experimented on two plants of _lobelia fulgens_, brought from separate places, and found[ ] that their pollen was good, for he fertilised with it _l. cardinalis_ and _syphilitica_; their ovules were likewise good, for they were fertilised by the pollen of these same two species; but these two plants of _l. fulgens_ could not be fertilised by their own pollen, as can generally be effected with perfect ease with this species. again, the pollen of a plant of _verbascum nigrum_ grown in a pot was found by gärtner[ ] capable of fertilising _v. lychnitis_ and _v. austriacum_; the ovules could be fertilised by the pollen of _v. thapsus_; but the flowers could not be fertilised by their own pollen. kölreuter, also,[ ] gives the case of three { } garden plants of _verbascum phoeniceum_, which bore during two years many flowers; these he successfully fertilised by the pollen of no less than four distinct species, but they produced not a seed with their own apparently good pollen; subsequently these same plants, and others raised from seed, assumed a strangely fluctuating condition, being temporarily sterile on the male or female side, or on both sides, and sometimes fertile on both sides; but two of the plants were perfectly fertile throughout the summer. it appears[ ] that certain flowers on certain plants of _lilium candidum_ can be fertilised more easily by pollen from a distinct individual than by their own. so, again, with the varieties of the potato. tinzmann,[ ] who made many trials with this plant, says that pollen from another variety sometimes "exerts a powerful influence, and i have found sorts of potatoes which would not bear seed from impregnation with the pollen of their own flowers, would bear it when impregnated with other pollen." it does not, however, appear to have been proved that the pollen which failed to act on the flower's own stigma was in itself good. in the genus passiflora it has long been known that several species do not produce fruit, unless fertilised by pollen taken from distinct species: thus, mr. mowbray[ ] found that he could not get fruit from _p. alata_ and _racemosa_ except by reciprocally fertilising them with each other's pollen. similar facts have been observed in germany and france;[ ] and i have received two authentic accounts of _p. quadrangularis_, which never produced fruit with its own pollen, but would do so freely when fertilised in one case with the pollen of _p. coerulea_, and in another case with that of _p. edulis_. so again, with respect to _p. laurifolia_, a cultivator of much experience has recently remarked[ ] that the flowers "must be fertilised with the pollen of _p. coerulea_, or of some other common kind, as their own pollen will not fertilise them." but the fullest details on this subject have been given by mr. scott:[ ] plants of _passiflora racemosa_, _coerulea_, and _alata_ flowered profusely during many years in the botanic gardens of edinburgh, and, though repeatedly fertilised by mr. scott and by others with their own pollen, never produced any seed; yet this occurred at once with all three species when they were crossed together in various ways. but in the case of _p. coerulea_, three plants, two of which grew in the botanic gardens, were all rendered fertile, merely by impregnating the one with pollen of the other. the same result was attained in the same manner with _p. alata_, but only with one plant out of three. as so many self-sterile species have been mentioned, it may be stated that in the case of _p. gracilis_, which is an annual, the flowers are nearly as fertile with their own pollen as with that from a distinct plant; thus sixteen flowers { } spontaneously self-fertilised produced fruit, each containing on an average . seed, whilst fruit from fourteen crossed flowers contained . seed. returning to _p. alata_, i have received ( ) some interesting details from mr. robinson munro. three plants, including one in england, have already been mentioned which were inveterately self-sterile, and mr. munro informs me of several others which, after repeated trials during many years, have been found in the same predicament. at some other places, however, this species fruits readily when fertilised with its own pollen. at taymouth castle there is a plant which was formerly grafted by mr. donaldson on a distinct species, name unknown, and ever since the operation it has produced fruit in abundance by its own pollen; so that this small and unnatural change in the state of this plant has restored its self-fertility! some of the seedlings from the taymouth castle plant were found to be not only sterile with their own pollen, but with each other's pollen, and with the pollen of distinct species. pollen from the taymouth plant failed to fertilise certain plants of the same species, but was successful on one plant in the edinburgh botanic gardens. seedlings were raised from this latter union, and some of their flowers were fertilised by mr. munro with their own pollen; but they were found to be as self-impotent as the mother-plant had always proved, except when fertilised by the grafted taymouth plant, and except, as we shall see, when fertilised by her own seedlings. for mr. munro fertilised eighteen flowers on the self-impotent mother-plant with pollen from these her own self-impotent seedlings, and obtained, remarkable as the fact is, eighteen fine capsules full of excellent seed! i have met with no case in regard to plants which shows so well as this of _p. alata_, on what small and mysterious causes complete fertility or complete sterility depends. the facts hitherto given relate to the much-lessened or completely destroyed fertility of pure species when impregnated with their own pollen, in comparison with their fertility when impregnated by distinct individuals or distinct species; but closely analogous facts have been observed with hybrids. herbert states[ ] that having in flower at the same time nine hybrid hippeastrums, of complicated origin, descended from several species, he found that "almost every flower touched with pollen from another cross produced seed abundantly, and those which were touched with their own pollen either failed entirely, or formed slowly a pod of inferior size, with fewer seeds." in the 'horticultural journal' he adds that, "the admission of the pollen of another cross-bred hippeastrum (however complicated the cross) to any _one_ flower of the number, is almost sure to check the fructification of the others." in a letter written to me in , dr. herbert says that he had already tried these experiments during five consecutive years, and he subsequently repeated them, with the same invariable result. { } he was thus led to make an analogous trial on a pure species, namely, on the _hippeastrum aulicum_, which he had lately imported from brazil: this bulb produced four flowers, three of which were fertilised by their own pollen, and the fourth by the pollen of a triple cross between _h. bulbulosum_, _reginæ_, and _vittatum_; the result was, that "the ovaries of the three first flowers soon ceased to grow, and after a few days perished entirely: whereas the pod impregnated by the hybrid made vigorous and rapid progress to maturity, and bore good seed, which vegetated freely." this is, indeed, as herbert remarks, "a strange truth," but not so strange as it then appeared. as a confirmation of these statements, i may add that mr. m. mayes,[ ] after much experience in crossing the species of amaryllis (hippeastrum), says, "neither the species nor the hybrids will, we are well aware, produce seed so abundantly from their own pollen as from that of others." so, again, mr. bidwell, in new south wales,[ ] asserts that _amaryllis belladonna_ bears many more seeds when fertilised by the pollen of _brunswigia_ (_amaryllis_ of some authors) _josephinæ_ or of _b. multiflora_, than when fertilised by its own pollen. mr. beaton dusted four flowers of a cyrtanthus with their own pollen, and four with the pollen of _vallota_ (_amaryllis_) _purpurea_; on the seventh day "those which received their own pollen slackened their growth, and ultimately perished; those which were crossed with the vallota held on."[ ] these latter cases, however, relate to uncrossed species, like those before given with respect to passiflora, orchids, &c., and are here referred to only because the plants belong to the same group of amaryllidaceæ. in the experiments on the hybrid hippeastrums, if herbert had found that the pollen of two or three kinds alone had been more efficient on certain kinds than their own pollen, it might have been argued that these, from their mixed parentage, had a closer mutual affinity than the others; but this explanation is inadmissible, for the trials were made reciprocally backwards and forwards on nine different hybrids; and a cross, whichever way taken, always proved highly beneficial. i can add a striking and analogous case from experiments made by the rev. a. rawson, of bromley common, with some complex hybrids of gladiolus. this skilful horticulturist possessed a number of french varieties, differing from each other only in the colour and size of the flowers, all descended from gandavensis, a well-known old hybrid, said to be descended from _g. natalensis_ by the pollen of _g. oppositiflorus_.[ ] mr. rawson, after repeated trials, found that none of the varieties would set seed with their own pollen, although { } taken from distinct plants of the same variety, which had, of course, been propagated by bulbs, but that they all seeded freely with pollen from any other variety. to give two examples: ophir did not produce a capsule with its own pollen, but when fertilised with that of janire, brenchleyensis, vulcain, and linné, it produced ten fine capsules; but the pollen of ophir was good, for when linné was fertilised by it seven capsules were produced. this later variety, on the other hand, was utterly barren with its own pollen, which we have seen was perfectly efficient on ophir. altogether, mr. rawson, in the year , fertilised twenty-six flowers borne by four varieties with pollen taken from other varieties, and every single flower produced a fine seed-capsule; whereas fifty-two flowers on the same plants, fertilised at the same time with their own pollen, did not yield a single seed-capsule. mr. rawson fertilised, in some cases, the alternate flowers, and in other cases all those down one side of the spike, with pollen of other varieties, and the remaining flowers with their own pollen; i saw these plants when the capsules were nearly mature, and their curious arrangement at once brought full conviction to the mind that an immense advantage had been derived from crossing these hybrids. lastly, i have heard from dr. e. bornet, of antibes, who has made numerous experiments in crossing the species of cistus, but as not yet published the results, that, when any of these hybrids are fertile, they may be said to be, in regard to function, dioecious; "for the flowers are always sterile when the pistil is fertilised by pollen taken from the same flower or from flowers on the same plant. but they are often fertile if pollen be employed from a distinct individual of the same hybrid nature, or from a hybrid made by a reciprocal cross." _conclusion._--the facts just given, which show that certain plants are self-sterile, although both sexual elements are in a fit state for reproduction when united with distinct individuals of the same or other species, appear at first sight opposed to all analogy. the sexual elements of the same flower have become, as already remarked, differentiated in relation to each other, almost like those of two distinct species. with respect to the species which, whilst living under their natural conditions, have their reproductive organs in this peculiar state, we may conclude that it has been naturally acquired for the sake of effectually preventing self-fertilisation. the case is closely analous with dimorphic and trimorphic plants, which can be fully fertilised only by plants belong to the opposite form, and not, as in the foregoing cases, in differently by any other plant. some of these dimorphic plants are completely sterile with pollen taken from the same plant or from the same { } form. it is interesting to observe the graduated series from plants which, when fertilised by their own pollen, yield the full number of seed, but with the seedlings a little dwarfed in stature--to plants which when self-fertilised yield few seeds--to those with yield none--and, lastly, to those in which the plant's own pollen and stigma act on each other like poison. this peculiar state of the reproductive organs, when occurring in certain individuals alone, is evidently abnormal; and as it chiefly affects exotic plants, or indigenous plants cultivated in pots, we may attribute it to some change in the conditions of life, acting on the plants themselves or on their parents. the self-impotent _passiflora alata_, which recovered its self-fertility after having been grafted on a distinct stock, shows how small a change is sufficient to act powerfully on the reproductive system. the possibility of a plant becoming under culture self-impotent is interesting as throwing light on the occurrence of this same condition in natural species. a cultivated plant in this state generally remains so during its whole life; and from this fact we may infer that the state is probably congenital. kölreuter, however, has described some plants of verbascum which varied in this respect even during the same season. as in all the normal cases, and in many, probably in most, of the abnormal cases, any two self-impotent plants can reciprocally fertilize each other, we may infer that a very slight difference in the nature of their sexual elements suffices to give fertility; but in other instances, as with some passifloras and the hybrid gladioli, a greater degree of differentiation appears to be necessary, for with these plants fertility is gained only by the union of distinct species, or of hybrids of distinct parentage. these facts all point to the same general conclusion, namely, that good is derived from a cross between individuals, which either innately, or from exposure to dissimilar conditions, have come to differ in sexual constitution. exotic animals confined in menageries are sometimes in nearly the same state as the above-described self-impotent plants; for, as we shall see in the following chapter, certain monkeys, the larger carnivora, several finches, geese, and pheasants, cross together, quite as freely as, or even more freely than, the individuals of the same species breed together. cases will, { } also, be given of sexual incompatibility between certain male and female domesticated animals, which, nevertheless, are fertile when matched with any other individual of the same kind. in the early part of this chapter it was shown that the crossing of distinct forms, whether closely or distantly allied, gives increased size and constitutional vigour, and, except in the case of crossed species, increased fertility, to the offspring. the evidence rests on the universal testimony of breeders (for it should be observed that i am not here speaking of the evil results of close interbreeding), and is practically exemplified in the higher value of cross-bred animals for immediate consumption. the good results of crossing have also been demonstrated, in the case of some animals and of numerous plants, by actual weight and measurement. although animals of pure blood will obviously be deteriorated by crossing, as far as their characteristic qualities are concerned, there seems to be no exception to the rule that advantages of the kind just mentioned are thus gained, even when there has not been any previous close interbreeding. the rule applies to all animals, even to cattle and sheep, which can long resist breeding in-and-in between the nearest blood-relations. it applies to individuals of the same sub-variety but of distinct families, to varieties or races, to sub-species, as well as to quite distinct species. in this latter case, however, whilst size, vigour, precocity, and hardiness are, with rare exceptions, gained, fertility, in a greater or less degree, is lost; but the gain cannot be exclusively attributed to the principle of compensation; for there is no close parallelism between the increased size and vigour of the offspring and their sterility. moreover it has been clearly proved that mongrels which are perfectly fertile gain these same advantages as well as sterile hybrids. the evil consequences of long-continued close interbreeding are not so easily recognised as the good effects from crossing, for the deterioration is gradual. nevertheless it is the general opinion of those who have had most experience, especially with animals which propagate quickly, that evil does inevitably follow sooner or later, but at different rates with different animals. no doubt a false belief may widely prevail like a superstition; yet it is difficult to suppose that so many acute and original { } observers have all been deceived at the expense of much cost and trouble. a male animal may sometimes be paired with his daughter, granddaughter, and so on, even for seven generations, without any manifest bad result; but the experiment has never been tried of matching brothers and sisters, which is considered the closest form of interbreeding, for an equal number of generations. there is good reason to believe that by keeping the members of the same family in distinct bodies, especially if exposed to somewhat different conditions of life, and by occasionally crossing these families, the evil results may be much diminished, or quite eliminated. these results are loss of constitutional vigour, size, and fertility; but there is no necessary deterioration in the general form of the body, or in other good qualities. we have seen that with pigs first-rate animals have been produced after long-continued close interbreeding, though they had become extremely infertile when paired with their near relations. the loss of fertility, when it occurs, seems never to be absolute, but only relative to animals of the same blood; so that this sterility is to a certain extent analogous with that of self-impotent plants which cannot be fertilised by their own pollen, but are perfectly fertile with pollen of any other plant of the same species. the fact of infertility of this peculiar nature being one of the results of long-continued interbreeding, shows that interbreeding does not act merely by combining and augmenting various morbid tendencies common to both parents; for animals with such tendencies, if not at the time actually ill, can generally propagate their kind. although offspring descended from the nearest blood-relations are not necessarily deteriorated in structure, yet some authors[ ] believe that they are eminently liable to malformations; and this is not improbable, as everything which lessens the vital powers acts in this manner. instances of this kind have been recorded in the case of pigs, bloodhounds, and some other animals. finally, when we consider the various facts now given which plainly show that good follows from crossing, and less plainly { } that evil follows from close interbreeding, and when we bear in mind that throughout the whole organic world elaborate provision has been made for the occasional union of distinct individuals, the existence of a great law of nature is, if not proved, at least rendered in the highest degree probable; namely, that the crossing of animals and plants which are not closely related to beach other is highly beneficial or even necessary, and that interbreeding prolonged during many generations is highly injurious. * * * * * { } chapter xviii. on the advantages and disadvantages of changed conditions of life: sterility from various causes. on the good derived from slight changes in the conditions of life--sterility from changed conditions, in animals, in their native country and in menageries--mammals, birds, and insects--loss of secondary sexual characters and of instincts--causes of sterility--sterility of domesticated animals from changed conditions--sexual incompatibility of individual animals--sterility of plants from changed conditions of life--contabescence of the anthers--monstrosities as a cause of sterility--double flowers--seedless fruit--sterility from the excessive development of the organs of vegetation--from long-continued propagation by buds--incipient sterility the primary cause of double flowers and seedless fruit. _on the good derived from slight changes in the conditions of life._--in considering whether any facts were known which might throw light on the conclusion arrived at in the last chapter, namely, that benefits ensue from crossing, and that it is a law of nature that all organic beings should occasionally cross, it appeared to me probable that the good derived from slight changes in the conditions of life, from being an analogous phenomenon, might serve this purpose. no two individuals, and still less no two varieties, are absolutely alike in constitution and structure; and when the germ of one is fertilised by the male element of another, we may believe that it is acted on in a somewhat similar manner as an individual when exposed to slightly changed conditions. now, every one must have observed the remarkable influence on convalescents of a change of residence, and no medical man doubts the truth of this fact. small farmers who hold but little land are convinced that their cattle derive great benefit from a change of pasture. in the case of plants, the evidence is strong that a great advantage is derived from exchanging seeds, tubers, bulbs, and cuttings from one soil or place to another as different as possible. { } the belief that plants are thus benefited, whether or not well founded, has been firmly maintained from the time of columella, who wrote shortly after the christian era, to the present day; and it now prevails in england, france, and germany.[ ] a sagacious observer, bradley, writing in ,[ ] says, "when we once become masters of a good sort of seed, we should at least put it into two or three hands, where the soils and situations are as different as possible; and every year the parties should change with one another; by which means, i find the goodness of the seed will be maintained for several years. for want of this use many farmers have failed in their crops and been great losers." he then gives his own practical experience on this head. a modern writer[ ] asserts, "nothing can be more clearly established in agriculture than that the continual growth of any one variety in the same district makes it liable to deterioration either in quality or quantity." another writer states that he sowed close together in the same field two lots of wheat-seed, the product of the same original stock, one of which had been grown on the same land, and the other at a distance, and the difference in favour of the crop from the latter seed was remarkable. a gentleman in surrey who has long made it his business to raise wheat to sell for seed, and who has constantly realised in the market higher prices than others, assures me that he finds it indispensable continually to change his seed; and that for this purpose he keeps two farms differing much in soil and elevation. with respect to the tubers of the potato, i find that at the present day the practice of exchanging sets is almost everywhere followed. the great growers of potatoes in lancashire formerly used to get tubers from scotland, but they found that "a change from the moss-lands, and _vice versâ_, was generally sufficient." in former times in france the crop of potatoes in the vosges had become reduced in the course of fifty or sixty years in the proportion from - to - bushels; and the famous oberlin attributed the surprising good which he effected in large part to changing the sets.[ ] a well-known practical gardener, mr. robson[ ] positively states that he has himself witnessed decided advantage from obtaining bulbs of the onion, tubers of the potato, and various seeds, all of the same kind, from different soils and distant parts of england. he further states that with { } plants propagated by cuttings, as with the pelargonium, and especially the dahlia, manifest advantage is derived from getting plans of the same variety, which have been cultivated in another place; or, "where the extent of the place allows, to take cuttings from one description of soil to plant on another, so as to afford the change that seems so necessary to the well-being of the plants." he maintains that after a time an exchange of this nature is "forced on the grower, whether he be prepared for it or not." similar remarks have been made by another excellent gardener, mr. fish, namely, that cuttings of the same variety of calceolaria, which he obtained from a neighbour, "showed much greater vigour than some of his own that were treated in exactly the same manner," and he attributed this solely to his own plants having become "to a certain extent worn out or tired of their quarters." something of this kind apparently occurs in grafting and budding fruit-trees; for, according to mr. abbey, grafts or buds generally take on a distinct variety or even species, or on a stock previously grafted, with greater facility than on stocks raised from seeds of the variety which is to be grafted; and he believes this cannot be altogether explained by the stocks in question being better adapted to the soil and climate of the place. it should, however, be added, that varieties grafted or budded on very distinct kinds, though they may take more readily and grow at first more vigorously than when grafted on closely allied stocks, afterwards often become unhealthy. i have studied m. tessier's careful and elaborate experiments,[ ] made to disprove the common belief that good is derived from a change of seed; and he certainly shows that the same seed may with care be cultivated on the same farm (it is not stated whether on exactly the same soil) for ten consecutive years without loss. another excellent observer, colonel le couteur,[ ] has come to the same conclusion; but then he expressly adds, if the same seed be used, "that which is grown on land manured from the mixen one year becomes seed for land prepared with lime, and that again becomes seed for land dressed with ashes, then for land dressed with mixed manure, and so on." but this in effect is a systematic exchange of seed, within the limits of the same farm. on the whole the belief, which has long been held by many skilful cultivators, that good follows from exchanging seed, tubers, &c., seems to be fairly well founded. considering the small size of most seeds, it seems hardly credible that the advantage thus derived can be due to the seeds obtaining in one soil some chemical element deficient in the other soil. as plants after once germinating naturally become fixed to the same spot, it might have been anticipated that they would show the good effects of a change more plainly than animals, which continually wander about; and this apparently is the { } case. life depending on, or consisting in, an incessant play of the most complex forces, it would appear that their action is in some way stimulated by slight changes in the circumstances to which each organism is exposed. all forces throughout nature, as mr. herbert spencer[ ] remarks, tend towards an equilibrium, and for the life of each being it is necessary that this tendency should be checked. if these views and the foregoing facts can be trusted, they probably throw light, on the one hand, on the good effects of crossing the breed, for the germ will be thus slightly modified or acted on by new forces; and on the other hand, on the evil effects of close interbreeding prolonged during many generations, during which the germ will be acted on by a male having almost identically the same constitution. _sterility from changed conditions of life._ i will now attempt to show that animals and plants, when removed from their natural conditions, are often rendered in some degree infertile or completely barren; and this occurs even when the conditions have not been greatly changed. this conclusion is not necessarily opposed to that at which we have just arrived, namely, that lesser changes of other kinds are advantageous to organic beings. our present subject is of some importance, from having an intimate connexion with the causes of variability. indirectly it perhaps bears on the sterility of species when crossed: for as, on the one hand, slight changes in the conditions of life are favourable to plants and animals, and the crossing of varieties adds to the size, vigour, and fertility of their offspring; so, on the other hand, certain other changes in the conditions of life cause sterility; and as this likewise ensues from crossing much-modified forms or species, we have a parallel and double series of facts, which apparently stand in close relation to each other. it is notorious that many animals, though perfectly tamed, { } refuse to breed in captivity. isidore geoffroy st. hilaire[ ] consequently has drawn a broad distinction between tamed animals which will not breed under captivity, and truly domesticated animals which breed freely--generally more freely, as shown in the sixteenth chapter, than in a state of nature. it is possible and generally easy to tame most animals; but experience has shown that it is difficult to get them to breed regularly, or even at all. i shall discuss this subject in detail; but will give only those cases which seem most illustrative. my materials are derived from notices scattered through various works, and especially from a report, drawn up for me by the kindness of the officers of the zoological society of london, which has especial value, as it records all the cases, during nine years from - , in which the animals were seen to couple but produced no offspring, as well as the cases in which they never, as far as known, coupled. this ms. report i have corrected by the annual reports subsequently published. many facts are given on the breeding of the animals in that magnificent work, 'gleanings from the menageries of knowsley hall,' by dr. gray. i made, also, particular inquiries from the experienced keeper of the birds in the old surrey zoological gardens. i should premise that a slight change in the treatment of animals sometimes makes a great difference in their fertility; and it is probable that the results observed in different menageries would differ. indeed some animals in our zoological gardens have become more productive since the year . it is, also, manifest from f. cuvier's account of the jardin des plantes,[ ] that the animals formerly bred much less freely there than with us; for instance, in the duck tribe, which is highly prolific, only one species had at that period produced young. the most remarkable cases, however, are afforded by animals kept in their native country, which, though perfectly tamed, quite healthy, and allowed some freedom, are absolutely incapable of breeding. rengger,[ ] who in paraguay particularly attended to this subject, specifies six quadrupeds in this condition; and he mentions two or three others which most rarely { } breed. mr. bates, in his admirable work on the amazons, strongly insists on similar cases;[ ] and he remarks, that the fact of thoroughly tamed native mammals and birds not breeding when kept by the indians, cannot be wholly accounted for by their negligence or indifference, for the turkey is valued by them, and the fowl has been adopted by the remotest tribes. in almost every part of the world--for instance, in the interior of africa, and in several of the polynesian islands--the natives are extremely fond of taming the indigenous quadrupeds and birds; but they rarely or never succeed in getting them to breed. the most notorious case of an animal not breeding in captivity is that of the elephant. elephants are kept in large numbers in their native indian home, live to old age, and are vigorous enough for the severest labour; yet, with one or two exceptions, they have never been known even to couple, though both males and females have their proper periodical seasons. if, however, we proceed a little eastward to ava, we hear from mr. crawfurd[ ] that their "breeding in the domestic state, or at least in the half-domestic state in which the female elephants are generally kept, is of every-day occurrence;" and mr. crawfurd informs me that he believes that the difference must be attributed solely to the females being allowed to roam the forests with some degree of freedom. the captive rhinoceros, on the other hand, seems from bishop heber's account[ ] to breed in india far more readily than the elephant. four wild species of the horse genus have bred in europe, though here exposed to a great change in their natural habits of life; but the species have generally been crossed one with another. most of the members of the pig family breed readily in our menageries: even the red river hog (_potamochoerus penicillatus_), from the sweltering plains of west africa, has bred twice in the zoological gardens. here also the peccary (_dicotyles torquatus_) has bred several times; but another species, the _d. labiatus_, though rendered so tame as to be half-domesticated, breeds so rarely in its native country of paraguay, that according to rengger[ ] the fact requires confirmation. mr. bates remarks that the tapir, though often kept tame in amazonia by the indians, never breeds. ruminants generally breed quite freely in england, though brought from widely different climates, as may be seen in the annual reports of the zoological gardens, and in the gleanings from lord derby's menagerie. the carnivora, with the exception of the plantigrade division, generally breed (though with capricious exceptions) almost as freely as ruminants. many species of felidæ have bred in various menageries, although imported from various climates and closely confined. mr. bartlett, the present superintendent of the zoological gardens,[ ] remarks that the lion appears to breed more frequently and to bring forth more young at a birth than any other species of the family. he adds that the tiger has rarely bred; { } "but there are several well-authenticated instances of the female tiger breeding with the lion." strange as the fact may appear, many animals under confinement unite with distinct species and produce hybrids quite as freely as, or even more freely than, with their own species. on inquiring from dr. falconer and others, it appears that the tiger when confined in india does not breed, though it has been known to couple. the cheetah (_felis jubata_) has never been known by mr. bartlett to breed in england, but it has bred at frankfort; nor does it breed in india, where it is kept in large numbers for hunting; but no pains would be taken to make them breed, as only those animals which have hunted for themselves in a state of nature are serviceable and worth training.[ ] according to rengger, two species of wild cats in paraguay, though thoroughly tamed, have never bred. although so many of the felidæ breed readily in the zoological gardens, yet conception by no means always follows union: in the nine-year report, various species are specified which were observed to couple seventy-three times, and no doubt this must have passed many times unnoticed; yet from the seventy-three unions only fifteen births ensued. the carnivora in the zoological gardens were formerly less freely exposed to the air and cold than at present, and this change of treatment, as i was assured by the former superintendent, mr. miller, greatly increased their fertility. mr. bartlett, and there cannot be a more capable judge, says, "it is remarkable that lions breed more freely in travelling collections than in the zoological gardens; probably the constant excitement and irritation produced by moving from place to place, or change of air, may have considerable influence in the matter." many members of the dog family breed readily when confined. the dhole is one of the most untameable animals in india, yet a pair kept there by dr. falconer produced young. foxes, on the other hand, rarely breed, and i have never heard of such an occurrence with the european fox: the silver fox of north america (_canis argentatus_), however, has bred several times in the zoological gardens. even the otter has bred there. every one knows how readily the semi-domesticated ferret breeds, though shut up in miserably small cages; but other species of viverra and paradoxurus absolutely refuse to breed in the zoological gardens. the genetta has bred both here and in the jardin des plantes, and produced hybrids. the _herpestes fasciatus_ has likewise bred; but i was formerly assured that the _h. griseus_, though many were kept in the gardens, never bred. the plantigrade carnivora breed under confinement much less freely, without our being able to assign any reason, than other members of the group. in the nine-year report it is stated that the bears had been seen in the zoological gardens to couple freely, but previously to had most rarely conceived. in the reports published since this date three species have produced young (hybrids in one case), and, wonderful to relate, the white polar bear has produced young. the badger (_meles taxus_) has bred several times in the gardens; but i have not heard of this { } occurring elsewhere in england, and the event must be very rare, for an instance in germany has been thought worth recording.[ ] in paraguay the native nasua, though kept in pairs during many years and perfectly tamed, has never been known, according to rengger, to breed or show any sexual passion; nor, as i hear from mr. bates, does this animal, or the cercoleptes, breed in the region of the amazons. two other plantigrade genera, procyon and gulo, though often kept tame in paraguay, never breed there. in the zoological gardens species of nasua and procyon have been seen to couple; but they did not produce young. as domesticated rabbits, guinea-pigs, and white mice breed so abundantly when closely confined under various climates, it might have been thought that most other members of the rodent order would have bred in captivity, but this is not the case. it deserves notice, as showing how the capacity to breed sometimes goes by affinity, that the one native rodent of paraguay, which there breeds _freely_ and has yielded successive generations, is the _cavia aperea_; and this animal is so closely allied to the guinea-pig, that it has been erroneously thought to be the parent-form.[ ] in the zoological gardens, some rodents have coupled, but have never produced young; some have neither coupled nor bred; but a few have bred, as the porcupine more than once, the barbary mouse, lemming, chinchilla, and the agouti (_dasyprocta aguti_), several times. this latter animal has also produced young in paraguay, though they were born dead and ill-formed; but in amazonia, according to mr. bates, it never breeds, though often kept tame about the houses. nor does the paca (_coelogenys paca_) breed there. the common hare when confined has, i believe, never bred in europe;[ ] though, according to a recent statement, it has crossed with the rabbit. i have never heard of the dormouse breeding in confinement. but squirrels offer a more curious case: with one exception, no species has ever bred in the zoological gardens, yet as many as fourteen individuals of _s. palmarum_ were kept together during several years. the _s. cinerea_ has been seen to couple, but it did not produce young; nor has this species, when rendered extremely tame in its native country, north america, been ever known to breed.[ ] at lord derby's menagerie squirrels of many kinds were kept in numbers, but mr. thompson, the superintendent, told me that none had ever bred there, or elsewhere as far as he knew. i have never heard of the english squirrel breeding in confinement. but the species which has bred more than once in the zoological gardens is the one which perhaps might have been least expected, namely, the flying squirrel (_sciuropterus volucella_): it has, also, bred several times { } near birmingham; but the female never produced more than two young at a birth, whereas in its native american home she bears from three to six young.[ ] monkeys, in the nine-year report from the zoological gardens, are stated to unite most freely, but during this period, though many individuals were kept, there were only seven births. i have heard of one american monkey alone, the ouistiti, breeding in europe.[ ] a macacus, according to flourens, bred in paris; and more than one species of this genus has produced young in london, especially the _macacus rhesus_, which everywhere shows a special capacity to breed under confinement. hybrids have been produced both in paris and london from this same genus. the arabian baboon, or _cynocephalus hamadryas_,[ ] and a cercopithecus have bred in the zoological gardens, and the latter species at the duke of northumberland's. several members of the family of lemurs have produced hybrids in the zoological gardens. it is much more remarkable that monkeys very rarely breed when confined in their native country; thus the cay (_cebus azaræ_) is frequently and completely tamed in paraguay, but rengger[ ] says that it breeds so rarely, that he never saw more than two females which had produced young. a similar observation has been made with respect to the monkeys which are frequently tamed by the aborigines in brazil.[ ] in the region of the amazons, these animals are so often kept in a tame state, that mr. bates in walking through the streets of parà counted thirteen species; but, as he asserts, they have never been known to breed in captivity.[ ] _birds._ birds offer in some respects better evidence than quadrupeds, from their breeding more rapidly and being kept in greater numbers. we have seen that carnivorous animals are more fertile under confinement than most other mammals. the reverse holds good with carnivorous birds. it is said[ ] that as many as eighteen species have been used in europe for hawking, and several others in persia and india;[ ] they have been kept in their native country in the finest condition, and have been flown during six, eight, or nine years;[ ] yet there is no record of their having ever produced young. as these birds were formerly caught whilst young, at great expense, being imported from iceland, norway, and sweden, there can { } be little doubt that, if possible, they would have been propagated. in the jardin des plantes, no bird of prey has been known to couple.[ ] no hawk, vulture, or owl has ever produced fertile eggs in the zoological gardens, or in the old surrey gardens, with the exception, in the former place on one occasion, of a condor and a kite (_milvus niger_). yet several species, namely, the _aquila fusca_, _haliætus leucocephalus_, _falco tinnunculus_, _f. subbuteo_, and _buteo vulgaris_, have been seen to couple in the zoological gardens. mr. morris[ ] mentions as a unique fact that a kestrel (_falco tinnunculus_) bred in an aviary. the one kind of owl which has been known to couple in the zoological gardens was the eagle owl (_bubo maximus_); and this species shows a special inclination to breed in captivity; for a pair at arundel castle, kept more nearly in a state of nature "than ever fell to the lot of an animal deprived of its liberty,"[ ] actually reared their young. mr. gurney has given another instance of this same owl breeding in confinement; and he records the case of a second species of owl, the _strix passerina_, breeding in captivity.[ ] of the smaller graminivorous birds, many kinds have been kept tame in their native countries, and have lived long; yet, as the highest authority on cage-birds[ ] remarks, their propagation is "uncommonly difficult." the canary-bird shows that there is no inherent difficulty in these birds breeding freely in confinement; and audubon says[ ] that the _fringilla_ (_spiza_) _ciris_ of north america breeds as perfectly as the canary. the difficulty with the many finches which have been kept in confinement is all the more remarkable as more than a dozen species could be named which have yielded hybrids with the canary; but hardly any of these, with the exception of the siskin (_fringilla spinus_), have reproduced their own kind. even the bullfinch (_loxia pyrrhula_) has bred as frequently with the canary, though belonging to a distinct genus, as with its own species.[ ] with respect to the skylark (_alauda arvensis_), i have heard of birds living for seven years in an aviary, which never produced young; and a great london bird-fancier assured me that he had never known an instance of their breeding; nevertheless one case has been recorded.[ ] in the nine-year report from the zoological society, twenty-four incessorial species are enumerated which had not bred, and of these only four were known to have coupled. parrots are singularly long-lived birds; and humboldt mentions the curious fact of a parrot in south america, which spoke the language of { } an extinct indian tribe, so that this bird preserved the sole relic of a lost language. even in this country there is reason to believe[ ] that parrots have lived to the age of nearly one hundred years; yet, though many have been kept in europe, they breed so rarely that the event has been thought worth recording in the gravest publications.[ ] according to bechstein[ ] the african _psittacus erithacus_ breeds oftener than any other species: the _p. macoa_ occasionally lays fertile eggs, but rarely succeeds in hatching them; this bird, however, has the instinct of incubation sometimes so strongly developed, that it will hatch the eggs of fowls or pigeons. in the zoological gardens and in the old surrey gardens some few species have coupled, but, with the exception of three species of parrakeets, none have bred. it is a much more remarkable fact that in guiana parrots of two kinds, as i am informed by sir e. schomburgk, are often taken from the nests by the indians and reared in large numbers; they are so tame that they fly freely about the houses, and come when called to be fed, like pigeons; yet he has never heard of a single instance of their breeding.[ ] in jamaica, a resident naturalist, mr. r. hill,[ ] says, "no birds more readily submit to human dependence than the parrot-tribe, but no instance of a parrot breeding in this tame life has been known yet." mr. hill specifies a number of other native birds kept tame in the west indies, which never breed in this state. the great pigeon family offers a striking contrast with parrots: in the nine-year report thirteen species are recorded as having bred, and, what is more noticeable, only two were seen to couple without any result. since the above date every annual report gives many cases of various pigeons breeding. the two magnificent crowned pigeons (_goura coronata_ and _victoriæ_) produced hybrids; nevertheless, of the former species more than a dozen birds were kept, as i am informed by mr. crawfurd, in a park at penang, under a perfectly well-adapted climate, but never once bred. the _columba migratoria_ in its native country, north america, invariably lays two eggs, but in lord derby's menagerie never more than one. the same fact has been observed with the _c. leucocephala_.[ ] gallinaceous birds of many genera likewise show an eminent capacity for breeding under captivity. this is particularly the case with pheasants; yet our english species seldom lays more than ten eggs in confinement; whilst from eighteen to twenty is the usual number in the wild state.[ ] with the gallinaceæ, as with all other orders, there are marked and { } inexplicable exceptions in regard to the fertility of certain species and genera under confinement. although many trials have been made with the common partridge, it has rarely bred, even when reared in large aviaries; and the hen will never hatch her own eggs.[ ] the american tribe of guans or cracidæ are tamed with remarkable ease, but are very shy breeders in this country;[ ] but with care various species were formerly made to breed rather freely in holland.[ ] birds of this tribe are often kept in a perfectly tamed condition in their native country by the indians, but they never breed.[ ] it might have been expected that grouse from their habits of life would not have bred in captivity, more especially as they are said soon to languish and die.[ ] but many cases are recorded of their breeding: the capercailzie (_tetrao urogallus_) has bred in the zoological gardens; it breeds without much difficulty when confined in norway, and in russia five successive generations have been reared: _tetrao tetrix_ has likewise bred in norway; _t. scoticus_ in ireland; _t. umbellus_ at lord derby's; and _t. cupido_ in north america. it is scarcely possible to imagine a greater change in habits than that which the members of the ostrich family must suffer, when cooped up in small enclosures under a temperate climate, after freely roaming over desert and tropical plains or entangled forests. yet almost all the kinds, even the mooruk (_casuarius bennettii_) from new ireland, has frequently produced young in the various european menageries. the african ostrich, though perfectly healthy and living long in the south of france, never lays more than from twelve to fifteen eggs, though in its native country it lays from twenty-five to thirty.[ ] here we have another instance of fertility impaired, but not lost, under confinement, as with the flying squirrel, the hen-pheasant, and two species of american pigeons. most waders can be tamed, as the rev. e. s. dixon informs me, with remarkable facility; but several of them are short-lived under confinement, so that their sterility in this state is not surprising. the cranes breed more readily than other genera: _grus montigresia_ has bred several times in paris and in the zoological gardens, as has _g. cinerea_ at the latter place, and _g. antigone_ at calcutta. of other members of this great order, _tetrapteryx paradisea_ has bred at knowsley, a porphyrio in sicily, and the _gallinula chloropus_ in the zoological gardens. on the other hand, several { } birds belonging to this order will not breed in their native country, jamaica; and the psophia, though often kept by the indians of guiana about their houses, "is seldom or never known to breed."[ ] no birds breed with such complete facility under confinement as the members of the great duck family; yet, considering their aquatic and wandering habits, and the nature of their food, this could not have been anticipated. even some time ago above two dozen species had bred in the zoological gardens; and m. selys-longchamps has recorded the production of hybrids from forty-four different members of the family; and to these professor newton has added a few more cases.[ ] "there is not," says mr. dixon,[ ] "in the wide world, a goose which is not in the strict sense of the word domesticable;" that is, capable of breeding under confinement; but this statement is probably too bold. the capacity to breed sometimes varies in individuals of the same species; thus audubon[ ] kept for more than eight years some wild geese (_anser canadensis_), but they would not mate; whilst other individuals of the same species produced young during the second year. i know of but one instance in the whole family of a species which absolutely refuses to breed in captivity, namely, the _dendrocygna viduata_, although, according to sir r. schomburgk,[ ] it is easily tamed, and is frequently kept by the indians of guiana. lastly, with respect to gulls, though many have been kept in the zoological gardens and in the old surrey gardens, no instance was known before the year of their coupling or breeding; but since that period the herring gull (_larus argentatus_) has bred many times in the zoological gardens and at knowsley. there is reason to believe that insects are affected by confinement like the higher animals. it is well known that the sphingidæ rarely breed when thus treated. an entomologist[ ] in paris kept twenty-five specimens of _saturnia pyri_, but did not succeed in getting a single fertile egg. a number of females of _orthosia munda_ and of _mamestra suasa_ reared in confinement were unattractive to the males.[ ] mr. newport kept nearly a hundred individuals of two species of vanessa, but not one paired; this, however, might have been due to their habit of coupling on the wing.[ ] mr. atkinson could never succeed in india in making the tarroo silk-moth breed in confinement.[ ] it appears that a number of moths, especially the sphingidæ, when hatched in the autumn out of their proper season, { } are completely barren; but this latter case is still involved in some obscurity.[ ] independently of the fact of many animals under confinement not coupling, or, if they couple, not producing young, there is evidence of another kind, that their sexual functions are thus disturbed. for many cases have been recorded of the loss by male birds when confined of their characteristic plumage. thus the common linnet (_linota cannabina_) when caged does not acquire the fine crimson colour on its breast, and one of the buntings (_emberiza passerina_) loses the black on its head. a pyrrhula and an oriolus have been observed to assume the quiet plumage of the hen-bird; and the _falco albidus_ returned to the dress of an earlier age.[ ] mr. thomson, the superintendent of the knowsley menagerie, informed me that he had often observed analogous facts. the horns of a male deer (_cervus canadensis_) during the voyage from america were badly developed; but subsequently in paris perfect horns were produced. when conception takes place under confinement, the young are often born dead, or die soon, or are ill-formed. this frequently occurs in the zoological gardens, and, according to rengger, with native animals confined in paraguay. the mother's milk often fails. we may also attribute to the disturbance of the sexual functions the frequent occurrence of that monstrous instinct which leads the mother to devour her own offspring,--a mysterious case of perversion, as it at first appears. sufficient evidence has now been advanced to prove that animals when first confined are eminently liable to suffer in their reproductive systems. we feel at first naturally inclined to attribute the result to loss of health, or at least to loss of vigour; but this view can hardly be admitted when we reflect how healthy, long-lived, and vigorous many animals are under { } captivity, such as parrots, and hawks when used for hawking, chetahs when used for hunting, and elephants. the reproductive organs themselves are not diseased; and the diseases, from which animals in menageries usually perish, are not those which in any way affect their fertility. no domestic animal is more subject too disease than the sheep, yet it is remarkably prolific. the failure of animals to breed under confinement has been sometimes attributed exclusively to a failure in their sexual instincts: this may occasionally come into play, but there is no obvious reason why this instinct should be especially liable to be affected with perfectly tamed animals, except indeed indirectly through the reproductive system itself being disturbed. moreover, numerous cases have been given of various animals which couple freely under confinement, but never conceive; or, if they conceive and produce young, these are fewer in number than is natural to the species. in the vegetable kingdom instinct of course can play no part; and we shall presently see that plants when removed from their natural conditions are affected in nearly the same manner as animals. change of climate cannot be the cause of the loss of fertility, for, whilst many animals imported into europe from extremely different climates breed freely, many others when confined in their native land are completely sterile. change of food cannot be the chief cause; for ostriches, ducks, and many other animals, which must have undergone a great change in this respect, breed freely. carnivorous birds when confined are extremely sterile; whilst most carnivorous mammals, except plantigrades, are moderately fertile. nor can the amount of food be the cause; for a sufficient supply will certainly be given to valuable animals; and there is no reason to suppose that much more food would be given to them, than to our choice domestic productions which retain their full fertility. lastly, we may infer from the case of the elephant, chetah, various hawks, and of many animals which are allowed to lead an almost free life in their native land, that want of exercise is not the sole cause. it would appear that any change in the habits of life, whatever these habits may be, if great enough, tends to affect in an inexplicable manner the powers of reproduction. the result { } depends more on the constitution of the species than on the nature of the change; for certain whole groups are affected more than others; but exceptions always occur, for some species in the most fertile groups refuse to breed, and some in the most sterile groups breed freely. those animals which usually breed freely under confinement, rarely breed, as i was assured, in the zoological gardens, within a year or two after their first importation. when an animal which is generally sterile under confinement happens to breed, the young apparently do not inherit this power; for had this been the case, various quadrupeds and birds, which are valuable for exhibition, would have become common. dr. broca even affirms[ ] that many animals in the jardin des plantes, after having produced young for three or four successive generations, become sterile; but this may be the result of too close interbreeding. it is a remarkable circumstance that many mammals and birds have produced hybrids under confinement quite as readily as, or even more readily than, they have procreated their own kind. of this fact many instances have been given;[ ] and we are thus reminded of those plants which when cultivated refuse to be fertilised by their own pollen, but can easily be fertilised by that of a distinct species. finally, we must conclude, limited as the conclusion is, that changed conditions of life have an especial power of acting injuriously on the reproductive system. the whole case is quite peculiar, for these organs, though not diseased, are thus rendered incapable of performing their proper functions, or perform them imperfectly. _sterility of domesticated animals from changed conditions._--with respect to domesticated animals, as their domestication mainly depends on the accident of their breeding freely under captivity, we ought not to expect that their reproductive system would be affected by any moderate degree of change. those orders of quadrupeds and birds, of which the wild species breed most readily in our menageries, have afforded us the greatest number of domesticated productions. savages in most parts of the world are fond of taming animals;[ ] and if any of these regularly produced { } young, and were at the same time useful, they would be at once domesticated. if, when their masters migrated into other countries, they were in addition found capable of withstanding various climates, they would be still more valuable; and it appears that the animals which breed readily in captivity can generally withstand different climates. some few domesticated animals, such as the reindeer and camel, offer an exception to this rule. many of our domesticated animals can bear with undiminished fertility the most unnatural conditions; for instance, rabbits, guinea-pigs, and ferrets breed in miserably confined hutches. few european dogs of any kind withstand without degeneration the climate of india; but as long as they survive, they retain, as i hear from mr. falconer, their fertility; so it is, according to dr. daniell, with english dogs taken to sierra leone. the fowl, a native of the hot jungles of india, becomes more fertile than its parent-stock in every quarter of the world, until we advance as far north as greenland and northern siberia, where this bird will not breed. both fowls and pigeons, which i received during the autumn direct from sierra leone, were at once ready to couple.[ ] i have, also, seen pigeons breeding as freely as the common kinds within a year after their importation from the upper nile. the guinea-fowl, an aboriginal of the hot and dry deserts of africa, whilst living under our damp and cool climate, produces a large supply of eggs. nevertheless, our domesticated animals under new conditions occasionally show signs of lessened fertility. roulin asserts that in the hot valleys of the equatorial cordillera sheep are not fully fecund;[ ] and according to lord somerville,[ ] the merino-sheep which he imported from spain were not at first perfectly fertile. it is said[ ] that mares brought up on dry food in the stable, and turned out to grass, do not at first breed. the peahen, as we have seen, is said not to lay so many eggs in england as in india. it was long before the canary-bird was fully fertile, and even now first-rate breeding birds are not common.[ ] in the hot and dry province of delhi, the eggs of the turkey, as i hear from dr. falconer, though placed under a hen, are extremely liable to fail. according to roulin, geese taken within a recent period to the lofty plateau of bogota, at first laid seldom, and then only a few eggs; of these scarcely a fourth were hatched, and half the young birds died: in the second generation they were more fertile; and when roulin wrote they were becoming as { } fertile as our geese in europe. in the philippine archipelago the goose, it is asserted, will not breed or even lay eggs.[ ] a more curious case is that of the fowl, which, according to roulin, when first introduced would not breed at cusco in bolivia, but subsequently became quite fertile; and the english game fowl, lately introduced, had not as yet arrived a its full fertility, for to raise two or three chickens from a nest of eggs was thought fortunate. in europe close confinement has a marked effect on the fertility of the fowl: it has been found in france that with fowls allowed considerable freedom only twenty per cent. of the eggs failed; when allowed less freedom forty per cent. failed; and in close confinement sixty out of the hundred were not hatched.[ ] so we see that unnatural and changed conditions of life produce some effect on the fertility of our most thoroughly domesticated animals, in the same manner, though in a far less degree, as with captive wild animals. it is by no means rare to find certain males and females which will not breed together, though both are known to be perfectly fertile with other males and females. we have no reason to suppose that this is caused by these animals having been subjected to any change in their habits of life; therefore such cases are hardly related to our present subject. the cause apparently lies in an innate sexual incompatibility of the pair which are matched. several instances have been communicated to me by mr. w. c. spooner (well known for his essay on cross-breeding), by mr. eyton of eyton, by mr. wicksted and othe breeders, and especially by mr. waring of chelsfield, in relation to horses, cattle, pigs, foxhounds, other dogs, and pigeons.[ ] in these cases, females, which either previously or subsequently were proved to be fertile, failed to breed with certain males, with whom it was particularly desired to match them. a change in the constitution of the female may sometimes have occurred before she was put to the second male; but in other cases this explanation is hardly tenable, for a female, known not to be barren, has been unsuccessfully paired seven or eight times with the same male likewise known to be perfectly fertile. with cart-mares, which sometimes will not breed with stallions of pure blood, but subsequently have bred with cart-stallions, mr. spooner is inclined to attribute the failure to the lesser sexual power of the race-horse. but i have heard from the greatest breeder of race-horses at the present day, through mr. waring, that "it frequently occurs with a mare to be put several times during one or two seasons to a particular stallion of acknowledged power, and yet prove barren; the mare afterwards breeding at once with some other horse." these facts are worth recording, as they show, like so many previous facts, on what slight constitutional differences the fertility of an animal often depends. { } _sterility of plants from changed conditions of life, and from other causes._ in the vegetable kingdom cases of sterility frequently occur, analogous with those previously given in the animal kingdom. but the subject is obscured by several circumstances, presently to be discussed, namely, the contabescence of the anthers, as gärtner has named a certain affection--monstrosities--doubleness of the flower--much-enlarged fruit--and long-continued or excessive propagation by buds. it is notorious that many plants in our gardens and hot-houses, though preserved in the most perfect health, rarely or never produce seed. i do not allude to plants which run to leaves, from being kept too damp, or too warm, or too much manured; for these do not produce the reproductive individual or flower, and the case may be wholly different. nor do i allude to fruit not ripening from want of heat, or rotting from too much moisture. but many exotic plants, with their ovules and pollen appearing perfectly sound, will not set any seed. the sterility in many cases, as i know from my own observation, is simply due to the absence of the proper insects for carrying the pollen to the stigma. but after excluding the several cases just specified, there are many plants in which the reproductive system has been seriously affected by the altered conditions of life to which they have been subjected. it would be tedious to enter on many details. linnæus long ago observed[ ] that alpine plants, although naturally laded with seed, produce either few or none when cultivated in gardens. but exceptions often occur: the _draba sylvestris_, one of our most thoroughly alpine plants, multiplies itself by seed in mr. h. c. watson's garden, near london; and kerner, who has particularly attended to the cultivation of alpine plants, found that various kinds, when cultivated, spontaneously sowed themselves.[ ] many plants which naturally grow in peat-earth are entirely sterile in our gardens. i have noticed the same fact with several liliaceous plants, which nevertheless grew vigorously. too much manure renders some kinds utterly sterile, as i have myself observed. the tendency to sterility from this cause runs in families; thus, according to gärtner,[ ] it is hardly possible to give too much manure to most gramineæ, cruciferæ, and leguminosæ, whilst succulent and bulbous-rooted plants are easily affected. extreme poverty of soil is less { } apt to induce sterility; but dwarfed plants of _trifolium minus_ and _repens_, growing on a lawn often mown and never manured, did not produce any seed. the temperature of the soil, and the season at which plants are watered, often have a marked effect on their fertility, as was observed by kölreuter in the case of mirabilis.[ ] mr. scott in the botanic gardens of edinburgh observed that _oncidium divaricatum_ would not set seed when grown in a basket in which it throve, but was capable of fertilisation in a pot where it was a little damper. _pelargonium fulgidum_, for many years after its introduction, seeded freely; it then became sterile; now it is fertile[ ] if kept in a dry stove during the winter. other varieties of pelargonium are sterile and others fertile without our being able to assign any cause. very slight changes in the position of a plant, whether planted on a bank or at its base, sometimes make all the difference in its producing seed. temperature apparently has a much more powerful influence on the fertility of plants than on that of animals. nevertheless it is wonderful what changes some few plants will withstand with undiminished fertility: thus the _zephyranthes candida_, a native of the moderately warm banks of the plata, sows itself in the hot dry country near lima, and in yorkshire resists the severest frosts, and i have seen seeds gathered from pods which had been covered with snow during three weeks.[ ] _berberis wallichii_, from the hot khasia range in india, is uninjured by our sharpest frosts, and ripens its fruit under our cool summers. nevertheless i presume we must attribute to change of climate the sterility of many foreign plants; thus the persian and chinese lilacs (_syringa persica_ and _chinensis_), though perfectly hardly, never here produce a seed; the common lilac (_s. vulgaris_) seeds with us moderately well, but in parts of germany the capsules never contain seed.[ ] some of the cases, given in the last chapter, of self-impotent plants, which are fertile both on the male and female side when united with distinct individuals or species, might have been here introduced; for as this peculiar form of sterility generally occurs with exotic plants or with endemic plants cultivated in pots, and as it disappeared in the _passiflora alata_ when grafted, we may conclude that in these cases it is the result of the treatment to which the plants or their parents have been exposed. the liability of plants to be affected in their fertility by slightly changed conditions is the more remarkable, as the pollen when once in process of formation is not easily injured; a plant may be transplanted, or a branch with flower-buds be cut off and placed in water, and the pollen will be matured. pollen, also, when once mature, may be kept for weeks or even months.[ ] the female organs are more sensitive, for gärtner[ ] found that dicotyledonous plants, when carefully removed so that they did not in the least flag, could seldom be fertilised; this occurred even with potted { } plants if the roots had grown out of the hole at the bottom. in some few cases, however, as with digitalis, transplantation did not prevent fertilisation; and according to the testimony of mawz, _brassica rapa_, when pulled up by its roots and placed in water, ripened its seed. flower-stems of several monocotyledonous plants when cut off and placed in water likewise produce seed. but in these cases i presume that the flowers had been already fertilised, for herbert[ ] found with the crocus that the plants might be removed or mutilated after the act of fertilisation, and would still perfect their seeds; but that, if transplanted before being fertilised, the application of pollen was powerless. plants which have been long cultivated can generally endure with undiminished fertility various and great changes; but not in most cases so great a change of climate as domesticated animals. it is remarkable that many plants under these circumstances are so much affected that the proportions and the nature of their chemical ingredients are modified, yet their fertility is unimpaired. thus, as dr. falconer informs me, there is a great difference in the character of the fibre in hemp, in the quantity of oil in the seed of the linum, in the proportion of narcotin to morphine in the poppy, in gluten to starch in wheat, when these plants are cultivated on the plains and on the mountains of india; nevertheless, they all remain fully fertile. _contabescence._--gärtner has designated by this term a peculiar condition of the anthers in certain plants, in which they are shrivelled, or become brown and tough, and contain no good pollen. when in this state they exactly resemble the anthers of the most sterile hybrids. gärtner,[ ] in his discussion on this subject, has shown that plants of many orders are occasionally thus affected; but the caryophyllaceæ and liliaceæ suffer most, and to these orders, i think, the ericaceæ may be added. contabescence varies in degree, but on the same plant all the flowers are generally affected to nearly the same extent. the anthers are affected at a very early period in the flower-bud, and remain in the same state (with one recorded exception) during the life of the plant. the affection cannot be cured by any change of treatment, and is propagated by layers, cuttings, &c., and perhaps even by seed. in contabescent plants the female organs are seldom affected, or merely become precocious in their development. the cause of this affection is doubtful, and is different in different cases. until i read gärtner's discussion i attributed it, as apparently did herbert, to the unnatural treatment of the plants; but its permanence under changed conditions, and the female organs not being affected, seem incompatible with this view. the fact of several endemic plants becoming contabescent in our gardens seems, at first sight, equally incompatible with this view; but kölreuter believes that this is the result of their transplantation. the contabescent plants of dianthus and verbascum, found wild by wiegmann, grew on a dry and sterile bank. the fact that exotic { } plants are eminently liable to this affection also seems to show that it is in some manner caused by their unnatural treatment. in some instances, as with silene, gärtner's view seems the most probable, namely, that it is caused by an inherent tendency in the species to become dioecious. i can add another cause, namely, the illegitimate unions of reciprocally dimorphic or trimorphic plants, for i have observed seedlings of three species of primula and of _lythrum salicaria_, which had been raised from plants illegitimately fertilised by their own-form pollen, with some or all their anthers in a contabescent state. there is perhaps an additional cause, namely, self-fertilisation; for many plants of dianthus and lobelia, which had been raised from self-fertilised seeds, had their anthers in this state; but these instances are not conclusive, as both genera are liable from other causes to this affection. cases of an opposite nature likewise occur, namely, plants with the female organs struck with sterility, whilst the male organs remain perfect. _dianthus japonicus_, a passiflora, and nicotiana, have been described by gärtner[ ] as being in this unusual condition. _monstrosities as a cause of sterility._--great deviations of structure, even when the reproductive organs themselves are not seriously affected, sometimes cause plants to become sterile. but in other cases plants may become monstrous to an extreme degree and yet retain their full fertility. gallesio, who certainly had great experience,[ ] often attributes sterility to this cause; but it may be suspected that in some of his cases sterility was the cause, and not the result, of the monstrous growths. the curious st. valery apple, although it bears fruit, rarely produces seed. the wonderfully anomalous flowers of _begonia frigida_, formerly described, though they appear fit for fructification, are sterile.[ ] species of primulæ, in which the calyx is brightly coloured, are said[ ] to be often sterile, though i have known them to be fertile. on the other hand, verlot gives several cases of proliferous flowers which can be propagated by seed. this was the case with a poppy, which had become monopetalous by the union of its petals.[ ] another extraordinary poppy, with the stamens replaced by numerous small supplementary capsules, likewise reproduces itself by seed. this has also occurred with a plant of _saxifraga geum_, in which a series of adventitious carpels, bearing ovules on their margins, had been developed between the stamens and the normal carpels.[ ] lastly, with respect to peloric flowers, which depart wonderfully from the natural structure,--those of _linaria vulgaris_ seem generally to be more or less sterile, whilst those before described of _antirrhinum majus_, when artificially fertilised with their own pollen, are perfectly { } fertile, though sterile when left to themselves, for bees are unable to crawl into the narrow tubular flower. the peloric flowers of _corydalis solida_, according to godron,[ ] are barren; whilst those of gloxinia are well known to yield plenty of seed. in our greenhouse pelargoniums, the central flower of the truss is often peloric, and mr. masters informs me that he tried in vain during several years to get seed from these flowers. i likewise made many vain attempts, but sometimes succeeded in fertilising them with pollen from a normal flower of another variety; and conversely i several times fertilised ordinary flowers with peloric pollen. only once i succeeded in raising a plant from a peloric flower fertilised by pollen from a peloric flower borne by another variety; but the plant, it may be added, presented nothing particular in its structure. hence we may conclude that no general rule can be laid down; but any great deviation from the normal structure, even when the reproductive organs themselves are not seriously affected, certainly often leads to sexual impotence. _double flowers._--when the stamens are converted into petals, the plant becomes on the male side sterile; when both stamens and pistils are thus changed, the plant becomes completely barren. symmetrical flowers having numerous stamens and petals are the most liable to become double, as perhaps follows from all multiple organs being the most subject to variability. but flowers furnished with only a few stamens, and others which are asymmetrical in structure, sometimes become double, as we see with the double gorse or ulex, petunia, and antirrhinum. the compositæ bear what are called double flowers by the abnormal development of the corolla of their central florets. doubleness is sometimes connected with prolification,[ ] or the continued growth of the axis of the flower. doubleness is strongly inherited. no one has produced, as lindley remarks,[ ] double flowers by promoting the perfect health of the plant. on the contrary, unnatural conditions of life favour their production. there is some reason to believe that seeds kept during many years, and seeds believed to be imperfectly fertilised, yield double flowers more freely than fresh and perfectly fertilised seed.[ ] long-continued cultivation in rich soil seems to be the commonest exciting cause. a double narcissus and a double _anthemis nobilis_, transplanted into very poor soil, have been observed to become single;[ ] and i have seen a completely double white primrose rendered permanently single by being divided and transplanted whilst in full flower. it has been observed by professor morren that doubleness of the flowers and variegation of the leaves are antagonistic states; but so many exceptions to the rule have lately been recorded,[ ] that, though general, it cannot be looked at as invariable. { } variegation seems generally to result from a feeble or atrophied condition of the plant, and a large proportion of the seedlings raised from parents both of which are variegated usually perish at an early age; hence we may perhaps infer that doubleness, which is the antagonistic state, commonly arises from a plethoric condition. on the other hand, extremely poor soil sometimes, though rarely, appears to cause doubleness: i formerly described[ ] some completely double, bud-like, flowers produced in large numbers by stunted wild plants of _gentiana amarella_ growing on a poor chalky bank. i have also noticed a distinct tendency to doubleness in the flowers of a ranunculus, horse-chesnut, and bladder-nut (_ranunculus repens_, _Æsculus pavia_, and _staphylea_), growing under very unfavourable conditions. professor lehman[ ] found several wild plants growing near a hot spring with double flowers. with respect to the cause of doubleness, which arises, as we see, under widely different circumstances, i shall presently attempt to show that the most probable view is that unnatural conditions first give a tendency to sterility, and that then, on the principle of compensation, as the reproductive organs do not perform their proper functions, they either become developed into petals, or additional petals are formed. this view has lately been supported by mr. laxton,[ ] who advances the case of some common peas, which, after long-continued heavy rain, flowered a second time, and produced double flowers. _seedless fruit._--many of our most valuable fruits, although consisting in a homological sense of widely different organs, are either quite sterile, or produce extremely few seeds. this is notoriously the case with our best pears, grapes, and figs, with the pine-apple, banana, bread-fruit, pomegranate, azarole, date-palms, and some members of the orange-tribe. poorer varieties of these same fruits either habitually or occasionally yield seed.[ ] most horticulturists look at the great size and anomalous development of the fruit as the cause, and sterility as the result; but the opposite view, as we shall presently see, is more probable. _sterility from the excessive development of the organs of growth or vegetation._--plants which from any cause grow too luxuriantly, and produce leaves, stems, runners, suckers, tubers, bulbs, &c., in excess, sometimes do not flower, or if they flower do not yield seed. to make european vegetables under the hot climate of india yield seed, it is necessary to check their growth; and, when one-third grown, they are taken up, and their stems and { } tap-roots are cut or mutilated.[ ] so it is with hybrids; for instance, prof. lecoq[ ] had three plants of mirabilis, which, though they grew luxuriantly and flowered, were quite sterile; but after beating one with a stick until a few branches alone were left, these at once yielded good seed. the sugar-cane, which grows vigorously and produces a large supply of succulent stems, never, according to various observers, bears seed in the west indies, malaga, india, cochin china, or the malay archipelago.[ ] plants which produce a large number of tubers are apt to be sterile, as occurs, to a certain extent, with the common potato; and mr. fortune informs me that the sweet potato (_convolvulus batatas_) in china never, as far as he has seen, yields seed. dr. royle remarks[ ] that in india the _agave vivipara_, when grown in rich soil, invariably produces bulbs, but no seeds; whilst a poor soil and dry climate leads to an opposite result. in china, according to mr. fortune, an extraordinary number of little bulbs are developed in the axils of the leaves of the yam, and this plant does not bear seed. whether in these cases, as in those of double flowers and seedless fruit, sexual sterility from changed conditions of life is the primary cause which leads to the excessive development of the organs of vegetation, is doubtful; though some evidence might be advanced in favour of this view. it is perhaps a more probable view that plants which propagate themselves largely by one method, namely by buds, have not sufficient vital power or organised matter for the other method of sexual generation. several distinguished botanists and good practical judges believe that long-continued propagation by cuttings, runners, tubers, bulbs, &c., independently of any excessive development of these parts, is the cause of many plants failing to produce flowers and of others failing to produce fertile flowers,--it is as if they had lost the habit of sexual generation.[ ] that many plants when thus propagated are sterile there can be no doubt, but whether the long continuance of this form of propagation is the actual cause of their sterility, i will not venture, from the want of sufficient evidence, to express an opinion. that plants may be propagated for long periods by buds, without the aid of sexual generation, we may safely infer from this being the case with many plants which must have long survived in a state of nature. as i have had occasion before to allude to this subject, i will here give such cases as i have collected. many alpine plants ascend mountains beyond the height at which they can produce seed.[ ] certain species of { } poa and festuca, when growing on mountain-pastures, propagate themselves, as i hear from mr. bentham, almost exclusively by bulblets. kalm gives a more curious instance[ ] of several american trees, which grow so plentifully in marshes or in thick woods, that they are certainly well adapted for these stations, yet scarcely ever produce seeds; but when accidentally growing on the outside of the marsh or wood, are loaded with seed. the common ivy is found in northern sweden and russia, but flowers and fruits only in the southern provinces. the _acorus calamus_ extends over a large portion of the globe, but so rarely perfects its fruit that this has been seen but by few botanists.[ ] the _hypericum calycinum_, which propagates itself so freely in our shrubberies by rhizomas and is naturalised in ireland, blossoms profusely, but sets no seed; nor did it set any when fertilised in my garden by pollen from plants growing at a distance. the _lysimachia nummularia_, which is furnished with long runners, so seldom produces seed-capsules, that prof. decaisne,[ ] who has especially attended to this plant, has never seen it in fruit. the _carex rigida_ often fails to perfect its seed in scotland, lapland, greenland, germany, and new hampshire in the united states.[ ] the periwinkle (_vinca minor_), which spreads largely by runners, is said scarcely ever to produce fruit in england;[ ] but this plant requires insect-aid for its fertilisation, and the proper insects may be absent or rare. the _jussiæa grandiflora_ has become naturalised in southern france, and has spread by its rhizomas so extensively as to impede the navigation of the waters, but never produces fertile seed.[ ] the horse-radish (_cochlearia armoracia_) spreads pertinaciously and is naturalised in various parts of europe; though it bears flowers, these rarely produce capsules: professor caspary also informs me that he has watched this plant since , but has never seen its fruit; nor is this surprising, as he finds scarcely a grain of good pollen. the common little _ranunculus ficaria_ rarely, and some say never, bears seed in england, france, or switzerland; but in i observed seeds on several plants growing near my house. according to m. chatin, there are two forms of this ranunculus; and it is the bulbiferous form which does not yield seed from producing no pollen.[ ] other cases { } analogous with the foregoing could be given; for instance, some kinds of mosses and lichens have never been seen to fructify in france. some of these endemic and naturalised plants are probably rendered sterile from excessive multiplication by buds, and their consequent incapacity to produce and nourish seed. but the sterility of others more probably depends on the peculiar conditions under which they live, as in the case of the ivy in the northern parts of europe, and of the trees in the swamps of the united states; yet these plants must be in some respects eminently well adapted for the stations which they occupy, for they hold their places against a host of competitors. finally, when we reflect on the sterility which accompanies the doubling of flowers,--the excessive development of fruit,--and a great increase in the organs of vegetation, we must bear in mind that the whole effect has seldom been caused at once. an incipient tendency is observed, and continued selection completes the work, as is known to be the case with our double flowers and best fruits. the view which seems the most probable, and which connects together all the foregoing facts and brings them within our present subject, is, that changed and unnatural conditions of life first give a tendency to sterility; and in consequence of this, the organs of reproduction being no longer able fully to perform their proper functions, a supply of organised matter, not required for the development of the seed, flows either into these same organs and renders them foliaceous, or into the fruit, stems, tubers, &c., increasing their size and succulency. but i am far from wishing to deny that there exists, independently of any incipient sterility, an antagonism between the two forms of reproduction, namely, by seed and by buds, when either is carried to an extreme degree. that incipient sterility plays an important part in the doubling of flowers, and in the other cases just specified, i infer chiefly from the following facts. when fertility is lost from a wholly different cause, namely, from hybridism, there is a strong tendency, as gärtner[ ] affirms, for flowers to become double, and this tendency is inherited. moreover it is notorious that with hybrids the male organs become sterile before the female organs, and with double flowers the stamens first become { } foliaceous. this latter fact is well shown by the male flowers of dioecious plants, which, according to gallesio,[ ] first become double. again, gärtner[ ] often insists that the flowers of even utterly sterile hybrids, which do not produce any seed, generally yield perfect capsules or fruit,--a fact which has likewise been repeatedly observed by naudin with the cucurbitaceæ; so that the production of fruit by plants rendered sterile through any other and distinct cause is intelligible. kölreuter has also expressed his unbounded astonishment at the size and development of the tubers in certain hybrids; and all experimentalists[ ] have remarked on the strong tendency in hybrids to increase by roots, runners, and suckers. seeing that hybrid plants, which from their nature are more or less sterile, thus tend to produce double flowers; that they have the parts including the seed, that is the fruit, perfectly developed, even when containing no seed; that they sometimes yield gigantic roots; that they almost invariably tend to increase largely by suckers and other such means;--seeing this, and knowing, from the many facts given in the earlier parts of this chapter, that almost all organic beings when exposed to unnatural conditions tend to become more or less sterile, it seems much the most probable view that with cultivated plants sterility is the exciting cause, and double flowers, rich seedless fruit, and in some cases largely-developed organs of vegetation, &c., are the indirect results--these results having been in most cases largely increased through continued selection by man. * * * * * { } chapter xix. summary of the four last chapters, with remarks on hybridism. on the effects of crossing--the influence of domestication on fertility--close interbreeding--good and evil results from changed conditions of life--varieties when crossed not invariably fertile--on the difference in fertility between crossed species and varieties--conclusions with respect to hybridism--light thrown on hybridism by the illegitimate progeny of dimorphic and trimorphic plants--sterility of crossed species due to differences confined to the reproductive system--not accumulated through natural selection--reasons why domestic varieties are not mutually sterile--too much stress has been laid on the difference in fertility between crossed species and crossed varieties--conclusion. it was shown in the fifteenth chapter that when individuals of the same variety, or even of a distinct variety, are allowed freely to intercross, uniformity of character is ultimately acquired. some few characters, however, are incapable of fusion, but these are unimportant, as they are almost always of a semi-monstrous nature, and have suddenly appeared. hence, to preserve our domesticated breeds true, or to improve them by methodical selection, it is obviously necessary that they should be kept separate. nevertheless, through unconscious selection, a whole body of individuals may be slowly modified, as we shall see in a future chapter, without separating them into distinct lots. domestic races have often been intentionally modified by one or two crosses, made with some allied race, and occasionally even by repeated crosses with very distinct races; but in almost all such cases, long-continued and careful selection has been absolutely necessary, owing to the excessive variability of the crossed offspring, due to the principle of reversion. in a few instances, however, mongrels have retained a uniform character from their first production. when two varieties are allowed to cross freely, and one is { } much more numerous than the other, the former will ultimately absorb the latter. should both varieties exist in nearly equal numbers, it is probable that a considerable period would elapse before the acquirement of a uniform character; and the character ultimately acquired would largely depend on prepotency of transmission, and on the conditions of life; for the nature of these conditions would generally favour one variety more than another, so that a kind of natural selection would come into play. unless the crossed offspring were slaughtered by man without the least discrimination, some degree of unmethodical selection would likewise come into action. from these several considerations we may infer, that when two or more closely allied species first came into the possession of the same tribe, their crossing will not have influenced, in so great a degree as has often been supposed, the character of the offspring in future times; although in some cases it probably has had a considerable effect. domestication, as a general rule, increases the prolificness of animals and plants. it eliminates the tendency to sterility which is common to species when first taken from a state of nature and crossed. on this latter head we have no direct evidence; but as our races of dogs, cattle, pigs, &c., are almost certainly descended from aboriginally distinct stocks, and as these races are now fully fertile together, or at least incomparably more fertile than most species when crossed, we may with much confidence accept this conclusion. abundant evidence has been given that crossing adds to the size, vigour, and fertility of the offspring. this holds good when there has been no previous close interbreeding. it applies to the individuals of the same variety but belonging to different families, to distinct varieties, sub-species, and partially even to species. in the latter case, though size is often gained, fertility is lost; but the increased size, vigour, and hardiness of many hybrids cannot be accounted for solely on the principle of compensation from the inaction of the reproductive system. certain plants, both of pure and hybrid origin, though perfectly healthy, have become self-impotent, apparently from the unnatural conditions to which they have been exposed; and such plants, as well as others in their normal state, can be stimulated to { } fertility only by crossing them with other individuals of the same species or even of a distinct species. on the other hand, long-continued close interbreeding between the nearest relations diminishes the constitutional vigour, size, and fertility of the offspring; and occasionally leads to malformations, but not necessarily to general deterioration of form or structure. this failure of fertility shows that the evil results of interbreeding are independent of the augmentation of morbid tendencies common to both parents, though this augmentation no doubt is often highly injurious. our belief that evil follows from close interbreeding rests to a large extent on the experience of practical breeders, especially of those who have reared many animals of the kinds which can be propagated quickly; but it likewise rests on several carefully recorded experiments. with some animals close interbreeding may be carried on for a long period with impunity by the selection of the most vigorous and healthy individuals; but sooner or later evil follows. the evil, however, comes on so slowly and gradually that it easily escapes observation, but can be recognised by the almost instantaneous manner in which size, constitutional vigour, and fertility are regained when animals that have long been interbred are crossed with a distinct family. these two great classes of facts, namely, the good derived from crossing, and the evil from close interbreeding, with the consideration of the innumerable adaptations throughout nature for compelling, or favouring, or at least permitting, the occasional union of distinct individuals, taken together, lead to the conclusion that it is a law of nature that organic beings shall not fertilise themselves for perpetuity. this law was first plainly hinted at in , with respect to plants, by andrew knight,[ ] and, not long afterwards, that sagacious observer kölreuter, after showing how well the malvaceæ are adapted for { } crossing, asks, "an id aliquid in recessu habeat, quod hujuscemodi flores nunquam proprio suo pulvere, sed semper eo aliarum suæ speciei impregnentur, merito quæritur? certe natura nil facit frustra." although we may demur to kölreuter's saying that nature does nothing in vain, seeing how many organic beings retain rudimentary and useless organs, yet undoubtedly the argument from the innumerable contrivances, which favour the crossing of distinct individuals of the same species, is of the greatest weight. the most important result of this law is that it leads to uniformity of character in the individuals of the same species. in the case of certain hermaphrodites, which probably intercross only at long intervals of time, and with unisexual animals inhabiting somewhat separated localities, which can only occasionally come into contact and pair, the greater vigour and fertility of the crossed offspring will ultimately prevail in giving uniformity of character to the individuals of the same species. but when we go beyond the limits of the same species, free intercrossing is barred by the law of sterility. in searching for facts which might throw light on the cause of the good effects from crossing, and of the evil effects from close interbreeding, we have seen that, on the one hand, it is a widely prevalent and ancient belief that animals and plants profit from slight changes in their condition of life; and it would appear that the germ, in a somewhat analogous manner, is more effectually stimulated by the male element, when taken from a distinct individual, and therefore slightly modified in nature, than when taken from a male having the same identical constitution. on the other hand, numerous facts have been given, showing that when animals are first subjected to captivity, even in their native land, and although allowed much liberty, their reproductive functions are often greatly impaired or quite annulled. some groups of animals are more affected than others, but with apparently capricious exceptions in every group. some animals never or rarely couple: some couple freely, but never or rarely conceive. the secondary male characters, the maternal functions and instincts, are occasionally affected. with plants, when first subjected to cultivation, analogous facts have been observed. we probably owe our double flowers, rich seedless { } fruits, and in some cases greatly developed tubers, &c., to incipient sterility of the above nature combined with a copious supply of nutriment. animals which have long been domesticated, and plants which have long been cultivated, can generally withstand with unimpaired fertility great changes in their conditions of life; though both are sometimes slightly affected. with animals the somewhat rare capacity of breeding freely under confinement has mainly determined, together with their utility, the kinds which have been domesticated. we can in no case precisely say what is the cause of the diminished fertility of an animal when first captured, or of a plant when first cultivated; we can only infer that it is caused by a change of some kind in the natural conditions of life. the remarkable susceptibility of the reproductive system to such changes,--a susceptibility not common to any other organ,--apparently has an important bearing on variability, as we shall see in a future chapter. it is impossible not to be struck with the double parallelism between the two classes of facts just alluded to. on the one hand, slight changes in the conditions of life, and crosses between slightly modified forms or varieties, are beneficial as far as prolificness and constitutional vigour are concerned. on the other hand, changes in the conditions greater in degree, or of a different nature, and crosses between forms which have been slowly and greatly modified by natural means,--in other words, between species,--are highly injurious, as far as the reproductive system is concerned, and in some few instances as far as constitutional vigour is concerned. can this parallelism be accidental? does it not rather indicate some real bond of connection? as a fire goes out unless it be stirred up, so the vital forces are always tending, according to mr. herbert spencer, to a state of equilibrium, unless disturbed and renovated through the action of other forces. in some few cases varieties tend to keep distinct, by breeding at different periods, by great differences in size, or by sexual preference,--in this latter respect more especially resembling species in a state of nature. but the actual crossing of varieties, far from diminishing, generally adds to the fertility of both the first union and the mongrel offspring. whether all { } the most widely distinct domestic varieties are invariably quite fertile when crossed, we do not positively know; much time and trouble would be requisite for the necessary experiments, and many difficulties occur, such as the descent of the various races from aboriginally distinct species, and the doubts whether certain forms ought to be ranked as species or varieties. nevertheless, the wide experience of practical breeders proves that the great majority of varieties, even if some should hereafter prove not to be indefinitely fertile _inter se_, are far more fertile when crossed, than the vast majority of closely allied natural species. a few remarkable cases have, however, been given on the authority of excellent observers, showing that with plants certain forms, which undoubtedly must be ranked as varieties, yield fewer seeds when crossed than is natural to the parent-species. other varieties have had their reproductive powers so far modified that they are either more or less fertile than are their parents, when crossed with a distinct species. nevertheless, the fact remains indisputable that domesticated varieties of animals and of plants, which differ greatly from each other in structure, but which are certainly descended from the same aboriginal species, such as the races of the fowl, pigeon, many vegetables, and a host of other productions, are extremely fertile when crossed; and this seems to make a broad and impassable barrier between domestic varieties and natural species. but, as i will now attempt to show, the distinction is not so great and overwhelmingly important as it at first appears. _on the difference in fertility between varieties and species when crossed._ this work is not the proper place for fully treating the subject of hybridism, and i have already given in my 'origin of species' a moderately full abstract. i will here merely enumerate the general conclusions which may be relied on, and which bear on our present point. _firstly_, the laws governing the production of hybrids are identical, or nearly identical, in the animal and vegetable kingdoms. _secondly_, the sterility of distinct species when first united, { } and that of their hybrid offspring, graduates, by an almost infinite number of steps, from zero, when the ovule is never impregnated and a seed-capsule is never formed, up to complete fertility. we can only escape the conclusion that some species are fully fertile when crossed, by determining to designate as varieties all the forms which are quite fertile. this high degree of fertility is, however, rare. nevertheless plants, which have been exposed to unnatural conditions, sometimes become modified in so peculiar a manner, that they are much more fertile when crossed by a distinct species than when fertilised by their own pollen. success in effecting a first union between two species, and the fertility of their hybrids, depends in an eminent degree on the conditions of life being favourable. the innate sterility of hybrids of the same parentage and raised from the same seed-capsule often differs much in degree. _thirdly_, the degree of sterility of a first cross between two species does not always run strictly parallel with that of their hybrid offspring. many cases are known of species which can be crossed with ease, but yield hybrids excessively sterile; and conversely some which can be crossed with great difficulty, but produce fairly fertile hybrids. this is an inexplicable fact, on the view that species have been specially endowed with mutual sterility in order to keep them distinct. _fourthly_, the degree of sterility often differs greatly in two species when reciprocally crossed; for the first will readily fertilise the second; but the latter is incapable, after hundreds of trials, of fertilising the former. hybrids produced from reciprocal crosses between the same two species, likewise sometimes differ in their degree of sterility. these cases also are utterly inexplicable on the view of sterility being a special endowment. _fifthly_, the degree of sterility of first crosses and of hybrids runs, to a certain extent, parallel with the general or systematic affinity of the forms which are united. for species belonging to distinct genera can rarely, and those belonging to distinct families can never, be crossed. the parallelism, however, is far from complete; for a multitude of closely allied species will not unite, or unite with extreme difficulty, whilst other species, widely different from each other, can be crossed with perfect facility. nor does the difficulty depend on ordinary { } constitutional differences, for annual and perennial plants, deciduous and evergreen trees, plants flowering at different seasons, inhabiting different stations, and naturally living under the most opposite climates, can often be crossed with ease. the difficulty or facility apparently depends exclusively on the sexual constitution of the species which are crossed; or on their sexual elective affinity, _i. e._ _wahlverwandtschaft_ of gärtner. as species rarely or never become modified in one character, without being at the same time modified in many, and as systematic affinity includes all visible resemblances and dissimilarities, any difference in sexual constitution between two species would naturally stand in more or less close relation with their systematic position. _sixthly_, the sterility of species when first crossed, and that of hybrids, may possibly depend to a certain extent on distinct causes. with pure species the reproductive organs are in a perfect condition, whilst with hybrids they are often plainly deteriorated. a hybrid embryo which partakes of the constitution of its father and mother is exposed to unnatural conditions, as long as it is nourished within the womb, or egg, or seed of the mother-form; and as we know that unnatural conditions often induce sterility, the reproductive organs of the hybrid might at this early age be permanently affected. but this cause has no bearing on the infertility of first unions. the diminished number of the offspring from first unions may often result, as is certainly sometimes the case, from the premature death of most of the hybrid embryos. but we shall immediately see that a law of an unknown nature apparently exists, which causes the offspring from unions, which are infertile, to be themselves more or less infertile; and this at present is all that can be said. _seventhly_, hybrids and mongrels present, with the one great exception of fertility, the most striking accordance in all other respects; namely, in the laws of their resemblance to their two parents, in their tendency to reversion, in their variability, and in being absorbed through repeated crosses by either parent-form. since arriving at the foregoing conclusions, condensed from my former work, i have been led to investigate a subject which throws considerable light on hybridism, namely, the fertility of { } reciprocally dimorphic and trimorphic plants, when illegitimately united. i have had occasion several times to allude to these plants, and i may here give a brief abstract[ ] of my observations. several plants belonging to distinct orders present two forms, which exist in about equal numbers, and which differ in no respect except in their reproductive organs; one form having a long pistil with short stamens, the other a short pistil with long stamens; both with differently sized pollen-grains. with trimorphic plants there are three forms likewise differing in the lengths of their pistils and stamens, in the size and colour of the pollen-grains, and in some other respects; and as in each of the three forms there are two sets of stamens, there are altogether six sets of stamens and three kinds of pistils. these organs are so proportioned in length to each other that, in any two of the forms, half the stamens in each stand on a level with the stigma of the third form. now i have shown, and the result has been confirmed by other observers, that, in order to obtain full fertility with these plants, it is necessary that the stigma of the one form should be fertilised by pollen taken from the stamens of corresponding height in the other form. so that with dimorphic species two unions, which may be called legitimate, are fully fertile, and two, which may be called illegitimate, are more or less infertile. with trimorphic species six unions are legitimate or fully fertile, and twelve are illegitimate or more or less infertile. the infertility which may be observed in various dimorphic and trimorphic plants, when they are illegitimately fertilised, that is, by pollen taken from stamens not corresponding in height with the pistil, differs much in degree, up to absolute and utter sterility; just in the same manner as occurs in crossing distinct species. as the degree of sterility in the latter case depends in an eminent degree on the conditions of life being more or less favourable, so i have found it with illegitimate unions. it is well known that if pollen of a distinct species be placed on the stigma of a flower, and its own pollen be afterwards, even { } after a considerable interval of time, placed on the same stigma, its action is so strongly prepotent that it generally annihilates the effect of the foreign pollen; so it is with the pollen of the several forms of the same species, for legitimate pollen is strongly prepotent over illegitimate pollen, when both are placed on the same stigma. i ascertained this by fertilising several flowers, first illegitimately, and twenty-four hours afterwards legitimately, with pollen taken from a peculiarly coloured variety, and all the seedlings were similarly coloured; this shows that the legitimate pollen, though applied twenty-four hours subsequently, had wholly destroyed or prevented the action of the previously applied illegitimate pollen. again, as, in making reciprocal crosses between the same two species, there is occasionally a great difference in the result, so something analogous occurs with dimorphic plants; for a short-styled cowslip (_p. veris_) yields more seed when fertilised by the long-styled form, and less seed when fertilised by its own form, compared with a long-styled cowslip when fertilised in the two corresponding methods. in all these respects the forms of the same undoubted species, when illegitimately united, behave in exactly the same manner as do two distinct species when crossed. this led me carefully to observe during four years many seedlings, raised from several illegitimate unions. the chief result is that these illegitimate plants, as they may be called, are not fully fertile. it is possible to raise from dimorphic species, both long-styled and short-styled illegitimate plants, and from trimorphic plants all three illegitimate forms. these can then be properly united in a legitimate manner. when this is done, there is no apparent reason why they should not yield as many seeds as did their parents when legitimately fertilised. but such is not the case; they are all infertile, but in various degrees; some being so utterly and incurably sterile that they did not yield during four seasons a single seed or even seed-capsule. these illegitimate plants, which are so sterile, although united with each other in a legitimate manner, may be strictly compared with hybrids when crossed _inter se_, and it is well known how sterile these latter generally are. when, on the other hand, a hybrid is crossed with either pure parent-species, the sterility is usually much lessened: and so it is when an illegitimate plant is fertilised by { } a legitimate plant. in the same manner as the sterility of hybrids does not always run parallel with the difficulty of making the first cross between the two parent species, so the sterility of certain illegitimate plants was unusually great, whilst the sterility of the union from which they were derived was by no means great. with hybrids raised from the same seed-capsule the degree of sterility is innately variable, so it is in a marked manner with illegitimate plants. lastly, many hybrids are profuse and persistent flowerers, whilst other and more sterile hybrids produce few flowers, and are weak, miserable dwarfs; exactly similar cases occur with the illegitimate offspring of various dimorphic and trimorphic plants. altogether there is the closest identity in character and behaviour between illegitimate plants and hybrids. it is hardly an exaggeration to maintain that the former are hybrids, but produced within the limits of the same species by the improper union of certain forms, whilst ordinary hybrids are produced from an improper union between so-called distinct species. we have already seen that there is the closest similarity in all respects between first illegitimate unions, and first crosses between distinct species. this will perhaps be made more fully apparent by an illustration: we may suppose that a botanist found two well-marked varieties (and such occur) of the long-styled form of the trimorphic _lythrum salicaria_, and that he determined to try by crossing whether they were specifically distinct. he would find that they yielded only about one-fifth of the proper number of seed, and that they behaved in all the other above-specified respects as if they had been two distinct species. but to make the case sure, he would raise plants from his supposed hybridised seed, and he would find that the seedlings were miserably dwarfed and utterly sterile, and that they behaved in all other respects like ordinary hybrids. he might then maintain that he had actually proved, in accordance with the common view, that his two varieties were as good and as distinct species as any in the world; but he would be completely mistaken. the facts now given on dimorphic and trimorphic plants are important, because they show us, firstly, that the physiological { } test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of specific distinction; secondly, because we may conclude that there must be some unknown law or bond connecting the infertility of illegitimate unions with that of their illegitimate offspring, and we are thus led to extend this view to first crosses and hybrids; thirdly, because we find, and this seems to me of especial importance, that with trimorphic plants three forms of the same species exist, which when crossed in a particular manner are infertile, and yet these forms differ in no respect from each other, except in their reproductive organs,--as in the relative length of the stamens and pistils, in the size, form, and colour of the pollen-grains, in the structure of the stigma, and in, the number and size of the seeds. with these differences and no others, either in organisation or constitution, we find that the illegitimate unions and the illegitimate progeny of these three forms are more or less sterile, and closely resemble in a whole series of relations the first unions and hybrid offspring of distinct species. from this we may infer that the sterility of species when crossed and of their hybrid progeny is likewise in all probability exclusively due to differences confined to the reproductive system. we have indeed been brought to a similar conclusion by observing that the sterility of crossed species does not strictly coincide with their systematic affinity, that is, with the sum of their external resemblances; nor does it coincide with their similarity in general constitution. but we are more especially led to this same conclusion by considering reciprocal crosses, in which the male of one species cannot be united, or can be united with extreme difficulty, with the female of a second species, whilst the converse cross can be effected with perfect facility; for this difference in the facility of making reciprocal crosses, and in the fertility of their offspring, must be attributed either to the male or female element in the first species having been differentiated with reference to the sexual element of the second species in a higher degree than in the converse case. in so complex a subject as hybridism it is of considerable importance thus to arrive at a definitive conclusion, namely, that the sterility which almost invariably follows the union of distinct { } species depends exclusively on differences in their sexual constitution. * * * * * on the principle which makes it necessary for man, whilst he is selecting and improving his domestic varieties, to keep them separate, it would clearly be advantageous to varieties in a state of nature, that is to incipient species, if they could be kept from blending, either through sexual aversion, or by becoming mutually sterile. hence it at one time appeared to me probable, as it has to others, that this sterility might have been acquired through natural selection. on this view we must suppose that a shade of lessened fertility first spontaneously appeared, like any other modification, in certain individuals of a species when crossed with other individuals of the same species; and that successive slight degrees of infertility, from being advantageous, were slowly accumulated. this appears all the more probable, if we admit that the structural differences between the forms of dimorphic and trimorphic plants, as the length and curvature of the pistil, &c., have been co-adapted through natural selection; for if this be admitted, we can hardly avoid extending the same conclusion to their mutual infertility. sterility moreover has been acquired through natural selection for other and widely different purposes, as with neuter insects in reference to their social economy. in the case of plants, the flowers on the circumference of the truss in the guelder-rose (_viburnum opulus_) and those on the summit of the spike in the feather-hyacinth (_muscari comosum_) have been rendered conspicuous, and apparently in consequence sterile, in order that insects might easily discover and visit the other flowers. but when we endeavour to apply the principle of natural selection to the acquirement by distinct species of mutual sterility, we meet with great difficulties. in the first place, it may be remarked that separate regions are often inhabited by groups of species or by single species, which when brought together and crossed are found to be more or less sterile; now it could clearly have been of no advantage to such separated species to have been rendered mutually sterile, and consequently this could not have been effected through natural selection; but it may perhaps be argued, that, if a species were rendered sterile with { } some one compatriot, sterility with other species would follow as a necessary consequence. in the second place, it is as much opposed to the theory of natural selection, as to the theory of special creation, that in reciprocal crosses the male element of one form should have been rendered utterly impotent on a second form, whilst at the same time the male element of this second form is enabled freely to fertilise the first form; for this peculiar state of the reproductive system could not possibly be advantageous to either species. in considering the probability of natural selection having come into action in rendering species mutually sterile, one great difficulty will be found to lie in the existence of many graduated steps from slightly lessened fertility to absolute sterility. it may be admitted, on the principle above explained, that it would profit an incipient species if it were rendered in some slight degree sterile when crossed with its parent-form or with some other variety; for thus fewer bastardised and deteriorated offspring would be produced to commingle their blood with the new species in process of formation. but he who will take the trouble to reflect on the steps by which this first degree of sterility could be increased through natural selection to that higher degree which is common to so many species, and which is universal with species which have been differentiated to a generic or family rank, will find the subject extraordinarily complex. after mature reflection it seems to me that this could not have been effected through natural selection; for it could have been of no direct advantage to an individual animal to breed badly with another individual of a different variety, and thus leave few offspring; consequently such individuals could not have been preserved or selected. or take the case of two species which in their present state, when crossed, produce few and sterile offspring; now, what is there which could favour the survival of those individuals which happened to be endowed in a slightly higher degree with mutual infertility and which thus approached by one small step towards absolute sterility? yet an advance of this kind, if the theory of natural selection be brought to bear, must have incessantly occurred with many species, for a multitude are mutually quite barren. with sterile neuter insects we have reason to { } believe that modifications in their structure have been slowly accumulated by natural selection, from an advantage having been thus indirectly given to the community to which they belonged over other communities of the same species; but an individual animal, if rendered slightly sterile when crossed with some other variety, would not thus in itself gain any advantage, or indirectly give any advantage to its nearest relatives or to other individuals of the same variety, leading to their preservation. i infer from these considerations that, as far as animals are concerned, the various degrees of lessened fertility which occur with species when crossed cannot have been slowly accumulated by means of natural selection. with plants, it is possible that the case may be somewhat different. with many kinds, insects constantly carry pollen from neighbouring plants to the stigmas of each flower; and with some species this is effected by the wind. now, if the pollen of a variety, when deposited on the stigma of the same variety, should become by spontaneous variation in ever so slight a degree prepotent over the pollen of other varieties, this would certainly be an advantage to the variety; for its own pollen would thus obliterate the effects of the pollen of other varieties, and prevent deterioration of character. and the more prepotent the variety's own pollen could be rendered through natural selection, the greater the advantage would be. we know from the researches of gärtner that, with species which are mutually sterile, the pollen of each is always prepotent on its own stigma over that of the other species; but we do not know whether this prepotency is a consequence of the mutual sterility, or the sterility a consequence of the prepotency. if the latter view be correct, as the prepotency became stronger through natural selection, from being advantageous to a species in process of formation, so the sterility consequent on prepotency would at the same time be augmented; and the final result would be various degrees of sterility, such as occurs with existing species. this view might be extended to animals, if the female before each birth received several males, so that the sexual element of the prepotent male of her own variety obliterated the effects of the access of previous males belonging to other varieties; but we have no reason to believe, at least { } with terrestrial animals, that this is the ease; as most males and females pair for each birth, and some few for life. on the whole we may conclude that with animals the sterility of crossed species has not been slowly augmented through natural selection; and as this sterility follows the same general laws in the vegetable as in the animal kingdom, it is improbable, though apparently possible, that with plants crossed species should have been rendered sterile by a different process. from this consideration, and remembering that species which have never co-existed in the same country, and which therefore could not have received any advantage from having been rendered mutually infertile, yet are generally sterile when crossed; and bearing in mind that in reciprocal crosses between the same two species there is sometimes the widest difference in their sterility, we must give up the belief that natural selection has come into play. as species have not been rendered mutually infertile through the accumulative action of natural selection, and as we may safely conclude, from the previous as well as from other and more general considerations, that they have not been endowed through an act of creation with this quality, we must infer that it has arisen incidentally during their slow formation in connection with other and unknown changes in their organisation. by a quality arising incidentally, i refer to such cases as different species of animals and plants being differently affected by poisons to which they are not naturally exposed; and this difference in susceptibility is clearly incidental on other and unknown differences in their organisation. so again the capacity in different kinds of trees to be grafted on each other, or on a third species, differs much, and is of no advantage to these trees, but is incidental on structural or functional differences in their woody tissues. we need not feel surprise at sterility incidentally resulting from crosses between distinct species,--the modified descendants of a common progenitor,--when we bear in mind how easily the reproductive system is affected by various causes--often by extremely slight changes in the conditions of life, by too close interbreeding, and by other agencies. it is well to bear in mind such cases, as that of the _passiflora alata_, which recovered its self-fertility from { } being grafted on a distinct species--the cases of plants which normally or abnormally are self-impotent, but can readily be fertilised by the pollen of a distinct species--and lastly the cases of individual domesticated animals which evince towards each other sexual incompatibility. * * * * * we now at last come to the immediate point under discussion: how is it that, with some few exceptions in the case of plants, domesticated varieties, such as those of the dog, fowl, pigeon, several fruit-trees, and culinary vegetables, which differ from each other in external characters more than many species, are perfectly fertile when crossed, or even fertile in excess, whilst closely allied species are almost invariably in some degree sterile? we can, to a certain extent, give a satisfactory answer to this question. passing over the fact that the amount of external difference between two species is no sure guide to their degree of mutual sterility, so that similar differences in the case of varieties would be no sure guide, we know that with species the cause lies exclusively in differences in their sexual constitution. now the conditions to which domesticated animals and cultivated plants have been subjected, have had so little tendency towards modifying the reproductive system in a manner leading to mutual sterility, that we have good grounds for admitting the directly opposite doctrine of pallas, namely, that such conditions generally eliminate this tendency; so that the domesticated descendants of species, which in their natural state would have been in some degree sterile when crossed, become perfectly fertile together. with plants, so far is cultivation from giving a tendency towards mutual sterility, that in several well-authenticated cases, already often alluded to, certain species have been affected in a very different manner, for they have become self-impotent, whilst still retaining the capacity of fertilising, and being fertilised by, distinct species. if the pallasian doctrine of the elimination of sterility through long-continued domestication be admitted, and it can hardly be rejected, it becomes in the highest degree improbable that similar circumstances should commonly both induce and eliminate the same tendency; though in certain cases, with species having a peculiar constitution, sterility might occasionally be thus { } induced. thus, as i believe, we can understand why with domesticated animals varieties have not been produced which are mutually sterile; and why with plants only a few such cases have been observed, namely, by gärtner, with certain varieties of maize and verbascum, by other experimentalists with varieties of the gourd and melon, and by kölreuter with one kind of tobacco. with respect to varieties which have originated in a state of nature, it is almost hopeless to expect to prove by direct evidence that they have been rendered mutually sterile; for if even a trace of sterility could be detected, such varieties would at once be raised by almost every naturalist to the rank of distinct species. if, for instance, gärtner's statement were fully confirmed, that the blue and red-flowered forms of the pimpernel (_anagallis arvensis_) are sterile when crossed, i presume that all the botanists who now maintain on various grounds that these two forms are merely fleeting varieties, would at once admit that they were specifically distinct. the real difficulty in our present subject is not, as it appears to me, why domestic varieties have not become mutually infertile when crossed, but why this has so generally occurred with natural varieties as soon as they have been modified in a sufficient and permanent degree to take rank as species. we are far from precisely knowing the cause; nor is this surprising, seeing how profoundly ignorant we are in regard to the normal and abnormal action of the reproductive system. but we can see that species, owing to their struggle for life with numerous competitors, must have been exposed to more uniform conditions during long periods of time, than have been domestic varieties; and this may well make a wide difference in the result. for we know how commonly wild animals and plants, when taken from their natural conditions and subjected to captivity, are rendered sterile; and the reproductive functions of organic beings which have always lived and been slowly modified under natural conditions would probably in like manner be eminently sensitive to the influence of an unnatural cross. domesticated productions, on the other hand, which, as shown by the mere fact of their domestication, were not originally highly sensitive to changes in their conditions of life, and which can now generally resist { } with undiminished fertility repeated changes of conditions, might be expected to produce varieties, which would be little liable to have their reproductive powers injuriously affected by the act of crossing with other varieties which had originated in a like manner. certain naturalists have recently laid too great stress, as it appears to me, on the difference in fertility between varieties and species when crossed. some allied species of trees cannot be grafted on each other,--all varieties can be so grafted. some allied animals are affected in a very different manner by the same poison, but with varieties no such case until recently was known, but now it has been proved that immunity from certain poisons stands in some cases in correlation with the colour of the hair. the period of gestation generally differs much with distinct species, but with varieties until lately no such difference had been observed. the time required for the germination of seeds differs in an analogous manner, and i am not aware that any difference in this respect has as yet been detected with varieties. here we have various physiological differences, and no doubt others could be added, between one species and another of the same genus, which do not occur, or occur with extreme rarity, in the case of varieties; and these differences are apparently wholly or in chief part incidental on other constitutional differences, just in the same manner as the sterility of crossed species is incidental on differences confined to the sexual system. why, then, should these latter differences, however serviceable they may indirectly be in keeping the inhabitants of the same country distinct, be thought of such paramount importance, in comparison with other incidental and functional differences? no sufficient answer to this question can be given. hence the fact that the most distinct domestic varieties are, with rare exceptions, perfectly fertile when crossed, and produce fertile offspring, whilst closely allied species are, with rare exceptions, more or less sterile, is not nearly so formidable an objection as it appears at first to the theory of the common descent of allied species. * * * * * { } chapter xx. selection by man. selection a difficult art--methodical, unconscious, and natural selection--results of methodical selection--care taken in selection--selection with plants--selection carried on by the ancients, and by semi-civilized people--unimportant characters often attended to--unconscious selection--as circumstances slowly change, so have our domesticated animals changed through the action of unconscious selection--influence of different breeders on the same sub-variety--plants as affected by unconscious selection--effects of selection as shown by the great amount of difference in the parts most valued by man. the power of selection, whether exercised by man, or brought into play under nature through the struggle for existence and the consequent survival of the fittest, absolutely depends on the variability of organic beings. without variability nothing can be effected; slight individual differences, however, suffice for the work, and are probably the sole differences which are effective in the production of new species. hence our discussion on the causes and laws of variability ought in strict order to have preceded our present subject, as well as the previous subjects of inheritance, crossing, &c.; but practically the present arrangement has been found the most convenient. man does not attempt to cause variability; though he unintentionally effects this by exposing organisms to new conditions of life, and by crossing breeds already formed. but variability being granted, he works wonders. unless some degree of selection be exercised, the free commingling of the individuals of the same variety soon obliterates, as we have previously seen, the slight differences which may arise, and gives to the whole body of individuals uniformity of character. in separated districts, long-continued exposure to different conditions of life may perhaps produce new races without the aid of selection; but to this difficult subject { } of the direct action of the conditions of life we shall in a future chapter recur. when animals or plants are born with some conspicuous and firmly inherited new character, selection is reduced to the preservation of such individuals, and to the subsequent prevention of crosses; so that nothing more need be said on the subject. but in the great majority of cases a new character, or some superiority in an old character, is at first faintly pronounced, and is not strongly inherited; and then the full difficulty of selection is experienced. indomitable patience, the finest powers of discrimination, and sound judgment must be exercised during many years. a clearly predetermined object must be kept steadily in view. few men are endowed with all these qualities, especially with that of discriminating very slight differences; judgment can be acquired only by long experience; but if any of these qualities be wanting, the labour of a life may be thrown away. i have been astonished when celebrated breeders, whose skill and judgment have been proved by their success at exhibitions, have shown me their animals, which appeared all alike, and have assigned their reasons for matching this and that individual. the importance of the great principle of selection mainly lies in this power of selecting scarcely appreciable differences, which nevertheless are found to be transmissible, and which can be accumulated until the result is made manifest to the eyes of every beholder. the principle of selection may be conveniently divided into three kinds. _methodical selection_ is that which guides a man who systematically endeavours to modify a breed according to some predetermined standard. _unconscious selection_ is that which follows from men naturally preserving the most valued and destroying the less valued individuals, without any thought of altering the breed; and undoubtedly this process slowly works great changes. unconscious selection graduates into methodical, and only extreme cases can be distinctly separated; for he who preserves a useful or perfect animal will generally breed from it with the hope of getting offspring of the same character; but as long as he has not a predetermined purpose to improve the breed, he may be said to be selecting { } unconsciously.[ ] lastly, we have _natural selection_, which implies that the individuals which are best fitted for the complex, and in the course of ages changing conditions to which they are exposed, generally survive and procreate their kind. with domestic productions, with which alone we are here strictly concerned, natural selection comes to a certain extent into action, independently of, and even in opposition to, the will of man. * * * * * _methodical selection._--what man has effected within recent times in england by methodical selection is clearly shown by our exhibitions of improved quadrupeds and fancy birds. with respect to cattle, sheep, and pigs, we owe their great improvement to a long series of well-known names--bakewell, colling, ellman, bates, jonas webb, lords leicester and western, fisher hobbs, and others. agricultural writers are unanimous on the power of selection: any number of statements to this effect could be quoted; a few will suffice. youatt, a sagacious and experienced observer, writes,[ ] the principle of selection is "that which enables the agriculturist, not only to modify the character of his flock, but to change it altogether." a great breeder of shorthorns[ ] says, "in the anatomy of the shoulder modern breeders have made great improvements on the ketton shorthorns by correcting the defect in the knuckle or shoulder-joint, and by laying the top of the shoulder more snugly into the crop, and thereby filling up the hollow behind it.... the eye has its fashion at different periods: at one time the eye high and outstanding from the head, and at another time the sleepy eye sunk into the head; but these extremes have merged into the medium of a full, clear, and prominent eye with a placid look." again, hear what an excellent judge of pigs[ ] says: "the legs { } should be no longer than just to prevent the animal's belly from trailing on the ground. the leg is the least profitable portion of the hog, and we therefore require no more of it than is absolutely necessary for the support of the rest." let any one compare the wild-boar with any improved breed, and he will see how effectually the legs have been shortened. few persons, except breeders, are aware of the systematic care taken in selecting animals, and of the necessity of having a clear and almost prophetic vision into futurity. lord spencer's skill and judgment were well known; and he writes,[ ] "it is therefore very desirable, before any man commences to breed either cattle or sheep, that he should make up his mind to the shape and qualities he wishes to obtain, and steadily pursue this object." lord somerville, in speaking of the marvellous improvement of the new leicester sheep, effected by bakewell and his successors, says, "it would seem as if they had first drawn a perfect form, and then given it life." youatt[ ] urges the necessity of annually drafting each flock, as many animals will certainly degenerate "from the standard of excellence, which the breeder has established in his own mind." even with a bird of such little importance as the canary, long ago ( - ) rules were established, and a standard of perfection was fixed, according to which the london fanciers tried to breed the several sub-varieties.[ ] a great winner of prizes at the pigeon-shows,[ ] in describing the short-faced almond tumbler, says, "there are many first-rate fanciers who are particularly partial to what is called the goldfinch-beak, which is very beautiful; others say, take a full-size round cherry, then take a barley-corn, and judiciously placing and thrusting it into the cherry, form as it were your beak; and that is not all, for it will form a good head and beak, provided, as i said before, it is judiciously done; others take an oat; but as i think the goldfinch-beak the handsomest, i would advise the inexperienced fancier to get the head of a goldfinch, and keep it by him for his observation." wonderfully different as is the beak of the rock-pigeon and goldfinch, undoubtedly, as far as { } external shape and proportions are concerned, the end has been nearly gained. not only should our animals be examined with the greatest care whilst alive, but, as anderson remarks,[ ] their carcases should be scrutinised, "so as to breed from the descendants of such only as, in the language of the butcher, cut up well." the "grain of the meat" in cattle, and its being well marbled with fat,[ ] and the greater or less accumulation of fat in the abdomen of our sheep, have been attended to with success. so with poultry, a writer,[ ] speaking of cochin-china fowls, which are said to differ much in the quality of their flesh, says, "the best mode is to purchase two young brother-cocks, kill, dress, and serve up one; if he be indifferent, similarly dispose of the other, and try again; if, however, he be fine and well-flavoured, his brother will not be amiss for breeding purposes for the table." the great principle of the division of labour has been brought to bear on selection. in certain districts[ ] "the breeding of bulls is confined to a very limited number of persons, who by devoting their whole attention to this department, are able from year to year to furnish a class of bulls which are steadily improving the general breed of the district." the rearing and letting of choice rams has long been, as is well known, a chief source of profit to several eminent breeders. in parts of germany this principle is carried with merino sheep to an extreme point.[ ] "so important is the proper selection of breeding animals considered, that the best flock-masters do not trust to their own judgment, or to that of their shepherds, but employ persons called 'sheep-classifiers,' who make it their special business to attend to this part of the management of several flocks, and thus to preserve, or if possible to improve, the best qualities of both parents in the lambs." in saxony, "when the lambs are weaned, each in his turn is placed upon a table that his wool and form may be minutely observed. { } the finest are selected for breeding and receive a first mark. when they are one year old, and prior to shearing them, another close examination of those previously marked takes place: those in which no defect can be found receive a second mark, and the rest are condemned. a few months afterwards a third and last scrutiny is made; the prime rams and ewes receive a third and final mark, but the slightest blemish is sufficient to cause the rejection of the animal." these sheep are bred and valued almost exclusively for the fineness of their wool; and the result corresponds with the labour bestowed on their selection. instruments have been invented to measure accurately the thickness of the fibres; and "an austrian fleece has been produced of which twelve hairs equalled in thickness one from a leicester sheep." throughout the world, wherever silk is produced, the greatest care is bestowed on selecting the cocoons from which the moths for breeding are to be reared. a careful cultivator[ ] likewise examines the moths themselves, and destroys those that are not perfect. but what more immediately concerns us is that certain families in france devote themselves to raising eggs for sale.[ ] in china, near shanghai, the inhabitants of two small districts have the privilege of raising eggs for the whole surrounding country, and that they may give up their whole time to this business, they are interdicted by law from producing silk.[ ] the care which successful breeders take in matching their birds is surprising. sir john sebright, whose fame is perpetuated by the "sebright bantam," used to spend "two and three days in examining, consulting, and disputing with a friend which were the best of five or six birds."[ ] mr. bult, whose pouter-pigeons won so many prizes and were exported to north america under the charge of a man sent on purpose, told me that he always deliberated for several days before he matched each pair. hence we can understand the advice of an eminent fancier, who writes,[ ] "i would here particularly guard { } you against having too great a variety of pigeons, otherwise you will know a little of all, but nothing about one as it ought to be known." apparently it transcends the power of the human intellect to breed all kinds: "it is possible that there may be a few fanciers that have a good general knowledge of fancy pigeons; but there are many more who labour under the delusion of supposing they know what they do not." the excellence of one sub-variety, the almond tumbler, lies in the plumage, carriage, head, beak, and eye; but it is too presumptuous in the beginner to try for all these points. the great judge above quoted says, "there are some young fanciers who are over-covetous, who go for all the above five properties at once; they have their reward by getting nothing." we thus see that breeding even fancy pigeons is no simple art: we may smile at the solemnity of these precepts, but he who laughs will win no prizes. what methodical selection has effected for our animals is sufficiently proved, as already remarked, by our exhibitions. so greatly were the sheep belonging to some of the earlier breeders, such as bakewell and lord western, changed, that many persons could not be persuaded that they had not been crossed. our pigs, as mr. corringham remarks,[ ] during the last twenty years have undergone, through rigorous selection together with crossing, a complete metamorphosis. the first exhibition for poultry was held in the zoological gardens in ; and the improvement effected since that time has been great. as mr. baily, the great judge, remarked to me, it was formerly ordered that the comb of the spanish cock should be upright, and in four or five years all good birds had upright combs; it was ordered that the polish cock should have no comb or wattles, and now a bird thus furnished would be at once disqualified; beards were ordered, and out of fifty-seven pens lately ( ) exhibited at the crystal palace, all had beards. so it has been in many other cases. but in all cases the judges order only what is occasionally produced and what can be improved and rendered constant by selection. the steady increase of weight during the last few years in our { } fowls, turkeys, ducks, and geese is notorious; "six-pound ducks are now common, whereas four pounds was formerly the average." as the actual time required to make a change has not often been recorded, it may be worth mentioning that it took mr. wicking thirteen years to put a clean white head on an almond tumbler's body, "a triumph," says another fancier, "of which he may be justly proud."[ ] mr. tollet, of betley hall, selected cows, and especially bulls, descended from good milkers, for the sole purpose of improving his cattle for the production of cheese; he steadily tested the milk with the lactometer, and in eight years he increased, as i was informed by him, the product in the proportion of four to three. here is a curious case[ ] of steady but slow progress, with the end not as yet fully attained: in a race of silkworms was introduced into france, in which one hundred out of the thousand failed to produce white cocoons; but now, after careful selection during sixty-five generations, the proportion of yellow cocoons has been reduced to thirty-five in the thousand. with plants selection has been followed with the same good results as with animals. but the process is simpler, for plants in the great majority of cases bear both sexes. nevertheless, with most kinds it is necessary to take as much care to prevent crosses as with animals or unisexual plants; but with some plants, such as peas, this care does not seem to be necessary. with all improved plants, excepting of course those which are propagated by buds, cuttings, &c., it is almost indispensable to examine the seedlings and destroy those which depart from the proper type. this is called "roguing," and is, in fact, a form of selection, like the rejection of inferior animals. experienced horticulturists and agriculturists incessantly urge every one to preserve the finest plants for the production of seed. although plants often present much more conspicuous variations than animals, yet the closest attention is generally requisite to detect each slight and favourable change. mr. masters relates[ ] how "many a patient hour was devoted," whilst he was { } young, to the detection of differences in peas intended for seed. mr. barnet[ ] remarks that the old scarlet american strawberry was cultivated for more than a century without producing a single variety; and another writer observes how singular it was that when gardeners first began to attend to this fruit it began to vary; the truth no doubt being that it had always varied, but that, until slight varieties were selected and propagated by seed, no conspicuous result was obtained. the finest shades of difference in wheat have been discriminated and selected with almost as much care, as we see in colonel le couteur's works, as in the case of the higher animals; but with our cereals the process of selection has seldom or never been long continued. it may be worth while to give a few examples of methodical selection with plants; but in fact the great improvement of all our anciently cultivated plants may be attributed to selection long carried on, in part methodically, and in part unconsciously. i have shown in a former chapter how the weight of the gooseberry has been increased by systematic selection and culture. the flowers of the heartsease have been similarly increased in size and regularity of outline. with the cineraria, mr. glenny[ ] "was bold enough, when the flowers were ragged and starry and ill defined in colour, to fix a standard which was then considered outrageously high and impossible, and which, even if reached, it was said, we should be no gainers by, as it would spoil the beauty of the flowers. he maintained that he was right; and the event has proved it to be so." the doubling of flowers has several times been effected by careful selection: the rev. w. williamson,[ ] after sowing during several years seed of _anemone coronaria_, found a plant with one additional petal; he sowed the seed of this, and by perseverance in the same course obtained several varieties with six or seven rows of petals. the single scotch rose was doubled, and yielded eight good varieties in nine or ten years.[ ] the canterbury bell (_campanula medium_) was doubled by careful selection in four generations.[ ] in four years mr. buckman,[ ] by culture and { } careful selection, converted parsnips, raised from wild seed, into a new and good variety. by selection during a long course of years, the early maturity of peas has been hastened from ten to twenty-one days.[ ] a more curious case is offered by the beet-plant, which, since its cultivation in france, has almost exactly doubled its yield of sugar. this has been effected by the most careful selection; the specific gravity of the roots being regularly tested, and the best roots saved for the production of seed.[ ] _selection by ancient and semi-civilised people._ in attributing so much importance to the selection of animals and plants, it may be objected that methodical selection would not have been carried on during ancient times. a distinguished naturalist considers it as absurd to suppose that semi-civilised people should have practised selection of any kind. undoubtedly the principle has been systematically acknowledged and followed to a far greater extent within the last hundred years than at any former period, and a corresponding result has been gained; but it would be a great error to suppose, as we shall immediately see, that its importance was not recognised and acted on during the most ancient times, and by semi-civilised people. i should premise that many facts now to be given only show that care was taken in breeding; but when this is the case, selection is almost sure to be practised to a certain extent. we shall hereafter be enabled better to judge how far selection, when only occasionally carried on, by a few of the inhabitants of a country, will slowly produce a great effect. in a well-known passage in the thirtieth chapter of genesis, rules are given for influencing, as was then thought possible, the colour of sheep; and speckled and dark breeds are spoken of as being kept separate. by the time of david the fleece was likened to snow. youatt,[ ] who has discussed all the passages in relation to breeding in the old testament, concludes that { } at this early period "some of the best principles of breeding must have been steadily and long pursued." it was ordered, according to moses, that "thou shalt not let thy cattle gender with a diverse kind;" but mules were purchased,[ ] so that at this early period other nations must have crossed the horse and ass. it is said[ ] that erichthonius, some generations before the trojan war, had many brood-mares, "which by his care and judgment in the choice of stallions produced a breed of horses superior to any in the surrounding countries." homer (book v.) speaks of Æneas's horses as bred from mares which were put to the steeds of laomedon. plato, in his 'republic,' says to glaucus, "i see that you raise at your house a great many dogs for the chase. do you take care about breeding and pairing them? among animals of good blood, are there not always some which are superior to the rest?" to which glaucus answers in the affirmative.[ ] alexander the great selected the finest indian cattle to send to macedonia to improve the breed.[ ] according to pliny,[ ] king pyrrhus had an especially valuable breed of oxen; and he did not suffer the bulls and cows to come together till four years old, that the breed might not degenerate. virgil, in his georgics (lib. iii.), gives as strong advice as any modern agriculturist could do, carefully to select the breeding stock; "to note the tribe, the lineage, and the sire; whom to reserve for husband of the herd;"--to brand the progeny;--to select sheep of the purest white, and to examine if their tongues are swarthy. we have seen that the romans kept pedigrees of their pigeons, and this would have been a senseless proceeding had not great care been taken in breeding them. columella gives detailed instructions about breeding fowls: "let the breeding hens therefore be of a choice colour, a robust body, square-built, full-breasted, with large heads, with upright and bright-red combs. those are believed to be the best bred which have five toes."[ ] according to tacitus, the celts attended to the races of their domestic animals; { } and cæsar states that they paid high prices to merchants for fine imported horses.[ ] in regard to plants, virgil speaks of yearly culling the largest seeds; and celsus says, "where the corn and crop is but small, we must pick out the best ears of corn, and of them lay up our seed separately by itself."[ ] coming down the stream of time, we may be brief. at about the beginning of the ninth century charlemagne expressly ordered his officers to take great care of his stallions; and if any proved bad or old, to forewarn him in good time before they were put to the mares.[ ] even in a country so little civilised as ireland during the ninth century, it would appear from some ancient verses,[ ] describing a ransom demanded by cormac, that animals from particular places, or having a particular character, were valued. thus it is said,-- two pigs of the pigs of mac lir, a ram and ewe both round and red, i brought with me from aengus. i brought with me a stallion and a mare from the beautiful stud of manannan, a bull and a white cow from druim cain. athelstan, in , received as a present from germany, running-horses; and he prohibited the exportation of english horses. king john imported "one hundred chosen stallions from flanders."[ ] on june th, , the prince of wales wrote to the archbishop of canterbury, begging for the loan of any choice stallion, and promising its return at the end of the season.[ ] there are numerous records at ancient periods in english history of the importation of choice animals of various kinds, and of foolish laws against their exportation. in the reigns of henry vii. and viii. it was ordered that the magistrates, at michaelmas, should scour the heaths and commons, and destroy all mares beneath a certain size.[ ] some of our earlier kings passed laws against the slaughtering rams of any good breed before they were seven years old, so that they { } might have time to breed. in spain cardinal ximenes issued, in , regulations on the _selection_ of good rams for breeding.[ ] the emperor akbar khan before the year is said to have "wonderfully improved" his pigeons by crossing the breeds; and this necessarily implies careful selection. about the same period the dutch attended with the greatest care to the breeding of these birds. belon in says that good managers in france examined the colour of their goslings in order to get geese of a white colour and better kinds. markham in tells the breeder "to elect the largest and goodliest conies," and enters into minute details. even with respect to seeds of plants for the flower-garden, sir j. hanmer writing about the year [ ] says, in "choosing seed, the best seed is the most weighty, and is had from the lustiest and most vigorous stems;" and he then gives rules about leaving only a few flowers on plants for seed; so that even such details were attended to in our flower-gardens two hundred years ago. in order to show that selection has been silently carried on in places where it would not have been expected, i may add that in the middle of the last century, in a remote part of north america, mr. cooper improved by careful selection all his vegetables, "so that they were greatly superior to those of any other person. when his radishes, for instance, are fit for use, he takes ten or twelve that he most approves, and plants them at least yards from others that blossom at the same time. in the same manner he treats all his other plants, varying the circumstances according to their nature."[ ] in the great work on china published in the last century by the jesuits, and which is chiefly compiled from ancient chinese encyclopædias, it is said that with sheep "improving the breed consists in choosing with particular care the lambs which are destined for propagation, in nourishing them well, and in keeping the flocks separate." the same principles were applied by the chinese to various plants and fruit-trees.[ ] an { } imperial edict recommends the choice of seed of remarkable size; and selection was practised even by imperial hands, for it is said that the ya-mi, or imperial rice, was noticed at an ancient period in a field by the emperor khang-hi, was saved and cultivated in his garden, and has since become valuable from being the only kind which will grow north of the great wall.[ ] even with flowers, the tree pæony (_p. moutan_) has been cultivated, according to chinese traditions, for years; between and varieties have been raised, which are cherished like tulips formerly were by the dutch.[ ] turning now to semi-civilised people and to savages: it occurred to me, from what i had seen of several parts of south america, where fences do not exist, and where the animals are of little value, that there would be absolutely no care in breeding or selecting them; and this to a large extent is true. roulin,[ ] however, describes in colombia a naked race of cattle, which are not allowed to increase, on account of their delicate constitution. according to azara[ ] horses are often born in paraguay with curly hair; but, as the natives do not like them, they are destroyed. on the other hand, azara states that a hornless bull, born in , was preserved and propagated its race. i was informed of the existence in banda oriental of a breed with reversed hair; and the extraordinary niata cattle first appeared and have since been kept distinct in la plata. hence certain conspicuous variations have been preserved, and others have been habitually destroyed, in these countries, which are so little favourable for careful selection. we have also seen that the inhabitants sometimes introduce cattle on their estates to prevent the evil effects of close interbreeding. on the other hand, i have heard on reliable authority that the gauchos of the pampas never take any pains in selecting the best bulls or stallions for breeding; and this probably accounts for the cattle and horses being remarkably uniform in character throughout the immense range of the argentine republic. looking to the old world, in the sahara desert "the touareg is as careful in the selection of his breeding mahari { } (a fine race of the dromedary) as the arab is in that of his horse. the pedigrees are handed down, and many a dromedary can boast a genealogy far longer than the descendants of the darley arabian."[ ] according to pallas the mongolians endeavour to breed the yaks or horse-tailed buffaloes with white tails, for these are sold to the chinese mandarins as fly-flappers; and moorcroft, about seventy years after pallas, found that white-tailed animals were still selected for breeding.[ ] we have seen in the chapter on the dog that savages in different parts of north america and in guiana cross their dogs with wild canidæ, as did the ancient gauls, according to pliny. this was done to give their dogs strength and vigour, in the same way as the keepers in large warrens now sometimes cross their ferrets (as i have been informed by mr. yarrell) with the wild polecat, "to give them more devil." according to varro, the wild ass was formerly caught and crossed with the tame animal to improve the breed, in the same manner as at the present day the natives of java sometimes drive their cattle into the forests to cross with the wild banteng (_bos sondaicus_).[ ] in northern siberia, among the ostyaks the dogs vary in markings in different districts, but in each place they are spotted black and white in a remarkably uniform manner;[ ] and from this fact alone we may infer careful breeding, more especially as the dogs of one locality are famed throughout the country for their superiority. i have heard of certain tribes of esquimaux who take pride in their teams of dogs being uniformly coloured. in guiana, as sir r. schomburgk informs me,[ ] the dogs of the turuma indians are highly valued and extensively bartered: the price of a good one is the same as that given for a wife: they are kept in a sort of cage, and the indians "take great care when the female is in season to prevent her uniting with a dog of an inferior description." the indians told sir robert that, if a dog proved bad or useless, { } he was not killed, but was left to die from sheer neglect. hardly any nation is more barbarous than the fuegians, but i hear from mr. bridges, the catechist to the mission, that, "when these savages have a large, strong, and active bitch, they take care to put her to a fine dog, and even take care to feed her well, that her young may be strong and well favoured." in the interior of africa, negroes, who have not associated with white men, show great anxiety to improve their animals: they "always choose the larger and stronger males for stock:" the malakolo were much pleased at livingstone's promise to send them a bull, and some bakalolo carried a live cock all the way from loanda into the interior.[ ] further south on the same continent, andersson states that he has known a damara give two fine oxen for a dog which struck his fancy. the damaras take great delight in having whole droves of cattle of the same colour, and they prize their oxen in proportion to the size of their horns. "the namaquas have a perfect mania for a uniform team; and almost all the people of southern africa value their cattle next to their women, and take a pride in possessing animals that look high-bred." "they rarely or never make use of a handsome animal as a beast of burden."[ ] the power of discrimination which these savages possess is wonderful, and they can recognise to which tribe any cattle belong. mr. andersson further informs me that the natives frequently match a particular bull with a particular cow. the most curious case of selection by semi-civilised people, or indeed by any people, which i have found recorded, is that given by garcilazo de la vega, a descendant of the incas, as having been practised in peru before the country was subjugated by the spaniards.[ ] the incas annually held great hunts, when all the wild animals were driven from an immense circuit to a central point. the beasts of prey were first destroyed as injurious. the wild guanacos and vicunas were sheared; the old males and females killed, and the others set at liberty. the various kinds of deer were examined; the old males and females { } were likewise killed; "but the young females, with a certain number of males, selected from the most beautiful and strong," were given their freedom. here, then, we have selection by man aiding natural selection. so that the incas followed exactly the reverse system of that which our scottish sportsmen are accused of following, namely, of steadily killing the finest stags, thus causing the whole race to degenerate.[ ] in regard to the domesticated llamas and alpacas, they were separated in the time of the incas according to colour; and if by chance one in a flock was born of the wrong colour, it was eventually put into another flock. in the genus auchenia there are four forms,--the guanaco and vicuna, found wild and undoubtedly distinct species; the llama and alpaca, known only in a domesticated condition. these four animals appear so different, that most professed naturalists, especially those who have studied these animals in their native country, maintain that they are specifically distinct, notwithstanding that no one pretends to have seen a wild llama or alpaca. mr. ledger, however, who has closely studied these animals both in peru and during their exportation to australia, and who has made many experiments on their propagation, adduces arguments[ ] which seem to me conclusive, that the llama is the domesticated descendant of the guanaco, and the alpaca of the vicuna. and now that we know that these animals many centuries ago were systematically bred and selected, there is nothing surprising in the great amount of change which they have undergone. it appeared to me at one time probable that, though ancient and semi-civilised people might have attended to the improvement of their more useful animals in essential points, yet that they would have disregarded unimportant characters. but human nature is the same throughout the world: fashion everywhere reigns supreme, and man is apt to value whatever he may chance to possess. we have seen that in south america the niata cattle, which certainly are not made useful by their shortened faces and upturned nostrils, have been preserved. the damaras of south africa value their cattle for uniformity { } of colour and enormously long horns. the mongolians value their yaks for their white tails. and i shall now show that there is hardly any peculiarity in our most useful animals which, from fashion, superstition, or some other motive, has not been valued, and consequently preserved. with respect to cattle, "an early record," according to youatt,[ ] "speaks of a hundred white cows with red ears being demanded as a compensation by the princes of north and south wales. if the cattle were of a dark or black colour, were to be presented." so that colour was attended to in wales before its subjugation by england. in central africa, an ox that beats the ground with its tail is killed; and in south africa some of the damaras will not eat the flesh of a spotted ox. the kaffirs value an animal with a musical voice; and "at a sale in british kaffraria the low of a heifer excited so much admiration that a sharp competition sprung up for her possession, and she realised a considerable price."[ ] with respect to sheep, the chinese prefer rams without horns; the tartars prefer them with spirally wound horns, because the hornless are thought to lose courage.[ ] some of the damaras will not eat the flesh of hornless sheep. in regard to horses, at the end of the fifteenth century animals of the colour described as _liart pommé_ were most valued in france. the arabs have a proverb, "never buy a horse with four white feet, for he carries his shroud with him;"[ ] the arabs also, as we have seen, despise dun-coloured horses. so with dogs, xenophon and others at an ancient period were prejudiced in favour of certain colours; and "white or slate-coloured hunting dogs were not esteemed."[ ] turning to poultry, the old roman gourmands thought that the liver of a white goose was the most savoury. in paraguay black-skinned fowls are kept because they are thought to be more productive, and their flesh the most proper for invalids.[ ] in guiana, as i am informed by sir r. schomburgk, the aborigines will not eat the flesh or eggs of the fowl, but two { } races are kept distinct merely for ornament. in the philippines, no less than nine sub-varieties of the game cock are kept and named, so that they must be separately bred. at the present time in europe, the smallest peculiarities are carefully attended to in our most useful animals, either from fashion, or as a mark of purity of blood. many examples could be given, two will suffice. "in the western counties of england the prejudice against a white pig is nearly as strong as against a black one in yorkshire." in one of the berkshire sub-breeds, it is said, "the white should be confined to four white feet, a white spot between the eyes, and a few white hairs behind each shoulder." mr. saddler possessed "three hundred pigs, every one of which was marked in this manner."[ ] marshall, towards the close of the last century, in speaking of a change in one of the yorkshire breeds of cattle, says the horns have been considerably modified, as "a clean, small, sharp horn has been _fashionable_ for the last twenty years."[ ] in a part of germany the cattle of the race de gfoehl are valued for many good qualities, but they must have horns of a particular curvature and tint, so much so that mechanical means are applied if they take a wrong direction; but the inhabitants "consider it of the highest importance that the nostrils of the bull should be flesh-coloured, and the eyelashes light; this is an indispensable condition. a calf with blue nostrils would not be purchased, or purchased at a very low price."[ ] therefore let no man say that any point or character is too trifling to be methodically attended to and selected by breeders. * * * * * _unconscious selection._--by this term i mean, as already more than once explained, the preservation by man of the most valued, and the destruction of the least valued individuals, without any conscious intention on his part of altering the breed. it is difficult to offer direct proofs of the results which follow from this kind of selection; but the indirect evidence is abundant. in fact, except that in the one case man acts intentionally, and in the other unintentionally, there is little difference between { } methodical and unconscious selection. in both cases man preserves the animals which are most useful or pleasing to him, and destroys or neglects the others. but no doubt a far more rapid result follows from methodical than from unconscious selection. the "roguing" of plants by gardeners, and the destruction by law in henry viii.'s reign of all under-sized mares, are instances of a process the reverse of selection in the ordinary sense of the word, but leading to the same general result. the influence of the destruction of individuals having a particular character is well shown by the necessity of killing every lamb with a trace of black about it, in order to keep the flock white; or again, by the effects on the average height of the men of france of the destructive wars of napoleon, by which many tall men were killed, the short ones being left to be the fathers of families. this at least is the conclusion of those who have closely studied the subject of the conscription; and it is certain that since napoleon's time the standard for the army has been lowered two or three times. unconscious selection so blends into methodical that it is scarcely possible to separate them. when a fancier long ago first happened to notice a pigeon with an unusually short beak, or one with the tail-feathers unusually developed, although he bred from these birds with the distinct intention of propagating the variety, yet he could not have intended to make a short-faced tumbler or a fantail, and was far from knowing that he had made the first step towards this end. if he could have seen the final result, he would have been struck with astonishment, but, from what we know of the habits of fanciers, probably not with admiration. our english carriers, barbs, and short-faced tumblers have been greatly modified in the same manner, as we may infer both from the historical evidence given in the chapters on the pigeon, and from the comparison of birds brought from distant countries. so it has been with dogs; our present fox-hounds differ from the old english hound; our greyhounds have become lighter; the wolf-dog, which belonged to the greyhound class, has become extinct; the scotch deer-hound has been modified, and is now rare. our bulldogs differ from those which were formerly used for baiting bulls. our pointers and newfoundlands do not { } closely resemble any native dog now found in the countries whence they were brought, these changes have been effected partly by crosses; but in every case the result has been governed by the strictest selection. nevertheless there is no reason to suppose that man intentionally and methodically made the breeds exactly what they now are. as our horses became fleeter, and the country more cultivated and smoother, fleeter fox-hounds were desired and produced, but probably without any one distinctly foreseeing what they would become. our pointers and setters, the latter almost certainly descended from large spaniels, have been greatly modified in accordance with fashion and the desire for increased speed. wolves have become extinct, deer have become rarer, bulls are no longer baited, and the corresponding breeds of the dog have answered to the change. but we may feel almost sure that when, for instance, bulls were no longer baited, no man said to himself, i will now breed my dogs of smaller size, and thus create the present race. as circumstances changed, men unconsciously and slowly modified their course of selection. with race-horses selection for swiftness has been followed methodically, and our horses can now easily beat their progenitors. the increased size and different appearance of the english race-horse led a good observer in india to ask, "could any one in this year of , looking at our race-horses, conceive that they were the result of the union of the arab horse and the african mare?"[ ] this change has, it is probable, been largely effected through unconscious selection, that is, by the general wish to breed as fine horses as possible in each generation, combined with training and high feeding, but without any intention to give to them their present appearance. according to youatt,[ ] the introduction in oliver cromwell's time of three celebrated eastern stallions speedily affected the english breed; "so that lord harleigh, one of the old school, complained that the great horse was fast disappearing." this is an excellent proof how carefully selection must have been attended to; for without such care, all traces of so small an infusion of eastern blood would soon have been absorbed and { } lost. notwithstanding that the climate of england has never been esteemed particularly favourable to the horse, yet long-continued selection, both methodical and unconscious, together with that practised by the arabs during a still longer and earlier period, has ended in giving us the best breed of horses in the world. macaulay[ ] remarks, "two men whose authority on such subjects was held in great esteem, the duke of newcastle and sir john fenwick, pronounced that the meanest hack ever imported from tangier would produce a finer progeny than could be expected from the best sire of our native breed. they would not readily have believed that a time would come when the princes and nobles of neighbouring lands would be as eager to obtain horses from england as ever the english had been to obtain horses from barbary." the london dray-horse, which differs so much in appearance from any natural species, and which from its size has so astonished many eastern princes, was probably formed by the heaviest and most powerful animals having been selected during many generations in flanders and england, but without the least intention or expectation of creating a horse such as we now see. if we go back to an early period of history, we behold in the antique greek statues, as schaaffhausen has remarked,[ ] a horse equally unlike a race or dray horse, and differing from any existing breed. the results of unconscious selection, in an early stage, are well shown in the difference between the flocks descended from the same stock, but separately reared by careful breeders. youatt gives an excellent instance of this fact in the sheep belonging to messrs. buckley and burgess, which "have been purely bred from the original stock of mr. bakewell for upwards of fifty years. there is not a suspicion existing in the mind of any one at all acquainted with the subject that the owner of either flock has deviated in any one instance from the pure blood of mr. bakewell's flock; yet the difference between the sheep possessed by these two gentlemen is so great, that they have the appearance of being quite different varieties."[ ] i have seen several analogous and { } well-marked cases with pigeons: for instance, i had a family of barbs, descended from those long bred by sir j. sebright, and another family long bred by another fancier, and the two families plainly differed from each other. nathusius--and a more competent witness could not be cited--observes that, though the shorthorns are remarkably uniform inn appearance (except in colouring), yet that the individual character and wishes of each breeder become impressed on his cattle, so that different herds differ slightly from each other.[ ] the hereford cattle assumed their present well-marked character soon after the year , through careful selection by mr. tomkins,[ ] and the breed has lately split into two strains--one strain having a white face, and differing slightly, it is said,[ ] in some other points; but there is no reason to believe that this split, the origin of which is unknown, was intentionally made; it may with much more probability be attributed to different breeders having attended to different points. so again, the berkshire breed of swine in the year had greatly changed from what it had been in ; and since at least two distinct sub-breeds have borne this same name.[ ] when we bear in mind how rapidly all animals increase, and that some must be annually slaughtered and some saved for breeding, then, if the same breeder during a long course of years deliberately settles which shall be saved and which shall be killed, it is almost inevitable that his individual frame of mind will influence the character of his stock, without his having had any intention to modify the breed or form a new strain. unconscious selection in the strictest sense of the word, that is, the saving of the more useful animals and the neglect or slaughter of the less useful, without any thought of the future, must have gone on occasionally from the remotest period and amongst the most barbarous nations. savages often suffer from famines, and are sometimes expelled by war from their own homes. in such cases it can hardly be doubted that they would save their most useful animals. when the fuegians { } are hard pressed by want, they kill their old women for food rather than their dogs; for, as we were assured, "old women no use--dogs catch otters." the same sound sense would surely lead them to preserve their more useful dogs when still harder pressed by famine. mr. oldfield, who has seen so much of the aborigines of australia, informs me that "they are all very glad to get a european kangaroo dog, and several instances have been known of the father killing his own infant that the mother might suckle the much-prized puppy." different kinds of dogs would be useful to the australian for hunting opossums and kangaroos, and to the fuegian for catching fish and otters; and the occasional preservation in the two countries of the most useful animals would ultimately lead to the formation of two widely distinct breeds. * * * * * with plants, from the earliest dawn of civilisation, the best variety which at each period was known would generally have been cultivated and its seeds occasionally sown; so that there will have been some selection from an extremely remote period, but without any prefixed standard of excellence or thought of the future. we at the present day profit by a course of selection occasionally and unconsciously carried on during thousands of years. this is proved in an interesting manner by oswald heer's researches on the lake-inhabitants of switzerland, as given in a former chapter; for he shows that the grain and seed of our present varieties of wheat, barley, oats, peas, beans, lentils, and poppy, exceed in size those which were cultivated in switzerland during the neolithic and bronze periods. these ancient people, during the neolithic period, possessed also a crab considerably larger than that now growing wild on the jura.[ ] the pears described by pliny were evidently extremely inferior in quality to our present pears. we can realise the effects of long-continued selection and cultivation in another way, for would any one in his senses expect to raise a first-rate apple from the seed of a truly wild crab, or a luscious melting pear from the wild pear? alphonse de candolle informs me that he has lately seen on an ancient mosaic at rome a representation of { } the melon; and as the romans, who were such gourmands, are silent on this fruit, he infers that the melon has been greatly ameliorated since the classical period. coming to later times, buffon,[ ] on comparing the flowers, fruit, and vegetables which were then cultivated, with some excellent drawings made a hundred and fifty years previously, was struck with surprise at the great improvement which had been effected; and remarks that these ancient flowers and vegetables would now be rejected, not only by a florist but by a village gardener. since the time of buffon the work of improvement has steadily and rapidly gone on. every florist who compares our present flowers with those figured in books published not long since, is astonished at the change. a well-known amateur,[ ] in speaking of the varieties of pelargonium raised by mr. garth only twenty-two years before, remarks, "what a rage they excited: surely we had attained perfection, it was said; and now not one of the flowers of those days will be looked at. but none the less is the debt of gratitude which we owe to those who saw what was to be done, and did it." mr. paul, the well-known horticulturist, in writing of the same flower,[ ] says he remembers when young being delighted with the portraits in sweet's work; "but what are they in point of beauty compared with the pelargoniums of this day? here again nature did not advance by leaps; the improvement was gradual, and, if we had neglected those very gradual advances, we must have foregone the present grand results." how well this practical horticulturist appreciates and illustrates the gradual and accumulative force of selection! the dahlia has advanced in beauty in a like manner; the line of improvement being guided by fashion, and by the successive modifications which the flower slowly underwent.[ ] a steady and gradual change has been noticed in many other flowers: thus an old florist,[ ] after describing the leading varieties of the pink which were grown in , adds, "the pinks of those days would now be scarcely grown as border-flowers." the improvement of { } so many flowers and the number of the varieties which have been raised is all the more striking when we hear that the earliest known flower-garden in europe, namely at padua, dates only from the year .[ ] * * * * * _effects of selection, as shown by the parts most valued by man presenting the greatest amount of difference._--the power of long-continued selection, whether methodical or unconscious, or both combined, is well shown in a general way, namely, by the comparison of the differences between the varieties of distinct species, which are valued for different parts, such as for the leaves, or stems, or tubers, the seed, or fruit, or flowers. whatever part man values most, that part will be found to present the greatest amount of difference. with trees cultivated for their fruit, sageret remarks that the fruit is larger than in the parent-species, whilst with those cultivated for the seed, as with nuts, walnuts, almonds, chesnuts, &c., it is the seed itself which is larger; and he accounts for this fact by the fruit in the one case, and by the seed in the other, having been carefully attended to and selected during many ages. gallesio has made the same observation. godron insists on the diversity of the tuber in the potato, of the bulb in the onion, and of the fruit in the melon; and on the close similarity in these same plants of the other parts.[ ] in order to judge how far my own impression on this subject was correct, i cultivated numerous varieties of the same species close to each other. the comparison of the amount of difference between widely different organs is necessarily vague; i will therefore give the results in only a few cases. we have previously seen in the ninth chapter how greatly the varieties of the cabbage differ in their foliage and stems, which are the selected parts, and how closely they resembled each other in their flowers, capsules, and seeds. in seven varieties of the radish, the roots differed greatly in colour and shape, but no difference { } whatever could be detected in their foliage, flowers, or seeds. now what a contrast is presented, if we compare the flowers of the varieties of these two plants with those of any species cultivated in our flower-gardens for ornament; or if we compare their seeds with those of the varieties of maize, peas, beans, &c., which are valued and cultivated for their seeds. in the ninth chapter it was shown that the varieties of the pea differ but little except in the tallness of the plant, moderately in the shape of the pod, and greatly in the pea itself, and these are all selected points. the varieties, however, of the _pois sans parchemin_ differ much more in their pods, and these are eaten and valued. i cultivated twelve varieties of the common bean; one alone, the dwarf fan, differed considerably in general appearance; two differed in the colour of their flowers, one being an albino, and the other being wholly instead of partially purple; several differed considerably in the shape and size of the pod, but far more in the bean itself, and this is the valued and selected part. toker's bean, for instance, is twice-and-a-half as long and broad as the horse-bean, and is much thinner and of a different shape. the varieties of the gooseberry, as formerly described, differ much in their fruit, but hardly perceptibly in their flowers or organs of vegetation. with the plum, the differences likewise appear to be greater in the fruit than in the flowers or leaves. on the other hand, the seed of the strawberry, which corresponds with the fruit of the plum, differs hardly at all; whilst every one knows how greatly the fruit--that is, the enlarged receptacle--differs in the several varieties. in apples, pears, and peaches the flowers and leaves differ considerably, but not, as far as i can judge, in proportion with the fruit. the chinese double-flowering peaches, on the other hand, show that varieties of this tree have been formed, which differ more in the flower than in fruit. if, as is highly probable, the peach is the modified descendant of the almond, a surprising amount of change has been effected in the same species, in the fleshy covering of the former and in the kernels of the latter. when parts stand in such close relation to each other as the fleshy covering of the fruit (whatever its homological nature may be) and the seed, when one part is modified, so generally is the other, but by no means necessarily in the same degree. with { } the plum-tree, for instance, some varieties produce plums which are nearly alike, but include stones extremely dissimilar in shape; whilst conversely other varieties produce dissimilar fruit with barely distinguishable stones; and generally the stones, though they have never been subjected to selection, differ greatly in the several varieties of the plum. in other cases organs which are not manifestly related, through some unknown bond vary together, and are consequently liable, without any intention on man's part, to be simultaneously acted on by selection. thus the varieties of the stock (matthiola) have been selected solely for the beauty of their flowers, but the seeds differ greatly in colour and somewhat in size. varieties of the lettuce have been selected solely on account of their leaves, yet produce seeds which likewise differ in colour. generally, through the law of correlation, when a variety differs greatly from its fellow-varieties in any one character, it differs to a certain extent in several other characters. i observed this fact when i cultivated together many varieties of the same species, for i used first to make a list of the varieties which differed most from each other in their foliage and manner of growth, afterwards of those that differed most in their flowers, then in their seed-capsules, and lastly in their mature seed; and i found that the same names generally occurred in two, three, or four of the successive lists. nevertheless the greatest amount of difference between the varieties was always exhibited, as far as i could judge, by that part or organ for which the plant was cultivated. when we bear in mind that each plant was at first cultivated because useful to man, and that its variation was a subsequent, often a long subsequent, event, we cannot explain the greater amount of diversity in the valuable parts by supposing that species endowed with an especial tendency to vary in any particular manner, were originally chosen. we must attribute the result to the variations in these parts having been successively preserved, and thus continually augmented; whilst other variations, excepting such as inevitably appeared through correlation, were neglected and lost. hence we may infer that most plants might be made, through long-continued selection, to yield races as different from each other in any character { } as they now are in those parts for which they are valued and cultivated. with animals we see something of the same kind; but they have not been domesticated in sufficient number or yielded sufficient varieties for a fair comparison. sheep are valued for their wool, and the wool differs much more in the several races than the hair in cattle. neither sheep, goats, european cattle, nor pigs are valued for their fleetness or strength; and we do not possess breeds differing in these respects like the race-horse and dray-horse. but fleetness and strength are valued in camels and dogs; and we have with the former the swift dromedary and heavy camel; with the latter the greyhound and mastiff. but dogs are valued even in a higher degree for their mental qualities and senses; and every one knows how greatly the races differ in these respects. on the other hand, where the dog is valued solely to serve for food, as in the polynesian islands and china, it is described as an extremely stupid animal.[ ] blumenbach remarks that "many dogs, such as the badger-dog, have a build so marked and so appropriate for particular purposes, that i should find it very difficult to persuade myself that this astonishing figure was an accidental consequence of degeneration."[ ] but had blumenbach reflected on the great principle of selection, he would not have used the term degeneration, and he would not have been astonished that dogs and other animals should become excellently adapted for the service of man. on the whole we may conclude that whatever part or character is most valued--whether the leaves, stems, tubers, bulbs, flowers, fruit, or seed of plants, or the size, strength, fleetness, hairy covering, or intellect of animals--that character will almost invariably be found to present the greatest amount of difference both in kind and degree. and this result may be safely attributed to man having preserved during a long course of generations the variations which were useful to him, and neglected the others. i will conclude this chapter by some remarks on an important subject. with animals such as the giraffe, of which { } the whole structure is admirably co-ordinated for certain purposes, it has been supposed that all the parts must have been simultaneously modified; and it has been argued that, on the principle of natural selection, this is scarcely possible. but in thus arguing, it has been tacitly assumed that the variations must have been abrupt and great. no doubt, if the neck of a ruminant were suddenly to become greatly elongated, the fore limbs and back would have to be simultaneously strengthened and modified; but it cannot be denied that an animal might have its neck, or head, or tongue, or fore-limbs elongated a very little without any corresponding modification in other parts of the body; and animals thus slightly modified would, during a dearth, have a slight advantage, and be enabled to browse on higher twigs, and thus survive. a few mouthfuls more or less every day would make all the difference between life and death. by the repetition of the same process, and by the occasional intercrossing of the survivors, there would be some progress, slow and fluctuating though it would be, towards the admirably co-ordinated structure of the giraffe. if the short-faced tumbler-pigeon, with its small conical beak, globular head, rounded body, short wings, and small feet--characters which appear all in harmony--had been a natural species, its whole structure would have been viewed as well fitted for its life; but in this case we know that inexperienced breeders are urged to attend to point after point, and not to attempt improving the whole structure at the same time. look at the greyhound, that perfect image of grace, symmetry, and vigour; no natural species can boast of a more admirably co-ordinated structure, with its tapering head, slim body, deep chest, tucked-up abdomen, rat-like tail, and long muscular limbs, all adapted for extreme fleetness, and for running down weak prey. now, from what we see of the variability of animals, and from what we know of the method which different men follow in improving their stock--some chiefly attending to one point, others to another point, others again correcting defects by crosses, and so forth--we may feel assured that if we could see the long line of ancestors of a first-rate greyhound, up to its wild wolf-like progenitor, we should behold an infinite number of the finest gradations, sometimes in one character and sometimes in another, but all leading towards our { } present perfect type. by small and doubtful steps such as these, nature, as we may confidently believe, has progressed on her grand march of improvement and development. a similar line of reasoning is as applicable to separate organs as to the whole organisation. a writer[ ] has recently maintained that "it is probably no exaggeration to suppose that, in order to improve such an organ as the eye at all, it must be improved in ten different ways at once. and the improbability of any complex organ being produced and brought to perfection in any such way is an improbability of the same kind and degree as that of producing a poem or a mathematical demonstration by throwing letters at random on a table." if the eye were abruptly and greatly modified, no doubt many parts would have to be simultaneously altered, in order that the organ should remain serviceable. but is this the case with smaller changes? there are persons who can see distinctly only in a dull light, and this condition depends, i believe, on the abnormal sensitiveness of the retina, and is known to be inherited. now, if a bird, for instance, received some great advantage from seeing well in the twilight, all the individuals with the most sensitive retina would succeed best and be the most likely to survive; and why should not all those which happened to have the eye itself a little larger, or the pupil capable of greater dilatation, be likewise preserved, whether or not these modifications were strictly simultaneous? these individuals would subsequently intercross and blend their respective advantages. by such slight successive changes, the eye of a diurnal bird would be brought into the condition of that of an owl, which has often been advanced as an excellent instance of adaptation. short-sight, which is often inherited, permits a person to see distinctly a minute object at so near a distance that it would be indistinct to ordinary eyes; and here we have a capacity which might be serviceable under certain conditions, abruptly gained. the fuegians on board the { } beagle could certainly see distant objects more distinctly than our sailors with all their long practice; i do not know whether this depends on nervous sensitiveness or on the power of adjustment in the focus; but this capacity for distant vision might, it is probable, be slightly augmented by successive modifications of either kind. amphibious animals, which are enabled to see both in the water and in the air, require and possess, as m. plateau has shown,[ ] eyes constructed on the following plan: "the cornea is always flat, or at least much flattened in front of the crystalline and over a space equal to the diameter of that lens, whilst the lateral portions may be much curved." the crystalline is very nearly a sphere, and the humours have nearly the same density as water. now, as a terrestrial animal slowly became more and more aquatic in its habits, very slight changes, first in the curvature of the cornea or crystalline, and then in the density of the humours, or conversely, might successively occur, and would be advantageous to the animal whilst under water, without serious detriment to its power of vision in the air. it is of course impossible to conjecture by what steps the fundamental structure of the eye in the vertebrata was originally acquired, for we know absolutely nothing about this organ in the first progenitors of the class. with respect to the lowest animals in the scale, the transitional states through which the eye at first probably passed, can by the aid of analogy be indicated, as i have attempted to show in my 'origin of species.'[ ] * * * * * { } chapter xxi. selection, _continued_. natural selection as affecting domestic productions--characters which appear of trifling value often of real importance--circumstances favourable to selection by man--facility in preventing crosses, and the nature of the conditions--close attention and perseverance indispensable--the production of a large number of individuals especially favourable--when no selection is applied, distinct races are not formed--highly-bred animals liable to degeneration--tendency in man to carry the selection of each character to an extreme point, leading to divergence of character, rarely to convergence--characters continuing to vary in the same direction in which they have already varied--divergence of character, with the extinction of intermediate varieties, leads to distinctness in our domestic races--limit to the power of selection--lapse of time important--manner in which domestic races have originated--summary. _natural selection, or the survival of the fittest, as affecting domestic productions._--we know little on this head. but as animals kept by savages have to provide their own food, either entirely or to a large extent, throughout the year, it can hardly be doubted that, in different countries, varieties differing in constitution and in various characters would succeed best, and so be naturally selected. hence perhaps it is that the few domesticated animals kept by savages partake, as has been remarked by more than one writer, of the wild appearance of their masters, and likewise resemble natural species. even in long-civilised countries, at least in the wilder parts, natural selection must act on our domestic races. it is obvious that varieties, having very different habits, constitution, and structure, would succeed best on mountains and on rich lowland pastures. for example, the improved leicester sheep were formerly taken to the lammermuir hills; but an intelligent sheep-master reported that "our coarse lean pastures were unequal to the task of supporting such heavy-bodied sheep; and they gradually dwindled away into less and less bulk: { } each generation was inferior to the preceding one; and when the spring was severe, seldom more than two-thirds of the lambs survived the ravages of the storms."[ ] so with the mountain cattle of north wales and the hebrides, it has been found that they could not withstand being crossed with the larger and more delicate lowland breeds. two french naturalists, in describing the horses of circassia, remark that, subjected as they are to extreme vicissitudes of climate, having to search for scanty pasture, and exposed to constant danger from wolves, the strongest and most vigorous alone survive.[ ] every one must have been struck with the surpassing grace, strength, and vigour of the game-cock, with its bold and confident air, its long, yet firm neck, compact body, powerful and closely pressed wings, muscular thighs, strong beak massive at the base, dense and sharp spurs set low on the legs for delivering the fatal blow, and its compact, glossy, and mail-like plumage serving as a defence. now the english game-cock has not only been improved during many years by man's careful selection, but in addition, as mr. tegetmeier has remarked,[ ] by a kind of natural selection, for the strongest, most active and courageous birds have stricken down their antagonists in the cockpit, generation after generation, and have subsequently served as the progenitors of their kind. in great britain, in former times, almost every district had its own breed of cattle and sheep; "they were indigenous to the soil, climate, and pasturage of the locality on which they grazed: they seemed to have been formed for it and by it."[ ] but in this case we are quite unable to disentangle the effects of the direct action of the conditions of life,--of use or habit--of natural selection--and of that kind of selection which we have seen is occasionally and unconsciously followed by man even during the rudest periods of history. let us now look to the action of natural selection on special characters. although nature is difficult to resist, yet man often strives against her power, and sometimes, as we shall see, with { } success. from the facts to be given, it will also be seen that natural selection would powerfully affect many of our domestic productions if left unprotected. this is a point of much interest, for we thus learn that differences apparently of very slight importance would certainly determine the survival of a form when forced to struggle for its own existence. it may have occurred to some naturalists, as it formerly did to me, that, though selection acting under natural conditions would determine the structure of all important organs, yet that it could not affect characters which are esteemed by us of little importance; but this is an error to which we are eminently liable, from our ignorance of what characters are of real value to each living creature. when man attempts to breed an animal with some serious defect in structure, or in the mutual relation of parts, he will either partially or completely fail, or encounter much difficulty; and this is in fact a form of natural selection. we have seen that the attempt was once made in yorkshire to breed cattle with enormous buttocks, but the cows perished so often in bringing forth their calves, that the attempt had to be given up. in rearing short-faced tumblers, mr. eaton says,[ ] "i am convinced that better head and beak birds have perished in the shell than ever were hatched; the reason being that the amazingly short-faced bird cannot reach and break the shell with its beak, and so perishes." here is a more curious case, in which natural selection comes into play only at long intervals of time: during ordinary seasons the niata cattle can graze as well as others, but occasionally, as from to , the plains of la plata suffer from long-continued droughts and the pasture is burnt up; at such times common cattle and horses perish by the thousand, but many survive by browsing on twigs, reeds, &c.; this the niata cattle cannot so well effect from their upturned jaws and the shape of their lips; consequently, if not attended to, they perish before the other cattle. in colombia, according to roulin, there is a breed of nearly hairless cattle, called pelones; these succeed in their native hot district, but are found too tender for the cordillera; in this case, natural selection { } determines only the range of the variety. it is obvious that a host of artificial races could never survive in a state of nature;--such as italian greyhounds,--hairless and almost toothless turkish dogs,--fantail pigeons, which cannot fly well against a strong wind,--barbs with their vision impeded by their eye-wattle,--polish fowls with their vision impeded by their great topknots,--hornless bulls and rams which consequently cannot cope with other males, and thus have a poor chance of leaving offspring,--seedless plants, and many other such cases. colour is generally esteemed by the systematic naturalist as unimportant: let us, therefore, see how far it indirectly affects our domestic productions, and how far it would affect them if they were left exposed to the full force of natural selection. in a future chapter i shall have to show that constitutional peculiarities of the strangest kind, entailing liability to the action of certain poisons, are correlated with the colour of the skin. i will here give a single case, on the high authority of professor wyman; he informs me that, being surprised at all the pigs in a part of virginia being black, he made inquiries, and ascertained that these animals feed on the roots of the _lachnanthes tinctoria_, which colours their bones pink, and, excepting in the case of the black varieties, causes the hoofs to drop off. hence, as one of the squatters remarked, "we select the black members of the litter for raising, as they alone have a good chance of living." so that here we have artificial and natural selection working hand in hand. i may add that in the tarentino the inhabitants keep black sheep alone, because the _hypericum crispum_ abounds there; and this plant does not injure black sheep, but kills the white ones in about a fortnight's time.[ ] complexion, and liability to certain diseases, are believed to run together in man and the lower animals. thus white terriers suffer more than terriers of any other colour from the fatal distemper.[ ] in north america plum-trees are liable to a disease which downing[ ] believes is not caused by insects; the kinds bearing purple fruit are most affected, "and we have never known the green or yellow fruited varieties infected { } until the other sorts had first become filled with the knots." on the other hand, peaches in north america suffer much from a disease called the _yellows_, which seems to be peculiar to that continent, and "more than nine-tenths of the victims, when the disease first appeared, were the yellow-fleshed peaches. the white-fleshed kinds are much more rarely attacked; in some parts of the country never." in mauritius, the white sugar-canes have of late years been so severely attacked by a disease, that many planters have been compelled to give up growing this variety (although fresh plants were imported from china for trial), and cultivate only red canes.[ ] now, if these plants had been forced to struggle with other competing plants and enemies, there cannot be a doubt that the colour of the flesh or skin of the fruit, unimportant as these characters are considered, would have rigorously determined their existence. liability to the attacks of parasites is also connected with colour. it appears that white chickens are certainly more subject than dark-coloured chickens to the _gapes_, which is caused by a parasitic worm in the trachea.[ ] on the other hand, experience has shown that in france the caterpillars which produce white cocoons resist the deadly fungus better than those producing yellow cocoons.[ ] analogous facts have been observed with plants: a new and beautiful white onion, imported from france, though planted close to other kinds, was alone attacked by a parasitic fungus.[ ] white verbenas are especially liable to mildew.[ ] near malaga, during an early period of the vine-disease, the green sorts suffered most; "and red and black grapes, even when interwoven with the sick plants, suffered not at all." in france whole groups of varieties were comparatively free, and others, such as the chasselas, did not afford a single fortunate exception; but i do not know whether any correlation between colour and liability to disease was here observed.[ ] in a former chapter it was shown how curiously liable one variety of the strawberry is to mildew. { } it is certain that insects regulate in many cases the range and even the existence of the higher animals, whilst living under their natural conditions. under domestication light-coloured animals suffer most: in thuringia[ ] the inhabitants do not like grey, white, or pale cattle, because they are much more troubled by various kinds of flies than the brown, red, or black cattle. an albino negro, it has been remarked,[ ] was peculiarly sensitive to the bites of insects. in the west indies[ ] it is said that "the only horned cattle fit for work are those which have a good deal of black in them. the white are terribly tormented by the insects; and they are weak and sluggish in proportion to the white." in devonshire there is a prejudice against white pigs, because it is believed that the sun blisters them when turned out;[ ] and i knew a man who would not keep white pigs in kent, for the same reason. the scorching of flowers by the sun seems likewise to depend much on colour; thus, dark pelargoniums suffer most; and from various accounts it is clear that the cloth-of-gold variety will not withstand a degree of exposure to sunshine which other varieties enjoy. another amateur asserts that not only all dark-coloured verbenas, but likewise scarlets, suffer from the sun; "the paler kinds stand better, and pale blue is perhaps the best of all." so again with the heartsease (_viola tricolor_); hot weather suits the blotched sorts, whilst it destroys the beautiful markings of some other kinds.[ ] during one extremely cold season in holland all red-flowered hyacinths were observed to be very inferior in quality. it is believed by many agriculturists that red wheat is hardier in northern climates than white wheat.[ ] with animals, white varieties from being conspicuous are the most liable to be attacked by beasts and birds of prey. in parts of france and germany where hawks abound, persons are advised not to keep white pigeons; for, as parmentier says, "it { } is certain that in a flock the white always first fall victims to the kite." in belgium, where so many societies have been established for the flight of carrier-pigeons, white is the one colour which for the same reason is disliked.[ ] on the other hand, it is said that the sea-eagle (_falco ossifragus_, linn.) on the west coast of ireland picks out the black fowls, so that "the villagers avoid as much as possible rearing birds of that colour." m. daudin,[ ] speaking of white rabbits kept in warrens in russia, remarks that their colour is a great disadvantage, as they are thus more exposed to attack, and can be seen during bright nights from a distance. a gentleman in kent, who failed to stock his woods with a nearly white and hardy kind of rabbit, accounted in the same manner for their early disappearance. any one who will watch a white cat prowling after her prey will soon perceive under what a disadvantage she lies. the white tartarian cherry, "owing either to its colour being so much like that of the leaves, or to the fruit always appearing from a distance unripe," is not so readily attacked by birds as other sorts. the yellow-fruited raspberry, which generally comes nearly true by seed, "is very little molested by birds, who evidently are not fond of it; so that nets may be dispensed with in places where nothing else will protect the red fruit."[ ] this immunity, though a benefit to the gardener, would be a disadvantage in a state of nature both to the cherry and raspberry, as their dissemination depends on birds. i noticed during several winters that some trees of the yellow-berried holly, which were raised from seed from a wild tree found by my father, remained covered with fruit, whilst not a scarlet berry could be seen on the adjoining trees of the common kind. a friend informs me that a mountain-ash (_pyrus aucuparia_) growing in his garden bears berries which, though not differently coloured, are always devoured by birds before those on the other trees. this variety of the mountain-ash would thus be more freely disseminated, and the yellow-berried variety of the holly less freely, than the common varieties of these two trees. { } independently of colour, other trifling differences are sometimes found to be of importance to plants under cultivation, and would be of paramount importance if they had to fight their own battle and to struggle with many competitors. the thin-shelled peas, called _pois sans parchemin_, are attacked by birds[ ] much more than common peas. on the other hand, the purple-podded pea, which has a hard shell, escaped the attacks of tomtits (_parus major_) in my garden far better than any other kind. the thin-shelled walnut likewise suffers greatly from the tomtit.[ ] these same birds have been observed to pass over and thus favour the filbert, destroying only the other kinds of nuts which grew in the same orchard.[ ] certain varieties of the pear have soft bark, and these suffer severely from boring wood-beetles; whilst other varieties are known to resist their attacks much better.[ ] in north america the smoothness, or absence of down on the fruit, makes a great difference in the attacks of the weevil, "which is the uncompromising foe of all smooth stone-fruits;" and the cultivator "has the frequent mortification of seeing nearly all, or indeed often the whole crop, fall from the trees when half or two-thirds grown." hence the nectarine suffers more than the peach. a particular variety of the morello cherry, raised in north america, is without any assignable cause more liable to be injured by this same insect than other cherry-trees.[ ] from some unknown cause, the winter majetin apple enjoys the great advantage of not being infested by the coccus. on the other hand, a particular case has been recorded in which aphides confined themselves to the winter nelis pear, and touched no other kind in an extensive orchard.[ ] the existence of minute glands on the leaves of peaches, nectarines, and apricots, would not be esteemed by botanists as a character of the least importance, for they are present or absent in closely related sub-varieties, descended from the same parent-tree; yet there is good evidence[ ] that the { } absence of glands leads to mildew, which is highly injurious to these trees. a difference either in flavour or in the amount of nutriment in certain varieties causes them to be more eagerly attacked by various enemies than other varieties of the same species. bullfinches (_pyrrhula vulgaris_) injure our fruit-trees by devouring the flower-buds, and a pair of these birds have been seen "to denude a large plum-tree in a couple of days of almost every bud;" but certain varieties[ ] of the apple and thorn (_cratægus oxyacantha_) are more especially liable to be attacked. a striking instance of this was observed in mr. rivers's garden, in which two rows of a particular variety of plum[ ] had to be carefully protected, as they were usually stripped of all their buds during the winter, whilst other sorts growing near them escaped. the root (or enlarged stem) of laing's swedish turnip is preferred by hares, and therefore suffers more than other varieties. hares and rabbits eat down common rye before st. john's-day-rye, when both grow together.[ ] in the south of france, when an orchard of almond-trees is formed, the nuts of the bitter variety are sown, "in order that they may not be devoured by field-mice;"[ ] so we see the use of the bitter principle in almonds. other slight differences, which would be thought quite unimportant, are no doubt sometimes of great service both to plants and animals. the whitesmith's gooseberry, as formerly stated, produces its leaves later than other varieties, and, as the flowers are thus left unprotected, the fruit often fails. in one variety of the cherry, according to mr. rivers,[ ] the petals are much curled backwards, and in consequence of this the stigmas were observed to be killed by a severe frost; whilst at the same time, in another variety with incurved petals, the stigmas were not in the least injured. the straw of the fenton wheat is remarkably unequal in height; and a competent observer believes that this variety is highly productive, partly because the ears, from being distributed at various heights above the ground, { } are less crowded together. the same observer maintains that in the upright varieties the divergent awns are serviceable by breaking the shocks when the ears are dashed together by the wind.[ ] if several varieties of a plant are grown together, and the seed is indiscriminately harvested, it is clear that the hardier and more productive kinds will, by a sort of natural selection, gradually prevail over the others; this takes place, as colonel le couteur believes,[ ] in our wheat-fields, for, as formerly shown, no variety is quite uniform in character. the same thing, as i am assured by nurserymen, would take place in our flower-gardens, if the seed of the different varieties were not separately saved. when the eggs of the wild and tame duck are hatched together, the young wild ducks almost invariably perish, from being of smaller size and not getting their fair share of food.[ ] facts in sufficient number have now been given showing that natural selection often checks, but occasionally favours, man's power of selection. these facts teach us, in addition, a valuable lesson, namely, that we ought to be extremely cautious in judging what characters are of importance in a state of nature to animals and plants, which have to struggle from the hour of their birth to that of their death for existence,--their existence depending on conditions, about which we are profoundly ignorant. _circumstances favourable to selection by man._ the possibility of selection rests on variability, and this, as we shall see in the following chapters, mainly depends on changed conditions of life, but is governed by infinitely complex, and, to a great extent, unknown laws. domestication, even when long continued, occasionally causes but a small amount of variability, as in the case of the goose and turkey. the slight differences, however, which characterise each individual animal and plant would in most, probably in all cases, suffice for the production of distinct races through careful and prolonged selection. we see what selection, though acting on mere individual differences, can effect when families of cattle, sheep, { } pigeons, &c., of the same race, have been separately bred during a number of years by different men without any wish on their part to modify the breed. we see the same fact in the difference between hounds bred for hunting in different districts,[ ] and in many other such cases. in order that selection should produce any result, it is manifest that the crossing of distinct races must be prevented; hence facility in pairing, as with the pigeon, is highly favourable for the work; and difficulty in pairing, as with cats, prevents the formation of distinct breeds. on nearly the same principle the cattle of the small island of jersey have been improved in their milking qualities "with a rapidity that could not have been obtained in a widely extended country like france."[ ] although free crossing is a danger on the one side which every one can see, too close interbreeding is a hidden danger on the other side. unfavourable conditions of life overrule the power of selection. our improved heavy breeds of cattle and sheep could not have been formed on mountainous pastures; nor could dray-horses have been raised on a barren and inhospitable land, such as the falkland islands, where even the light horses of la plata rapidly decrease in size. nor could the wool of sheep have been much increased in length within the tropics; yet selection has kept merino sheep nearly true under diversified and unfavourable conditions of life. the power of selection is so great, that breeds of the dog, sheep, and poultry, of the largest and least size, long and short beaked pigeons, and other breeds with opposite characters, have had their characteristic qualities augmented, though treated in every way alike, being exposed to the same climate and fed on the same food. selection, however, is either checked or favoured by the effects of use or habit. our wonderfully-improved pigs could never have been formed if they had been forced to search for their own food; the english racehorse and greyhound could not have been improved up to their present high standard of excellence without constant training. as conspicuous deviations of structure occur rarely, the improvement of each breed is generally the result, as already { } remarked, of the selection of slight individual differences. hence the closest attention, the sharpest powers of observation, and indomitable perseverance, are indispensable. it is, also, highly important that many individuals of the breed which is to be improved should be raised; for thus there will be a better chance of the appearance of variations in the right direction, and individuals varying in an unfavourable manner may be freely rejected or destroyed. but that a large number of individuals should be raised, it is necessary that the conditions of life should favour the propagation of the species. had the peacock been bred as easily as the fowl, we should probably ere this have had many distinct races. we see the importance of a large number of plants, from the fact of nursery gardeners almost always beating amateurs in the exhibition of new varieties. in it was estimated[ ] that between and pelargoniums were annually raised from seed in england, yet a decidedly improved variety is rarely obtained. at messrs. carter's grounds, in essex, where such flowers as the lobelia, nemophila, mignonette, &c., are grown by the acre for seed, "scarcely a season passes without some new kinds being raised, or some improvement affected on old kinds."[ ] at kew, as mr. beaton remarks, where many seedlings of common plants are raised, "you see new forms of laburnums, spiræas, and other shrubs."[ ] so with animals: marshall,[ ] in speaking of the sheep in one part of yorkshire, remarks, "as they belong to poor people, and are mostly in small lots, they never can be improved." lord rivers, when asked how he succeeded in always having first-rate greyhounds, answered, "i breed many, and hang many." this, as another man remarks, "was the secret of his success; and the same will be found in exhibiting fowls,--successful competitors breed largely, and keep the best."[ ] it follows from this that the capacity of breeding at an early age and at short successive intervals, as with pigeons, rabbits, &c., facilitates selection; for the result is thus soon made visible, and perseverance in the work is encouraged. it can hardly be { } accidental that the great majority of the culinary and agricultural plants which have yielded numerous races are annuals or biennials, which therefore are capable of rapid propagation and thus of improvement. sea-kale, asparagus, common and jerusalem artichokes, potatoes, and onions, alone are perennials. onions are propagated like annuals, and of the other plants just specified, none, with the exception of the potato, have yielded more than one or two varieties. no doubt fruit-trees, which cannot be propagated quickly by seed, have yielded a host of varieties, though not permanent races; but these, judging from pre-historic remains, were produced at a later and more civilised epoch than the races of culinary and agricultural plants. a species may be highly variable, but distinct races will not be formed, if from any cause selection be not applied. the carp is highly variable, but it would be extremely difficult to select slight variations in fishes whilst living in their natural state, and distinct races have not been formed;[ ] on the other hand, a closely allied species, the gold-fish, from being reared in glass or open vessels, and from having been carefully attended to by the chinese, has yielded many races. neither the bee, which has been semi-domesticated from an extremely remote period, nor the cochineal insect, which was cultivated by the aboriginal mexicans, has yielded races; and it would be impossible to match the queen-bee with any particular drone, and most difficult to match cochineal insects. silk-moths, on the other hand, have been subjected to rigorous selection, and have produced a host of races. cats, which from their nocturnal habits cannot be selected for breeding, do not, as formerly remarked, yield distinct races in the same country. the ass in england varies much in colour and size; but it is an animal of little value, bred by poor people; consequently there has been no selection, and distinct races have not been formed. we must not attribute the inferiority of our asses to climate, for in india they are of even smaller size than in europe. but when selection is brought to bear on the ass, all is changed. near cordova, as i am informed (feb. ) by mr. w. e. webb, c.e., they are carefully bred, as much as l. having been paid for a stallion ass, { } and they have been immensely improved. in kentucky, asses have been imported (for breeding mules) from spain, malta, and france; these "seldom averaged more than fourteen hands high; but the kentuckians, by great care, have raised them up to fifteen hands, and sometimes even to sixteen. the prices paid for these splendid animals, for such they really are, will prove how much they are in request. one male, of great celebrity, was sold for upwards of one thousand pounds sterling." these choice asses are sent to cattle-shows, one day being given to their exhibition.[ ] analogous facts have been observed with plants: the nutmeg-tree in the malay archipelago is highly variable, but there has been no selection, and there are no distinct races.[ ] the common mignonette (_reseda odorata_), from bearing inconspicuous flowers, valued solely for their fragrance, "remains in the same unimproved condition as when first introduced."[ ] our common forest-trees are very variable, as may be seen in every extensive nursery-ground; but as they are not valued like fruit-trees, and as they seed late in life, no selection has been applied to them; consequently, as mr. patrick matthews remarks,[ ] they have not yielded distinct races, leafing at different periods, growing to different sizes, and producing timber fit for different purposes. we have gained only some fanciful and semi-monstrous varieties, which no doubt appeared suddenly as we now see them. some botanists have argued that plants cannot have so strong a tendency to vary as is generally supposed, because many species long grown in botanic gardens, or unintentionally cultivated year after year mingled with our corn crops, have not produced distinct races; but this is accounted for by slight variations not having been selected and propagated. let a plant which is now grown in a botanic garden, or any common weed, be cultivated on a large scale, and let a sharp-sighted gardener look out for each slight variety and sow the seed, and then, if distinct races are not produced, the argument will be valid. { } the importance of selection is likewise shown by considering special characters. for instance, with most breeds of fowls the form of the comb and the colour of the plumage have been attended to, and are eminently characteristic of each race; but in dorkings, fashion has never demanded uniformity of comb or colour; and the utmost diversity in these respects prevails. rose-combs, double-combs, cup-combs, &c., and colours of all kinds, may be seen in purely-bred and closely related dorking fowls, whilst other points, such as the general form of body, and the presence of an additional toe, have been attended to, and are invariably present. it has also been ascertained that colour can be fixed in this breed, as well as in any other.[ ] * * * * * during the formation or improvement of a breed, its members will always be found to vary much in those characters to which especial attention is directed, and of which each slight improvement is eagerly sought and selected. thus with short-faced tumbler-pigeons, the shortness of the beak, shape of head and plumage,--with carriers, the length of the beak and wattle,--with fantails, the tail and carriage,--with spanish fowls, the white face and comb,--with long-eared rabbits, the length of ear, are all points which are eminently variable. so it is in every case, and the large price paid for first-rate animals proves the difficulty of breeding them up to the highest standard of excellence. this subject has been discussed by fanciers,[ ] and the greater prizes given for highly improved breeds, in comparison with those given for old breeds which are not now undergoing rapid improvement, has been fully justified. nathusius makes[ ] a similar remark when discussing the less uniform character of improved shorthorn cattle and of the english horse, in comparison, for example, with the unennobled cattle of hungary, or with the horses of the asiatic steppes. this want of uniformity in the parts which at the time are undergoing selection, chiefly depends on the strength of the principle of reversion but it likewise depends to a certain extent on the continued { } variability of the parts which have recently varied. that the same parts do continue varying in the same manner we must admit, for, if it were not so, there could be no improvement beyond an early standard of excellence, and we know that such improvement is not only possible, but is of general occurrence. as a consequence of continued variability, and more especially of reversion, all highly improved races, if neglected or not subjected to incessant selection, soon degenerate. youatt gives a curious instance of this in some cattle formerly kept in glamorganshire; but in this case the cattle were not fed with sufficient care. mr. baker, in his memoir on the horse, sums up: "it must have been observed in the preceding pages that, whenever there has been neglect, the breed has proportionally deteriorated."[ ] if a considerable number of improved cattle, sheep, or other animals of the same race, were allowed to breed freely together, with no selection, but with no change in their condition of life, there can be no doubt that after a score or hundred generations they would be very far from excellent of their kind; but, from what we see of the many common races of dogs, cattle, fowls, pigeons, &c., which without any particular care have long retained nearly the same character, we have no grounds for believing that they would altogether depart from their type. it is a general belief amongst breeders that characters of all kinds become fixed by long-continued inheritance. but i have attempted to show in the fourteenth chapter that this belief apparently resolves itself into the following proposition, namely, that all characters whatever, whether recently acquired or ancient, tend to be transmitted, but that those which have already long withstood all counteracting influences, will, as a general rule, continue to withstand them, and consequently be faithfully transmitted. _tendency in man to carry the practice of selection to an extreme point._ it is an important principle that in the process of selection man almost invariably wishes to go to an extreme point. thus, in useful qualities, there is no limit to his desire to breed certain { } horses and dogs as fleet as possible, and others as strong as possible; certain kinds of sheep for extreme fineness, and others for extreme length of wool; and he wishes to produce fruit, grain, tubers, and other useful parts of plants, as large and excellent as possible. with animals bred for amusement, the same principle is even more powerful; for fashion, as we see even in our dress, always runs to extremes. this view has been expressly admitted by fanciers. instances were given in the chapters on the pigeon, but here is another: mr. eaton, after describing a comparatively new variety, namely, the archangel, remarks, "what fanciers intend doing with this bird i am at a loss to know, whether they intend to breed it down to the tumbler's head and beak, or carry it out to the carrier's head and beak; leaving it as they found it, is not progressing." ferguson, speaking of fowls, says, "their peculiarities, whatever they may be, must necessarily be fully developed: a little peculiarity forms nought but ugliness, seeing it violates the existing laws of symmetry." so mr. brent, in discussing the merits of the sub-varieties of the belgian canary-bird, remarks, "fanciers always go to extremes; they do not admire indefinite properties."[ ] this principle, which necessarily leads to divergence of character, explains the present state of various domestic races. we can thus see how it is that race-horses and dray-horses, greyhounds and mastiffs, which are opposed to each other in every character,--how varieties so distinct as cochin-china fowls and bantams, or carrier-pigeons with very long beaks, and tumblers with excessively short beaks, have been derived from the same stock. as each breed is slowly improved, the inferior varieties are first neglected and finally lost. in a few cases, by the aid of old records, or from intermediate varieties still existing in countries where other fashions have prevailed, we are enabled partially to trace the graduated changes through which certain breeds have passed. selection, whether methodical or unconscious, always tending towards an extreme point, together with the neglect and slow extinction of the intermediate and less-valued forms, is the key which unlocks the mystery how man has produced such wonderful results. { } in a few instances selection, guided by utility for a single purpose, has led to convergence of character. all the improved and different races of the pig, as nathusius has well shown,[ ] closely approach each other in character, in their shortened legs and muzzles, their almost hairless, large, rounded bodies, and small tusks. we see some degree of convergence in the similar outline of the body in well-bred cattle belonging to distinct races.[ ] i know of no other such cases. continued divergence of character depends on, and is indeed a clear proof, as previously remarked, of the same parts continuing to vary in the same direction. the tendency to mere general variability or plasticity of organisation can certainly be inherited, even from one parent, as has been shown by gärtner and kölreuter, in the production of varying hybrids from two species, of which one alone was variable. it is in itself probable that, when an organ has varied in any manner, it will again vary in the same manner, if the conditions which first caused the being to vary remain, as far as can be judged, the same. this is either tacitly or expressly admitted by all horticulturists: if a gardener observes one or two additional petals in a flower, he feels confident that in a few generations he will be able to raise a double flower, crowded with petals. some of the seedlings from the weeping moccas oak were so prostrate that they only crawled along the ground. a seedling from the fastigate or upright irish yew is described as differing greatly from the parent-form "by the exaggeration of the fastigate habit of its branches."[ ] mr. sheriff, who has been more successful than any other man in raising new kinds of wheat, remarks, "a good variety may safely be regarded as the forerunner of a better one."[ ] a great rose-grower, mr. rivers, has made the same remark with respect to roses. sageret,[ ] who had large experience, in speaking of the future progress of fruit-trees, observes that the most important principle is "that the more plants have departed from their original type, the more they tend to depart from it." there is apparently much truth in this { } remark; for we can in no other way understand the surprising amount of difference between varieties in the parts or qualities which are valued, whilst other parts retain nearly their original character. the foregoing discussion naturally leads to the question, what is the limit to the possible amount of variation in any part or quality, and, consequently, is there any limit to what selection can effect? will a race-horse ever be reared fleeter than eclipse? can our prize-cattle and sheep be still further improved? will a gooseberry ever weigh more than that produced by "london" in ? will the beet-root in france yield a greater percentage of sugar? will future varieties of wheat and other grain produce heavier crops than our present varieties? these questions cannot be positively answered; but it is certain that we ought to be cautious in answering by a negative. in some lines of variation the limit has probably been reached. youatt believes that the reduction of bone in some of our sheep has already been carried so far that it entails great delicacy of constitution.[ ] but seeing the great improvement within recent times in our cattle and sheep, and especially in our pigs; seeing the wonderful increase in weight in our poultry of all kinds during the last few years; he would be a bold man who would assert that perfection has been reached. eclipse perhaps may never be beaten until all our race-horses have been rendered swifter, through the selection of the best horses during many generations; and then the old eclipse may possibly be eclipsed; but, as mr. wallace has remarked, there must be an ultimate limit to the fleetness of every animal, whether under nature or domestication; and with the horse this limit has perhaps been reached. until our fields are better manured, it may be impossible for a new variety of wheat to yield a heavier crop. but in many cases those who are best qualified to judge do not believe that the extreme point has as yet been reached even with respect to characters which have already been carried to a high standard of perfection. for instance, the short-faced tumbler-pigeon has been greatly modified; nevertheless, according to mr. eaton,[ ] "the field is still as open for fresh competitors as it was one hundred years ago." over and over again it has been said that { } perfection had been attained with our flowers, but a higher standard has soon been reached. hardly any fruit has been more improved than the strawberry, yet a great authority remarks,[ ] "it must not be concealed that we are far from the extreme limits at which we may arrive." time is an important element in the formation of our domestic races, as it permits innumerable individuals to be born, and these when exposed to diversified conditions are rendered variable. methodical selection has been occasionally practised from an ancient period to the present day, even by semi-civilised people, and during former times will have produced some effect. unconscious selection will have been still more effective; for during a lengthened period the more valuable individual animals will occasionally have been saved, and the less valuable neglected. in the course, also, of time, different varieties, especially in the less civilised countries, will have been more or less modified through natural selection. it is generally believed, though on this head we have little or no evidence, that new characters in time become fixed; and after having long remained fixed it seems possible that under new conditions they might again be rendered variable. how great the lapse of time has been since man first domesticated animals and cultivated plants, we begin dimly to see. when the lake-buildings of switzerland were inhabited during the neolithic period, several animals were already domesticated and various plants cultivated. if we may judge from what we now see of the habits of savages, it is probable that the men of the earlier stone period--when many great quadrupeds were living which are now extinct, and when the face of the country was widely different from what it now is--possessed at least some few domesticated animals, although their remains have not as yet been discovered. if the science of language can be trusted, the art of ploughing and sowing the land was followed, and the chief animals had been already domesticated, at an epoch so immensely remote, that the sanskrit, greek, latin, gothic, celtic, and sclavonic languages had not as yet diverged from their common parent-tongue.[ ] { } it is scarcely possible to overrate the effects of selection occasionally carried on in various ways and places during thousands of generations. all that we know, and, in a still stronger degree, all that we do not know,[ ] of the history of the great majority of our breeds, even of our more modern breeds, agrees with the view that their production, through the action of unconscious and methodical selection, has been almost insensibly slow. when a man attends rather more closely than is usual to the breeding of his animals, he is almost sure to improve them to a slight extent. they are in consequence valued in his immediate neighbourhood, and are bred by others; and their characteristic features, whatever these may be, will then slowly but steadily be increased, sometimes by methodical and almost always by unconscious selection. at last a strain, deserving to be called a sub-variety, becomes a little more widely known, receives a local name, and spreads. the spreading will have been extremely slow during ancient and less civilised times, but now is rapid. by the time that the new breed had assumed a somewhat distinct character, its history, hardly noticed at the time, will have been completely forgotten; for, as low remarks,[ ] "we know how quickly the memory of such events is effaced." as soon as a new breed is thus formed, it is liable through the same process to break up into new strains and sub-varieties. for different varieties are suited for, and are valued under, different circumstances. fashion changes, but, should a fashion last for even a moderate length of time, so strong is the principle of inheritance, that some effect will probably be impressed on the breed. thus varieties go on increasing in number, and history shows us how wonderfully they have increased since the earliest records.[ ] as each new variety is produced, the earlier, intermediate, and less valuable forms will be neglected, and perish. when a breed, from not being valued, is kept in small numbers, its extinction almost inevitably follows sooner or later, either from accidental causes of destruction or from close interbreeding; and this is an event which, in the case of well-marked breeds, excites attention. the birth or production of a new domestic race is so slow a process that it { } escapes notice; its death or destruction is comparatively sudden, is often recorded, and when too late sometimes regretted. several authors have drawn a wide distinction between artificial and natural races. the latter are more uniform in character, possessing in a high degree the character of natural species, and are of ancient origin. they are generally found in less civilised countries, and have probably been largely modified by natural selection, and only to a small extent by man's unconscious and methodical selection. they have, also, during a long period, been directly acted on by the physical conditions of the countries which they inhabit. the so-called artificial races, on the other hand, are not so uniform in character; some have a semi-monstrous character, such as "the wry-legged terriers so useful in rabbit-shooting,"[ ] turnspit dogs, ancon sheep, niata oxen, polish fowls, fantail-pigeons, &c.; their characteristic features have generally been acquired suddenly, though subsequently increased in many cases by careful selection. other races, which certainly must be called artificial, for they have been largely modified by methodical selection and by crossing, as the english race-horse, terrier-dogs, the english game-cock, antwerp carrier-pigeons, &c., nevertheless cannot be said to have an unnatural appearance; and no distinct line, as it seems to me, can be drawn between natural and artificial races. it is not surprising that domestic races should generally present a different aspect from natural species. man selects and propagates modifications solely for his own use or fancy, and not for the creature's own good. his attention is struck by strongly marked modifications, which have appeared suddenly, due to some great disturbing cause in the organisation. he attends almost exclusively to external characters; and when he succeeds in modifying internal organs,--when for instance he reduces the bones and offal, or loads the viscera with fat, or gives early maturity, &c.,--the chances are strong that he will at the same time weaken the constitution. on the other hand, when an animal has to struggle throughout its life with many competitors and enemies, under circumstances inconceivably complex and liable to change, modifications of the most varied nature--in the internal organs as well as in external characters, in the { } functions and mutual relations of parts--will be rigorously tested, preserved, or rejected. natural selection often checks man's comparatively feeble and capricious attempts at improvement; and if this were not so, the result of his work, and of nature's work, would be even still more different. nevertheless, we must not overrate the amount of difference between natural species and domestic races; the most experienced naturalists have often disputed whether the latter are descended from one or from several aboriginal stocks, and this clearly shows that there is no palpable difference between species and races. domestic races propagate their kind far more truly, and endure for much longer periods, than most naturalists are willing to admit. breeders feel no doubt on this head; ask a man who has long reared shorthorn or hereford cattle, leicester or southdown sheep, spanish or game poultry, tumbler or carrier-pigeons, whether these races may not have been derived from common progenitors, and he will probably laugh you to scorn. the breeder admits that he may hope to produce sheep with finer or longer wool and with better carcases, or handsomer fowls, or carrier-pigeons with beaks just perceptibly longer to the practised eye, and thus be successful at an exhibition. thus far he will go, but no farther. he does not reflect on what follows from adding up during a long course of time many, slight, successive modifications; nor does he reflect on the former existence of numerous varieties, connecting the links in each divergent line of descent. he concludes, as was shown in the earlier chapters, that all the chief breeds to which he has long attended are aboriginal productions. the systematic naturalist, on the other hand, who generally knows nothing of the art of breeding, who does not pretend to know how and when the several domestic races were formed, who cannot have seen the intermediate gradations, for they do not now exist, nevertheless feels no doubt that these races are sprung from a single source. but ask him whether the closely allied natural species which he has studied may not have descended from a common progenitor, and he in his turn will perhaps reject the notion with scorn. thus the naturalist and breeder may mutually learn a useful lesson from each other. * * * * * _summary on selection by man._--there can be no doubt that { } methodical selection has effected and will effect wonderful results. it was occasionally practised in ancient times, and is still practised by semi-civilised people. characters of the highest importance, and others of trifling value, have been attended to, and modified. i need not here repeat what has been so often said on the part which unconscious selection has played: we see its power in the difference between flocks which have been separately bred, and in the slow changes, as circumstances have slowly changed, which many animals have undergone in the same country, or when transported into a foreign land. we see the combined effects of methodical and unconscious selection in the great amount of difference between varieties in those parts or qualities which are valued by man, in comparison with those which are not valued, and consequently have not been attended to. natural selection often determines man's power of selection. we sometimes err in imagining that characters, which are considered as unimportant by the systematic naturalist, could not be affected by the struggle for existence, and therefore be acted on by natural selection; but striking cases have been given, showing how great an error this is. the possibility of selection coming into action rests on variability; and this is mainly caused, as we shall hereafter see, by changes in the conditions of life. selection is sometimes rendered difficult, or even impossible, by the conditions being opposed to the desired character or quality. it is sometimes checked by the lessened fertility and weakened constitution which follow from long-continued close interbreeding. that methodical selection may be successful, the closest attention and discernment, combined with unwearied patience, are absolutely necessary; and these same qualities, though not indispensable, are highly serviceable in the case of unconscious selection. it is almost necessary that a large number of individuals should be reared; for thus there will be a fair chance of variations of the desired nature arising, and every individual with the slightest blemish or in any degree inferior may be freely rejected. hence length of time is an important element of success. thus, also, propagation at an early age and at short intervals favours the work. facility in pairing animals, or their inhabiting a confined area, is advantageous as a check to free crossing. whenever and { } wherever selection is not practised, distinct races are not formed. when any one part of the body or quality is not attended to, it remains either unchanged or varies in a fluctuating manner, whilst at the same time other parts and other qualities may become permanently and greatly modified. but from the tendency to reversion and to continued variability, those parts or organs which are now undergoing rapid improvement through selection, are likewise found to vary much. consequently highly-bred animals, when neglected, soon degenerate; but we have no reason to believe that the effects of long-continued selection would, if the conditions of life remained the same, be soon and completely lost. man always tends to go to an extreme point in the selection, whether methodical or unconscious, of all useful and pleasing qualities. this is an important principle, as it leads to continued divergence, and in some rare cases to convergence of character. the possibility of continued divergence rests on the tendency in each part or organ to go on varying in the same manner in which it has already varied; and that this occurs, is proved by the steady and gradual improvement of many animals and plants during lengthened periods. the principle of divergence of character, combined with the neglect and final extinction of all previous, less-valued, and intermediate varieties, explains the amount of difference and the distinctness of our several races. although we may have reached the utmost limit to which certain characters can be modified, yet we are far from having reached, as we have good reason to believe, the limit in the majority of cases. finally, from the difference between selection as carried on by man and by nature, we can understand how it is that domestic races often, though by no means always, differ in general aspect from closely allied natural species. throughout this chapter and elsewhere i have spoken of selection as the paramount power, yet its action absolutely depends on what we in our ignorance call spontaneous or accidental variability. let an architect be compelled to build an edifice with uncut stones, fallen from a precipice. the shape of each fragment may be called accidental; yet the shape of each has been determined by the force of gravity, the nature { } of the rock, and the slope of the precipice,--events and circumstances, all of which depend on natural laws; but there is no relation between these laws and the purpose for which each fragment is used by the builder. in the same manner the variations of each creature are determined by fixed and immutable laws; but these bear no relation to the living structure which is slowly built up through the power of selection, whether this be natural or artificial selection. if our architect succeeded in rearing a noble edifice, using the rough wedge-shaped fragments for the arches, the longer stones for the lintels, and so forth, we should admire his skill even in a higher degree than if he had used stones shaped for the purpose. so it is with selection, whether applied by man or by nature; for though variability is indispensably necessary, yet, when we look at some highly complex and excellently adapted organism, variability sinks to a quite subordinate position in importance in comparison with selection, in the same manner as the shape of each fragment used by our supposed architect is unimportant in comparison with his skill. * * * * * { } chapter xxii. causes of variability. variability does not necessarily accompany reproduction--causes assigned by various authors--individual differences--variability of every kind due to changed conditions of life--on the nature of such changes--climate, food, excess of nutriment--slight changes sufficient--effects of grafting on the variability of seedling-trees--domestic productions become habituated to changed conditions--on the accumulative action of changed conditions--close interbreeding and the imagination of the mother supposed to cause variability--crossing as a cause of the appearance of new characters--variability from the commingling of characters and from reversion--on the manner and period of action of the causes which either directly, or indirectly through the reproductive system, induce variability. we will now consider, as far as we can, the causes of the almost universal variability of our domesticated productions. the subject is an obscure one; but it may be useful to probe our ignorance. some authors, for instance dr. prosper lucas, look at variability as a necessary contingent on reproduction, and as much an aboriginal law, as growth or inheritance. others have of late encouraged, perhaps unintentionally, this view by speaking of inheritance and variability as equal and antagonistic principles. pallas maintained, and he has had some followers, that variability depends exclusively on the crossing of primordially distinct forms. other authors attribute the tendency to variability to an excess of food, and with animals to an excess relatively to the amount of exercise taken, or again to the effects of a more genial climate. that these causes are all effective is highly probable. but we must, i think, take a broader view, and conclude that organic beings, when subjected during several generations to any change whatever in their conditions, tend to vary; the kind of variation which ensues depending in a far higher degree on the nature or constitution of the being, than on the nature of the changed conditions. { } those authors who believe that it is a law of nature that each individual should differ in some slight degree from every other, may maintain, apparently with truth, that this is the fact, not only with all domesticated animals and cultivated plants, but likewise with all organic beings in a state of nature. the laplander by long practice knows and gives a name to each reindeer, though, as linnæus remarks, "to distinguish one from another among such multitudes was beyond my comprehension, for they were like ants on an ant-hill." in germany shepherds have won wagers by recognising each sheep in a flock of a hundred, which they had never seen until the previous fortnight. this power of discrimination, however, is as nothing compared to that which some florists have acquired. verlot mentions a gardener who could distinguish kinds of camellia, when not in flower; and it has been positively asserted that the famous old dutch florist voorhelm, who kept above varieties of the hyacinth, was hardly ever deceived in knowing each variety by the bulb alone. hence we must conclude that the bulbs of the hyacinth and the branches and leaves of the camellia, though appearing to an unpractised eye absolutely undistinguishable, yet really differ.[ ] as linnæus has compared the reindeer in number to ants, i may add that each ant knows its fellow of the same community. several times i carried ants of the same species (_formica rufa_) from one ant-hill to another, inhabited apparently by tens of thousands of ants; but the strangers were instantly detected and killed. i then put some ants taken from a very large nest into a bottle strongly perfumed with assafoetida, and after an interval of twenty-four hours returned them to their home; they were at first threatened by their fellows, but were soon recognised and allowed to pass. hence each ant certainly recognises, independently of odour, its fellow; and if all the ants of the same community have not some countersign or watchword, they must present to each other's senses some distinguishable character. { } the dissimilarity of brothers or sisters of the same family, and of seedlings from the same capsule, may be in part accounted for by the unequal blending of the characters of the two parents, and by the more or less complete recovery through reversion of ancestral characters on either side; but we thus only push the difficulty further back in time, for what made the parents or their progenitors different? hence the belief[ ] that an innate tendency to vary exists, independently of external conditions, seems at first sight probable. but even the seeds nurtured in the same capsule are not subjected to absolutely uniform conditions, as they draw their nourishment from different points; and we shall see in a future chapter that this difference sometimes suffices greatly to affect the character of the future plant. the less close similarity of the successive children of the same family in comparison with human twins, which often resemble each other in external appearance, mental disposition, and constitution, in so extraordinary a manner, apparently proves that the state of the parents at the exact period of conception, or the nature of the subsequent embryonic development, has a direct and powerful influence on the character of the offspring. nevertheless, when we reflect on the { } individual differences between organic beings in a state of nature, as shown by every wild animal knowing its mate; and when we reflect on the infinite diversity of the many varieties of our domesticated productions, we may well be inclined to exclaim, though falsely as i believe, that variability must be looked at as an ultimate fact, necessarily contingent on reproduction. those authors who adopt this latter view would probably deny that each separate variation has its own proper exciting cause. although we can seldom trace the precise relation between cause and effect, yet the considerations presently to be given lead to the conclusion that each modification must have its own distinct cause. when we hear of an infant born, for instance, with a crooked finger, a misplaced tooth, or other slight deviation of structure, it is difficult to bring the conviction home to the mind that such abnormal cases are the result of fixed laws, and not of what we blindly call accident. under this point of view the following case, which has been carefully examined and communicated to me by dr. william ogle, is highly instructive. two girls, born as twins, and in all respects extremely alike, had their little fingers on both hands crooked; and in both children the second bicuspid tooth in the upper jaw, of the second dentition, was misplaced; for these teeth, instead of standing in a line with the others, grew from the roof of the mouth behind the first bicuspids. neither the parents nor any other member of the family had exhibited any similar peculiarity. now, as both these children were affected in exactly the same manner by both deviations of structure, the idea of accident is at once excluded; and we are compelled to admit that there must have existed some precise and sufficient cause which, if it had occurred a hundred times, would have affected a hundred children. we will now consider the general arguments, which appear to me to have great weight, in favour of the view that variations of all kinds and degrees are directly or indirectly caused by the conditions of life to which each being, and more especially its ancestors, have been exposed. no one doubts that domesticated productions are more variable than organic beings which have never been removed from their { } natural conditions. monstrosities graduate so insensibly into mere variations that it is impossible to separate them; and all those who have studied monstrosities believe that they are far commoner with domesticated than with wild animals and plants;[ ] and in the case of plants, monstrosities would be equally noticeable in the natural as in the cultivated state. under nature, the individuals of the same species are exposed to nearly uniform conditions, for they are rigorously kept to their proper places by a host of competing animals and plants; they have, also, long been habituated to their conditions of life; but it cannot be said that they are subject to quite uniform conditions, and they are liable to a certain amount of variation. the circumstances under which our domestic productions are reared are widely different: they are protected from competition; they have not only been removed from their natural conditions and often from their native land, but they are frequently carried from district to district, where they are treated differently, so that they never remain during a considerable length of time exposed to closely similar conditions. in conformity with this, all our domesticated productions, with the rarest exceptions, vary far more than natural species. the hive-bee, which feeds itself and follows in most respects its natural habits of life, is the least variable of all domesticated animals, and probably the goose is the next least variable; but even the goose varies more than almost any wild bird, so that it cannot be affiliated with perfect certainty to any natural species. hardly a single plant can be named, which has long been cultivated and propagated by seed, that is not highly variable; common rye (_secale cereale_) has afforded fewer and less marked varieties than almost any other cultivated plant;[ ] but it may be doubted whether the variations of this, the least valuable of all our cereals, have been closely observed. bud-variation, which was fully discussed in a former chapter, shows us that variability may be quite independent of seminal reproduction, and likewise of reversion to long-lost ancestral characters. no one will maintain that the sudden appearance { } of a moss-rose on a provence-rose is a return to a former state, for mossiness of the calyx has been observed in no natural species; the same argument is applicable to variegated and laciniated leaves; nor can the appearance of nectarines on peach-trees be accounted for with any probability on the principle of reversion. but bud-variations more immediately concern us, as they occur far more frequently on plants which have been highly cultivated during a length of time, than on other and less highly cultivated plants; and very few well-marked instances have been observed with plants growing under strictly natural conditions. i have given one instance of an ash-tree growing in a gentleman's pleasure-grounds; and occasionally there may be seen, on beech and other trees, twigs leafing at a different period from the other branches. but our forest trees in england can hardly be considered as living under strictly natural conditions; the seedlings are raised and protected in nursery-grounds, and must often be transplanted into places where wild trees of the kind would not naturally grow. it would be esteemed a prodigy if a dog-rose growing in a hedge produced by bud-variation a moss-rose, or a wild bullace or wild cherry-tree yielded a branch bearing fruit of a different shape and colour from the ordinary fruit. the prodigy would be enhanced if these varying branches were found capable of propagation, not only by grafts, but sometimes by seed; yet analogous cases have occurred with many of our highly cultivated trees and herbs. these several considerations alone render it probable that variability of every kind is directly or indirectly caused by changed conditions of life. or, to put the case under another point of view, if it were possible to expose all the individuals of a species during many generations to absolutely uniform conditions of life, there would be no variability. _on the nature of the changes in the conditions of life which induce variability._ from a remote period to the present day, under climates and circumstances as different as it is possible to conceive, organic beings of all kinds, when domesticated or cultivated, have { } varied. we see this with the many domestic races of quadrupeds and birds belonging to different orders, with gold-fish and silkworms, with plants of many kinds, raised in various quarters of the world. in the deserts of northern africa the date-palm has yielded thirty-eight varieties; in the fertile plains of india it is notorious how many varieties of rice and of a host of other plants exist; in a single polynesian island, twenty-four varieties of the bread-fruit, the same number of the banana, and twenty-two varieties of the arum, are cultivated by the natives; the mulberry-tree in india and europe has yielded many varieties serving as food for the silkworm; and in china sixty-three varieties of the bamboo are used for various domestic purposes.[ ] these facts alone, and innumerable others could be added, indicate that a change of almost any kind in the conditions of life suffices to cause variability--different changes acting on different organisms. andrew knight[ ] attributed the variation of both animals and plants to a more abundant supply of nourishment, or to a more favourable climate, than that natural to the species. a more genial climate, however, is far from necessary; the kidney-bean, which is often injured by our spring frosts, and peaches, which require the protection of a wall, have varied much in england, as has the orange-tree in northern italy, where it is barely able to exist.[ ] nor can we overlook the fact, though not immediately connected with our present subject, that the plants and shells of the arctic regions are eminently variable.[ ] moreover, it does not appear that a change of climate, whether more or less genial, is one of the most potent causes of variability; for in regard to plants alph. de candolle, in his 'géographie { } botanique,' repeatedly shows that the native country of a plant, where in most cases it has been longest cultivated, is that where it has yielded the greatest number of varieties. it is doubtful whether a change in the nature of the food is a potent cause of variability. scarcely any domesticated animal has varied more than the pigeon or the fowl, but their food, especially that of highly-bred pigeons, is generally the same. nor can our cattle and sheep have been subjected to any great change in this respect. but in all these cases the food probably is much less varied in kind than that which was consumed by the species in its natural state.[ ] of all the causes which induce variability, excess of food, whether or not changed in nature, is probably the most powerful. this view was held with regard to plants by andrew knight, and is now held by schleiden, more especially in reference to the inorganic elements of the food.[ ] in order to give a plant more food it suffices in most cases to grow it separately, and thus prevent other plants robbing its roots. it is surprising, as i have often seen, how vigorously our common wild plants flourish when planted by themselves, though not in highly manured land. growing plants separately is, in fact, the first step in cultivation. we see the converse of the belief that excess of food induces variability in the following statement by a great raiser of seeds of all kinds.[ ] "it is a rule invariably with us, when we desire to keep a true stock of any one kind of seed, to grow it on poor land without dung; but when we grow for quantity, we act contrary, and sometimes have dearly to repent of it." in the case of animals the want of a proper amount of exercise, as bechstein has remarked, has perhaps played, independently of the direct effects of the disuse of any particular organ, an important part in causing variability. we can see in a vague manner that, when the organised and nutrient fluids of the body are not used during growth, or by the wear and tear of the tissues, { } they will be in excess; and as growth, nutrition, and reproduction are intimately allied processes, this superfluity might disturb the due and proper action of the reproductive organs, and consequently affect the character of the future offspring. but it may be argued that neither an excess of food nor a superfluity in the organised fluids of the body necessarily induces variability. the goose and the turkey have been well fed for many generations, yet have varied very little. our fruit-trees and culinary plants, which are so variable, have been cultivated from an ancient period, and, though they probably still receive more nutriment than in their natural state, yet they must have received during many generations nearly the same amount; and it might be thought that they would have become habituated to the excess. nevertheless, on the whole, knight's view, that excess of food is one of the most potent causes of variability, appears, as far as i can judge, probable. whether or not our various cultivated plants have received nutriment in excess, all have been exposed to changes of various kinds. fruit-trees are grafted on different stocks, and grown in various soils. the seeds of culinary and agricultural plants are carried from place to place; and during the last century the rotation of our crops and the manures used have been greatly changed. slight changes of treatment often suffice to induce variability. the simple fact of almost all our cultivated plants and domesticated animals having varied in all places and at all times, leads to this conclusion. seeds taken from common english forest-trees, grown under their native climate, not highly manured or otherwise artificially treated, yield seedlings which vary much, as may be seen in every extensive seed-bed. i have shown in a former chapter what a number of well marked and singular varieties the thorn (_cratægus oxyacantha_) has produced; yet this tree has been subjected to hardly any cultivation. in staffordshire i carefully examined a large number of two british plants, namely, _geranium phæum_ and _pyrenaicum_, which have never been highly cultivated. these plants had spread spontaneously by seed from a common garden into an open plantation; and the seedlings varied in almost every single character, both in their flowers and foliage, to a degree which { } i have never seen exceeded; yet they could not have been exposed to any great change in their conditions. with respect to animals, azara has remarked with much surprise,[ ] that, whilst the feral horses on the pampas are always of one of three colours, and the cattle always of a uniform colour, yet these animals, when bred on the unenclosed estancias, though kept in a state which can hardly be called domesticated, and apparently exposed to almost identically the same conditions as when they are feral, nevertheless display a great diversity of colour. so again in india several species of fresh-water fish are only so far treated artificially, that they are reared in great tanks; but this small change is sufficient to induce much variability.[ ] some facts on the effects of grafting, in regard to the variability of trees, deserve attention. cabanis asserts that when certain pears are grafted on the quince, their seeds yield more varieties than do the seeds of the same variety of pear when grafted on the wild pear.[ ] but as the pear and quince are distinct species, though so closely related that the one can be readily grafted and succeeds admirably on the other, the fact of variability being thus caused is not surprising; we are, however, here enabled to see the cause, namely, the different nature of the stock with its roots and the rest of the tree. several north american varieties of the plum and peach are well known to reproduce themselves truly by seed; but downing asserts,[ ] "that when a graft is taken from one of these trees and placed upon another stock, this grafted tree is found to lose its singular property of producing the same variety by seed, and becomes like all other worked trees;"--that is, its seedlings become highly variable. another case is worth giving: the lalande variety of the walnut-tree leafs between april th and may th, and its seedlings invariably inherit the same habit; whilst several other varieties of the walnut leaf in june. now, if seedlings are raised from the may-leafing lalande variety, grafted on another may-leafing variety, though both stock and graft have the same early habit of leafing, yet the seedlings leaf at various times, { } even as late as the th of june.[ ] such facts as these are well fitted to show, on what obscure and slight causes variability rests. i may here just allude to the appearance of new and valuable varieties of fruit-trees and of wheat in woods and waste places, which at first sight seems a most anomalous circumstance. in france a considerable number of the best pears have been discovered in woods; and this has occurred so frequently, that poiteau asserts that "improved varieties of our cultivated fruits rarely originate with nurserymen."[ ] in england, on the other hand, no instance of a good pear having been found wild has been recorded; and mr. rivers informs me that he knows of only one instance with apples, namely, the bess poole, which was discovered in a wood in nottinghamshire. this difference between the two countries may be in part accounted for by the more favourable climate of france, but chiefly from the great number of seedlings which spring up there in the woods. i infer that this is the case from a remark made by a french gardener,[ ] who regards it as a national calamity that such a number of pear-trees are periodically cut down for firewood, before they have borne fruit. the new varieties which thus spring up in the woods, though they cannot have received any excess of nutriment, will have been exposed to abruptly changed conditions, but whether this is the cause of their production is very doubtful. these varieties, however, are probably all descended[ ] from old cultivated kinds growing in adjoining orchards,--a circumstance which will account for their variability; and out of a vast number of varying trees there will always be a good chance of the appearance of a valuable kind. in north america, where fruit-trees frequently spring up in waste places, the washington pear was found in a hedge, and the emperor peach in a wood.[ ] with respect to wheat, some writers have spoken[ ] as if it were an ordinary event for new varieties to be found in waste places; the fenton wheat was certainly discovered growing on a pile of basaltic detritus in a quarry, but in such a situation the plant would probably receive a sufficient amount { } of nutriment. the chidham wheat was raised from an ear found _on_ a hedge; and hunter's wheat was discovered _by_ the roadside in scotland, but it is not said that this latter variety grew where it was found.[ ] whether our domestic productions would ever become so completely habituated to the conditions under which they now live, as to cease varying, we have no sufficient means for judging. but, in fact, our domestic productions are never exposed for a great length of time to uniform conditions, and it is certain that our most anciently cultivated plants, as well as animals, still go on varying, for all have recently undergone marked improvement. in some few cases, however, plants have become habituated to new conditions. thus metzger, who cultivated in germany during many years numerous varieties of wheat, brought from different countries,[ ] states that some kinds were at first extremely variable, but gradually, in one instance after an interval of twenty-five years, became constant; and it does not appear that this resulted from the selection of the more constant forms. * * * * * _on the accumulative action of changed conditions of life._--we have good grounds for believing that the influence of changed conditions accumulates, so that no effect is produced on a species until it has been exposed during several generations to continued cultivation or domestication. universal experience shows us that when new flowers are first introduced into our gardens they do not vary; but ultimately all, with the rarest exceptions, vary to a greater or less extent. in a few cases the requisite number of generations, as well as the successive steps in the progress of variation, have been recorded, as in the often-quoted instance of the dahlia.[ ] after several years' culture the zinnia has only lately ( ) begun to vary in any great degree. "in the first seven or eight years of high cultivation the swan river daisy (_brachycome iberidifolia_) kept to its original colour; it then varied into lilac and purple and other minor shades."[ ] analogous facts have been recorded with the scotch rose. in discussing the variability of plants several experienced horticulturists have spoken to the { } same general effect. mr. salter[ ] remarks, "every one knows that the chief difficulty is in breaking through the original form and colour of the species, and every one will be on the look-out for any natural sport, either from seed or branch; that being once obtained, however trifling the change may be, the result depends upon himself." m. de jonghe, who has had so much success in raising new varieties of pears and strawberries,[ ] remarks with respect to the former, "there is another principle, namely, that the more a type has entered into a state of variation, the greater is its tendency to continue doing so; and the more it has varied from the original type, the more it is disposed to vary still farther." we have, indeed, already discussed this latter point when treating of the power which man possesses, through selection, of continually augmenting in the same direction each modification; for this power depends on continued variability of the same general kind. the most celebrated horticulturist in france, namely, vilmorin,[ ] even maintains that, when any particular variation is desired, the first step is to get the plant to vary in any manner whatever, and to go on selecting the most variable individuals, even though they vary in the wrong direction; for the fixed character of the species being once broken, the desired variation will sooner or later appear. as nearly all our animals were domesticated at an extremely remote epoch, we cannot, of course, say whether they varied quickly or slowly when first subjected to new conditions. but dr. bachman[ ] states that he has seen turkeys raised from the eggs of the wild species lose their metallic tints and become spotted with white in the third generation. mr. yarrell many years ago informed me that the wild ducks bred on the ponds in st. james's park, which had never been crossed, as it is believed, with domestic ducks, lost their true plumage after a few generations. an excellent observer,[ ] who has often reared birds from the eggs of the wild duck, and who took precautions { } that there should be no crossing with domestic breeds, has given, as previously stated, full details on the changes which they gradually undergo. he found that he could not breed these wild ducks true for more than five or six generations, "as they then proved so much less beautiful. the white collar round the neck of the mallard became much broader and more irregular, and white feathers appeared in the ducklings' wings." they increased also in size of body; their legs became less fine, and they lost their elegant carriage. fresh eggs were then procured from wild birds; but again the same result followed. in these cases of the duck and turkey we see that animals, like plants, do not depart from their primitive type until they have been subjected during several generations to domestication. on the other hand, mr. yarrell informed me that the australian dingos, bred in the zoological gardens, almost invariably produced in the first generation puppies marked with white and other colours; but these introduced dingos had probably been procured from the natives, who keep them in a semi-domesticated state. it is certainly a remarkable fact that changed conditions should at first produce, as far as we can see, absolutely no effect; but that they should subsequently cause the character of the species to change. in the chapter on pangenesis i shall attempt to throw a little light on this fact. * * * * * returning now to the causes which are supposed to induce variability. some authors[ ] believe that close interbreeding gives this tendency, and leads to the production of monstrosities. in the seventeenth chapter some few facts were advanced, showing that monstrosities are, as it appears, occasionally thus caused; and there can be no doubt that close interbreeding induces lessened fertility and a weakened constitution; hence it may lead to variability: but i have not sufficient evidence on this head. on the other hand, close interbreeding, if not carried to an injurious extreme, far from causing variability, tends to fix the character of each breed. it was formerly a common belief, still held by some persons, that the imagination of the mother affects the child in { } the womb.[ ] this view is evidently not applicable to the lower animals, which lay unimpregnated eggs, or to plants. dr. william hunter, in the last century, told my father that during many years every woman in a large london lying-in hospital was asked before her confinement whether anything had specially affected her mind, and the answer was written down; and it so happened that in no one instance could a coincidence be detected between the woman's answer and any abnormal structure; but when she knew the nature of the structure, she frequently suggested some fresh cause. the belief in the power of the mother's imagination may perhaps have arisen from the children of a second marriage resembling the previous father, as certainly sometimes occurs, in accordance with the facts given in the eleventh chapter. * * * * * _crossing as a cause of variability._--in an early part of this chapter it was stated that pallas[ ] and a few other naturalists maintain that variability is wholly due to crossing. if this means that new characters never spontaneously appear in our domestic races, but that they are all directly derived from certain aboriginal species, the doctrine is little less than absurd; for it implies that animals like italian greyhounds, pug-dogs, bull-dogs, pouter and fantail pigeons, &c., were able to exist in a state of nature. but the doctrine may mean something widely different, namely, that the crossing of distinct species is the sole cause of the first appearance of new characters, and that without this aid man could not have formed his various breeds. as, however, new characters have appeared in certain cases by bud-variation, we may conclude with certainty that crossing is not necessary for variability. it is, moreover, almost certain that the breeds of various animals, such as of the rabbit, pigeon, duck, &c., and the varieties of several plants, are the modified descendants of a single wild species. nevertheless, it is probable that the crossing of two forms, when one or both have long been domesticated or cultivated, adds to the variability of the offspring, independently of the commingling of the characters derived from the two parent-forms; and this implies { } that new characters actually arise. but we must not forget the facts advanced in the thirteenth chapter, which clearly prove that the act of crossing often leads to the reappearance or reversion of long-lost characters; and in most cases it would be impossible to distinguish between the reappearance of ancient characters and the first appearance of new characters. practically, whether new or old, they would be new to the breed in which they reappeared. gärtner declares,[ ] and his experience is of the highest value on such a point, that, when he crossed native plants which had not been cultivated, he never once saw in the offspring any new character; but that from the odd manner in which the characters derived from the parents were combined, they sometimes appeared as if new. when, on the other hand, he crossed cultivated plants, he admits that new characters occasionally appeared, but he is strongly inclined to attribute their appearance to ordinary variability, not in any way to the cross. an opposite conclusion, however, appears to me the more probable. according to kölreuter, hybrids in the genus mirabilis vary almost infinitely, and he describes new and singular characters in the form of the seeds, in the colour of the anthers, in the cotyledons being of immense size, in new and highly peculiar odours, in the flowers expanding early in the season, and in their closing at night. with respect to one lot of these hybrids, he remarks that they presented characters exactly the reverse of what might have been expected from their parentage.[ ] prof. lecoq[ ] speaks strongly to the same effect in regard to this same genus, and asserts that many of the hybrids from _mirabilis jalapa_ and _multiflora_ might easily be mistaken for distinct species, and adds that they differed in a greater degree, than the other species of the genus, from _m. jalapa_. herbert, also, has described[ ] the offspring from a hybrid rhododendron as being "as _unlike all others_ in foliage, as if they had been a separate species." the common experience of floriculturists proves that the crossing and recrossing of distinct but allied plants, such as the species of petunia, calceolaria, fuchsia, verbena, &c., induces excessive variability; hence the appearance of quite new characters is probable. m. carrière[ ] has lately discussed this subject: he states that _erythrina cristagalli_ had been multiplied by seed for many years, but had not yielded any varieties: it was then crossed with the allied _e. herbacea_, and "the resistance was now overcome, and varieties were produced with flowers of extremely different size, form, and colour." from the general and apparently well-founded belief that the crossing { } of distinct species, besides commingling their characters, adds greatly to their variability, it has probably arisen that some botanists have gone so far as to maintain[ ] that, when a genus includes only a single species, this when cultivated never varies. the proposition made so broadly cannot be admitted; but it is probably true that the variability of cultivated monotypic genera is much less than that of genera including numerous species, and this quite independently of the effects of crossing. i have stated in my 'origin of species,' and in a future work shall more fully show, that the species belonging to small genera generally yield a less number of varieties in a state of nature than those belonging to large genera. hence the species of small genera would, it is probable, produce fewer varieties under cultivation than the already variable species of larger genera. although we have not at present sufficient evidence that the crossing of species, which have never been cultivated, leads to the appearance of new characters, this apparently does occur with species which have been already rendered in some degree variable through cultivation. hence crossing, like any other change in the conditions of life, seems to be an element, probably a potent one, in causing variability. but we seldom have the means of distinguishing, as previously remarked, between the appearance of really new characters and the reappearance of long-lost characters, evoked through the act of crossing. i will give an instance of the difficulty in distinguishing such cases. the species of datura may be divided into two sections, those having white flowers with green stems, and those having purple flowers with brown stems: now naudin[ ] crossed _datura lævis_ and _ferox_, both of which belong to the white section, and raised from them hybrids. of these hybrids, every one had brown stems and bore purple flowers; so that they resembled the species of the other section of the genus, and not their own two parents. naudin was so much astonished at this fact, that he was led carefully to observe both parent-species, and he discovered that the pure seedlings of _d. ferox_, immediately after germination, had dark purple stems, extending from the young roots up to the cotyledons, and that this tint remained ever afterwards as a ring round the base of the stem of the plant when old. now i have shown in the thirteenth chapter that the retention or exaggeration of an early character is so intimately related to reversion, that it evidently comes under the same principle. hence probably we ought to look at the purple flowers and brown stems of these hybrids, not as new characters due to variability, but as a return to the former state of some ancient progenitor. independently of the appearance of new characters from crossing, a few words may be added to what has been said in former chapters on the unequal combination and transmission of the characters proper to the two parent-forms. when two species or races are crossed, the offspring of { } the first generation are generally uniform, but subsequently they display an almost infinite diversity of character. he who wishes, says kölreuter,[ ] to obtain an endless number of varieties from hybrids should cross and recross them. there is also much variability when hybrids or mongrels are reduced or absorbed by repeated crosses with either pure parent-form; and a still higher degree of variability when three distinct species, and most of all when four species, are blended together by successive crosses. beyond this point gärtner,[ ] on whose authority the foregoing statements are made, never succeeded in effecting a union; but max wichura[ ] united six distinct species of willows into a single hybrid. the sex of the parent-species affects in an inexplicable manner the degree of variability of hybrids; for gärtner[ ] repeatedly found that when a hybrid was used as the father, and either one of the pure parent-species, or a third species, was used as the mother, the offspring were more variable than when the same hybrid was used as the mother, and either pure parent or the same third species as the father: thus seedlings from _dianthus barbatus_ crossed by the hybrid _d. chinensi-barbatus_ were more variable than those raised from this latter hybrid fertilised by the pure _d. barbatus_. max wichura[ ] insists strongly on an analogous result with his hybrid willows. again gärtner[ ] asserts that the degree of variability sometimes differs in hybrids raised from reciprocal crosses between the same two species; and here the sole difference is, that the one species is first used as the father and then as the mother. on the whole we see that, independently of the appearance of new characters, the variability of successive crossed generations is extremely complex, partly from the offspring partaking unequally of the characters of the two parent-forms, and more especially from their unequal tendency to revert to these same characters or to those of more ancient progenitors. * * * * * _on the manner and on the period of action of the causes which induce variability._--this is an extremely obscure subject, and we need here only briefly consider, firstly, whether inherited variations are caused by the organisation being directly acted on, or indirectly through the reproductive system; and secondly, at what period of life or growth they are primarily caused. we shall see in the two following chapters that various agencies, such as an abundant supply of food, exposure to a different climate, increased use or disuse of parts, &c., prolonged during several generations, certainly modify either the whole organisation or certain organs. this direct action of changed conditions perhaps comes into play much more frequently than can be proved, and it is at least clear that in all cases of { } bud-variation the action cannot have been through the reproductive system. with respect to the part which the reproductive system takes in causing variability, we have seen in the eighteenth chapter that even slight changes in the conditions of life have a remarkable power in causing a greater or less degree of sterility. hence it seems not improbable that being generated though a system so easily affected should themselves be affected, or should fail to inherit, or inherit in excess, characters proper to their parents. we know that certain groups of organic beings, but with exceptions in each group, have their reproductive systems much more easily affected by changed conditions than other groups; for instance, carnivorous birds more readily than carnivorous mammals, and parrots more readily than pigeons; and this fact harmonizes with the apparently capricious manner and degree in which various groups of animals and plants vary under domestication. kölreuter[ ] was struck with the parallelism between the excessive variability of hybrids when crossed and recrossed in various ways,--these hybrids having their reproductive powers more or less affected,--and the variability of anciently cultivated plants. max wichura[ ] has gone one step farther, and shows that with many of our highly cultivated plants, such as the hyacinth, tulip, auricula, snapdragon, potato, cabbage, &c., which there is no reason to believe have been hybridized, the anthers contain many irregular pollen-grains, in the same state as in hybrids. he finds also in certain wild forms, the same coincidence between the state of the pollen and a high degree of variability, as in many species of rubus; but in _r. cæsius_ and _idæus_, which are not highly variable species, the pollen is sound. it is also notorious that many cultivated plants, such as the banana, pine-apple, breadfruit, and others previously mentioned, have their reproductive organs so seriously affected as to be generally quite sterile; and when they do yield seed, the seedlings, judging from the large number of cultivated races which exist, must be variable in an extreme degree. these facts indicate that there is some relation between the state of the reproductive organs and a tendency to variability; but we must not conclude that the relation is strict. although many of our highly cultivated plants may have their pollen in a deteriorated condition, yet, as we have previously seen, they yield more seed, and our anciently domesticated animals are more prolific, than the corresponding species in a state of nature. the peacock is almost the only bird which is believed to be less fertile under domestication than in its native state, and it has varied in a remarkably small degree. from these considerations it would seem that changes in the conditions of life lead either to sterility or to variability, or to both; and not that sterility induces variability. on the whole it is probable that any cause affecting the organs of reproduction would likewise affect their product,--that is, the offspring thus generated. { } the period of life at which the causes that induce variability act, is another obscure subject, which has been discussed by various authors.[ ] in some of the cases, to be given in the following chapter, of modifications from the direct action of changed conditions, which are inherited, there can be no doubt that the causes have acted on the mature or nearly mature animal. on the other hand, monstrosities, which cannot be distinctly separated from lesser variations, are often caused by the embryo being injured whilst in the mother's womb or in the egg. thus i. geoffroy st. hilaire[ ] asserts that poor women who work hard during their pregnancy, and the mothers of illegitimate children troubled in their minds and forced to conceal their state, are far more liable to give birth to monsters than women in easy circumstances. the eggs of the fowl when placed upright or otherwise treated unnaturally frequently produce monstrous chickens. it would, however, appear that complex monstrosities are induced more frequently during a rather late than during a very early period of embryonic life; but this may partly result from some one part, which has been injured during an early period, affecting by its abnormal growth other parts subsequently developed; and this would be less likely to occur with parts injured at a later period.[ ] when any part or organ becomes monstrous through abortion, a rudiment is generally left, and this likewise indicates that its development had already commenced. insects sometimes have their antennæ or legs in a monstrous condition, and yet the larvæ from which they are metamorphosed do not possess either antennæ or legs; and in those cases, as quatrefages[ ] believes, we are enabled to see the precise period at which the normal progress of development has been troubled. but the nature of the food given to a caterpillar sometimes affects the colours of the moth, without the caterpillar itself being affected; therefore it seems possible that other characters in the mature insect might be indirectly modified through the larvæ. there is no reason to suppose that organs which have been rendered monstrous have always been acted on during their development; the cause may have acted on the organisation at a much earlier stage. it is even probable that either the male or female sexual elements, or both, before their union, may be affected in such a manner as to lead to modifications in organs developed at a late period of life; in nearly the same manner as a child may inherit from his father a disease which does not appear until old age. in accordance with the facts above given, which prove that in many cases a close relation exists between variability and the sterility following from changed conditions, we may conclude that the exciting cause often acts at the earliest possible period, namely, on the sexual elements, before impregnation has taken place. that an affection of the female sexual element may induce variability we may likewise infer as probable from the occurrence of bud-variations; for a bud seems to be the analogue of an ovule. but the male element is apparently much oftener affected by changed { } conditions, at least in a visible manner, than the female element or ovule; and we know from gärtner's and wichura's statements that a hybrid used as the father and crossed with a pure species gives a greater degree of variability to the offspring, than does the same hybrid when used as the mother. lastly, it is certain that variability may be transmitted through either sexual element, whether or not originally excited in them, for kölreuter and gärtner[ ] found that when two species were crossed, if either one was variable, the offspring were rendered variable. * * * * * _summary._--from the facts given in this chapter, we may conclude that the variability of organic beings under domestication, although so general, is not an inevitable contingent on growth and reproduction, but results from the conditions to which the parents have been exposed. changes of any kind in the conditions of life, even extremely slight changes, often suffice to cause variability. excess of nutriment is perhaps the most efficient single exciting cause. animals and plants continue to be variable for an immense period after their first domestication; but the conditions to which they are exposed never long remain quite constant. in the course of time they can be habituated to certain changes, so as to become less variable; and it is possible that when first domesticated they may have been even more variable than at present. there is good evidence that the power of changed conditions accumulates; so that two, three, or more generations must be exposed to new conditions before any effect is visible. the crossing of distinct forms, which have already become variable, increases in the offspring the tendency to further variability, by the unequal commingling of the characters of the two parents, by the reappearance of long-lost characters, and by the appearance of absolutely new characters. some variations are induced by the direct action of the surrounding conditions on the whole organisation, or on certain parts alone, and other variations are induced indirectly through the reproductive system being affected in the same manner as is so common with organic beings when removed from their natural conditions. the causes which induce variability act on the mature organism, on the embryo, and, as we have good reason to believe, on both sexual elements before impregnation has been effected. * * * * * { } chapter xxiii. direct and definite action of the external conditions of life. slight modifications in plants from the definite action of changed conditions in size, colour, chemical properties, and in the state of the tissues--local diseases--conspicuous modifications from changed climate or food, etc.--plumage of birds affected by peculiar nutriment, and by the inoculation of poison--land-shells--modifications of organic beings in a state of nature through the definite action of external conditions--comparison of american and european trees--galls--effects of parasitic fungi--considerations opposed to the belief in the potent influence of changed external conditions--parallel series of varieties--amount of variation does not correspond with the degree of change in the conditions--bud-variation--monstrosities produced by unnatural treatment--summary. if we ask ourselves why this or that character has been modified under domestication, we are, in most cases lost in utter darkness. many naturalists, especially of the french school, attribute every modification to the "monde ambiant," that is, to changed climate, with all its diversities of heat and cold, dampness and dryness, light and electricity, to the nature of the soil, and to varied kinds and amount of food. by the term definite action, as used in this chapter, i mean an action of such a nature that, when many individuals of the same variety are exposed during several generations to any change in their physical conditions of life, all, or nearly all the individuals, are modified in the same manner. a new sub-variety would thus be produced without the aid of selection. i do not include under the term of definite action the effects of habit or of the increased use and disuse of various organs. modifications of this nature, no doubt, are definitely caused by the conditions to which the beings are subjected; but they depend much less on the nature of the conditions than on the laws of growth; hence they are included under a distinct head in the { } following chapter. we know, however, far too little of the causes and laws of variation to make a sound classification. the direct action of the conditions of life, whether leading to definite or indefinite results, is a totally distinct consideration from the effects of natural selection; for natural selection depends on the survival under various and complex circumstances of the best-fitted individuals, but has no relation whatever to the primary cause of any modification of structure. i will first give in detail all the facts which i have been able to collect, rendering it probable that climate, food, &c., have acted so definitely and powerfully on the organisation of our domesticated productions, that they have sufficed to form new sub-varieties or races, without the aid of selection by man or of natural selection. i will then give the facts and considerations opposed to this conclusion, and finally we will weigh, as fairly as we can, the evidence on both sides. when we reflect that distinct races of almost all our domesticated animals exist in each kingdom of europe, and formerly even in each district of england, we are at first strongly inclined to attribute their origin to the definite action of the physical conditions of each country; and this has been the conclusion of many authors. but we should bear in mind that man annually has to choose which animals shall be preserved for breeding, and which shall be slaughtered. we have also seen that both methodical and unconscious selection were formerly practised, and are now occasionally practised by the most barbarous races, to a much greater extent than might have been anticipated. hence it is very difficult to judge how far the difference in conditions between, for instance, the several districts in england, could have sufficed without the aid of selection to modify the breeds which have been reared in each. it may be argued that, as numerous wild animals and plants have ranged during many ages throughout great britain, and still retain the same character, the difference in conditions between the several districts could not have modified in so marked a manner the various native races of cattle, sheep, pigs, and horses. the same difficulty of distinguishing between selection and the definite effects of the conditions of life, is encountered in a still higher degree when we compare closely allied natural { } forms, inhabiting two countries, such as north america and europe, which do not differ greatly in climate, nature of soil, &c., for in this case natural selection will inevitably and rigorously have acted during a long succession of ages. from the importance of the difficulty just alluded to, it will be advisable to give as large a body of facts as possible, showing that extremely slight differences in treatment, either in different parts of the same country, or during different seasons, certainly cause an appreciable effect, at least on varieties which are already in an unstable condition. ornamental flowers are good for this purpose, as they are highly variable, and are carefully observed. all floriculturists are unanimous that certain varieties are affected by very slight differences in the nature of the artificial compost in which they are grown, and by the natural soil of the district, and by the season. thus, a skilful judge, in writing on carnations and picotees,[ ] asks "where can admiral curzon be seen possessing the colour, size, and strength which it has in derbyshire? where can flora's garland be found equal to those at slough? where do high-coloured flowers revel better than at woolwich and birmingham? yet in no two of these districts do the same varieties attain an equal degree of excellence, although each may be receiving the attention of the most skilful cultivators." the same writer then recommends every cultivator to keep five different kinds of soil and manure, "and to endeavour to suit the respective appetites of the plants you are dealing with, for without such attention all hope of general success will be vain." so it is with the dahlia:[ ] the lady cooper rarely succeeds near london, but does admirably in other districts; the reverse holds good with other varieties; and again, there are others which succeed equally well in various situations. a skilful gardener[ ] states that he procured cuttings of an old and well-known variety (pulchella) of verbena, which from having been propagated in a different situation presented a slightly different shade of colour; the two varieties were afterwards multiplied by cuttings, being carefully kept distinct; but in the second year they could hardly be distinguished, and in the third year no one could distinguish them. the nature of the season has an especial influence on certain varieties of the dahlia: in two varieties were pre-eminently good, and the next year these same two were pre-eminently bad. a famous amateur[ ] asserts that in many varieties of the rose came so untrue in character, "that it was hardly possible to recognise them, and the thought was not seldom entertained that the grower had lost his tally." the same amateur[ ] states that in two-thirds of his auriculas produced central trusses of flowers, and these are remarkable from not keeping true; { } and he adds that in some seasons certain varieties of this plant all prove good, and the next season all prove bad; whilst exactly the reverse happens with other varieties. in the editor of the 'gardener's chronicle'[ ] remarked how singular it was that this year many calceolarias tended to assume a tubular form. with heartsease[ ] the blotched sorts do not acquire their proper character until hot weather sets in; whilst other varieties lose their beautiful marks as soon as this occurs. analogous facts have been observed with leaves: mr. beaton asserts[ ] that he raised at shrubland, during six years, twenty thousand seedlings from the punch pelargonium, and not one had variegated leaves; but at surbiton, in surrey, one-third, or even a greater proportion, of the seedlings from this same variety were more or less variegated. the soil of another district in surrey has a strong tendency to cause variegation, as appears from information given me by sir f. pollock. verlot[ ] states that the variegated strawberry retains its character as long as grown in a dryish soil, but soon loses it when planted in fresh and humid soil. mr. salter, who is well known for his success in cultivating variegated plants, informs me that rows of strawberries were planted in his garden in , in the usual way; and at various distances in one row, several plants simultaneously became variegated, and what made the case more extraordinary, all were variegated in precisely the same manner. these plants were removed, but during the three succeeding years other plants in the same row became variegated, and in no instance were the plants in any adjoining row affected. the chemical qualities, odours, and tissues of plants are often modified by a change which seems to us slight. the hemlock is said not to yield conicine in scotland. the root of the _aconitum napellus_ becomes innocuous in frigid climates. the medicinal properties of the digitalis are easily affected by culture. the rhubarb flourishes in england, but does not produce the medicinal substance which makes the plant so valuable in chinese tartary. as the _pistacia lentiscus_ grows abundantly in the south of france, the climate must suit it, but it yields no mastic. the _laurus sassafras_ in europe loses the odour proper to it in north america.[ ] many similar facts could be given, and they are remarkable because it might have been thought that definite chemical compounds would have been little liable to change either in quality or quantity. the wood of the american locust-tree (_robinia_) when grown in england is nearly worthless, as is that of the oak-tree when grown at the cape of good hope.[ ] hemp and flax, as i hear from dr. falconer, flourish and yield plenty of seed on the plains of india, but their fibres are brittle { } and useless. hemp, on the other hand, fails to produce in england that resinous matter which is so largely used in india as an intoxicating drug. the fruit of the melon is greatly influenced by slight differences in culture and climate. hence it is generally a better plan, according to naudin, to improve an old kind than to introduce a new one into any locality. the seed of the persian melon produces near paris fruit inferior to the poorest market kinds, but at bordeaux yields delicious fruit.[ ] seed is annually brought from thibet to kashmir,[ ] and produces fruit weighing from four to ten pounds, but plants raised from seed saved in kashmir next year give fruit weighing only from two to three pounds. it is well known that american varieties of the apple produce in their native land magnificent and brightly-coloured fruit, but in england of poor quality and a dull colour. in hungary there are many varieties of the kidney-bean, remarkable for the beauty of their seeds, but the rev. m. j. berkeley[ ] found that their beauty could hardly ever be preserved in england, and in some cases the colour was greatly changed. we have seen in the ninth chapter, with respect to wheat, what a remarkable effect transportal from the north to the south of france, and reversely, produced on the weight of the grain. when man can perceive no change in plants or animals which have been exposed to a new climate or to different treatment, insects can sometimes perceive a marked change. the same species of cactus has been carried to india from canton, manilla, mauritius, and from the hot-houses of kew, and there is likewise a so-called native kind, formerly introduced from south america; all these plants are alike in appearance, but the cochineal insect flourishes only on the native kind, on which it thrives prodigiously.[ ] humboldt remarks[ ] that white men "born in the torrid zone walk barefoot with impunity in the same apartment where a european, recently landed, is exposed to the attacks of the _pulex penetrans_." this insect, the too well-known chigoe, must therefore be able to distinguish what the most delicate chemical analysis fails to distinguish, namely, a difference between the blood or tissues of a european and those of a white man born in the country. but the discernment of the chigoe is not so surprising as it at first appears; for { } according to liebig[ ] the blood of men with different complexions, though inhabiting the same country, emits a different odour. diseases peculiar to certain localities, heights, or climates, may be here briefly noticed, as showing the influence of external circumstances on the human body. diseases confined to certain races of man do not concern us, for the constitution of the race may play the more important part, and this may have been determined by unknown causes. the plica polonica stands, in this respect, in a nearly intermediate position; for it rarely affects germans, who inhabit the neighbourhood of the vistula, where so many poles are grievously affected; and on the other hand, it does not affect russians, who are said to belong to the same original stock with the poles.[ ] the elevation of a district often governs the appearance of diseases; in mexico the yellow fever does not extend above mètres; and in peru, people are affected with the _verugas_ only between and mètres above the sea; many other such cases could be given. a peculiar cutaneous complaint, called the _bouton d'alep_, affects in aleppo and some neighbouring districts almost every native infant, and some few strangers; and it seems fairly well established that this singular complaint depends on drinking certain waters. in the healthy little island of st. helena the scarlet-fever is dreaded like the plague; analogous facts have been observed in chili and mexico.[ ] even in the different departments of france it is found that the various infirmities which render the conscript unfit for serving in the army, prevail with remarkable inequality, revealing, as boudin observes, that many of them are endemic, which otherwise would never have been suspected.[ ] any one who will study the distribution of disease will be struck with surprise at what slight differences in the surrounding circumstances govern the nature and severity of the complaints by which man is at least temporarily affected. the modifications as yet referred to have been extremely slight, and in most cases have been caused, as far as we can judge, by equally slight changes in the conditions. but can it be safely maintained that such changed conditions, if acting during a long series of generations, would not produce a marked effect? it is commonly believed that the people of the united states differ in appearance from the parent anglo-saxon race; and selection cannot have come into action within so short a period. a good observer[ ] states that a general absence of fat, { } a thin and elongated neck, stiff and lank hair, are the chief characteristics. the change in the nature of the hair is supposed to be caused by the dryness of the atmosphere. if immigration into the united states were now stopped, who can say that the character of the whole people would not be greatly modified in the course of two or three thousand years? the direct and definite action of changed conditions, in contradistinction to the accumulation of indefinite variations, seems to me so important that i will give a large additional body of miscellaneous facts. with plants, a considerable change of climate sometimes produces a conspicuous result. i have given in detail in the ninth chapter the most remarkable case known to me, namely, that in germany several varieties of maize brought from the hotter parts of america were transformed in the course of only two or three generations. dr. falconer informs me that he has seen the english ribston-pippin apple, a himalayan oak, prunus and pyrus, all assume in the hotter parts of india a fastigate or pyramidal habit; and this fact is the more interesting, as a chinese tropical species of pyrus naturally has this habit of growth. although in these cases the changed manner of growth seems to have been directly caused by the great heat, we know that many fastigate trees have originated in their temperate homes. in the botanic gardens of ceylon the apple-tree[ ] "sends out numerous runners under ground, which continually rise into small stems, and form a growth around the parent-tree." the varieties of the cabbage which produce heads in europe fail to do so in certain tropical countries.[ ] the _rhododendron ciliatum_ produced at kew flowers so much larger and paler-coloured than those which it bears on its native himalayan mountain, that dr. hooker[ ] would hardly have recognised the species by the flowers alone. many similar facts with respect to the colour and size of flowers could be given. the experiments of vilmorin and buckman on carrots and parsnips prove that abundant nutriment produces a definite and inheritable effect on the so-called roots, with scarcely any change in other parts of the plant. alum directly influences the colour of the flowers of the hydrangea.[ ] dryness seems generally to favour the hairyness or villosity of plants. gärtner found that hybrid verbascums became extremely woolly when grown in pots. mr. masters, on the other hand, states that the _opuntia leucotricha_ "is well clothed with beautiful white hairs when grown in a damp heat; but in a dry heat exhibits none of this peculiarity."[ ] slight variations of many kinds, not worth specifying in detail, are retained only as { } long as plants are grown in certain soils, of which sageret[ ] gives from his own experience some instances. odart, who insists strongly on the permanence of the varieties of the grape, admits[ ] that some varieties, when grown under a different climate or treated differently, vary in an extremely slight degree, as in the tint of the fruit and in the period of ripening. some authors have denied that grafting causes even the slightest difference in the scion; but there is sufficient evidence that the fruit is sometimes slightly affected in size and flavour, the leaves in duration, and the flowers in appearance.[ ] with animals there can be no doubt, from the facts given in the first chapter, that european dogs deteriorate in india, not only in their instincts but in structure; but the changes which they undergo are of such a nature, that they may be partly due to reversion to a primitive form, as in the case of feral animals. in parts of india the turkey becomes reduced in size, "with the pendulous appendage over the beak enormously developed."[ ] we have seen how soon the wild duck, when domesticated, loses its true character, from the effects of abundant or changed food, or from taking little exercise. from the direct action of a humid climate and poor pasture the horse rapidly decreases in size in the falkland islands. from information which i have received, this seems likewise to be the case to a certain extent with sheep in australia. climate definitely influences the hairy covering of animals; in the west indies a great change is produced in the fleece of sheep, in about three generations. dr. falconer states[ ] that the thibet mastiff and goat, when brought down from the himalaya to kashmir, lose their fine wool. at angora not only goats, but shepherd-dogs and cats, have fine fleecy hair, and mr. ainsworth[ ] attributes the thickness of the fleece to the severe winters, and its silky lustre to the hot summers. burnes states positively[ ] that the karakool sheep lose their peculiar black curled fleeces when removed into any other country. even within the limits of england, i have been assured that with two breeds of sheep the wool was slightly changed by the flocks being pastured in different localities.[ ] it has been asserted on good authority[ ] that horses kept during several years in the deep coal-mines of belgium become covered with velvety hair, almost like that on the mole. these cases probably stand in close relation to the natural change of coat in winter and summer. naked varieties of several domestic animals have occasionally appeared; but there is no reason to { } believe that this is in any way related to the nature of the climate to which they have been exposed.[ ] it appears at first sight probable that the increased size, the tendency to fatten, the early maturity and altered forms of our improved cattle, sheep, and pigs, have directly resulted from their abundant supply of food. this is the opinion of many competent judges, and probably is to a great extent true. but as far as form is concerned, we must not overlook the equal or more potent influence of lessened use on the limbs and lungs. we see, moreover, as far as size is concerned, that selection is apparently a more powerful agent than a large supply of food, for we can thus only account for the existence, as remarked to me by mr. blyth, of the largest and smallest breeds of sheep in the same country, of cochin-china fowls and bantams, of small tumbler and large runt pigeons, all kept together and supplied with abundant nourishment. nevertheless there can be little doubt that our domesticated animals have been modified, independently of the increased or lessened use of parts, by the conditions to which they have been subjected, without the aid of selection. for instance, prof. rütimeyer[ ] shows that the bones of all domesticated quadrupeds can be distinguished from those of wild animals by the state of their surface and general appearance. it is scarcely possible to read nathusius's excellent 'vorstudien,'[ ] and doubt that, with the highly improved races of the pig, abundant food has produced a conspicuous effect on the general form of the body, on the breadth of the head and face, and even on the teeth. nathusius rests much on the case of a purely bred berkshire pig, which when two months old became diseased in its digestive organs, and was preserved for observation until nineteen months old; at this age it had lost several characteristic features of the breed, and had acquired a long, narrow head, of large size relatively to its small body, and elongated legs. but in this case and in some others we ought not to assume that, because certain characters are lost, perhaps through reversion, under one course of treatment, therefore that they had been at first directly produced by an opposite course. in the case of the rabbit, which has become feral on the island of porto santo, we are at first strongly tempted to attribute the whole change--the greatly reduced size, the altered tints of the fur, and the loss of certain characteristic marks--to the definite action of the new conditions to which it has been exposed. but in all such cases we have to consider in addition the tendency to reversion to progenitors more or less remote, and the natural selection of the finest shades of difference. the nature of the food sometimes either definitely induces certain peculiarities, or stands in some close relation with them. pallas long ago asserted that the fat-tailed sheep of siberia degenerated and lost their enormous tails when removed from certain saline pastures; and recently { } erman[ ] states that this occurs with the kirgisian sheep when brought to orenburgh. it is well known that hemp-seed causes bullfinches and certain other birds to become black. mr. wallace has communicated to me some much more remarkable facts of the same nature. the natives of the amazonian region feed the common green parrot (_chrysotis festiva_, linn.) with the fat of large siluroid fishes, and the birds thus treated become beautifully variegated with red and yellow feathers. in the malayan archipelago, the natives of gilolo alter in an analogous manner the colours of another parrot, namely, the _lorius garrulus_, linn., and thus produce the _lori rajah_ or king-lory. these parrots in the malay islands and south america, when fed by the natives on natural vegetable food, such as rice and plantains, retain their proper colours. mr. wallace has, also, recorded[ ] a still more singular fact. "the indians (of s. america) have a curious art by which they change the colours of the feathers of many birds. they pluck out those from the part they wish to paint, and inoculate the fresh wound with the milky secretion from the skin of a small toad. the feathers grow of a brilliant yellow colour, and on being plucked out, it is said, grow again of the same colour without any fresh operation." bechstein[ ] does not entertain any doubt that seclusion from light affects, at least temporarily, the colours of cage-birds. it is well known that the shells of land-mollusca are affected by the abundance of lime in different districts. isidore geoffroy st. hilaire[ ] gives the case of _helix lactea_, which has recently been carried from spain to the south of france and to the rio plata, and in both these countries now presents a distinct appearance, but whether this has resulted from food or climate is not known. with respect to the common oyster, mr. f. buckland informs me that he can generally distinguish the shells from different districts; young oysters brought from wales and laid down in beds where "_natives_" are indigenous, in the short space of two months begin to assume the "native" character. m. costa[ ] has recorded a much more remarkable case of the same nature, namely, that young shells taken from the shores of england and placed in the mediterranean, at once altered their manner of growth and formed prominent diverging rays, like those on the shells of the proper mediterranean oyster. the same individual shell, showing both forms of growth, was exhibited before a society in paris. lastly, it is well known that caterpillars fed on different food sometimes either themselves acquire a different colour or produce moths different in colour.[ ] { } it would be travelling beyond my proper limits here to discuss how far organic beings in a state of nature are definitely modified by changed conditions. in my 'origin of species' i have given a brief abstract of the facts bearing on this point, and have shown the influence of light on the colours of birds, and of residence near the sea on the lurid tints of insects, and on the succulency of plants. mr. herbert spencer[ ] has recently discussed with much ability this whole subject on broad and general grounds. he argues, for instance, that with all animals the external and internal tissues are differently acted on by the surrounding conditions, and they invariably differ in intimate structure. so again the upper and lower surfaces of true leaves, as well as of stems and petioles, when these assume the function and occupy the position of leaves, are differently circumstanced with respect to light, &c., and apparently in consequence differ in structure. but, as mr. herbert spencer admits, it is most difficult in all such cases to distinguish between the effects of the definite action of physical conditions and the accumulation through natural selection of inherited variations which are serviceable to the organism, and which have arisen independently of the definite action of these conditions. although we are not here concerned with organic beings in a state of nature, yet i may call attention to one case. mr. meehan,[ ] in a remarkable paper, compares twenty-nine kinds of american trees, belonging to various orders, with their nearest european allies, all grown in close proximity in the same garden and under as nearly as possible the same conditions. in the american species mr. meehan finds, with the rarest exceptions, that the leaves fall earlier in the season, and assume before falling a brighter tint; that they are less deeply toothed or serrated; that the buds are smaller; that the trees are more diffuse in growth and have fewer branchlets; and, lastly, that the seeds are smaller--all in comparison with the corresponding european species. now, considering that these trees belong to distinct orders, it is out of the question that the peculiarities just specified should have been inherited in the one continent from one progenitor, and in the other from another progenitor; and considering that the trees inhabit widely different stations, these peculiarities can hardly be supposed to be of any special { } service to the two series in the old and new worlds; therefore these peculiarities cannot have been naturally selected. hence we are led to infer that they have been definitely caused by the long-continued action of the different climate of the two continents on the trees. _galls._--another class of facts, not relating to cultivated plants, deserves attention. i allude to the production of galls. every one knows the curious, bright-red, hairy productions on the wild rose-tree, and the various different galls produced by the oak. some of the latter resemble fruit, with one face as rosy as the rosiest apple. these bright colours can be of no service either to the gall-forming insect or to the tree, and probably are the direct result of the action of the light, in the same manner as the apples of nova scotia or canada are brighter coloured than english apples. the strongest upholder of the doctrine that organic beings are created beautiful to please mankind would not, i presume, extend this view to galls. according to osten sacken's latest revision, no less than fifty-eight kinds of galls are produced on the several species of oak, by cynips with its sub-genera; and mr. b. d. walsh[ ] states that he can add many others to the list. one american species of willow, the _salix humilis_, bears ten distinct kinds of galls. the leaves which spring from the galls of various english willows differ completely in shape from the natural leaves. the young shoots of junipers and firs, when punctured by certain insects, yield monstrous growths like flowers and cones; and the flowers of some plants become from the same cause wholly changed in appearance. galls are produced in every quarter of the world; of several sent to me by mr. thwaites from ceylon, some were as symmetrical as a composite flower when in bud, others smooth and spherical like a berry; some protected by long spines, others clothed with yellow wool formed of long cellular hairs, others with regularly tufted hairs. in some galls the internal structure is simple, but in others it is highly complex; thus m. lucaze-duthiers[ ] has figured in the common ink-gall no less than seven concentric layers, composed of distinct tissue, { } namely, the epidermic, sub-epidermic, spongy, intermediate, and the hard protective layer formed of curiously thickened woody cells, and, lastly, the central mass abounding with starch-granules on which the larvæ feed. galls are produced by insects of various orders, but the greater number by species of cynips. it is impossible to read m. lucaze-duthier's discussion and doubt that the poisonous secretion of the insect causes the growth of the gall, and every one knows how virulent is the poison secreted by wasps and bees, which belong to the same order with cynips. galls grow with extraordinary rapidity, and it is said that they attain their full size in a few days;[ ] it is certain that they are almost completely developed before the larvæ are hatched. considering that many gall-insects are extremely small, the drop of secreted poison must be excessively minute; it probably acts on one or two cells alone, which, being abnormally stimulated, rapidly increase by a process of self-division. galls, as mr. walsh[ ] remarks, afford good, constant, and definite characters, each kind keeping as true to form as does any independent organic being. this fact becomes still more remarkable when we hear that, for instance, seven out of the ten different kinds of galls produced on _salix humilis_ are formed by gall-gnats (_cecidomyidæ_) which, "though essentially distinct species, yet resemble one another so closely that in almost all cases it is difficult, and in some cases impossible, to distinguish the full-grown insects one from the other."[ ] for in accordance with a wide-spread analogy we may safely infer that the poison secreted by insects so closely allied would not differ much in nature; yet this slight difference is sufficient to induce widely different results. in some few cases the same species of gall-gnat produces on distinct species of willows galls which cannot be distinguished; the _cynips fecundatrix_, also, has been known to produce on the turkish oak, to which it is not properly attached, exactly the same kind of gall as on the european oak.[ ] these latter facts apparently prove that the nature of the poison is a much more powerful { } agent in determining the form of the gall than the specific character of the tree which is acted on. as the poisonous secretion of insects belonging to various orders has the special power of affecting the growth of various plants;--as a slight difference in the nature of the poison suffices to produce widely different results;--and lastly, as we know that the chemical compounds secreted by plants are eminently liable to be modified by changed conditions of life, we may believe it possible that various parts of a plant might be modified through the agency of its own altered secretions. compare, for instance, the mossy and viscid calyx of a moss-rose, which suddenly appears through bud-variation on a provence-rose, with the gall of red moss growing from the inoculated leaf of a wild rose, with each filament symmetrically branched like a microscopical spruce-fir, bearing a glandular tip and secreting odoriferous gummy matter.[ ] or compare, on the one hand, the fruit of the peach, with its hairy skin, fleshy covering, hard shell and kernel, and on the other hand one of the more complex galls with its epidermic, spongy, and woody layers, surrounding tissue loaded with starch granules. these normal and abnormal structures manifestly present a certain degree of resemblance. or, again, reflect on the cases above given of parrots which have had their plumage brightly decorated through some change in their blood, caused by having been fed on certain fishes, or locally inoculated with the poison of a toad. i am far from wishing to maintain that the moss-rose or the hard shell of the peach-stone or the bright colours of birds are actually due to any chemical change in the sap or blood; but these cases of galls and of parrots are excellently adapted to show us how powerfully and singularly external agencies may affect structure. with such facts before us, we need feel no surprise at the appearance of any modification in any organic being. i may, also, here allude to the remarkable effects which parasitic fungi sometimes produce on plants. reissek[ ] has described a thesium, affected by an oecidium, which was greatly modified, and assumed some of the { } characteristic features of certain allied species, or even genera. suppose, says reissek, "the condition originally caused by the fungus to become constant in the course of time, the plant would, if found growing wild, be considered as a distinct species or even as belonging to a new genus." i quote this remark to show how profoundly, yet in how natural a manner, this plant must have been modified by the parasitic fungus. _facts and considerations opposed to the belief that the conditions of life act in a potent manner in causing definite modifications of structure._ i have alluded to the slight differences in species when naturally living in distinct countries under different conditions; and such differences we feel at first inclined, probably to a limited extent with justice, to attribute to the definite action of the surrounding conditions. but it must be borne in mind that there are a far greater number of animals and plants which range widely and have been exposed to great diversities of conditions, yet remain nearly uniform in character. some authors, as previously remarked, account for the varieties of our culinary and agricultural plants by the definite action of the conditions to which they have been exposed in the different parts of great britain; but there are about plants[ ] which are found in every single english county; these plants must have been exposed for an immense period to considerable differences of climate and soil, yet do not differ. so, again, some birds, insects, other animals, and plants range over large portions of the world, yet retain the same character. notwithstanding the facts previously given on the occurrence of highly peculiar local diseases and on the strange modifications of structure in plants caused by the inoculated poison of insects, and other analogous cases; still there are a multitude of variations--such as the modified skull of the niata ox and bulldog, the long horns of caffre cattle, the conjoined toes of the solid-hoofed swine, the immense crest and protuberant skull of polish fowls, the crop of the pouter-pigeon, and a host of other such cases--which we can hardly attribute to the definite action, in the sense before specified, of the external conditions of life. no doubt in every case there must have been some exciting cause; but as we see innumerable individuals exposed to nearly the same conditions, and one alone is affected, we may conclude that the constitution of the individual is of far higher { } importance than the conditions to which it has been exposed. it seems, indeed, to be a general rule that conspicuous variations occur rarely, and in one individual alone out of many thousands, though all may have been exposed, as far as we can judge, to nearly the same conditions. as the most strongly marked variations graduate insensibly into the most trifling, we are led by the same train of thought to attribute each slight variation much more to innate differences of constitution, however caused, than to the definite action of the surrounding conditions. we are led to the same conclusion by considering the cases, formerly alluded to, of fowls and pigeons, which have varied and will no doubt go on varying in directly opposite ways, though kept during many generations under nearly the same conditions. some, for instance, are born with their beaks, wings, tails, legs, &c., a little longer, and others with these same parts a little shorter. by the long-continued selection of such slight individual differences, which occur in birds kept in the same aviary, widely different races could certainly be formed; and long-continued selection, important as is the result, does nothing but preserve the variations which appear to us to arise spontaneously. in these cases we see that domesticated animals vary in an indefinite number of particulars, though treated as uniformly as is possible. on the other hand, there are instances of animals and plants, which, though exposed to very different conditions, both under nature and domestication, have varied in nearly the same manner. mr. layard informs me that he has observed amongst the caffres of south africa a dog singularly like an arctic esquimaux dog. pigeons in india present nearly the same wide diversities of colour as in europe; and i have seen chequered and simply barred pigeons, and pigeons with blue and white loins, from sierra leone, madeira, england, and india. new varieties of flowers are continually raised in different parts of great britain, but many of these are found by the judges at our exhibitions to be almost identical with old varieties. a vast number of new fruit-trees and culinary vegetables have been produced in north america: these differ from european varieties in the same general manner as the several varieties raised in europe differ from each other; and no one has ever pretended that the climate of america has given to the many american varieties any general character by which they can be recognised. nevertheless, from the facts previously advanced on the authority of mr. meehan with respect to american and european forest-trees, it would be rash to affirm that varieties raised in the two countries would not in the course of ages assume a distinctive character. mr. masters has recorded a striking fact[ ] bearing on this subject: he raised numerous plants of _hybiscus syriacus_ from seed collected in south carolina and the holy land, where the parent-plants must have been exposed to considerably different conditions; yet the seedlings from both localities broke into two similar strains, one with obtuse leaves and purple or crimson flowers, and the other with elongated leaves and more or less pink flowers. { } we may, also, infer the prepotent influence of the constitution of the organism over the definite action of the conditions of life, from the several cases given in the earlier chapters of parallel series of varieties,--an important subject, hereafter to be more fully discussed. sub-varieties of the several kinds of wheat, gourds, peaches, and other plants, and to a certain limited extent sub-varieties of the fowl, pigeon, and dog, have been shown either to resemble or to differ from each other in a closely corresponding and parallel manner. in other cases, a variety of one species resembles a distinct species; or the varieties of two distinct species resemble each other. although these parallel resemblances no doubt often result from reversion to the former characters of a common progenitor; yet in other cases, when new characters first appear, the resemblance must be attributed to the inheritance of a similar constitution, and consequently to a tendency to vary in the same manner. we see something of a similar kind in the same monstrosity appearing and reappearing many times in the same animal, and, as dr. maxwell masters has remarked to me, in the same plant. we may at least conclude thus far, that the amount of modification which animals and plants have undergone under domestication, does not correspond with the degree to which they have been subjected to changed circumstances. as we know the parentage of domesticated birds far better than of most quadrupeds, we will glance through the list. the pigeon has varied in europe more than almost any other bird; yet it is a native species, and has not been exposed to any extraordinary change of conditions. the fowl has varied equally, or almost equally, with the pigeon, and is a native of the hot jungles of india. neither the peacock, a native of the same country, nor the guinea-fowl, an inhabitant of the dry deserts of africa, has varied at all, or only in colour. the turkey, from mexico, has varied but little. the duck, on the other hand, a native of europe, has yielded some well-marked races; and as this is an aquatic bird, it must have been subjected to a far more serious change in its habits than the pigeon or even the fowl, which nevertheless have varied in a much higher degree. the goose, a native of europe and aquatic like the duck, has varied less than any other domesticated bird, except the peacock. bud-variation is, also, important under our present point of view. in some few cases, as when all the eyes or buds on the same tuber of the potato, or all the fruit on the same plum-tree, or all the flowers on the same plant, have suddenly varied in the same manner, it might be argued that the { } variation had been definitely caused by some change in the conditions to which the plants had been exposed; yet, in other cases, such an admission is extremely difficult. as new characters sometimes appear by bud-variation, which do not occur in the parent-species or in any allied species, we may reject, at least in these cases, the idea that they are due to reversion. now it is well worth while to reflect maturely on some striking case of bud-variation, for instance that of the peach. this tree has been cultivated by the million in various parts of the world, has been treated differently, grown on its own roots and grafted on various stocks, planted as a standard, against a wall, and under glass; yet each bud of each sub-variety keeps true to its kind. but occasionally, at long intervals of time, a tree in england, or under the widely-different climate of virginia, produces a single bud, and this yields a branch which ever afterwards bears nectarines. nectarines differ, as every one knows, from peaches in their smoothness, size, and flavour; and the difference is so great, that some botanists have maintained that they are specifically distinct. so permanent are the characters thus suddenly acquired, that a nectarine produced by bud-variation has propagated itself by seed. to guard against the supposition that there is some fundamental distinction between bud and seminal variation, it is well to bear in mind that nectarines have likewise been produced from the stone of the peach; and, reversely, peaches from the stone of the nectarine. now is it possible to conceive external conditions more closely alike than those to which the buds on the same tree are exposed? yet one bud alone, out of the many thousands borne by the same tree, has suddenly without any apparent cause produced a nectarine. but the case is even stronger than this, for the same flower-bud has yielded a fruit, one-half or one-quarter a nectarine, and the other half or three-quarters a peach. again, seven or eight varieties of the peach have yielded by bud-variation nectarines: the nectarines thus produced, no doubt, differ a little from each other; but still they are nectarines. of course there must be some cause, internal or external, to excite the peach-bud to change its nature; but i cannot imagine a class of facts better adapted to force on our minds the conviction that what we call the external conditions of life are quite insignificant in { } relation to any particular variation, in comparison with the organisation or constitution of the being which varies. it is known from the labours of geoffroy st. hilaire, and recently from those of dareste and others, that eggs of the fowl, if shaken, placed upright, perforated, covered in part with varnish, &c., produce monstrous chickens. now these monstrosities may be said to be directly caused by such unnatural conditions, but the modifications thus induced are not of a definite nature. an excellent observer, m. camille dareste,[ ] remarks "that the various species of monstrosities are not determined by specific causes; the external agencies which modify the development of the embryo act solely in causing a perturbation--a perversion in the normal course of development." he compares the result to what we see in illness: a sudden chill, for instance, affects one individual alone out of many, causing either a cold, or sore-throat, rheumatism, or inflammation of the lungs or pleura. contagious matter acts in an analogous manner.[ ] we may take a still more specific instance: seven pigeons were struck by rattle-snakes;[ ] some suffered from convulsions; some had their blood coagulated, in others it was perfectly fluid; some showed ecchymosed spots on the heart, others on the intestines, &c.; others again showed no visible lesion in any organ. it is well known that excess in drinking causes different diseases in different men; but men living under a cold and tropical climate are differently affected:[ ] and in this case we see the definite influence of opposite conditions. the foregoing facts apparently give us as good an idea as we are likely for a long time to obtain, how in many cases external conditions act directly, though not definitely, in causing modifications of structure. * * * * * _summary._--there can be no doubt, from the facts given in the early part of this chapter, that extremely slight changes in { } the conditions of life sometimes act in a definite manner on our already variable domesticated productions; and, as the action of changed conditions in causing general or indefinite variability is accumulative, so it may be with their definite action. hence it is possible that great and definite modifications of structure may result from altered conditions acting during a long series of generations. in some few instances a marked effect has been produced quickly on all, or nearly all, the individuals which have been exposed to some considerable change of climate, food, or other circumstance. this has occurred, and is now occurring, with european men in the united states, with european dogs in india, with horses in the falkland islands, apparently with various animals at angora, with foreign oysters in the mediterranean, and with maize grown in europe from tropical seed. we have seen that the chemical compounds secreted by plants and the state of their tissues are readily affected by changed conditions. in some cases a relation apparently exists between certain characters and certain conditions, so that if the latter be changed the character is lost--as with cultivated flowers, with some few culinary plants, with the fruit of the melon, with fat-tailed sheep, and other sheep having peculiar fleeces. the production of galls, and the change of plumage in parrots when fed on peculiar food or when inoculated by the poison of a toad, prove to us what great and mysterious changes in structure and colour may be the definite result of chemical changes in the nutrient fluids or tissues. we have also reason to believe that organic beings in a state of nature may be modified in various definite ways by the conditions to which they have been long exposed, as in the case of american trees in comparison with their representatives in europe. but in all such cases it is most difficult to distinguish between the definite results of changed conditions, and the accumulation through natural selection of serviceable variations which have arisen independently of the nature of the conditions. if, for instance, a plant had to be modified so as to become fitted to inhabit a humid instead of an arid station, we have no reason to believe that variations of the right kind would occur more frequently if the parent-plant inhabited a station a little more { } humid than usual. whether the station was unusually dry or humid, variations adapting the plant in a slight degree for directly opposite habits of life would occasionally arise, as we have reason to believe from what we know in other cases. in most, perhaps in all cases, the organisation or constitution of the being which is acted on, is a much more important element than the nature of the changed conditions, in determining the nature of the variation. we have evidence of this in the appearance of nearly similar modifications under different conditions, and of different modifications under apparently nearly the same conditions. we have still better evidence of this in closely parallel varieties being frequently produced from distinct races, or even distinct species, and in the frequent recurrence of the same monstrosity in the same species. we have also seen that the degree to which domesticated birds have varied, does not stand in any close relation with the amount of change to which they have been subjected. to recur once again to bud-variations. when we reflect on the millions of buds which many trees have produced, before some one bud has varied, we are lost in wonder what the precise cause of each variation can be. let us recall the case given by andrew knight of the forty-year-old tree of the yellow magnum bonum plum, an old variety which has been propagated by grafts on various stocks for a very long period throughout europe and north america, and on which a single bud suddenly produced the red magnum bonum. we should also bear in mind that distinct varieties, and even distinct species,--as in the case of peaches, nectarines, and apricots,--of certain roses and camellias,--although separated by a vast number of generations from any progenitor in common, and although cultivated under diversified conditions, have yielded by bud-variation closely analogous varieties. when we reflect on these facts we become deeply impressed with the conviction that in such cases the nature of the variation depends but little on the conditions to which the plant has been exposed, and not in any especial manner on its individual character, but much more on the general nature or constitution, inherited from some remote progenitor, of the whole group of allied beings to which the plant belongs. we are thus driven to conclude that in most { } cases the conditions of life play a subordinate part in causing any particular modification; like that which a spark plays, when a mass of combustibles bursts into flame--the nature of the flame depending on the combustible matter, and not on the spark. no doubt each slight variation must have its efficient cause; but it is as hopeless an attempt to discover the cause of each as to say why a chill or a poison affects one man differently from another. even with modifications resulting from the definite action of the conditions of life, when all or nearly all the individuals, which have been similarly exposed, are similarly affected, we can rarely see the precise relation between cause and effect. in the next chapter it will be shown that the increased use or disuse of various organs, produces an inherited effect. it will further be seen that certain variations are bound together by correlation and other laws. beyond this we cannot at present explain either the causes or manner of action of variation. finally, as indefinite and almost illimitable variability is the usual result of domestication and cultivation, with the same part or organ varying in different individuals in different or even in directly opposite ways; and as the same variation, if strongly pronounced, usually recurs only after long intervals of time, any particular variation would generally be lost by crossing, reversion, and the accidental destruction of the varying individuals, unless carefully preserved by man. hence, although it must be admitted that new conditions of life do sometimes definitely affect organic beings, it may be doubted whether well-marked races have often been produced by the direct action of changed conditions without the aid of selection either by man or nature. * * * * * { } chapter xxiv. laws of variation--use and disuse, etc. nisus formativus, or the co-ordinating power of the organisation--on the effects of the increased use and disuse of organs--changed habits of life--acclimatisation with animals and plants--various methods by which this can be effected--arrests of development--rudimentary organs. in this and the two following chapters i shall discuss, as well as the difficulty of the subject permits, the several laws which govern variability. these may be grouped under the effects of use and disuse, including changed habits and acclimatisation--arrests of development--correlated variation--the cohesion of homologous parts--the variability of multiple parts--compensation of growth--the position of buds with respect to the axis of the plant--and lastly, analogous variation. these several subjects so graduate into each other that their distinction is often arbitrary. it may be convenient first briefly to discuss that co-ordinating and reparative power which is common, in a higher or lower degree, to all organic beings, and which was formerly designated by physiologists as the _nisus formativus_. blumenbach and others[ ] have insisted that the principle which permits a hydra, when cut into fragments, to develop itself into two or more perfect animals, is the same with that which causes a wound in the higher animals to heal by a cicatrice. such cases as that of the hydra are evidently analogous with the spontaneous division or fissiparous generation of the lowest animals, and likewise with the budding of plants. between these extreme cases and that of a mere cicatrice we have every gradation. spallanzani,[ ] by cutting off the legs and tail of a salamander, got in the course of three months six crops of these members; so that perfect bones were reproduced by one animal during one season. at whatever { } point the limb was cut off, the deficient part, and no more, was exactly reproduced. even with man, as we have seen in the twelfth chapter, when treating of polydactylism, the entire limb whilst in an embryonic state, and supernumerary digits, are occasionally, though imperfectly, reproduced after amputation. when a diseased bone has been removed, a new one sometimes "gradually assumes the regular form, and all the attachments of muscles, ligaments, &c., become as complete as before."[ ] this power of regrowth does not, however, always act perfectly: the reproduced tail of a lizard differs in the forms of the scales from the normal tail: with certain orthopterous insects the large hind legs are reproduced of smaller size:[ ] the white cicatrice which in the higher animals unites the edges of a deep wound is not formed of perfect skin, for elastic tissue is not produced till long afterwards.[ ] "the activity of the _nisus formativus_," says blumenbach, "is in an inverse ratio to the age of the organised body." to this may be added that its power is greater in animals the lower they are in the scale of organisation; and animals low in the scale correspond with the embryos of higher animals belonging to the same class. newport's observations[ ] afford a good illustration of this fact, for he found that "myriapods, whose highest development scarcely carries them beyond the larvæ of perfect insects, can regenerate limbs and antennæ up to the time of their last moult;" and so can the larvæ of true insects, but not the mature insect. salamanders correspond in development with the tadpoles or larvæ of the tailless batrachians, and both possess to a large extent the power of regrowth; but not so the mature tailless batrachians. absorption often plays an important part in the repairs of injuries. when a bone is broken, and does not unite, the ends are absorbed and rounded, so that a false joint is formed; or if the ends unite, but overlap, the projecting parts are removed.[ ] but absorption comes into action, as virchow remarks, during the normal growth of bones; parts which are solid during youth become hollowed out for the medullary tissue as the bone increases in size. in trying to understand the many well-adapted cases of regrowth when aided by absorption, we should remember that most parts of the organisation, even whilst retaining the same form, undergo constant renewal; so that a part which was not renewed would naturally be liable to complete absorption. some cases, usually classed under the so-called _nisus formativus_, at first appear to come under a distinct head; for not only are old structures reproduced, but structures which appear new are formed. thus, after inflammation "false membranes," furnished with blood-vessels, lymphatics, and nerves, are developed; or a foetus escapes from the fallopian tubes, and falls into the abdomen, "nature pours out a quantity of plastic lymph, which forms itself into organised membrane, richly supplied with blood-vessels," and the foetus is nourished for a time. in certain cases of { } hydrocephalus the open and dangerous spaces in the skull are filled up with new bones, which interlock by perfect serrated sutures.[ ] but most physiologists, especially on the continent, have now given up the belief in plastic lymph or blastema, and virchow[ ] maintains that every structure, new or old, is formed by the proliferation of pre-existing cells. on this view false membranes, like cancerous or other tumours, are merely abnormal developments of normal growths; and we can thus understand how it is that they resemble adjoining structures; for instance, that "false membrane in the serous cavities acquires a covering of epithelium exactly like that which covers the original serous membrane; adhesions of the iris may become black apparently from the production of pigment-cells like those of the uvea."[ ] no doubt the power of reparation, though not always quite perfect, is an admirable provision, ready for various emergencies, even for those which occur only at long intervals of time.[ ] yet this power is not more wonderful than the growth and development of every single creature, more especially of those which are propagated by fissiparous generation. this subject has been here noticed, because we may infer that, when any part or organ is either greatly increased in size or wholly suppressed through variation and continued selection, the co-ordinating power of the organisation will continually tend to bring all the parts again into harmony with each other. _on the effects of the increased use and disuse of organs._ it is notorious, and we shall immediately adduce proofs, that increased use or action strengthens muscles, glands, sense-organs, &c.; and that disuse, on the other hand, weakens them. i have not met with any clear explanation of this fact in works on physiology. mr. herbert spencer[ ] maintains that when muscles are much used, or when intermittent pressure is applied to the epidermis, an excess of nutritive matter exudes from the vessels, and that this gives additional development to the adjoining parts. that an increased flow of blood towards an organ leads to its greater development is probable, if not certain. mr. paget[ ] thus accounts for the long, thick, and dark-coloured hair which occasionally grows, even in young children, near old-standing inflamed surfaces or fractured bones. when hunter { } inserted the spur of a cock into the comb, which is well supplied with blood-vessels, it grew in one case in a spiral direction to a length of six inches, and in another case forward, like a horn, so that the bird could not touch the ground with its beak. but whether mr. herbert spencer's view of the exudation of nutritive matter due to increased movement and pressure, will fully account for the augmented size of bones, ligaments, and especially of internal glands and nerves, seems doubtful. according to the interesting observations of m. sedillot,[ ] when a portion of one bone of the leg or fore-arm of an animal is removed and is not replaced by growth, the associated bone enlarges till it attains a bulk equal to that of the two bones, of which it has to perform the functions. this is best exhibited in dogs in which the tibia has been removed; the companion bone, which is naturally almost filiform and not one-fifth the size of the other, soon acquires a size equal to or greater than the tibia. now, it is at first difficult to believe that increased weight acting on a straight bone could, by alternately increased and diminished pressure, cause nutritive matter to exude from the vessels which permeate the periosteum. nevertheless, the observations adduced by mr. spencer,[ ] on the strengthening of the bowed bones of rickety children, along their concave sides, leads to the belief that this is possible. mr. h. spencer has also shown that the ascent of the sap in trees is aided by the rocking movement caused by the wind; and the sap strengthens the trunk "in proportion to the stress to be borne; since the more severe and the more repeated the strains, the greater must be the exudation from the vessels into the surrounding tissue, and the greater the thickening of this tissue by secondary deposits."[ ] but woody trunks may be formed of hard tissue without their having been subjected to any movement, as we see with ivy closely attached to old walls. in all these cases, it is very difficult to disentangle the effects of long-continued selection from those consequent on the increased action or movement of the part. mr. h. spencer[ ] acknowledges this difficulty, and gives as an instance the spines { } or thorns of trees, and the shells of nuts. here we have extremely hard woody tissue without the possibility of any movement to cause exudation, and without, as far as we can see, any other directly exciting cause; and as the hardness of these parts is of manifest service to the plant, we may look at the result as probably due to the selection of so-called spontaneous variations. every one knows that hard work thickens the epidermis on the hands; and when we hear that with infants long before their birth the epidermis is thicker on the palms and soles of the feet than on any other part of the body, as was observed with admiration by albinus,[ ] we are naturally inclined to attribute this to the inherited effects of long-continued use or pressure. we are tempted to extend the same view even to the hoofs of quadrupeds; but who will pretend to determine how far natural selection may have aided in the formation of structures of such obvious importance to the animal? that use strengthens the muscles may be seen in the limbs of artisans who follow different trades; and when a muscle is strengthened, the tendons, and the crests of bone to which they are attached, become enlarged; and this must likewise be the case with the blood-vessels and nerves. on the other hand, when a limb is not used, as by eastern fanatics, or when the nerve supplying it with nervous power is effectually destroyed, the muscles wither. so again, when the eye is destroyed the optic nerve becomes atrophied, sometimes even in the course of a few months.[ ] the proteus is furnished with branchiæ as well as with lungs: and schreibers[ ] found that when the animal was compelled to live in deep water the branchiæ were developed to thrice their ordinary size, and the lungs were partially atrophied. when, on the other hand, the animal was compelled to live in shallow water, the lungs became larger and more vascular, whilst the branchiæ disappeared in a more or less complete degree. such modifications as these are, however, of comparatively little value for us, as we do not actually know that they tend to be inherited. in many cases there is reason to believe that the lessened use of various organs has affected the corresponding parts in the offspring. but there is no good evidence that this ever follows in the course of a single generation. it appears, as in the case of general or indefinite variability, that several generations must be subjected to changed habits for any appreciable result. our domestic fowls, ducks, and geese have almost lost, not { } only in the individual but in the race, their power of flight; for we do not see a chicken, when frightened, take flight like a young pheasant. hence i was led carefully to compare the limb-bones of fowls, ducks, pigeons, and rabbits, with the same bones in the wild parent-species. as the measurements and weights were fully given in the earlier chapters, i need here only recapitulate the results. with domestic pigeons, the length of the sternum, the prominence of its crest, the length of the scapulæ and furcula, the length of the wings as measured from tip to tip of the radius, are all reduced relatively to the same parts in the wild pigeon. the wing and tail feathers, however, are increased in length, but this may have as little connection with the use of the wings or tail, as the lengthened hair on a dog with the amount of exercise which the breed has habitually taken. the feet of pigeons, except in the long-beaked races, are reduced in size. with fowls the crest of the sternum is less prominent, and is often distorted or monstrous; the wing-bones have become lighter relatively to the leg-bones, and are apparently a little shorter in comparison with those of the parent-form, the _gallus bankiva_. with ducks, the crest of the sternum is affected in the same manner as in the foregoing cases: the furcula, coracoids, and scapulæ are all reduced in weight relatively to the whole skeleton: the bones of the wings are shorter and lighter, and the bones of the legs longer and heavier, relatively to each other, and relatively to the whole skeleton, in comparison with the same bones in the wild-duck. the decreased weight and size of the bones, in the foregoing cases, is probably the indirect result of the reaction of the weakened muscles on the bones. i failed to compare the feathers of the wings of the tame and wild duck; but gloger[ ] asserts that in the wild duck the tips of the wing-feathers reach almost to the end of the tail, whilst in the domestic duck they often hardly reach to its base. he remarks, also, on the greater thickness of the legs, and says that the swimming membrane between the toes is reduced; but i was not able to detect this latter difference. with the domesticated rabbit the body, together with the whole skeleton, is generally larger and heavier than in the wild animal, and the leg-bones are heavier in due proportion; but whatever standard of comparison be taken, neither the leg-bones nor the scapulæ have increased in length proportionally with the increased dimensions of the rest of the skeleton. the skull has become in a marked manner narrower, and, from the measurements of its capacity formerly given, we may conclude, that this narrowness results from the decreased size of the brain, consequent on the mentally inactive life led by these closely-confined animals. we have seen in the eighth chapter that silk-moths, which have been kept during many centuries closely confined, emerge from their cocoons with their wings distorted, incapable of flight, often greatly reduced in size, or even, according to quatrefages, quite rudimentary. this condition of the wings may be largely owing to the same kind of monstrosity which often affects wild lepidoptera when artificially reared from the cocoon; or it may { } be in part due to an inherent tendency, which is common to the females of many bombycidæ, to have their wings in a more or less rudimentary state; but part of the effect may probably be attributed to long-continued disuse. from the foregoing facts there can be no doubt that certain parts of the skeleton in our anciently domesticated animals, have been modified in length and weight by the effects of decreased or increased use; but they have not been modified, as shown in the earlier chapters, in shape or structure. we must, however, be cautious in extending this latter conclusion to animals living a free life; for these will occasionally be exposed during successive generations to the severest competition. with wild animals it would be an advantage in the struggle for life that every superfluous and useless detail of structure should be removed or absorbed; and thus the reduced bones might ultimately become changed in structure. with highly-fed domesticated animals, on the other hand, there is no economy of growth; nor any tendency to the elimination of trifling and superfluous details of structure. turning now to more general observations, nathusius has shown that, with the improved races of the pig, the shortened legs and snout, the form of the articular condyles of the occiput, and the position of the jaws with the upper canine teeth projecting in a most anomalous manner in front of the lower canines, may be attributed to these parts not having been fully exercised. for the highly-cultivated races do not travel in search of food, nor root up the ground with their ringed muzzles. these modifications of structure, which are all strictly inherited, characterise several improved breeds, so that they cannot have been derived from any single domestic or wild stock.[ ] with respect to cattle, professor tanner has remarked that the lungs and liver in the improved breeds "are found to be considerably reduced in size when compared with those possessed by animals having perfect liberty;"[ ] and the reduction of these organs affects the general shape of the body. the cause of the reduced lungs in highly-bred animals which take little exercise is { } obvious; and perhaps the liver may be affected by the nutritious and artificial food on which they largely subsist. it is well known that, when an artery is tied, the anastomosing branches, from being forced to transmit more blood, increase in diameter; and this increase cannot be accounted for by mere extension, as their coats gain in strength. mr. herbert spencer[ ] has argued that with plants the flow of sap from the point of supply to the growing part first elongates the cells in this line; and that the cells then become confluent, thus forming the ducts; so that, on this view, the vessels in plants are formed by the mutual reaction of the flowing sap and cellular tissue. dr. w. turner has remarked,[ ] with respect to the branches of arteries, and likewise to a certain extent with nerves, that the great principle of compensation frequently comes into play; for "when two nerves pass to adjacent cutaneous areas, an inverse relation as regards size may subsist between them; a deficiency in one may be supplied by an increase in the other, and thus the area of the former may be trespassed on by the latter nerve." but how far in these cases the difference in size in the nerves and arteries is due to original variation, and how far to increased use or action, is not clear. in reference to glands, mr. paget observes that "when one kidney is destroyed the other often becomes much larger, and does double work."[ ] if we compare the size of the udders and their power of secretion in cows which have been long domesticated, and in certain goats in which the udders nearly touch the ground, with the size and power of secretion of these organs in wild or half-domesticated animals, the difference is great. a good cow with us daily yields more than five gallons, or forty pints of milk, whilst a first-rate animal, kept, for instance, by the damaras of south africa,[ ] "rarely gives more than two or three pints of milk daily, and, should her calf be taken from her, she absolutely refuses to give any." we may attribute the excellence of our cows, and of certain goats, partly to the continued selection of the best milking animals, and partly to the inherited effects of the increased action, through man's art, of the secreting glands. it is notorious, as was remarked in the twelfth chapter, that short-sight is inherited; and if we compare watchmakers or engravers with, for instance, sailors, we can hardly doubt that vision continually directed towards a near object permanently affects the structure of the eye. veterinarians are unanimous that horses become affected with spavins, splints, ringbones, &c., from being shod, and from travelling on hard roads, and they are almost equally unanimous that these injuries are transmitted. formerly horses were not shod in north carolina, and it has been asserted that they did not then suffer from these diseases of the legs and feet.[ ] { } our domesticated quadrupeds are all descended, as far as is known, from species having erect ears; yet few kinds can be named, of which at least one race has not drooping ears. cats in china, horses in parts of russia, sheep in italy and elsewhere, the guinea-pig in germany, goats and cattle in india, rabbits, pigs, and dogs in all long-civilised countries, have dependent ears. with wild animals, which constantly use their ears like funnels to catch every passing sound, and especially to ascertain the direction whence it comes, there is not, as mr. blyth has remarked, any species with drooping ears except the elephant. hence the incapacity to erect the ears is certainly in some manner the result of domestication; and this incapacity has been attributed by various authors[ ] to disuse, for animals protected by man are not compelled habitually to use their ears. col. hamilton smith[ ] states that in ancient effigies of the dog, "with the exception of one egyptian instance, no sculpture of the earlier grecian era produces representations of hounds with completely drooping ears; those with them half pendulous are missing in the most ancient; and this character increases, by degrees, in the works of the roman period." godron also has remarked that "the pigs of the ancient egyptians had not their ears enlarged and pendent."[ ] but it is remarkable that the drooping of the ears, though probably the effect of disuse, is not accompanied by any decrease in size; on the contrary, when we remember that animals so different as fancy rabbits, certain indian breeds of the goat, our petted spaniels, bloodhounds, and other dogs, have enormously elongated ears, it would appear as if disuse actually caused an increase in length. with rabbits, the drooping of the much elongated ears has affected even the structure of the skull. the tail of no wild animal, as remarked to me by mr. blyth, is curled; whereas pigs and some races of dogs have their tails much curled. this deformity, therefore, appears to be the result of domestication, but whether in any way connected with the lessened use of the tail is doubtful. { } the epidermis on our hands is easily thickened, as every one knows, by hard work. in a district of ceylon the sheep have "horny callosities that defend their knees, and which arise from their habit of kneeling down to crop the short herbage, and this distinguishes the jaffna flocks from those of other portions of the island;" but it is not stated whether this peculiarity is inherited.[ ] the mucous membrane which lines the stomach is continuous with the external skin of the body; therefore it is not surprising that its texture should be affected by the nature of the food consumed, but other and more interesting changes likewise follow. hunter long ago observed that the muscular coat of the stomach of a gull (_larus tridactylus_) which had been fed for a year chiefly on grain was thickened; and, according to dr. edmondston, a similar change periodically occurs in the shetland islands in the stomach of the _larus argentatus_, which in the spring frequents the corn-fields and feeds on the seed. the same careful observer has noticed a great change in the stomach of a raven which had been long fed on vegetable food. in the case of an owl (_strix grallaria_) similarly treated, menetries states that the form of the stomach was changed, the inner coat became leathery, and the liver increased in size. whether these modifications in the digestive organs would in the course of generations become inherited is not known.[ ] the increased or diminished length of the intestines, which apparently results from changed diet, is a more remarkable case, because it is characteristic of certain animals in their domesticated condition, and therefore must be inherited. the complex absorbent system, the blood-vessels, nerves, and muscles, are necessarily all modified together with the intestines. according to daubenton, the intestines of the domestic cat are one-third longer than those of the wild cat of europe; and although this species is not the parent-stock of the domestic animal, yet, as isidore geoffroy has remarked, the several species { } of cats are so closely allied that the comparison is probably a fair one. the increased length appears to be due to the domestic cat being less strictly carnivorous in its diet than any wild feline species; i have seen a french kitten eating vegetables as readily as meat. according to cuvier, the intestines of the domesticated pig exceed greatly in proportionate length those of the wild boar. in the tame and wild rabbit the change is of an opposite nature, and probably results from the nutritious food given to the tame rabbit.[ ] * * * * * _changed habits of life, independently of the use or disuse of particular organs._--this subject, as far as the mental powers of animals are concerned, so blends into instinct, on which i shall treat in a future work, that i will here only remind the reader of the many cases which occur under domestication, and which are familiar to every one--for instance the tameness of our animals--the pointing or retrieving of dogs--their not attacking the smaller animals kept by man--and so forth. how much of these changes ought to be attributed to inherited habit, and how much to the selection of individuals which have varied in the desired manner, irrespectively of the special circumstances under which they have been kept, can seldom be told. we have already seen that animals may be habituated to a changed diet; but a few additional instances may here be given. in the polynesian islands and in china the dog is fed exclusively on vegetable matter, and the taste for this kind of food is to a certain extent inherited.[ ] our sporting dogs will not touch the bones of game birds, whilst other dogs devour them with greediness. in some parts of the world sheep have been largely fed on fish. the domestic hog is fond of barley, the wild boar is said to disdain it; and the disdain is partially inherited, for some young wild pigs bred in captivity showed an aversion for this grain, whilst others of the same brood relished it.[ ] one of my relations bred some young pigs from { } a chinese sow by a wild alpine boar; they lived free in the park, and were so tame that they came to the house to be fed; but they would not touch swill, which was devoured by the other pigs. an animal when once accustomed to an unnatural diet, which can generally be effected only during youth, dislikes its proper food, as spallanzani found to be the case with a pigeon which had been long fed on meat. individuals of the same species take to new food with different degrees of readiness; one horse, it is stated, soon learned to eat meat, whilst another would have perished from hunger rather than have partaken of it.[ ] the caterpillars of the _bombyx hesperus_ feed in a state of nature on the leaves of the _café diable_, but, after having been reared on the ailanthus, they would not touch the _café diable_, and actually died of hunger.[ ] it has been found possible to accustom marine fish to live in fresh water; but as such changes in fish, and other marine animals, have been chiefly observed in a state of nature, they do not properly belong to our present subject. the period of gestation and of maturity, as shown in the earlier chapters,--the season and the frequency of the act of breeding,--have all been greatly modified under domestication. with the egyptian goose the rate of change in the season has been recorded.[ ] the wild drake pairs with one female, the domestic drake is polygamous. certain breeds of fowls have lost the habit of incubation. the paces of the horse, and the manner of flight in certain breeds of the pigeon, have been modified, and are inherited. the voice differs much in certain fowls and pigeons. some breeds are clamorous and others silent, as in the call and common duck, or in the spitz and pointer dog. every one knows how dogs differ from each other in their manner of hunting, and in their ardour after different kinds of game or vermin. with plants the period of vegetation is easily changed and is inherited, as in the case of summer and winter wheat, barley, { } and vetches; but to this subject we shall immediately return under acclimatisation. annual plants sometimes become perennial under a new climate, as i hear from dr. hooker is the case with the stock and mignonette in tasmania. on the other hand, perennials sometimes become annuals, as with the ricinus in england, and as, according to captain mangles, with many varieties of the heartsease. von berg[ ] raised from seed of _verbascum phoenicium_, which is usually a biennial, both annual and perennial varieties. some deciduous bushes become evergreen in hot countries.[ ] rice requires much water, but there is one variety in india which can be grown without irrigation.[ ] certain varieties of the oat and of our other cereals are best fitted for certain soils.[ ] endless similar facts could be given in the animal and vegetable kingdoms. they are noticed here because they illustrate analogous differences in closely allied natural species, and because such changed habits of life, whether due to use and disuse, or to the direct action of external conditions, or to so-called spontaneous variation, would be apt to lead to modifications of structure. * * * * * _acclimatisation._--from the previous remarks we are naturally led to the much disputed subject of acclimatisation. there are two distinct questions: do varieties descended from the same species differ in their power of living under different climates? and secondly, if they so differ, how have they become thus adapted? we have seen that european dogs do not succeed well in india, and it is asserted,[ ] that no one has succeeded in there keeping the newfoundland long alive; but then it may be argued, probably with truth, that these northern breeds are specifically distinct from the native dogs which flourish in india. the same remark may be made with respect to different breeds of sheep, of which, according to youatt,[ ] not one brought "from a torrid climate lasts out the second year," in the zoological gardens. but sheep are capable of some degree of acclimatisation, for merino sheep bred at the cape of good hope have been found { } far better adapted for india than those imported from england.[ ] it is almost certain that the breeds of the fowl are descended from the same species; but the spanish breed, which there is good reason to believe originated near the mediterranean,[ ] though so fine and vigorous in england, suffers more from frost than any other breed. the arrindy silk-moth introduced from bengal, and the ailanthus moth from the temperate province of shan tung, in china, belong to the same species, as we may infer from their identity in the caterpillar, cocoon, and mature states;[ ] yet they differ much in constitution: the indian form "will flourish only in warm latitudes," the other is quite hardy and withstands cold and rain. plants are more strictly adapted to climate than are animals. the latter when domesticated withstand such great diversities of climate, that we find nearly the same species in tropical and temperate countries; whilst the cultivated plants are widely dissimilar. hence a larger field is open for inquiry in regard to the acclimatisation of plants than of animals. it is no exaggeration to say that with almost every plant which has long been cultivated varieties exist, which are endowed with constitutions fitted for very different climates; i will select only a few of the more striking cases, as it would be tedious to give all. in north america numerous fruit-trees have been raised, and in horticultural publications,--for instance, in downing,--lists are given of the varieties which are best able to withstand the severe climate of the northern states and canada. many american varieties of the pear, plum, and peach are excellent in their own country, but until recently hardly one was known that succeeded in england; and with apples,[ ] not one succeeds. though the american varieties can withstand a severer winter than ours, the summer here is not hot enough. fruit-trees have originated in europe as in america with different constitutions, but they are not here much noticed, as the same nurserymen do not supply a wide area. the forelle pear flowers early, and when the flowers have just set, and this is the critical period, they have been observed, both in france and england, to withstand with complete impunity a frost of ° and even ° fahr., which killed the flowers, whether fully expanded or in bud, of all other kinds of pears.[ ] this power in the flower of resisting cold and afterwards producing fruit does not invariably depend, as we know on good authority,[ ] on general constitutional vigour. { } in proceeding northward, the number of varieties which are enabled to resist the climate rapidly decreases, as may be seen in the list of the varieties of the cherry, apple, and pear, which can be cultivated in the neighbourhood of stockholm.[ ] near moscow, prince troubetzkoy planted for experiment in the open ground several varieties of the pear, but one alone, the _poire sans pepins_, withstood the cold of winter.[ ] we thus see that our fruit-trees, like distinct species of the same genus, certainly differ from each other in their constitutional adaptation to different climates. with the varieties of many plants, the adaptation to climate is often very close. thus it has been proved by repeated trials "that few if any of the english varieties of wheat are adapted for cultivation in scotland;"[ ] but the failure in this case is at first only in the quantity, though ultimately in the quality, of the grain produced. the rev. j. m. berkeley sowed wheat-seed from india, and got "the most meagre ears," on land which would certainly have yielded a good crop from english wheat.[ ] in these cases varieties have been carried from a warmer to a cooler climate; in the reverse case, as "when wheat was imported directly from france into the west indian islands, it produced either wholly barren spikes or furnished with only two or three miserable seeds, while west indian seed by its side yielded an enormous harvest."[ ] here is another case of close adaptation to a slightly cooler climate; a kind of wheat which in england may be used indifferently either as a winter or summer variety, when sown under the warmer climate of grignan, in france, behaved exactly as if it had been a true winter wheat.[ ] botanists believe that all the varieties of maize belong to the same species; and we have seen that in north america, in proceeding northward, the varieties cultivated in each zone produce their flowers and ripen their seed within shorter and shorter periods. so that the tall, slowly maturing southern varieties do not succeed in new england, and the new english varieties do not succeed in canada. i have not met with any statement that the southern varieties are actually injured or killed by a degree of cold which the northern varieties withstand with impunity, though this is probable; but the production of early flowering and early seeding varieties deserves to be considered as one form of acclimatisation. hence it has been found possible, according to kalm, to cultivate maize further and further northwards in america. in europe, also, as we learn from the evidence given by alph. de candolle, the culture of maize has extended since the end of the last century thirty leagues north of its former boundary.[ ] on the authority of the great linnæus,[ ] i may quote an { } analogous case, namely, that in sweden tobacco raised from home-grown seed ripens its seed a month sooner and is less liable to miscarry than plants raised from foreign seed. with the vine, differently from the maize, the line of practical culture has retreated a little southward since the middle ages;[ ] but this seems due to commerce, including that of wine, being now freer or more easy. nevertheless the fact of the vine not having spread northward shows that acclimatisation has made no progress during several centuries. there is, however, a marked difference in the constitution of the several varieties,--some being hardy, whilst others, like the muscat of alexandria, require a very high temperature to come to perfection. according to labat,[ ] vines taken from france to the west indies succeed with extreme difficulty, whilst those imported from madeira, or the canary islands, thrive admirably. gallesio gives a curious account of the naturalisation of the orange in italy. daring many centuries the sweet orange was propagated exclusively by grafts, and so often suffered from frosts that it required protection. after the severe frost of , and more especially after that of , so many trees were destroyed that seedlings from the sweet orange were raised, and, to the surprise of the inhabitants, their fruit was found to be sweet. the trees thus raised were larger, more productive, and hardier than the former kinds; and seedlings are now continually raised. hence gallesio concludes that much more was effected for the naturalisation of the orange in italy by the accidental production of new kinds during a period of about sixty years, than had been effected by grafting old varieties during many ages.[ ] i may add that risso[ ] describes some portuguese varieties of the orange as extremely sensitive to cold, and as much tenderer than certain other varieties. the peach was known to theophrastus, b.c.[ ] according to the authorities quoted by dr. f. rolle,[ ] it was tender when first introduced into greece, and even in the island of rhodes only occasionally bore fruit. if this be correct, the peach, in spreading during the last two thousand years over the middle parts of europe, must have become much hardier. at the present day different varieties differ much in hardiness: some french varieties will not succeed in england; and near paris, the _pavie de bonneuil_ does not ripen its fruit till very late, even when grown on a wall; "it is, therefore, only fit for a very hot southern climate."[ ] i will briefly give a few other cases. a variety of _magnolia grandiflora_, raised by m. roy, withstands cold several degrees lower than that which any other variety can resist. with camellias there is much difference in hardiness. one particular variety of noisette rose withstood the severe frost of "untouched and hale amidst a universal destruction of other { } noisettes." in new york the "irish yew is quite hardy, but the common yew is liable to be cut down." i may add that there are varieties of the sweet potato (_convolvulus batatas_) which are suited for warmer, as well as for colder, climates.[ ] the plants as yet mentioned have been found capable of resisting an unusual degree of cold or heat, when fully grown. the following cases refer to plants whilst young. in a large bed of young araucarias of the same age, growing close together and equally exposed, it was observed,[ ] after the unusually severe winter of - , that, "in the midst of the dying, numerous individuals remained on which the frost had absolutely made no kind of impression." dr. lindley, after alluding to this and other similar cases, remarks, "among the lessons which the late formidable winter has taught us, is that, even in their power of resisting cold, individuals of the same species of plants are remarkably different." near salisbury, there was a sharp frost on the night of may th, , and all the french beans (_phaseolus vulgaris_) in a bed were killed except about one in thirty, which completely escaped.[ ] on the same day of the month, but in the year , there was a severe frost in kent, and two rows of scarlet-runners (_p. multiflorus_) in my garden, containing plants of the same age and equally exposed, were all blackened and killed except about a dozen plants. in an adjoining row of "fulmer's dwarf bean" (_p. vulgaris_), one single plant escaped. a still more severe frost occurred four days afterwards, and of the dozen plants which had previously escaped only three survived; these were not taller or more vigorous than the other young plants, but they escaped completely, with not even the tips of their leaves browned. it was impossible to behold these three plants, with their blackened, withered, and dead brethren all round them, and not see at a glance that they differed widely in constitutional power of resisting frost. this work is not the proper place to show that wild plants { } of the same species, naturally growing at different altitudes or under different latitudes, become to a certain extent acclimatised, as is proved by the different behaviour of their seedlings when raised in england. in my 'origin of species' i have alluded to some cases, and i could add others. one instance must suffice: mr. grigor, of forres,[ ] states that seedlings of the scotch fir (_pinus sylvestris_), raised from seed from the continent and from the forests of scotland, differ much. "the difference is perceptible in one-year-old, and more so in two-year-old seedlings; but the effects of the winter on the second year's growth almost uniformly makes those from the continent quite brown, and so damaged, that by the month of march they are quite unsaleable, while the plants from the native scotch pine, under the same treatment, and standing alongside, although considerably shorter, are rather stouter and quite green, so that the beds of the one can be known from the other when seen from the distance of a mile." closely similar facts have been observed with seedling larches. hardy varieties would alone be valued or noticed in europe; whilst tender varieties, requiring more warmth, would generally be neglected; but such occasionally arise. thus loudon[ ] describes a cornish variety of the elm which is almost an evergreen, and of which the shoots are often killed by the autumnal frosts, so that its timber is of little value. horticulturists know that some varieties are much more tender than others: thus all the varieties of the broccoli are more tender than cabbages; but there is much difference in this respect in the sub-varieties of the broccoli; the pink and purple kinds are a little hardier than the white cape broccoli, "but they are not to be depended on after the thermometer falls below ° fahr.:" the walcheren broccoli is less tender than the cape, and there are several varieties which will stand much severer cold than the walcheren.[ ] cauliflowers seed more freely in india than cabbages.[ ] to give one instance with flowers: eleven plants raised from a hollyhock, called the _queen of the whites_,[ ] were found to be much more tender than various other seedlings. it may be presumed that all tender varieties would succeed better under a climate warmer than ours. with fruit-trees, it is well known that certain varieties, for instance of the peach, stand forcing in a hot-house better than others; and this shows { } either pliability of organisation or some constitutional difference. the same individual cherry-tree, when forced, has been observed during successive years gradually to change its period of vegetation.[ ] few pelargoniums can resist the heat of a stove, but _alba multiflora_ will, as a most skilful gardener asserts, "stand pine-apple top and bottom heat the whole winter, without looking any more drawn than if it had stood in a common greenhouse; and _blanche fleur_ seems as if it had been made on purpose for growing in winter, like many bulbs, and to rest all summer."[ ] there can hardly be a doubt that the _alba multiflora_ pelargonium must have a widely different constitution from that of most other varieties of this plant; it would probably withstand even an equatorial climate. we have seen that according to labat the vine and wheat require acclimatisation in order to succeed in the west indies. similar facts have been observed at madras: "two parcels of mignonette-seed, one direct from europe, the other saved at bangalore (of which the mean temperature is much below that of madras) were sown at the same time: they both vegetated equally favourably, but the former all died off a few days after they appeared above ground; the latter still survive, and are vigorous healthy plants." so again, "turnip and carrot seed saved at hyderabad are found to answer better at madras than seed from europe or from the cape of good hope."[ ] mr. j. scott, of the calcutta botanic gardens, informs me that seeds of the sweet-pea (_lathyrus odoratus_) imported from england produce plants, with thick, rigid stems and small leaves, which rarely blossom and never yield seed; plants raised from french seed blossom sparingly, but all the flowers are sterile; on the other hand, plants raised from sweet-peas grown near darjeeling in upper india, but originally derived from england, can be successfully cultivated on the plains of india; for they flower and seed profusely, and their stems are lax and scandent. in some of the foregoing cases, as dr. hooker has remarked to me, the greater success may perhaps be attributed to the seeds having been more fully ripened under a more favourable climate; but this view can hardly be extended to so many cases, including plants, which, from being cultivated under a climate hotter than their native one, become fitted for a still hotter climate. we may therefore safely conclude that plants can to a certain extent become accustomed to a climate either hotter or colder than their own; although these latter cases have been more frequently observed. we will now consider the means by which acclimatisation may be effected, namely, through the spontaneous appearance of varieties having a different constitution, and through the effects of use or habit. in regard to the first process, there is no evidence that a change in the constitution of the { } offspring necessarily stands in any direct relation with the nature of the climate inhabited by the parents. on the contrary, it is certain that hardy and tender varieties of the same species appear in the same country. new varieties thus spontaneously arising become fitted to slightly different climates in two different ways; firstly, they may have the power, either as seedlings or when full-grown, of resisting intense cold, as with the moscow pear, or of resisting intense heat, as with some kinds of pelargonium, or the flowers may withstand severe frost, as with the forelle pear. secondly, plants may become adapted to climates widely different from their own, from flowering and fruiting either earlier or later in the season. in both these cases the power of acclimatisation by man consists simply in the selection and preservation of new varieties. but without any direct intention on his part of securing a hardier variety, acclimatisation may be unconsciously effected by merely raising tender plants from seed, and by occasionally attempting their cultivation further and further northwards, as in the case of maize, the orange, and the peach. how much influence ought to be attributed to inherited habit or custom in the acclimatisation of animals and plants is a much more difficult question. in many cases natural selection can hardly have failed to have come into play and complicated the result. it is notorious that mountain sheep resist severe weather and storms of snow which would destroy lowland breeds; but then mountain sheep have been thus exposed from time immemorial, and all delicate individuals will have been destroyed, and the hardiest preserved. so with the arrindy silk-moths of china and india; who can tell how far natural selection may have taken a share in the formation of the two races, which are now fitted for such widely different climates? it seems at first probable that the many fruit-trees, which are so well fitted for the hot summers and cold winters of north america, in contrast with their poor success under our climate, have become adapted through habit; but when we reflect on the multitude of seedlings annually raised in that country, and that none would succeed unless born with a fitting constitution, it is possible that mere habit may have done nothing towards their acclimatisation. on the other hand, when we { } hear that merino sheep, bred during no great number of generations at the cape of good hope--that some european plants raised during only a few generations in the cooler parts of india, withstand the hotter parts of that country much better than the sheep or seeds imported directly from england, we must attribute some influence to habit. we are led to the same conclusion when we hear from naudin[ ] that the races of melons, squashes, and gourds, which have long been cultivated in northern europe, are comparatively more precocious, and need much less heat for maturing their fruit, than the varieties of the same species recently brought from tropical regions. in the reciprocal conversion of summer and winter wheat, barley, and vetches into each other, habit produces a marked effect in the course of a very few generations. the same thing apparently occurs with the varieties of maize, which, when carried from the southern to the northern states of america, or into germany, soon become accustomed to their new homes. with vine-plants taken to the west indies from madeira, which are said to succeed better than plants brought directly from france, we have some degree of acclimatisation in the individual, independently of the production of new varieties by seed. the common experience of agriculturists is of some value, and they often advise persons to be cautious in trying in one country the productions of another. the ancient agricultural writers of china recommend the preservation and cultivation of the varieties peculiar to each country. during the classical period, columella wrote, "vernaculum pecus peregrino longe præstantius est."[ ] i am aware that the attempt to acclimatise either animals or plants has been called a vain chimæra. no doubt the attempt in most cases deserves to be thus called, if made independently of the production of new varieties endowed with a different constitution. habit, however much prolonged, rarely produces any effect on a plant propagated by buds; it apparently acts only through successive seminal generations. { } the laurel, bay, laurestinus, &c., and the jerusalem artichoke, which are propagated by cuttings or tubers, are probably now as tender in england as when first introduced; and this appears to be the case with the potato, which until recently was seldom multiplied by seed. with plants propagated by seed, and with animals, there will be little or no acclimatisation unless the hardier individuals are either intentionally or unconsciously preserved. the kidney-bean has often been advanced as an instance of a plant which has not become hardier since its first introduction into britain. we hear, however, on excellent authority,[ ] that some very fine seed, imported from abroad, produced plants "which blossomed most profusely, but were nearly all but abortive, whilst plants grown alongside from english seed podded abundantly;" and this apparently shows some degree of acclimatisation in our english plants. we have also seen that seedlings of the kidney-bean occasionally appear with a marked power of resisting frost; but no one, as far as i can hear, has ever separated such hardy seedlings, so as to prevent accidental crossing, and then gathered their seed, and repeated the process year after year. it may, however, be objected with truth that natural selection ought to have had a decided effect on the hardiness of our kidney-beans; for the tenderest individuals must have been killed during every severe spring, and the hardier preserved. but it should be borne in mind that the result of increased hardiness would simply be that gardeners, who are always anxious for as early a crop as possible, would sow their seed a few days earlier than formerly. now, as the period of sowing depends much on the soil and elevation of each district, and varies with the season; and as new varieties have often been imported from abroad, can we feel sure that our kidney-beans are not somewhat hardier? i have not been able, by searching old horticultural works, to answer this question satisfactorily. on the whole the facts now given show that, though habit does something towards acclimatisation, yet that the spontaneous appearance of constitutionally different individuals is a far more effective agent. as no single instance has been recorded, either with animals or plants, of hardier individuals { } having been long and steadily selected, though such selection is admitted to be indispensable for the improvement of any other character, it is not surprising that man has done little in the acclimatisation of domesticated animals and cultivated plants. we need not, however, doubt that under nature new races and new species would become adapted to widely different climates, by spontaneous variation, aided by habit, and regulated by natural selection. _arrests of development: rudimentary and aborted organs._ these subjects are here introduced because there is reason to believe that rudimentary organs are in many cases the result of disuse. modifications of structure from arrested development, so great or so serious as to deserve to be called monstrosities, are of common occurrence, but, as they differ much from any normal structure, they require here only a passing notice. when a part or organ is arrested during its embryonic growth, a rudiment is generally left. thus the whole head may be represented by a soft nipple-like projection, and the limbs by mere papillæ. these rudiments of limbs are sometimes inherited, as has been observed in a dog.[ ] many lesser anomalies in our domesticated animals appear to be due to arrested development. what the cause of the arrest may be, we seldom know, except in the case of direct injury to the embryo within the egg or womb. that the cause does not generally act at a very early embryonic period we may infer from the affected organ seldom being wholly aborted,--a rudiment being generally preserved. the external ears are represented by mere vestiges in a chinese breed of sheep; and in another breed, the tail is reduced "to a little button, suffocated, in a manner, by fat."[ ] in tailless dogs and cats a stump is left; but i do not know whether it includes at an early embryonic age rudiments of all the caudal vertebræ. in certain breeds of fowls the comb and wattles are reduced to rudiments; in the cochin-china breed scarcely more than rudiments of spurs exist. with polled suffolk cattle, "rudiments of horns can often be felt at an early age;"[ ] and with species in a state of nature, the relatively greater development of rudimentary organs at an early period of life is highly characteristic of such organs. with hornless breeds of cattle and sheep; another and singular kind of rudiment has been observed, namely, minute dangling horns attached to the skin alone, and which are often shed and grow again. with hornless goats, according to desmarest,[ ] { } the bony protuberances which properly support the horns exist as mere rudiments. with cultivated plants it is far from rare to find the petals, stamens, and pistils represented by rudiments, like those observed in natural species. so it is with the whole seed in many fruits; thus near astrakhan there is a grape with mere traces of seeds, "so small and lying so near the stalk that they are not perceived in eating the grape."[ ] in certain varieties of the gourd, the tendrils, according to naudin, are represented by rudiments or by various monstrous growths. in the broccoli and cauliflower the greater number of the flowers are incapable of expansion, and include rudimentary organs. in the feather hyacinth (_muscari comosum_) the upper and central flowers are brightly coloured but rudimentary; under cultivation the tendency to abortion travels downwards and outwards, and all the flowers become rudimentary; but the abortive stamens and pistils are not so small in the lower as in the upper flowers. in the _viburnum opulus_, on the other hand, the outer flowers naturally have their organs of fructification in a rudimentary state, and the corolla is of large size; under cultivation, the change spreads to the centre, and all the flowers become affected; thus the well-known snow-ball bush is produced. in the compositæ, the so-called doubling of the flowers consists in the greater development of the corolla of the central florets, generally accompanied with some degree of sterility; and it has been observed[ ] that the progressive doubling invariably spreads from the circumference to the centre,--that is, from the ray florets, which so often include rudimentary organs, to those of the disc. i may add, as bearing on this subject, that, with asters, seeds taken from the florets of the circumference have been found to yield the greatest number of double flowers.[ ] in these several cases we have a natural tendency in certain parts to become rudimentary, and this under culture spreads either to, or from, the axis of the plant. it deserves notice, as showing how the same laws govern the changes which natural species and artificial varieties undergo, that in a series of species in the genus carthamus, one of the compositæ, a tendency in the seeds to the abortion of the pappus may be traced extending from the circumference to the centre of the disc: thus, according to a. de jussieu,[ ] the abortion is only partial in _carthamus creticus_, but more extended in _c. lanatus_; for in this species two or three alone of the central seeds are furnished with a pappus, the surrounding seeds being either quite naked or furnished with a few hairs; and lastly, in _c. tinctorius_, even the central seeds are destitute of pappus, and the abortion is complete. with animals and plants under domestication, when an organ disappears, leaving only a rudiment, the loss has generally been sudden, as with hornless and tailless breeds; and such cases may be ranked as inherited monstrosities. but in some few cases the loss has been gradual, and { } has been partly effected by selection, as with the rudimentary combs and wattles of certain fowls. we have also seen that the wings of some domesticated birds have been slightly reduced by disuse, and the great reduction of the wings in certain silk-moths, with mere rudiments left, has probably been aided by disuse. with species in a state of nature, rudimentary organs are so extremely common that scarcely one can be named which is wholly free from a blemish of this nature. such organs are generally variable, as several naturalists have observed; for, being useless, they are not regulated by natural selection, and they are more or less liable to reversion. the same rule certainly holds good with parts which have become rudimentary under domestication. we do not know through what steps under nature rudimentary organs have passed in being reduced to their present condition; but we so incessantly see in species of the same group the finest gradations between an organ in a rudimentary and perfect state, that we are led to believe that the passage must have been extremely gradual. it may be doubted whether a change of structure so abrupt as the sudden loss of an organ would ever be of service to a species in a state of nature; for the conditions to which all organisms are closely adapted usually change very slowly. even if an organ did suddenly disappear in some one individual by an arrest of development, intercrossing with the other individuals of the same species would cause it to reappear in a more or less perfect manner, so that its final reduction could only be effected by the slow process of continued disuse or natural selection. it is much more probable that, from changed habits of life, organs first become of less and less use, and ultimately superfluous; or their place may be supplied by some other organ; and then disuse, acting on the offspring through inheritance at corresponding periods of life, would go on reducing the organ; but as most organs could be of no use at an early embryonic period, they would not be affected by disuse; consequently they would be preserved at this stage of growth, and would remain as rudiments. in addition to the effects of disuse, the principle of economy of growth, already alluded to in this chapter, would lead to the still further reduction of all superfluous parts. with respect to the final and total suppression or abortion of any organ, another and distinct principle, which will be discussed in the chapter on pangenesis, probably takes a share in the work. with animals and plants reared by man there is no severe or recurrent struggle for existence, and the principle of economy will not come into action. so far, indeed, is this from being the case, that in some instances organs, which are naturally rudimentary in the parent-species, become partially redeveloped in the domesticated descendants. thus cows, like most other ruminants, properly have four active and two rudimentary mammæ; but in our domesticated animals, the latter occasionally become considerably developed and yield milk. the atrophied mammæ, which, in male domesticated animals, including man, have in some rare cases grown to full size and secreted milk, perhaps offer an analogous case. the hind feet of dogs include rudiments of a fifth toe, and in certain large breeds these toes, though still rudimentary, become considerably developed { } and are furnished with claws. in the common hen, the spurs and comb are rudimentary, but in certain breeds these become, independently of age or disease of the ovaria, well developed. the stallion has canine teeth, but the mare has only traces of the alveoli, which, as i am informed by the eminent veterinary mr. g. t. brown, frequently contain minute irregular nodules of bone. these nodules, however, sometimes become developed into imperfect teeth, protruding through the gums and coated with enamel; and occasionally they grow to a third or even a fourth of the length of the canines in the stallion. with plants i do not know whether the redevelopment of rudimentary organs occurs more frequently under culture than under nature. perhaps the pear-tree may be a case in point, for when wild it bears thorns, which though useful as a protection are formed of branches in a rudimentary condition, but, when the tree is cultivated, the thorns are reconverted into branches. finally, though organs which must be classed as rudimentary frequently occur in our domesticated animals and cultivated plants, these have generally been formed suddenly, through an arrest of development. they usually differ in appearance from the rudiments which so frequently characterise natural species. in the latter, rudimentary organs have been slowly formed through continued disuse, acting by inheritance at a corresponding age, aided by the principle of the economy of growth, all under the control of natural selection. with domesticated animals, on the other hand, the principle of economy is far from coming into action, and their organs, although often slightly reduced by disuse, are not thus almost obliterated with mere rudiments left. * * * * * { } chapter xxv. laws of variation, _continued_--correlated variability. explanation of term--correlation as connected with development--modifications correlated with the increased or decreased size of parts--correlated variation of homologous parts--feathered feet in birds assuming the structure of the wings--correlation between the head and the extremities--between the skin and dermal appendages--between the organs of sight and hearing--correlated modifications in the organs of plants--correlated monstrosities--correlation between the skull and ears--skull and crest of feathers--skull and horns--correlation of growth complicated by the accumulated effects of natural selection--colour as correlated with constitutional peculiarities. all the parts of the organisation are to a certain extent connected or correlated together; but the connexion may be so slight that it hardly exists, as with compound animals or the buds on the same tree. even in the higher animals various parts are not at all closely related; for one part may be wholly suppressed or rendered monstrous without any other part of the body being affected. but in some cases, when one part varies, certain other parts always, or nearly always, simultaneously vary; they are then subject to the law of correlated variation. formerly i used the somewhat vague expression of correlation of growth, which may be applied to many large classes of facts. thus, all the parts of the body are admirably coordinated for the peculiar habits of life of each organic being, and they may be said, as the duke of argyll insists in his 'reign of law,' to be correlated for this purpose. again, in large groups of animals certain structures always co-exist; for instance, a peculiar form of stomach with teeth of peculiar form, and such structures may in one sense be said to be correlated. but these cases have no necessary connexion with the law to be discussed in the present chapter; for we do not know that { } the initial or primary variations of the several parts were in any way related; slight modifications or individual differences may have been preserved, first in one and then in another part, until the final and perfectly co-adapted structure was acquired; but to this subject i shall presently recur. again, in many groups of animals the males alone are furnished with weapons, or are ornamented with gay colours; and these characters manifestly stand in some sort of correlation with the male reproductive organs, for when the latter are destroyed these characters disappear. but it was shown in the twelfth chapter that the very same peculiarity may become attached at any age to either sex, and afterwards be exclusively transmitted by the same sex at a corresponding age. in these cases we have inheritance limited by, or correlated with, both sex and age; but we have no reason for supposing that the original cause of the variation was necessarily connected with the reproductive organs, or with the age of the affected being. in cases of true correlated variation, we are sometimes able to see the nature of the connexion; but in most cases the bond is hidden from us, and certainly differs in different cases. we can seldom say which of two correlated parts first varies, and induces a change in the other; or whether the two are simultaneously produced by some distinct cause. correlated variation is an important subject for us; for when one part is modified through continued selection, either by man or under nature, other parts of the organisation will be unavoidably modified. from this correlation it apparently follows that, with our domesticated animals and plants, varieties rarely or never differ from each other by some single character alone. one of the simplest cases of correlation is that a modification which arises during an early stage of growth tends to influence the subsequent development of the same part, as well as of other and intimately connected parts. isidore geoffroy st. hilaire states[ ] that this may constantly be observed with monstrosities { } in the animal kingdom; and moquin-tandon[ ] remarks, that, as with plants the axis cannot become monstrous without in some way affecting the organs subsequently produced from it, so axial anomalies are almost always accompanied by deviations of structure in the appended parts. we shall presently see that with short-muzzled races of the dog certain histological changes in the basal elements of the bones arrest their development and shorten them, and this affects the position of the subsequently developed molar teeth. it is probable that certain modifications in the larvæ of insects would affect the structure of the mature insects. but we must be very careful not to extend this view too far, for, during the normal course of development, certain members in the same group of animals are known to pass through an extraordinary course of change, whilst other and closely allied members arrive at maturity with little change of structure. another simple case of correlation is that with the increased or decreased dimensions of the whole body, or of any particular part, certain organs are increased or diminished in number, or are otherwise modified. thus pigeon-fanciers have gone on selecting pouters for length of body, and we have seen that their vertebræ are generally increased in number, and their ribs in breadth. tumblers have been selected for their small bodies, and their ribs and primary wing-feathers are generally lessened in number. fantails have been selected for their large, widely-expanded tails, with numerous tail-feathers, and the caudal vertebræ are increased in size and number. carriers have been selected for length of beak, and their tongues have become longer, but not in strict accordance with the length of beak. in this latter breed and in others having large feet, the number of the scutellæ on the toes is greater than in the breeds with small feet. many similar cases could be given. in germany it has been observed that the period of gestation is longer in large-sized than in small-sized breeds of cattle. with our highly-improved animals of all kinds the period of maturity has advanced, both with respect to the full growth of the body and the period of reproduction; and, in correspondence with this, the teeth are now developed earlier than formerly, so that, { } to the surprise of agriculturists, the ancient rules for judging the age of an animal by the state of its teeth are no longer trustworthy.[ ] _correlated variation of homologous parts._--parts which are homologous tend to vary in the same manner; and this is what might have been expected, for such parts are identical in form and structure during an early period of embryonic development, and are exposed in the egg or womb to similar conditions. the symmetry, in most kinds of animals, of the corresponding or homologous organs on the right and left sides of the body, is the simplest case in point; but this symmetry sometimes fails, as with rabbits having only one ear, or stags with one horn, or with many-horned sheep which sometimes carry an additional horn on one side of their heads. with flowers which have regular corollas, the petals generally vary in the same manner, as we see in the same complicated and elegant pattern, on the flowers of the chinese pink; but with irregular flowers, though the petals are of course homologous, this symmetry often fails, as with the varieties of the _antirrhinum_ or snapdragon, or that variety of the kidney-bean (_phaseolus multiflorus_) which has a white standard-petal. in the vertebrata the front and hind limbs are homologous, and they tend to vary in the same manner, as we see in long and short-legged, or in thick and thin-legged races of the horse and dog. isidore geoffroy[ ] has remarked on the tendency of supernumerary digits in man to appear, not only on the right and left sides, but on the upper and lower extremities. meckel has insisted[ ] that, when the muscles of the arm depart in number or arrangement from their proper type, they almost always imitate those of the leg; and so conversely the varying muscles of the leg imitate the normal muscles of the arm. in several distinct breeds of the pigeon and fowl, the legs and the two outer toes are heavily feathered, so that in the trumpeter pigeon they appear like little wings. in the feather-legged bantam the "boots" or feathers, which grow from the outside of the leg and generally from the two outer toes, have, { } according to the excellent authority of mr. hewitt,[ ] been seen to exceed the wing-feathers in length, and in one case were actually nine and a half inches in length! as mr. blyth has remarked to me, these leg-feathers resemble the primary wing-feathers, and are totally unlike the fine down which naturally grows on the legs of some birds, such as grouse and owls. hence it may be suspected that excess of food has first given redundancy to the plumage, and then that the law of homologous variation has led to the development of feathers on the legs, in a position corresponding with those on the wing, namely, on the outside of the tarsi and toes. i am strengthened in this belief by the following curious case of correlation, which for a long time seemed to me utterly inexplicable, namely, that in pigeons of any breed, if the legs are feathered, the two outer toes are partially connected by skin. these two outer toes correspond with our third and fourth toes. now, in the wing of the pigeon or any other bird, the first and fifth digits are wholly aborted; the second is rudimentary and carries the so-called "bastard-wing;" whilst the third and fourth digits are completely united and enclosed by skin, together forming the extremity of the wing. so that in feather-footed pigeons, not only does the exterior surface support a row of long feathers, like wing-feathers, but the very same digits which in the wing are completely united by skin become partially united by skin in the feet; and thus by the law of the correlated variation of homologous parts we can understand the curious connection of feathered legs and membrane between the two outer toes. andrew knight[ ] has remarked that the face or head and the limbs vary together in general proportions. compare, for instance, the head and limbs of a dray and race-horse, or of a greyhound and mastiff. what a monster a greyhound would appear with the head of a mastiff! the _modern_ bulldog, however, has fine limbs, but this is a recently-selected character. from the measurements given in the sixth chapter, we clearly see that in all the breeds of the pigeon the length of the beak and the size of the feet are correlated. the view which, as before explained, seems the most probable is, that disuse in all cases tends { } to diminish the feet, the beak becoming at the same time through correlation shorter; but that in those few breeds in which length of beak has been a selected point, the feet, notwithstanding disuse, have through correlation increased in size. with the increased length of the beak in pigeons, not only the tongue increases in length, but likewise the orifice of the nostrils. but the increased length of the orifice of the nostrils perhaps stands in closer correlation with the development of the corrugated skin or wattle at the base of the beak; for when there is much wattle round the eyes, the eyelids are greatly increased or even doubled in length. there is apparently some correlation even in colour between the head and the extremities. thus with horses a large white star or blaze on the forehead is generally accompanied by white feet.[ ] with white rabbits and cattle, dark marks often co-exist on the tips of the ears and on the feet. in black and tan dogs of different breeds, tan-coloured spots over the eyes and tan-coloured feet almost invariably go together. these latter cases of connected colouring may be due either to reversion or to analogous variation,--subjects to which we shall hereafter return,--but this does not necessarily determine the question of their original correlation. if those naturalists are correct who maintain that the jaw-bones are homologous with the limb-bones, then we can understand why the head and limbs tend to vary together in shape and even in colour; but several highly competent judges dispute the correctness of this view. the lopping forwards and downwards of the immense ears of fancy rabbits is in part due to the disuse of the muscles, and in part to the weight and length of the ears, which have been increased by selection during many generations. now, with the increased size and changed direction of the ears, not only has the bony auditory meatus become changed in outline, direction, and greatly in size, but the whole skull has been slightly modified. this could be clearly seen in "half-lops"--that is, in rabbits with one ear alone lopping forward--for the opposite sides of their skulls were not strictly symmetrical. this seems to me a curious instance of correlation, between hard { } bones and organs so soft and flexible, as well as so unimportant under a physiological point of view, as the external ears. the result no doubt is largely due to mere mechanical action, that is, to the weight of the ears, on the same principle that the skull of a human infant is easily modified by pressure. the skin and the appendages of hair, feathers, hoofs, horns, and teeth, are homologous over the whole body. every one knows that the colour of the skin and that of the hair usually vary together; so that virgil advises the shepherd to look whether the mouth and tongue of the ram are black, lest the lambs should not be purely white. with poultry and certain ducks we have seen that the colour of the plumage stands in some connexion with the colour of the shell of the egg,--that is, with the mucous membrane which secretes the shell. the colour of the skin and hair, and the odour emitted by the glands of the skin, are said[ ] to be connected, even in the same race of men. generally the hair varies in the same way all over the body in length, fineness, and curliness. the same rule holds good with feathers, as we see with the laced and frizzled breeds both of fowls and pigeons. in the common cock the feathers on the neck and loins are always of a particular shape, called hackles: now in the polish breed, both sexes are characterised by a tuft of feathers on the head; but through correlation these feathers in the male always assume the form of hackles. the wing and tail-feathers, though arising from parts not homologous, vary in length together; so that long or short winged pigeons generally have long or short tails. the case of the jacobin-pigeon is more curious, for the wing and tail feathers are remarkably long; and this apparently has arisen in correlation with the elongated and reversed feathers on the back of the neck, which form the hood. the hoofs and hair are homologous appendages; and a careful observer, namely azara,[ ] states that in paraguay horses of various colours are often born with their hair curled and twisted like that on the head of a negro. this peculiarity is strongly inherited. but what is remarkable is that the hoofs of these horses "are absolutely like those of a mule." the hair also of the mane and tail is invariably much shorter than usual, being only from four { } to twelve inches in length; so that curliness and shortness of the hair are here, as with the negro, apparently correlated. with respect to the horns of sheep, youatt[ ] remarks that "multiplicity of horns is not found in any breed of much value: it is generally accompanied by great length and coarseness of the fleece." several tropical breeds of sheep, which are clothed with hair instead of wool, have horns almost like those of a goat. sturm[ ] expressly declares that in different races the more the wool is curled the more the horns are spirally twisted. we have seen in the third chapter, where other analogous facts have been given, that the parent of the mauchamp breed, so famous for its fleece, had peculiarly shaped horns. the inhabitants of angora assert[ ] that "only the white goats which have horns wear the fleece in the long curly locks that are so much admired; those which are not horned having a comparatively close coat." from these cases we may conclude that the hair or wool and the horns vary in a correlated manner. those who have tried hydropathy are aware that the frequent application of cold water stimulates the skin; and whatever stimulates the skin tends to increase the growth of the hair, as is well shown in the abnormal growth of hair near old inflamed surfaces. now, professor low[ ] is convinced that with the different races of british cattle thick skin and long hair depend on the humidity of the climate which they inhabit. we can thus see how a humid climate might act on the horns--in the first place directly on the skin and hair, and secondly by correlation on the horns. the presence or absence of horns, moreover, both in the case of sheep and cattle, acts, as will presently be shown, by some sort of correlation on the skull. with respect to hair and teeth, mr. yarrell[ ] found many of the teeth deficient in three hairless "_Ægyptian_" dogs, and in a hairless terrier. the incisors, canines, and premolars suffered most, but in one case all the teeth, except the large tubercular molar on each side, were deficient. with man several striking cases have been recorded[ ] of inherited baldness with { } inherited deficiency, either complete or partial, of the teeth. we see the same connexion in those rare cases in which the hair has been renewed in old age, for this has "usually been accompanied by a renewal of the teeth." i have remarked in a former part of this volume that the great reduction in the size of the tusks in domestic boars probably stands in close relation with their diminished bristles, due to a certain amount of protection; and that the reappearance of the tusks in boars, which have become feral and are fully exposed to the weather, probably depends on the reappearance of the bristles. i may add, though not strictly connected with our present point, that an agriculturist[ ] asserts that "pigs with little hair on their bodies are most liable to lose their tails, showing a weakness of the tegumental structure. it may be prevented by crossing with a more hairy breed." in the previous cases deficient hair, and teeth deficient in number or size, are apparently connected. in the following cases abnormally redundant hair, and teeth either deficient or redundant, are likewise connected. mr. crawfurd[ ] saw at the burmese court a man, thirty years old, with his whole body, except the hands and feet, covered with straight silky hair, which on the shoulders and spine was five inches in length. at birth the ears alone were covered. he did not arrive at puberty, or shed his milk teeth, until twenty years old; and at this period he acquired five teeth in the upper jaw, namely four incisors and one canine, and four incisor teeth in the lower jaw; all the teeth were small. this man had a daughter, who was born with hair within her ears; and the hair soon extended over her body. when captain yule[ ] visited the court, he found this girl grown up; and she presented a strange appearance with even her nose densely covered with soft hair. like her father, she was furnished with incisor teeth alone. the king had with difficulty bribed a man to marry her, and of her two children, one, a boy fourteen months old, had hair growing out of his ears, with a beard and moustache. this strange peculiarity had, therefore, been inherited for three generations, with the molar teeth deficient in the grandfather and mother; whether { } these teeth would likewise fail in the infant could not be told. here is another case communicated to me by mr. wallace on the authority of dr. purland, a dentist: julia pastrana, a spanish dancer, was a remarkably fine woman, but she had a thick masculine beard and a hairy forehead; she was photographed, and her stuffed skin was exhibited as a show; but what concerns us is, that she had in both the upper and lower jaw an irregular double set of teeth, one row being placed within the other, of which dr. purland took a cast. from the redundancy of the teeth her mouth projected, and her face had a gorilla-like appearance. these cases and those of the hairless dogs forcibly call to mind the fact, that the two orders of mammals--namely, the edentata and cetacea--which are the most abnormal in their dermal covering, are likewise the most abnormal either by deficiency or redundancy of teeth. the organs of sight and hearing are generally admitted to be homologous, both with each other and with the various dermal appendages; hence these parts are liable to be abnormally affected in conjunction. mr. white cowper says "that in all cases of double microphthalmia brought under his notice he has at the same time met with defective development of the dental system." certain forms of blindness seem to be associated with the colour of the hair; a man with black hair and a woman with light-coloured hair, both of sound constitution, married and had nine children, all of whom were born blind; of these children, five "with dark hair and brown iris were afflicted with amaurosis; the four others, with light-coloured hair and blue iris, had amaurosis and cataract conjoined." several cases could be given, showing that some relation exists between various affections of the eyes and ears; thus liebreich states that out of deaf-mutes in berlin, no less than fourteen suffered from the rare disease called pigmentary retinitis. mr. white cowper and dr. earle have remarked that inability to distinguish different colours, or colour-blindness, "is often associated with a corresponding inability to distinguish musical sounds."[ ] { } here is a more curious case: white cats, if they have blue eyes, are almost always deaf. i formerly thought that the rule was invariable, but i have heard of a few authentic exceptions. the first two notices were published in , and relate to english and persian cats: of the latter, the rev. w. t. bree possessed a female, and he states "that of the offspring produced at one and the same birth, such as, like the mother, were entirely white (with blue eyes) were, like her, invariably deaf; while those that had the least speck of colour on their fur, as invariably possessed the usual faculty of hearing."[ ] the rev. w. darwin fox informs me that he has seen more than a dozen instances of this correlation in english, persian, and danish cats; but he adds "that, if one eye, as i have several times observed, be not blue, the cat hears. on the other hand, i have never seen a white cat with eyes of the common colour that was deaf." in france dr. sichel[ ] has observed during twenty years similar facts; he adds the remarkable case of the iris beginning, at the end of four months, to grow dark-coloured, and then the cat first began to hear. this case of correlation in cats has struck many persons as marvellous. there is nothing unusual in the relation between blue eyes and white fur; and we have already seen that the organs of sight and hearing are often simultaneously affected. in the present instance the cause probably lies in a slight arrest of development in the nervous system in connection with the sense-organs. kittens during the first nine days, whilst their eyes are closed, appear to be completely deaf; i have made a great clanging noise with a poker and shovel close to their heads, both when they were asleep and awake, without producing any effect. the trial must not be made by shouting close to their ears, for they are, even when asleep, extremely sensitive to a breath of air. now, as long as the eyes continue closed, the iris is no doubt blue, for in all the kittens which i have seen this colour remains for some time after the eyelids open. hence, if we suppose the development of the organs of sight and hearing to be arrested at the stage of the closed eyelids, the eyes would { } remain permanently blue and the ears would be incapable of perceiving sound; and we should thus understand this curious case. as, however, the colour of the fur is determined long before birth, and as the blueness of the eyes and the whiteness of the fur are obviously connected, we must believe that some primary cause acts at an early period. the instances of correlated variability hitherto given have been chiefly drawn from the animal kingdom, and we will now turn to plants. leaves, sepals, petals, stamens, and pistils are all homologous. in double flowers we see that the stamens and pistils vary in the same manner, and assume the form and colour of the petals. in the double columbine (_aquilegia vulgaris_), the successive whorls of stamens are converted into cornucopias, which are enclosed within each other and resemble the petals. in hose-and-hose flowers the sepals mock the petals. in some cases the flowers and leaves vary together in tint: in all the varieties of the common pea, which have purple flowers, a purple mark may be seen on the stipules. in other cases the leaves and fruit and seeds vary together in colour, as in a curious pale-leaved variety of the sycamore, which has recently been described in france,[ ] and as in the purple-leaved hazel, in which the leaves, the husk of the nut, and the pellicle round the kernel are all coloured purple.[ ] pomologists can predict to a certain extent, from the size and appearance of the leaves of their seedlings, the probable nature of the fruit; for, as van mons remarks,[ ] variations in the leaves are generally accompanied by some modification in the flower, and consequently in the fruit. in the serpent melon, which has a narrow tortuous fruit above a yard in length, the stem of the plant, the peduncle of the female flower, and the middle lobe of the leaf, are all elongated in a remarkable manner. on the other hand, several varieties of cucurbita, which have dwarfed stems, all produce, as naudin remarks with surprise, leaves of the same peculiar shape. mr. g. maw informs me that all the varieties of the scarlet pelargoniums which have contracted or imperfect leaves have contracted flowers: the difference between { } "brilliant" and its parent "tom thumb" is a good instance of this. it may be suspected that the curious case described by risso,[ ] of a variety of the orange which produces on the young shoots rounded leaves with winged petioles, and afterwards elongated leaves on long but wingless petioles, is connected with the remarkable change in form and nature which the fruit undergoes during its development. in the following instance we have the colour and form of the petals apparently correlated, and both dependent on the nature of the season. an observer, skilled in the subject, writes,[ ] "i noticed, during the year , that every dahlia, of which the colour had any tendency to scarlet, was deeply notched--indeed to so great an extent as to give the petals the appearance of a saw; the indentures were, in some instances, more than a quarter of an inch deep." again, dahlias which have their petals tipped with a different colour from the rest are very inconstant, and during certain years some, or even all the flowers, become uniformly coloured; and it has been observed with several varieties,[ ] that when this happens the petals grow much elongated and lose their proper shape. this, however, may be due to reversion, both in colour and form, to the aboriginal species. * * * * * in this discussion on correlation, we have hitherto treated of cases in which we can partly understand the bond of connexion; but i will now give cases in which we cannot even conjecture, or can only very obscurely see, what is the nature of the bond. isidore geoffroy st. hilaire, in his work on monstrosities, insists,[ ] "que certaines anomalies coexistent rarement entr'elles, d'autres fréquemment, d'autres enfin presque constamment, malgré la différence très-grande de leur nature, et quoiqu'elles puissent paraître _complètement indépendantes_ les unes des autres." we see something analogous in certain diseases: thus i hear from mr. paget that in a rare affection of the { } renal capsules (of which the functions are unknown), the skin becomes bronzed; and in hereditary syphilis, both the milk and the second teeth assume a peculiar and characteristic form. professor rolleston, also, informs me that the incisor teeth are sometimes furnished with a vascular rim in correlation with intra-pulmonary deposition of tubercles. in other cases of phthisis and of cyanosis the nails and finger-ends become clubbed like acorns. i believe that no explanation has been offered of these and of many other cases of correlated disease. what can be more curious and less intelligible than the fact previously given, on the authority of mr. tegetmeier, that young pigeons of all breeds, which when mature have white, yellow, silver-blue, or dun-coloured plumage, come out of the egg almost naked; whereas pigeons of other colours when first born are clothed with plenty of down? white pea-fowls, as has been observed both in england and france,[ ] and as i have myself seen, are inferior in size to the common coloured kind; and this cannot be accounted for by the belief that albinism is always accompanied by constitutional weakness; for white or albino moles are generally larger than the common kind. to turn to more important characters: the niata cattle of the pampas are remarkable from their short foreheads, upturned muzzles, and curved lower jaws. in the skull the nasal and premaxillary bones are much shortened, the maxillaries are excluded from any junction with the nasals, and all the bones are slightly modified, even to the plane of the occiput. from the analogical case of the dog, hereafter to be given, it is probable that the shortening of the nasal and adjoining bones is the proximate cause of the other modifications in the skull, including the upward curvature of the lower jaw, though we cannot follow out the steps by which these changes have been effected. polish fowls have a large tuft of feathers on their heads; and their skulls are perforated by numerous holes, so that a pin can be driven into the brain without touching any bone. that this deficiency of bone is in some way connected with the tuft of feathers is clear from tufted ducks and geese likewise having { } perforated skulls. the case would probably be considered by some authors as one of balancement or compensation. in the chapter on fowls, i have shown that with polish fowls the tuft of feathers was probably at first small; by continued selection it became larger, and then rested on a fleshy or fibrous mass; and finally, as it became still larger, the skull itself became more and more protuberant until it acquired its present extraordinary structure. through correlation with the protuberance of the skull, the shape and even the relative connexion of the premaxillary and nasal bones, the shape of the orifice of the nostrils, the breadth of the frontal bone, the shape of the post-lateral processes of the frontal and squamosal bones, and the direction of the bony cavity of the ear, have all been modified. the internal configuration of the skull and the whole shape of the brain have likewise been altered in a truly marvellous manner. after this case of the polish fowl it would be superfluous to do more than refer to the details previously given on the manner in which the changed form of the comb, in various breeds of the fowl, has affected the skull, causing by correlation crests, protuberances, and depressions on its surface. with our cattle and sheep the horns stand in close connexion with the size of the skull, and with the shape of the frontal bones; thus cline[ ] found that the skull of a horned ram weighed five times as much as that of a hornless ram of the same age. when cattle become hornless, the frontal bones are "materially diminished in breadth towards the poll;" and the cavities between the bony plates "are not so deep, nor do they extend beyond the frontals."[ ] * * * * * it may be well here to pause and observe how the effects of correlated variability, of the increased use of parts, and of the accumulation through natural selection of so-called spontaneous variations, are in many cases inextricably commingled. we may borrow an illustration from mr. herbert spencer, who remarks that, when the irish elk acquired its gigantic horns, weighing above one hundred pounds, numerous co-ordinated { } changes of structure would have been indispensable,--namely, a thickened skull to carry the horns; strengthened cervical vertebræ, with strengthened ligaments; enlarged dorsal vertebræ to support the neck, with powerful fore-legs and feet; all these parts being supplied with proper muscles, blood-vessels, and nerves. how then could these admirably co-ordinated modifications of structure have been acquired? according to the doctrine which i maintain, the horns of the male elk were slowly gained through sexual selection,--that is, by the best-armed males conquering the worse-armed, and leaving a greater number of descendants. but it is not at all necessary that the several parts of the body should have simultaneously varied. each stag presents individual differences, and in the same district those which had slightly heavier horns, or stronger necks, or stronger bodies, or were the most courageous, would secure the greater number of does, and consequently leave a greater number of offspring. the offspring would inherit, in a greater or less degree, these same qualities, would occasionally intercross with each other, or with other individuals varying in some favourable manner; and of their offspring, those which were the best endowed in any respect would continue multiplying; and so onwards, always progressing, sometimes in one direction, and sometimes in another, towards the present excellently co-ordinated structure of the male elk. to make this clear, let us reflect on the probable steps, as shown in the twentieth chapter, by which our race and dray-horses have arrived at their present state of excellence; if we could view the whole series of intermediate forms between one of these animals and an early unimproved progenitor, we should behold a vast number of animals, not equally improved in each generation throughout their entire structure, but sometimes a little more in one point, and sometimes in another, yet on the whole gradually approaching in character to our present race or dray-horses, which are so admirably fitted in the one case for fleetness and in the other for draught. although natural selection would thus[ ] tend to give to the { } male elk its present structure, yet it is probable that the inherited influence of use has played an equal or more important part. as the horns gradually increased in weight, the muscles of the neck, with the bones to which they are attached, would increase in size and strength; and these parts would react on the body and legs. nor must we overlook the fact that certain parts of the skull and the extremities would, judging by analogy, tend from the first to vary in a correlated manner. the increased weight of the horns would also act directly on the skull, in the same manner as, when one bone is removed in the leg of a dog, the other bone, which has to carry the whole weight of the body, increases in thickness. but from the facts given with respect to horned and hornless cattle, it is probable that the horns and skull would immediately act on each other through the principle of correlation. lastly, the growth and subsequent wear and tear of the augmented muscles and bones would require an increased supply of blood, and consequently an increased supply of food; and this again would require increased powers of mastication, digestion, respiration, and excretion. _colour as correlated with constitutional peculiarities._ it is an old belief that with man there is a connexion between complexion and constitution; and i find that some of the best authorities believe in this to the present day.[ ] thus dr. beddoe by his tables shows[ ] that a relation exists between liability to consumption and the colour of the hair, eyes, and skin. it has been affirmed[ ] that, in the french army which invaded russia, soldiers having a dark complexion, from the { } southern parts of europe, withstood the intense cold better than those with lighter complexions from the north; but no doubt such statements are liable to error. in the second chapter on selection i have given several cases proving that with animals and plants differences in colour are correlated with constitutional differences, as shown by greater or less immunity from certain diseases, from the attacks of parasitic plants and animals, from burning by the sun, and from the action of certain poisons. when all the individuals of any one variety possess an immunity of this nature, we cannot feel sure that it stands in any sort of correlation with their colour; but when several varieties of the same species, which are similarly coloured, are thus characterised, whilst other coloured varieties are not thus favoured, we must believe in the existence of a correlation of this kind. thus in the united states purple-fruited plums of many kinds are far more affected by a certain disease than green or yellow-fruited varieties. on the other hand, yellow-fleshed peaches of various kinds suffer from another disease much more than the white-fleshed varieties. in the mauritius red sugar-canes are much less affected by a particular disease than the white canes. white onions and verbenas are the most liable to mildew; and in spain the green-fruited grapes suffered from the vine-disease more than other coloured varieties. dark-coloured pelargoniums and verbenas are more scorched by the sun than varieties of other colours. red wheats are believed to be hardier than white; whilst red-flowered hyacinths were more injured during one particular winter in holland than other coloured varieties. with animals, white terriers suffer most from the distemper, white chickens from a parasitic worm in their tracheæ, white pigs from scorching by the sun, and white cattle from flies; but the caterpillars of the silk-moth which yield white cocoons suffered in france less from the deadly parasitic fungus than those producing yellow silk. the cases of immunity from the action of certain vegetable poisons, in connexion with colour, are more interesting, and are at present wholly inexplicable. i have already given a remarkable instance, on the authority of professor wyman, of all the hogs, excepting those of a black colour, suffering severely in virginia from eating the root of the _lachnanthes tinctoria_. { } according to spinola and others,[ ] buckwheat (_polygonum fagopyrum_), when in flower, is highly injurious to white or white-spotted pigs, if they are exposed to the heat of the sun, but is quite innocuous to black pigs. by two accounts, the _hypericum crispum_ in sicily is poisonous to white sheep alone; their heads swell, their wool falls off, and they often die; but this plant, according to lecce, is poisonous only when it grows in swamps; nor is this improbable, as we know how readily the poisonous principle in plants is influenced by the conditions under which they grow. three accounts have been published in eastern prussia, of white and white-spotted horses being greatly injured by eating mildewed and honeydewed vetches; every spot of skin bearing white hairs becoming inflamed and gangrenous. the rev. j. rodwell informs me that his father turned out about fifteen cart-horses into a field of tares which in parts swarmed with black aphides, and which no doubt were honeydewed, and probably mildewed; the horses, with two exceptions, were chesnuts and bays with white marks on their faces and pasterns, and the white parts alone swelled and became angry scabs. the two bay horses with no white marks entirely escaped all injury. in guernsey, when horses eat fools' parsley (_Æthusa cynapium_) they are sometimes violently purged; and this plant "has a peculiar effect on the nose and lips, causing deep cracks and ulcers, particularly on horses with white muzzles."[ ] with cattle, independently of the action of any poison, cases have been published by youatt and erdt of cutaneous diseases with much constitutional disturbance (in one instance after exposure to a hot sun) affecting every single point which bore a white hair, but completely passing over other parts of the body. similar cases have been observed with horses.[ ] we thus see that not only do those parts of the skin which bear white hair differ in a remarkable manner from those bearing { } hair of any other colour, but that in addition some great, constitutional difference must stand in correlation with the colour of the hair; for in the above-mentioned cases, vegetable poisons caused fever, swelling of the head, as well as other symptoms, and even death, to all the white or white-spotted animals. * * * * * { } chapter xxvi. laws of variation, _continued_--summary. on the affinity and cohesion of homologous parts--on the variability of multiple and homologous parts--compensation of growth--mechanical pressure--relative position of flowers with respect to the axis of the plant, and of seeds in the capsule, as inducing variation--analogous or parallel varieties--summary of the three last chapters. _on the affinity of homologous parts._--this law was first generalised by geoffroy saint hilaire, under the expression of _la loi de l'affinité de soi pour soi_. it has been fully discussed and illustrated by his son, isidore geoffroy, with respect to monsters in the animal kingdom,[ ] and by moquin-tandon, with respect to monstrous plants. when similar or homologous parts, whether belonging to the same embryo or to two distinct embryos, are brought during an early stage of development into contact, they often blend into a single part or organ; and this complete fusion indicates some mutual affinity between the parts, otherwise they would simply cohere. whether any power exists which tends to bring homologous parts into contact seems more doubtful. the tendency to complete fusion is not a rare or exceptional fact. it is exhibited in the most striking manner by double monsters. nothing can be more extraordinary than the manner, as shown in various published plates, in which the corresponding parts of two embryos become intimately fused together. this is perhaps best seen in monsters with two heads, which are united, summit to summit, or face to face, or, janus-like, back to back, or obliquely side to side. in one instance of two heads united almost face to face, but a little obliquely, four ears were developed, and on one side a perfect face, which was manifestly formed by the union of two { } half-faces. whenever two bodies or two heads are united, each bone, muscle, vessel, and nerve on the line of junction seems to seek out its fellow, and becomes completely fused with it. lereboullet,[ ] who carefully studied the development of double monsters in fishes, observed in fifteen instances the steps by which two heads gradually became fused into one. in this and other such cases, no one, i presume, supposes that the two already formed heads actually blend together, but that the corresponding parts of each head grow into one during the further progress of development, accompanied as it always is with incessant absorption and renovation. double monsters were formerly thought to be formed by the union of two originally distinct embryos developed upon distinct vitelli; but now it is admitted that "their production is due to the spontaneous divarication of the embryonic mass into two halves;"[ ] this, however, is effected by different methods. but the belief that double monsters originate from the division of one germ, does not necessarily affect the question of subsequent fusion, or render less true the law of the affinity of homologous parts. the cautious and sagacious j. müller,[ ] when speaking of janus-like monsters, says, that "without the supposition that some kind of affinity or attraction is exerted between corresponding parts, unions of this kind are inexplicable." on the other hand, vrolik, and he is followed by others, disputes this conclusion, and argues from the existence of a whole series of monstrosities, graduating from a perfectly double monster to a mere rudiment of an additional digit, that "an excess of formative power" is the cause and origin of every monstrous duplicity. that there are two distinct classes of cases, and that parts may be doubled independently of the existence of two embryos, is certain; for a single embryo, or even a single adult animal, may produce doubled organs. thus valentin, as quoted by vrolik, injured the caudal extremity of an embryo, and three days afterwards it produced rudiments of a double pelvis and of double hind limbs. { } hunter and others have observed lizards with their tails reproduced and doubled. when bonnet divided longitudinally the foot of the salamander, several additional digits were occasionally formed. but neither these cases, nor the perfect series from a double monster to an additional digit, seem to me opposed to the belief that corresponding parts have a mutual affinity, and consequently tend to fuse together. a part may be doubled and remain in this state, or the two parts thus formed may afterwards through the law of affinity become blended; or two homologous parts in two separate embryos may, through the same principle, unite and form a single part. the law of the affinity and fusion of similar parts applies to the homologous organs of the same individual animal, as well as to double monsters. isidore geoffroy gives a number of instances of two or more digits, of two whole legs, of two kidneys, and of several teeth becoming symmetrically fused together in a more or less perfect manner. even the two eyes have been known to unite into a single eye, forming a cyclopean monster, as have the two ears, though naturally standing so far apart. as geoffroy remarks, these facts illustrate in an admirable manner the normal fusion of various organs which during an early embryonic period are double, but which afterwards always unite into a single median organ. organs of this nature are generally found in a permanently double condition in other members of the same class. these cases of normal fusion appear to me to afford the strongest support in favour of the present law. adjoining parts which are not homologous sometimes cohere; but this cohesion appears to result from mere juxtaposition, and not from mutual affinity. in the vegetable kingdom moquin-tandon[ ] gives a long list of cases, showing how frequently homologous parts, such as leaves, petals, stamens, and pistils, as well as aggregates of homologous parts, such as buds, flowers, and fruit, become blended into each other with perfect symmetry. it is interesting to examine a compound flower of this nature, formed of exactly double the proper number of sepals, petals, stamens, and pistils, with each whorl of organs circular, and with no trace left of the { } process of fusion. the tendency in homologous parts to unite during their early development, moquin-tandon considers as one of the most striking laws governing the production of monsters. it apparently explains a multitude of cases, both in the animal and vegetable kingdoms; it throws a clear light on many normal structures which have evidently been formed by the union of originally distinct parts, and it possesses, as we shall see in a future chapter, much theoretical interest. * * * * * _on the variability of multiple and homologous parts._--isidore geoffroy[ ] insists that, when any part or organ is repeated many times in the same animal, it is particularly liable to vary both in number and structure. with respect to number, the proposition may, i think, be considered as fully established; but the evidence is chiefly derived from organic beings living under their natural conditions, with which we are not here concerned. when the vertebræ, or teeth, or rays in the fins of fishes, or feathers in the tails of birds, or petals, stamens, pistils, and seeds in plants, are very numerous, the number is generally variable. the explanation of this simple fact is by no means obvious. with respect to the variability in structure of multiple parts, the evidence is not so decisive; but the fact, as far as it may be trusted, probably depends on multiple parts being of less physiological importance than single parts; consequently their perfect standard of structure has been less rigorously enforced by natural selection. * * * * * _compensation of growth, or balancement._--this law, as applied to natural species, was propounded by goethe and geoffroy st. hilaire at nearly the same time. it implies that, when much organised matter is used in building up some one part, other parts are starved and become reduced. several authors, especially botanists, believe in this law; others reject it. as far as i can judge, it occasionally holds good; but its importance has probably been exaggerated. it is scarcely possible to distinguish between the supposed effects of such compensation of growth, and the effects of long-continued selection, which { } may at the same time lead to the augmentation of one part and the diminution of another. there can be no doubt that an organ may be greatly increased without any corresponding diminution in the adjoining parts. to recur to our former illustration of the irish elk, it may be asked what part has suffered in consequence of the immense development of the horns? it has already been observed that the struggle for existence does not bear hard on our domesticated productions; consequently the principle of economy of growth will seldom affect them, and we ought not to expect to find frequent evidence of compensation. we have, however, some such cases. moquin-tandon describes a monstrous bean,[ ] in which the stipules were enormously developed, and the leaflets apparently in consequence completely aborted; this case is interesting, as it represents the natural condition of _lathyrus aphaca_, with its stipules of great size, and its leaves reduced to mere threads, which act as tendrils. de candolle[ ] has remarked that the varieties of _raphanus sativus_ which have small roots yield numerous seed, valuable from containing oil, whilst those with large roots are not productive in this latter respect; and so it is with _brassica asperifolia_. the varieties of the potato which produce tubers very early in the season rarely bear flowers; but andrew knight,[ ] by checking the growth of the tubers, forced the plants to flower. the varieties of _cucurbita pepo_ which produce large fruit yield, according to naudin, few in number; whilst those producing small fruit yield a vast number. lastly, i have endeavoured to show in the eighteenth chapter that with many cultivated plants unnatural treatment checks the full and proper action of the reproductive organs, and they are thus rendered more or less sterile; consequently, in the way of compensation, the fruit becomes greatly enlarged, and, in double flowers, the petals are greatly increased in number. with animals, it has been found difficult to produce cows which should first yield much milk, and afterwards be capable of { } fattening well. with fowls which have large topknots and beards the comb and wattles are generally much reduced in size. perhaps the entire absence of the oil-gland in fantail pigeons may be connected with the great development of their tails. * * * * * _mechanical pressure as a cause of modifications._--in some few cases there is reason to believe that mere mechanical pressure has affected certain structures. every one knows that savages alter the shape of their infants' skulls by pressure at an early age; but there is no reason to believe that the result is ever inherited. nevertheless vrolik and weber[ ] maintain that the shape of the human head is influenced by the shape of the mother's pelvis. the kidneys in different birds differ much in form, and st. ange[ ] believes that this is determined by the form of the pelvis, which again, no doubt, stands in close relation with their various habits of locomotion. in snakes, the viscera are curiously displaced, in comparison with their position in other vertebrates; and this has been attributed by some authors to the elongation of their bodies; but here, as in so many previous cases, it is impossible to disentangle any direct result of this kind from that consequent on natural selection. godron has argued[ ] that the normal abortion of the spur on the inner side of the flower in corydalis, is caused by the buds being closely pressed at a very early period of growth, whilst under ground, against each other and against the stem. some botanists believe that the singular difference in the shape both of the seed and corolla, in the interior and exterior florets in certain compositous and umbelliferous plants, is due to the pressure to which the inner florets are subjected; but this conclusion is doubtful. the facts just given do not relate to domesticated productions, and therefore do not strictly concern us. but here is a more appropriate case: h. müller[ ] has shown that in { } short-faced races of the dog some of the molar teeth are placed in a slightly different position from that which they occupy in other dogs, especially in those having elongated muzzles; and as he remarks, any inherited change in the arrangement of the teeth deserves notice, considering their classificatory importance. this difference in position is due to the shortening of certain facial bones, and the consequent want of space; and the shortening results from a peculiar and abnormal state of the basal cartilages of the bones. _relative position of flowers with respect to the axis, and of seeds in the capsule, as inducing variation._ in the thirteenth chapter various peloric flowers were described, and their production was shown to be due either to arrested development, or to reversion to a primordial condition. moquin-tandon has remarked that the flowers which stand on the summit of the main stem or of a lateral branch are more liable to become peloric than those on the sides;[ ] and he adduces, amongst other instances, that of _teucrium campanulatum_. in another labiate plant grown by me, viz. the _galeobdolon luteum_, the peloric flowers were always produced on the summit of the stem, where flowers are not usually borne. in pelargonium, a _single_ flower in the truss is frequently peloric, and when this occurs i have during several years invariably observed it to be the central flower. this is of such frequent occurrence that one observer[ ] gives the names of ten varieties flowering at the same time, in every one of which the central flower was peloric. occasionally more than one flower in the truss is peloric, and then of course the additional ones must be lateral. these flowers are interesting as showing how the whole structure is correlated. in the common pelargonium the upper sepal is produced into a nectary which coheres with the flower-peduncle; the two upper petals differ a little in shape from the three lower ones, and are marked with dark shades of colour; the stamens are graduated in length and upturned. in the peloric flowers, the nectary aborts; all the petals become alike both in shape and colour; the stamens are generally reduced in number and become straight, so that the whole flower resembles that of the allied genus erodium. the correlation between these changes is well shown when one of the two upper petals alone loses its dark mark, for in this case the nectary does not entirely abort, but is usually much reduced in length.[ ] { } morren has described[ ] a marvellous flask-shaped flower of the calceolaria, nearly four inches in length, which was almost completely peloric; it grew on the summit of the plant, with a normal flower on each side; prof. westwood also has described[ ] three similar peloric flowers, which all occupied a central position on the flower-branches. in the orchideous genus, phalænopsis, the terminal flower has been seen to become peloric. in a laburnum-tree i observed that about a fourth part of the racemes produced terminal flowers which had lost their papilionaceous structure. these were produced after almost all the other flowers on the same racemes had withered. the most perfectly pelorised examples had six petals, each marked with black striæ like those on the standard-petal. the keel seemed to resist the change more than the other petals. dutrochet has described[ ] an exactly similar case in france, and i believe these are the only two instances of pelorism in the laburnum which have been recorded. dutrochet remarks that the racemes on this tree do not properly produce a terminal flower, so that, as in the case of the galeobdolon, their position as well as their structure are both anomalies, which no doubt are in some manner related. dr. masters has briefly described another leguminous plant,[ ] namely, a species of clover, in which the uppermost and central flowers were regular or had lost their papilionaceous structure. in some of these plants the flower-heads were also proliferous. lastly, linaria produces two kinds of peloric flowers, one having simple petals, and the other having them all spurred. the two forms, as naudin remarks,[ ] not rarely occur on the same plant, but in this case the spurred form almost invariably stands on the summit of the spike. the tendency in the terminal or central flower to become peloric more frequently than other flowers, probably results from "the bud which stands on the end of a shoot receiving the most sap; it grows out into a stronger shoot than those situated lower down."[ ] i have discussed the connection between pelorism and a central position, partly because some few plants are known normally to produce a terminal flower different in structure from the lateral ones; but chiefly on account of the following case, in which we see a tendency to variability or to reversion connected with the same position. a great judge of auriculas[ ] states that when an auricula throws up a side bloom it is pretty sure to keep its character; but that if it grows from the centre or heart of the plant, whatever the colour of the edging ought to be, "it is just as likely to come in any other class as in the one to which it properly belongs." this is so notorious a { } fact, that some florists regularly pinch off the central trusses of flowers. whether in the highly improved varieties the departure of the central trusses from their proper type is due to reversion, i do not know. mr. dombrain insists that, whatever may be the commonest kind of imperfection in each variety, this is generally exaggerated in the central truss. thus one variety "sometimes has the fault of producing a little green floret in the centre of the flower," and in central blooms these become excessive in size. in some central blooms, sent to me by mr. dombrain, all the organs of the flower were rudimentary in structure, of minute size, and of a green colour, so that by a little further change all would have been converted into small leaves. in this case we clearly see a tendency to prolification--a term which, i may explain to those who have never attended to botany, means the production of a branch or flower, or head of flowers, out of another flower. now dr. masters[ ] states that the central or uppermost flower on a plant is generally the most liable to prolification. thus, in the varieties of the auricula, the loss of their proper character and a tendency to prolification, and in other plants a tendency to prolification and pelorism, are all connected together, and are due either to arrested development, or to reversion to a former condition. the following is a more interesting case; metzger[ ] cultivated in germany several kinds of maize brought from the hotter parts of america, and he found, as has been previously described, that in two or three generations the grains became greatly changed in form, size, and colour; and with respect to two races he expressly states that in the first generation, whilst the lower grains on each head retained their proper character, the uppermost grains already began to assume that character which in the third generation all the grains acquired. as we do not know the aboriginal parent of the maize, we cannot tell whether these changes are in any way connected with reversion. in the two following cases, reversion, as influenced by the position of the seed in the capsule, evidently acts. the blue imperial pea is the offspring of the blue prussian, and has larger seed and broader pods than its parent. now mr. masters, of canterbury, a careful observer and a raiser of new varieties of the pea, states[ ] that the blue imperial always has a strong tendency to revert to its parent-stock, and the reversion "occurs in this manner: the last (or uppermost) pea in the pod is frequently much smaller than the rest; and if these small peas are carefully collected and sown separately, very many more, in proportion, will revert to their origin, than those taken from the other parts of the pod." again m. chaté[ ] says that in raising seedling stocks he succeeds in getting eighty per cent. to bear double flowers, by leaving only a few of the secondary branches to seed; but in addition to this, "at the time of extracting the seeds, the upper portion of the pod is separated and { } placed aside, because it has been ascertained that the plants coming from the seeds situated in this portion of the pod, give eighty per cent. of single flowers." now the production of single-flowering plants from the seed of double-flowering plants is clearly a case of reversion. these latter facts, as well as the connection between a central position and pelorism and prolification, show in an interesting manner how small a difference--namely a little greater freedom in the flow of sap towards one part of the same plant--determines important changes of structure. * * * * * _analogous or parallel variation._--by this term i wish to express that similar characters occasionally make their appearance in the several varieties or races descended from the same species, and more rarely in the offspring of widely distinct species. we are here concerned, not as hitherto with the causes of variation, but with the results; but this discussion could not have been more conveniently introduced elsewhere. the cases of analogous variation, as far as their origin is concerned, may be grouped, disregarding minor subdivisions, under two main heads; firstly, those due to unknown causes having acted on organic beings with nearly the same constitution, and which consequently vary in an analogous manner; and secondly, those due to the reappearance of characters which were possessed by a more or less remote progenitor. but these two main divisions can often be only conjecturally separated, and graduate, as we shall presently see, into each other. under the first head of analogous variations, not due to reversion, we have the many cases of trees belonging to quite different orders which have produced pendulous and fastigate varieties. the beech, hazel, and barberry have given rise to purple-leaved varieties; and as bernhardi has remarked,[ ] a multitude of plants, as distinct as possible, have yielded varieties with deeply-cut or laciniated leaves. varieties descended from three distinct species of brassica have their stems, or so-called roots, enlarged into globular masses. the nectarine is the offspring of the peach; and the varieties of both these trees offer a remarkable parallelism in the fruit being white, red, or yellow fleshed--in being clingstones or freestones--in the flowers being large or small--in the leaves being serrated or crenated, furnished with globose or reniform glands, or quite destitute of glands. it should be remarked that each variety of the nectarine has not derived its character from a corresponding variety of the peach. the several varieties also of a closely allied genus, namely the apricot, differ from each other in nearly the same parallel manner. there is no reason { } to believe that in any of these cases long-lost characters have reappeared, and in most of them this certainly has not occurred. three species of cucurbita have yielded a multitude of races, which correspond so closely in character that, as naudin insists, they may be arranged in an almost strictly parallel series. several varieties of the melon are interesting from resembling in important characters other species, either of the same genus or of allied genera; thus, one variety has fruit so like, both externally and internally, the fruit of a perfectly distinct species, namely, the cucumber, as hardly to be distinguished from it; another has long cylindrical fruit twisting about like a serpent; in another the seeds adhere to portions of the pulp; in another the fruit, when ripe, suddenly cracks and falls into pieces; and all these highly remarkable peculiarities are characteristic of species belonging to allied genera. we can hardly account for the appearance of so many unusual characters by reversion to a single ancient form; but we must believe that all the members of the family have inherited a nearly similar constitution from an early progenitor. our cereal and many other plants offer similar cases. with animals we have fewer cases of analogous variation, independently of direct reversion. we see something of the kind in the resemblance between the short-muzzled races of the dog, such as the pug and bulldog; in feather-footed races of the fowl, pigeon, and canary-bird; in horses of the most different races presenting the same range of colour; in all black-and-tan dogs having tan-coloured eye-spots and feet, but in this latter case reversion may possibly have played a part. low has remarked[ ] that several breeds of cattle are "sheeted,"--that is, have a broad band of white passing round their bodies like a sheet; this character is strongly inherited and sometimes originates from a cross; it may be the first step in reversion to an original or early type, for, as was shown in the third chapter, white cattle with dark ears, feet, and tip of tail formerly existed, and now exist in a feral or semi-feral condition in several quarters of the world. under our second main division, namely, of analogous variations due to reversion, the best cases are afforded by animals, and by none better than by pigeons. in all the most distinct breeds sub-varieties occasionally appear coloured exactly like the parent rock-pigeon, with black wing-bars, white loins, banded tail, &c.; and no one can doubt that these characters are simply due to reversion. so with minor details; turbits properly have white tails, but occasionally a bird is born with a dark-coloured and banded tail; pouters properly have white primary wing-feathers, but not rarely a "sword-flighted" bird, that is, one with the few first primaries dark-coloured, appears; and in these cases we have characters proper to the rock-pigeon, but new to the breed, evidently appearing from reversion. in some domestic varieties the wing-bars, instead of being simply black, as in the rock-pigeon, are beautifully edged with different zones of colour, and they then present a striking analogy with the wing-bars in certain natural species of the same family, such as _phaps chalcoptera_; and this may probably be accounted for by { } all the forms descended from the same remote progenitor having a tendency to vary in the same manner. thus also we can perhaps understand the fact of some laugher-pigeons cooing almost like turtle-doves, and of several races having peculiarities in their flight, for certain natural species (viz. _c. torquatrix_ and _palumbus_) display singular vagaries in this respect. in other cases a race, instead of imitating in character a distinct species, resembles some other race; thus certain runts tremble and slightly elevate their tails, like fantails; and turbits inflate the upper part of their oesophagus, like pouter-pigeons. it is a common circumstance to find certain coloured marks persistently characterising all the species of a genus, but differing much in tint; and the same thing occurs with the varieties of the pigeon: thus, instead of the general plumage being blue with the wing-bars black, there are snow-white varieties with red bars, and black varieties with white bars; in other varieties the wing-bars, as we have seen, are elegantly zoned with different tints. the spot pigeon is characterised by the whole plumage being white, excepting the tail and a spot on the forehead; but these parts may be red, yellow, or black. in the rock-pigeon and in many varieties the tail is blue, with the outer edges of the outer feathers white; but in one sub-variety of the monk-pigeon we have a reversed variation, for the tail is white, except the outer edges of the outer feathers, which are black.[ ] with some species of birds, for instance with gulls, certain coloured parts appear as if almost washed out, and i have observed exactly the same appearance in the terminal dark tail-bar in certain pigeons, and in the whole plumage of certain varieties of the duck. analogous facts in the vegetable kingdom could be given. many sub-varieties of the pigeon have reversed and somewhat lengthened feathers on the back part of their heads, and this is certainly not due to reversion to the parent-species, which shows no trace of such structure; but when we remember that sub-varieties of the fowl, turkey, canary-bird, duck, and goose, all have topknots or reversed feathers on their heads; and when we remember that scarcely a single large natural group of birds can be named, in which some members have not a tuft of feathers on their heads, we may suspect that reversion to some extremely remote form has come into action. several breeds of the fowl have either spangled or pencilled feathers; and these cannot be derived from the parent-species, the _gallus bankiva_; though of course it is possible that an early progenitor of this species may have been spangled, and a still earlier or a later progenitor may have been pencilled. but as many gallinaceous birds are spangled or pencilled, it is a more probable view that the several domestic breeds of the fowl have acquired this kind of plumage from all the members of the family inheriting a tendency to vary in a like manner. the same principle may account for the ewes in certain breeds of sheep being hornless, like the females of some other hollow-horned ruminants; it may account for certain domestic cats having slightly-tufted ears, like those of the lynx; and for the skulls of domestic rabbits often differing from each { } other in the same characters by which the skulls of the various species of the genus lepus differ. i will only allude to one other case, already discussed. now that we know that the wild parent of the ass has striped legs, we may feel confident that the occasional appearance of stripes on the legs of the domestic ass is due to direct reversion; but this will not account for the lower end of the shoulder-stripe being sometimes angularly bent or slightly forked. so, again, when we see dun and other coloured horses with stripes on the spine, shoulders, and legs, we are led, from reasons formerly given, to believe that they reappear from direct reversion to the wild parent-horse. but when horses have two or three shoulder-stripes with one of them occasionally forked at the lower end, or when they have stripes on their faces, or as foals are faintly striped over nearly their whole bodies, with the stripes angularly bent one under the other on the forehead, or irregularly branched in other parts, it would be rash to attribute such diversified characters to the reappearance of those proper to the aboriginal wild horse. as three african species of the genus are much striped, and as we have seen that the crossing of the unstriped species often leads to the hybrid offspring being conspicuously striped--bearing also in mind that the act of crossing certainly causes the reappearance of long-lost characters--it is a more probable view that the above-specified stripes are due to reversion, not to the immediate wild parent-horse, but to the striped progenitor of the whole genus. i have discussed this subject of analogous variation at considerable length, because, in a future work on natural species, it will be shown that the varieties of one species frequently mock distinct species--a fact in perfect harmony with the foregoing cases, and explicable only on the theory of descent. secondly, because these facts are important from showing, as remarked in a former chapter, that each trifling variation is governed by law, and is determined in a much higher degree by the nature of the organisation, than by the nature of the conditions to which the varying being has been exposed. thirdly, because these facts are to a certain extent related to a more general law, namely, that which mr. b. d. walsh[ ] has called the "law of _equable variability_," or, as he explains it, "if any given character is very variable in one species of a group, it will tend to be variable in allied species; and if any given character is perfectly constant in one species of a group, it will tend to be constant in allied species." this leads me to recall a discussion in the chapter on selection, in which it was shown that with domestic races, which are { } now undergoing rapid improvement, those parts or characters which are the most valued vary the most. this naturally follows from recently selected characters continually tending to revert to their former less improved standard, and from their being still acted on by the same agencies, whatever these may be, which first caused the characters in question to vary. the same principle is applicable to natural species, for, as stated in my 'origin of species,' generic characters are less variable than specific characters; and the latter are those which have been modified by variation and natural selection, since the period when all the species belonging to the same genus branched off from a common progenitor, whilst generic characters are those which have remained unaltered from a much more remote epoch, and accordingly are now less variable. this statement makes a near approach to mr. walsh's law of equable variability. secondary sexual characters, it may be added, rarely serve to characterise distinct genera, for they usually differ much in the species of the same genus, and are highly variable in the individuals of the same species; we have also seen in the earlier chapters of this work how variable secondary sexual characters become under domestication. _summary of the three previous chapters, on the laws of variation._ in the twenty-third chapter we have seen that changed conditions occasionally act in a definite manner on the organisation, so that all, or nearly all, the individuals thus exposed become modified in the same manner. but a far more frequent result of changed conditions, whether acting directly on the organisation or indirectly through the reproductive system being affected is indefinite and fluctuating variability. in the three latter chapters we have endeavoured to trace some of the laws by which such variability is regulated. increased use adds the size of a muscle, together with the blood-vessels, nerves, ligaments, the crests of bone to which these are attached, the whole bone and other connected bones. so it is with various glands. increased functional activity strengthens the sense-organs. increased and intermittent pressure thickens the epidermis; and a change in the nature of the food sometimes modifies the coats of the stomach, and increases or { } decreases the length of the intestines. continued disuse, on the other hand, weakens and diminishes all parts of the organisation. animals which during many generations have taken but little exercise, have their lungs reduced in size, and as a consequence the bony fabric of the chest, and the whole form of the body, become modified. with our anciently domesticated birds, the wings have been little used, and they are slightly reduced; with their decrease, the crest of the sternum, the scapulæ, coracoids, and furcula, have all been reduced. with domesticated animals, the reduction of a part from disuse is never carried so far that a mere rudiment is left, but we have good reason to believe that this has often occurred under nature. the cause of this difference probably is that with domestic animals not only sufficient time has not been granted for so profound a change, but that, from not being exposed to a severe struggle for life, the principle of the economy of organisation does not come into action. on the contrary, we sometimes see that structures which are rudimentary in the parent-species become partially redeveloped in their domesticated progeny. when rudiments are formed or left under domestication, they are the result of a sudden arrest of development, and not of long-continued disuse with the absorption of all superfluous parts; nevertheless they are of interest, as showing that rudiments are the relics of organs once perfectly developed. corporeal, periodical, and mental habits, though the latter have been almost passed over in this work, become changed under domestication, and the changes are often inherited. such changed habits in any organic being, especially when living a free life, would often lead to the augmented or diminished use of various organs, and consequently to their modification. from long-continued habit, and more especially from the occasional birth of individuals with a slightly different constitution, domestic animals and cultivated plants become to a certain extent acclimatised, or adapted to a climate different from that proper to the parent-species. through the principle of correlated variability, when one part varies other parts vary,--either simultaneously, or one after the other. thus an organ modified during an early embryonic period affects other parts subsequently developed. when an { } organ, such as the beak, increases or decreases in length, adjoining or correlated parts, as the tongue and the orifice of the nostrils, tend to vary in the same manner. when the whole body increases or decreases in size, various parts become modified; thus with pigeons the ribs increase or decrease in number and breadth. homologous parts, which are identical during their early development and are exposed to similar conditions, tend to vary in the same or in some connected manner,--as in the case of the right and left sides of the body, of the front and hind limbs, and even of the head and limbs. so it is with the organs of sight and hearing; for instance, white cats with blue eyes are almost always deaf. there is a manifest relation throughout the body between the skin and its various appendages of hair, feathers, hoofs, horns, and teeth. in paraguay, horses with curly hair have hoofs like those of a mule; the wool and the horns of sheep vary together; hairless dogs are deficient in their teeth; men with redundant hair have abnormal teeth, either deficient or in excess. birds with long wing-feathers usually have long tail-feathers. when long feathers grow from the outside of the legs and toes of pigeons, the two outer toes are connected by membrane; for the whole leg tends to assume the structure of the wing. there is a manifest relation between a crest of feathers on the head and a marvellous amount of change in the skull of various fowls; and in a lesser degree, between the greatly elongated, lopping ears of rabbits and the structure of their skulls. with plants, the leaves, various parts of the flower, and the fruit, often vary together in a correlated manner. in some cases we find correlation without being able even to conjecture what is the nature of the connexion, as with various correlated monstrosities and diseases. this is likewise the case with the colour of the adult pigeon, in connexion with the presence of down on the young bird. numerous curious instances have been given of peculiarities of constitution, in correlation with colour, as shown by the immunity of individuals of some one colour from certain diseases, from the attacks of parasites, and from the action of certain vegetable poisons. correlation is an important subject; for with species, and in a lesser degree with domestic races, we continually find that { } certain parts have been greatly modified to serve some useful purpose; but we almost invariably find that other parts have likewise been more or less modified, without our being able to discover any advantage in the change. no doubt great caution is necessary in coming to this conclusion, for it is difficult to overrate our ignorance on the use of various parts of the organisation; but from what we have now seen, we may believe that many modifications are of no direct service, having arisen in correlation with other and useful changes. homologous parts during their early development evince an affinity for each other,--that is, they tend to cohere and fuse together much more readily than other parts. this tendency to fusion explains a multitude of normal structures. multiple and homologous organs are especially liable to vary in number and probably in form. as the supply of organised matter is not unlimited, the principle of compensation sometimes comes into action; so that, when one part is greatly developed, adjoining parts or functions are apt to be reduced; but this principle is probably of much less importance than the more general one of the economy of growth. through mere mechanical pressure hard parts occasionally affect soft adjoining parts. with plants the position of the flowers on the axis, and of the seeds in the capsule, sometimes leads, through a freer flow of sap, to changes of structure; but these changes are often due to reversion. modifications, in whatever manner caused, will be to a certain extent regulated by that co-ordinating power or _nisus formativus_, which is in fact a remnant of one of the forms of reproduction, displayed by many lowly organised beings in their power of fissiparous generation and budding. finally, the effects of the laws, which directly or indirectly govern variability, may be largely influenced by man's selection, and will so far be determined by natural selection that changes advantageous to any race will be favoured and disadvantageous changes checked. domestic races descended from the same species, or from two or more allied species, are liable to revert to characters derived from their common progenitor, and, as they have much in common in their constitutions, they are also liable under changed conditions to vary in the same manner; from these { } two causes analogous varieties often arise. when we reflect on the several foregoing laws, imperfectly as we understand them, and when we bear in mind how much remains to be discovered, we need not be surprised at the extremely intricate manner in which our domestic productions have varied, and still go on varying. * * * * * { } chapter xxvii. provisional hypothesis of pangenesis. preliminary remarks.--first part:--the facts to be connected under a single point of view, namely, the various kinds of reproduction--the direct action of the male element on the female--development--the functional independence of the elements or units of the body--variability--inheritance--reversion. second part:--statement of the hypothesis--how far the necessary assumptions are improbable--explanation by aid of the hypothesis of the several classes of facts specified in the first part--conclusion. in the previous chapters large classes of facts, such as those bearing on bud-variation, the various forms of inheritance, the causes and laws of variation, have been discussed; and it is obvious that these subjects, as well as the several modes of reproduction, stand in some sort of relation to each other. i have been led, or rather forced, to form a view which to a certain extent connects these facts by a tangible method. every one would wish to explain to himself, even in an imperfect manner, how it is possible for a character possessed by some remote ancestor suddenly to reappear in the offspring; how the effects of increased or decreased use of a limb can be transmitted to the child; how the male sexual element can act not solely on the ovule, but occasionally on the mother-form; how a limb can be reproduced on the exact line of amputation, with neither too much nor too little added; how the various modes of reproduction are connected, and so forth. i am aware that my view is merely a provisional hypothesis or speculation; but until a better one be advanced, it may be serviceable by bringing together a multitude of facts which are at present left disconnected by any efficient cause. as whewell, the historian of the inductive sciences, remarks:--"hypotheses may often be of service to science, when they involve a certain portion of incompleteness, and even of error." under this point of view i venture to advance the hypothesis of pangenesis, which { } implies that the whole organisation, in the sense of every separate atom or unit, reproduces itself. hence ovules and pollen-grains,--the fertilised seed or egg, as well as buds,--include and consist of a multitude of germs thrown off from each separate atom of the organism. in the first part i will enumerate as briefly as i can the groups of facts which seem to demand connection; but certain subjects, not hitherto discussed, must be treated at disproportionate length. in the second part the hypothesis will be given; and we shall see, after considering how far the necessary assumptions are in themselves improbable, whether it serves to bring under a single point of view the various facts. part i. reproduction may be divided into two main classes, namely, sexual and asexual. the latter is effected in many ways--by gemmation, that is by the formation of buds of various kinds, and by fissiparous generation, that is by spontaneous or artificial division. it is notorious that some of the lower animals, when cut into many pieces, reproduce so many perfect individuals: lyonnet cut a nais or freshwater worm into nearly forty pieces, and these all reproduced perfect animals.[ ] it is probable that segmentation could be carried much further in some of the protozoa, and with some of the lowest plants each cell will reproduce the parent-form. johannes müller thought that there was an important distinction between gemmation and fission; for in the latter case the divided portion, however small, is more perfectly organised; but most physiologists are now convinced that the two processes are essentially alike.[ ] prof. huxley remarks, "fission is little more than a peculiar { } mode of budding," and prof. h. j. clark, who has especially attended to this subject, shows in detail that there is sometimes "a compromise between self-division and budding." when a limb is amputated, or when the whole body is bisected, the cut extremities are said to bud forth; and as the papilla, which is first formed, consists of undeveloped cellular tissue like that forming an ordinary bud, the expression is apparently correct. we see the connection of the two processes in another way; for trembley observed that with the hydra the reproduction of the head after amputation was checked as soon as the animal began to bud.[ ] between the production, by fissiparous generation, of two or more complete individuals, and the repair of even a very slight injury, we have, as remarked in a former chapter, so perfect and insensible a gradation, that it is impossible to doubt that they are connected processes. between the power which repairs a trifling injury in any part, and the power which previously "was occupied in its maintenance by the continued mutation of its particles," there cannot be any great difference; and we may follow mr. paget in believing them to be the selfsame power. as at each stage of growth an amputated part is replaced by one in the same state of development, we must likewise follow mr. paget in admitting "that the powers of development from the embryo are identical with those exercised for the restoration from injuries: in other words, that the powers are the same by which perfection is first achieved, and by which, when lost, it is recovered."[ ] finally, we may conclude that the several forms of gemmation, and of fissiparous generation, the repair of injuries, the maintenance of each part in its proper state, and the growth or progressive development of the whole structure of the embryo, are all essentially the results of one and the same great power. _sexual generation._--the union of the two sexual elements seems to make a broad distinction between sexual and asexual reproduction. but the well-ascertained cases of parthenogenesis prove that the distinction is not really so great as it at first appears; for ovules occasionally, and even in some cases { } frequently, become developed into perfect beings, without the concourse of the male element. j. müller and others admit that ovules and buds have the same essential nature. certain bodies, which during their early development cannot be distinguished by any external character from true ovules, nevertheless must be classed as buds, for though formed within the ovarium they are incapable of fertilisation. this is the case with the germ-balls of the cecidomyide larvæ, as described by leuckart.[ ] ovules and the male element, before they become united, have, like buds, an independent existence.[ ] both have the power of transmitting every single character possessed by the parent-form. we see this clearly when hybrids are paired _inter se_, for the characters of either grandparent often reappear, either perfectly or by segments, in the progeny. it is an error to suppose that the male transmits certain characters and the female other characters; though no doubt, from unknown causes, one sex sometimes has a stronger power of transmission than the other. it has been maintained by some authors that a bud differs essentially from a fertilised germ, by always reproducing the perfect character of the parent-stock; whilst fertilised germs become developed into beings which differ, in a greater or less degree, from each other and from their parents. but there is no such broad distinction as this. in the eleventh chapter, numerous cases were given showing that buds occasionally grow into plants having new and strongly marked characters; and varieties thus produced can be propagated for a length of time by buds, and occasionally by seed. nevertheless, it must be admitted that beings produced sexually are much more liable to vary than those produced asexually; and of this fact a partial explanation will hereafter be attempted. the variability in both cases is determined by the same general causes, and is governed by the same laws. hence new varieties arising from buds cannot be distinguished from those arising from seed. although bud-varieties usually retain their character during { } successive bud-generations, yet they occasionally revert, even after a long series of bud-generations, to their former character. this tendency to reversion in buds is one of the most remarkable of the several points of agreement between the offspring from bud and seminal reproduction. there is, however, one difference between beings produced sexually and asexually, which is very general. the former usually pass in the course of their development from a lower to a higher grade, as we see in the metamorphoses of insects and in the concealed metamorphoses of the vertebrata; but this passage from a lower to a higher grade cannot be considered as a necessary accompaniment of sexual reproduction, for hardly anything of the kind occurs in the development of aphis amongst insects, or with certain crustaceans, cephalopods, or with any of the higher vascular plants. animals propagated asexually by buds or fission are on the other hand never known to undergo a retrogressive metamorphosis; that is, they do not first sink to a lower, before passing on to their higher and final stage of development. but during the act of asexual production or subsequently to it, they often advance in organisation, as we see in the many cases of "alternate generation." in thus speaking of alternate generation, i follow those naturalists who look at the process as essentially one of internal budding or of fissiparous generation. some of the lower plants, however, such as mosses and certain algæ, according to dr. l. radlkofer,[ ] when propagated asexually, do undergo a retrogressive metamorphosis. we can to a certain extent understand, as far as the final cause is concerned, why beings propagated by buds should so rarely retrogress during development; for with each organism the structure acquired at each stage of development must be adapted to its peculiar habits. now, with beings produced by gemmation,--and this, differently from sexual reproduction, may occur at any period of growth,--if there were places for the support of many individuals at some one stage of development, the simplest plan would be that they should be multiplied by gemmation at that stage, and not that they should first retrograde in their development to an earlier or simpler structure, which might not be fitted for the surrounding conditions. { } from the several foregoing considerations we may conclude that the difference between sexual and asexual generation is not nearly so great as it at first appears; and we have already seen that there is the closest agreement between gemmation, fissiparous generation, the repair of injuries, and ordinary growth or development. the capacity of fertilisation by the male element seems to be the chief distinction between an ovule and a bud; and this capacity is not invariably brought into action, as in the cases of parthenogenetic reproduction. we are here naturally led to inquire what the final cause can be of the necessity in ordinary generation for the concourse of the two sexual elements. seeds and ova are often highly serviceable as the means of disseminating plants and animals, and of preserving them during one or more seasons in a dormant state; but unimpregnated seeds or ova, and detached buds, would be equally serviceable for both purposes. we can, however, indicate two important advantages gained by the concourse of the two sexes, or rather of two individuals belonging to opposite sexes; for, as i have shown in a former chapter, the structure of every organism appears to be especially adapted for the concurrence, at least occasionally, of two individuals. in nearly the same manner as it is admitted by naturalists that hybridism, from inducing sterility, is of service in keeping the forms of life distinct and fitted for their proper places; so, when species are rendered highly variable by changed conditions of life, the free intercrossing of the varying individuals will tend to keep each form fitted for its proper place in nature; and crossing can be effected only by sexual generation, but whether the end thus gained is of sufficient importance to account for the first origin of sexual intercourse is very doubtful. secondly, i have shown, from the consideration of a large body of facts, that, as a slight change in the conditions of life is beneficial to each creature, so, in an analogous manner, is the change effected in the germ by sexual union with a distinct individual; and i have been led, from observing the many widely-extended provisions throughout nature for this purpose, and from the greater vigour of crossed organisms of all kinds, as proved by direct experiments, as well as from the evil effects of close interbreeding when long { } continued, to believe that the advantage thus gained is very great. besides these two important ends, there may, of course, be others, as yet unknown to us, gained by the concourse of the two sexes. why the germ, which before impregnation undergoes a certain amount of development, ceases to progress and perishes, unless it be acted on by the male element; and why conversely the male element, which is enabled to keep alive for even four or five years within the spermatheca of a female insect, likewise perishes, unless it acts on or unites with the germ, are questions which cannot be answered with any certainty. it is, however, possible that both sexual elements perish, unless brought into union, simply from including too little formative matter for independent existence and development; for certainly they do not in ordinary cases differ in their power of giving character to the embryo. this view of the importance of the quantity of formative matter seems probable from the following considerations. there is no reason to suspect that the spermatozoa or pollen-grains of the same individual animal or plant differ from each other; yet quatrefages has shown in the case of the teredo,[ ] as did formerly prevost and dumas with other animals, that more than one spermatozoon is requisite to fertilise an ovule. this has likewise been clearly proved by newport,[ ] who adds the important fact, established by numerous experiments, that, when a very small number of spermatozoa are applied to the ova of batrachians, they are only partially impregnated and the embryo is never fully developed: the first step, however, towards development, namely, the partial segmentation of the yelk, does occur to a greater or less extent, but is never completed up to granulation. the rate of the segmentation is likewise determined by the number of the spermatozoa. with respect to plants, nearly the same results were obtained by kölreuter and gärtner. this last careful observer found,[ ] after making successive trials on a malva with more and more pollen-grains, that even thirty grains did not fertilise a single seed; but when forty grains were applied to the { } stigma, a few seeds of small size were formed. the pollen-grains of mirabilis are extraordinarily large, and the ovarium contains only a single ovule; and these circumstances led naudin[ ] to make the following interesting experiments: a flower was fertilised by three grains and succeeded perfectly; twelve flowers were fertilised by two grains, and seventeen flowers by a single grain, and of these one flower alone in each lot perfected its seed; and it deserves especial notice that the plants produced by these two seeds never attained their proper dimensions, and bore flowers of remarkably small size. from these facts we clearly see that the quantity of the peculiar formative matter which is contained within the spermatozoa and pollen-grains is an all-important element in the act of fertilisation, not only in the full development of the seed, but in the vigour of the plant produced from such seed. we see something of the same kind in certain cases of parthenogenesis, that is, when the male element is wholly excluded; for m. jourdan[ ] found that, out of about , eggs laid by unimpregnated silk-moths, many passed through their early embryonic stages, showing that they were capable of self-development, but only twenty-nine out of the whole number produced caterpillars. therefore it is not an improbable view that deficient bulk or quantity in the formative matter, contained within the sexual elements, is the main cause of their not having the capacity of prolonged separate existence and development. the belief that it is the function of the spermatozoa to communicate life to the ovule seems a strange one, seeing that the unimpregnated ovule is already alive and continues for a considerable time alive. we shall hereafter see that it is probable that the sexual elements, or possibly only the female element, include certain primordial cells, that is, such as have undergone no differentiation, and which are not present in an active state in buds. _graft-hybrids._--when discussing in the eleventh chapter the curious case of the _cytisus adami_, facts were given which render it to a certain degree probable, in accordance with the belief of some distinguished botanists, that, when the tissues of two plants { } belonging to distinct species or varieties are intimately united, buds are afterwards occasionally produced which, like hybrids, combine the characters of the two united forms. it is certain that when trees with variegated leaves are grafted or budded on a common stock, the latter sometimes produces buds bearing variegated leaves; but this may perhaps be looked at as a case of inoculated disease. should it ever be proved that hybridised buds can be formed by the union of two distinct vegetative tissues, the essential identity of sexual and asexual reproduction would be shown in the most interesting manner; for the power of combining in the offspring the characters of both parents, is the most striking of all the functions of sexual generation. _direct action of the male element on the female._--in the chapter just referred to, i have given abundant proofs that foreign pollen occasionally affects the mother-plant in a direct manner. thus, when gallesio fertilised an orange-flower with pollen from the lemon, the fruit bore stripes of perfectly characterised lemon-peel: with peas, several observers have seen the colour of the seed-coats and even of the pod directly affected by the pollen of a distinct variety; so it has been with the fruit of the apple, which consists of the modified calyx and upper part of the flower-stalk. these parts in ordinary cases are wholly formed by the mother-plant. we here see the male element affecting and hybridising not that part which it is properly adapted to affect, namely the ovule, but the partially developed tissues of a distinct individual. we are thus brought half-way towards a graft-hybrid, in which the cellular tissue of one form, instead of its pollen, is believed to hybridise the tissues of a distinct form. i formerly assigned reasons for rejecting the belief that the mother-plant is affected through the intervention of the hybridised embryo; but even if this view were admitted, the case would become one of graft-hybridism, for the fertilised embryo and the mother-plant must be looked at as distinct individuals. with animals which do not breed until nearly mature, and of which all the parts are then fully developed, it is hardly possible that the male element should directly affect the female. but we have the analogous and perfectly well-ascertained case of the male element of a distinct form, as with the { } quagga and lord morton's mare, affecting the ovarium of the female, so that the ovules and offspring subsequently produced by her when impregnated by other males are plainly affected and hybridised by the first male. _development._--the fertilised germ reaches maturity by a vast number of changes: these are either slight and slowly effected, as when the child grows into the man, or are great and sudden, as with the metamorphoses of most insects. between these extremes we have, even within the same class, every gradation: thus, as sir j. lubbock has shown,[ ] there is an ephemerous insect which moults above twenty times, undergoing each time a slight but decided change of structure; and these changes, as he further remarks, probably reveal to us the normal stages of development which are concealed and hurried through, or suppressed, in most other insects. in ordinary metamorphoses, the parts and organs appear to become changed into the corresponding parts in the next stage of development; but there is another form of development, which has been called by professor owen metagenesis. in this case "the new parts are not moulded upon the inner surface of the old ones. the plastic force has changed its course of operation. the outer case, and all that gave form and character to the precedent individual, perish and are cast off; they are not changed into the corresponding parts of the new individual. these are due to a new and distinct developmental process," &c.[ ] metamorphosis, however, graduates so insensibly into metagenesis, that the two processes cannot be distinctly separated. for instance, in the last change which cirripedes undergo, the alimentary canal and some other organs are moulded on pre-existing parts; but the eyes of the old and the young animal are developed in entirely different parts of the body; the tips of the mature limbs are formed within the larval limbs, and may be said to be metamorphosed from them; but their basal portions and the whole thorax are developed in a plane actually at right angles to the limbs and thorax of the larva; and this { } may be called metagenesis. the metagenetic process is carried to an extreme degree in the development of some echinoderms, for the animal in the second stage of development is formed almost like a bud within the animal of the first stage, the latter being then cast off like an old vestment, yet sometimes still maintaining for a short period an independent vitality.[ ] if, instead of a single individual, several were to be thus developed metagenetically within a pre-existing form, the process would be called one of alternate generation. the young thus developed may either closely resemble the encasing parent-form, as with the larvæ of cecidomyia, or may differ to an astonishing degree, as with many parasitic worms and with jelly-fishes; but this does not make any essential difference in the process, any more than the greatness or abruptness of the change in the metamorphoses of insects. the whole question of development is of great importance for our present subject. when an organ, the eye for instance, is metagenetically formed in a part of the body where during the previous stage of development no eye existed, we must look at it as a new and independent growth. the absolute independence of new and old structures, which correspond in structure and function, is still more obvious when several individuals are formed within a previous encasing form, as in the cases of alternate generation. the same important principle probably comes largely into play even in the case of continuous growth, as we shall see when we consider the inheritance of modifications at corresponding ages. we are led to the same conclusion, namely, the independence of parts successively developed, by another and quite distinct group of facts. it is well known that many animals belonging to the same class, and therefore not differing widely from each other, pass through an extremely different course of development. thus certain beetles, not in any way remarkably different from others of the same order, undergo what has been called a hyper-metamorphosis--that is, they pass through an early stage wholly different from the ordinary grub-like larva. in the same sub-order of crabs, namely, the macroura, as fritz { } müller remarks, the river cray-fish is hatched under the same form which it ever afterwards retains; the young lobster has divided legs, like a mysis; the palæmon appears under the form of a zoea, and peneus under the nauplius-form; and how wonderfully these larval forms differ from each other, is known to every naturalist.[ ] some other crustaceans, as the same author observes, start from the same point and arrive at nearly the same end, but in the middle of their development are widely different from each other. still more striking cases could be given with respect to the echinodermata. with the medusæ or jelly-fishes professor allman observes, "the classification of the hydroida would be a comparatively simple task if, as has been erroneously asserted, generically-identical medusoids always arose from generically-identical polypoids; and on the other hand, that generically-identical polypoids always gave origin to generically-identical medusoids." so, again, dr. strethill wright remarks, "in the life-history of the hydroidæ any phase, planuloid, polypoid, or medusoid, may be absent."[ ] according to the belief now generally accepted by our best naturalists, all the members of the same order or class, the macrourous crustaceans for instance, are descended from a common progenitor. during their descent they have diverged much in structure, but have retained much in common; and this divergence and retention of character has been effected, though they have passed and still pass through marvellously different metamorphoses. this fact well illustrates how independent each structure must be from that which precedes and follows it in the course of development. _the functional independence of the elements or units of the body._--physiologists agree that the whole organism consists of a multitude of elemental parts, which are to a great extent independent of each other. each organ, says claude bernard,[ ] { } has its proper life, its autonomy; it can develop and reproduce itself independently of the adjoining tissues. the great german authority, virchow,[ ] asserts still more emphatically that each system, as the nervous or osseous system, or the blood, consists of an "enormous mass of minute centres of action.... every element has its own special action, and even though it derive its stimulus to activity from other parts, yet alone effects the actual performance of its duties.... every single epithelial and muscular fibre-cell leads a sort of parasitical existence in relation to the rest of the body.... every single bone-corpuscle really possesses conditions of nutrition peculiar to itself." each element, as mr. paget remarks, lives its appointed time, and then dies, and, after being cast off or absorbed, is replaced.[ ] i presume that no physiologist doubts that, for instance, each bone-corpuscle of the finger differs from the corresponding corpuscle in the corresponding joint of the toe; and there can hardly be a doubt that even those on the corresponding sides of the body differ, though almost identical in nature. this near approach to identity is curiously shown in many diseases in which the same exact points on the right and left sides of the body are similarly affected; thus mr. paget[ ] gives a drawing of a diseased pelvis, in which the bone has grown into a most complicated pattern, but "there is not one spot or line on one side which is not represented, as exactly as it would be in a mirror, on the other." many facts support this view of the independent life of each minute element of the body. virchow insists that a single bone-corpuscle or a single cell in the skin may become diseased. the spur of a cock, after being inserted into the eye of an ox, lived for eight years, and acquired a weight of grammes, or nearly fourteen ounces.[ ] the tail of a pig has been grafted into the middle of its back, and reacquired sensibility. dr. ollier[ ] inserted a piece of periosteum from the bone of a young dog under the skin of a rabbit, and true bone was developed. a multitude of similar facts could be given. the { } frequent presence of hairs and of perfectly developed teeth, even teeth of the second dentition, in ovarian tumours,[ ] are facts leading to the same conclusion. whether each of the innumerable autonomous elements of the body is a cell or the modified product of a cell, is a more doubtful question, even if so wide a definition be given to the term, as to include cell-like bodies without walls and without nuclei.[ ] professor lionel beale uses the term "germinal matter" for the contents of cells, taken in this wide acceptation, and he draws a broad distinction between germinal matter and "formed material" or the various products of cells.[ ] but the doctrine of _omnis cellula e cellulâ_ is admitted for plants, and is a widely prevalent belief with respect to animals.[ ] thus virchow, the great supporter of the cellular theory, whilst allowing that difficulties exist, maintains that every atom of tissue is derived from cells, and these from pre-existing cells, and these primarily from the egg, which he regards as a great cell. that cells, still retaining the same nature, increase by self-division or proliferation, is admitted by almost every one. but when an organism undergoes a great change of structure during development, the cells, which at each stage are supposed to be directly derived from previously-existing cells, must likewise be greatly changed in nature; this change is apparently attributed by the supporters of the cellular doctrine to some inherent power which the cells possess, and not to any external agency. another school maintains that cells and tissues of all kinds may be formed, independently of pre-existing cells, from plastic lymph or blastema; and this it is thought is well exhibited in the repair of wounds. as i have not especially attended to histology, it would be presumptuous in me to express an opinion on the two opposed doctrines. but every one appears to admit that the body consists of a multitude of "organic units,"[ ] { } each of which possesses its own proper attributes, and is to a certain extent independent of all others. hence it will be convenient to use indifferently the terms cells or organic units or simply units. _variability and inheritance._--we have seen in the twenty-second chapter that variability is not a principle co-ordinate with life or reproduction, but results from special causes, generally from changed conditions acting during successive generations. part of the fluctuating variability thus induced is apparently due to the sexual system being easily affected by changed conditions, so that it is often rendered impotent; and when not so seriously affected, it often fails in its proper function of transmitting truly the characters of the parents to the offspring. but variability is not necessarily connected with the sexual system, as we see from the cases of bud-variation; and although we may not be able to trace the nature of the connexion, it is probable that many deviations of structure which appear in sexual offspring result from changed conditions acting directly on the organisation, independently of the reproductive organs. in some instances we may feel sure of this, when all, or nearly all the individuals which have been similarly exposed are similarly and definitely affected--as in the dwarfed and otherwise changed maize brought from hot countries when cultivated in germany; in the change of the fleece in sheep within the tropics; to a certain extent in the increased size and early maturity of our highly-improved domesticated animals; in inherited gout from intemperance; and in many other such cases. now, as such changed conditions do not especially affect the reproductive organs, it seems mysterious on any ordinary view why their product, the new organic being, should be similarly affected. how, again, can we explain to ourselves the inherited effects of the use or disuse of particular organs? the domesticated duck flies less and walks more than the wild duck, and its limb-bones have become in a corresponding manner diminished and increased in comparison with those of the wild duck. a horse is trained to certain paces, and the colt inherits similar consensual movements. the domesticated rabbit becomes tame from close confinement; the dog intelligent from associating with man; the retriever is taught to fetch and carry: and these { } mental endowments and bodily powers are all inherited. nothing in the whole circuit of physiology is more wonderful. how can the use or disuse of a particular limb or of the brain affect a small aggregate of reproductive cells, seated in a distant part of the body, in such a manner that the being developed from these cells inherits the characters of either one or both parents? even an imperfect answer to this question would be satisfactory. sexual reproduction does not essentially differ, as we have seen, from budding or self-division, and these processes graduate through the repair of injuries into ordinary development and growth; it might therefore be expected that every character would be as regularly transmitted by all the methods of reproduction as by continued growth. in the chapters devoted to inheritance it was shown that a multitude of newly-acquired characters, whether injurious or beneficial, whether of the lowest or highest vital importance, are often faithfully transmitted--frequently even when one parent alone possesses some new peculiarity. it deserves especial attention that characters appearing at any age tend to reappear at a corresponding age. we may on the whole conclude that in all cases inheritance is the rule, and non-inheritance the anomaly. in some instances a character is not inherited, from the conditions of life being directly opposed to its development; in many instances, from the conditions incessantly inducing fresh variability, as with grafted fruit-trees and highly cultivated flowers. in the remaining cases the failure may be attributed to reversion, by which the child resembles its grandparents or more remote progenitors, instead of its parents. this principle of reversion is the most wonderful of all the attributes of inheritance. it proves to us that the transmission of a character and its development, which ordinarily go together and thus escape discrimination, are distinct powers; and these powers in some cases are even antagonistic, for each acts alternately in successive generations. reversion is not a rare event, depending on some unusual or favourable combination of circumstances, but occurs so regularly with crossed animals and plants, and so frequently with uncrossed breeds, that it is evidently an essential part of the principle of inheritance. we know that { } changed conditions have the power of evoking long-lost characters, as in the case of some feral animals. the act of crossing in itself possesses this power in a high degree. what can be more wonderful than that characters, which have disappeared during scores, or hundreds, or even thousands of generations, should suddenly reappear perfectly developed, as in the case of pigeons and fowls when purely bred, and especially when crossed; or as with the zebrine stripes on dun-coloured horses, and other such cases? many monstrosities come under this same head, as when rudimentary organs are redeveloped, or when an organ which we must believe was possessed by an early progenitor, but of which not even a rudiment is left, suddenly reappears, as with the fifth stamen in some scrophulariaceæ. we have already seen that reversion acts in bud-reproduction; and we know that it occasionally acts during the growth of the same individual animal, especially, but not exclusively, when of crossed parentage,--as in the rare cases described of individual fowls, pigeons, cattle, and rabbits, which have reverted as they advanced in years to the colours of one of their parents or ancestors. we are led to believe, as formerly explained, that every character which occasionally reappears is present in a latent form in each generation, in nearly the same manner as in male and female animals secondary characters of the opposite sex lie latent, ready to be evolved when the reproductive organs are injured. this comparison of the secondary sexual characters which are latent in both sexes, with other latent characters, is the more appropriate from the case recorded of the hen, which assumed some of the masculine characters, not of her own race, but of an early progenitor; she thus exhibited at the same time the redevelopment of latent characters of both kinds and connected both classes. in every living creature we may feel assured that a host of lost characters lie ready to be evolved under proper conditions. how can we make intelligible, and connect with other facts, this wonderful and common capacity of reversion,--this power of calling back to life long-lost characters? { } part ii. i have now enumerated the chief facts which every one would desire to connect by some intelligible bond. this can be done, as it seems to me, if we make the following assumptions; if the first and chief one be not rejected, the others, from being supported by various physiological considerations, will not appear very improbable. it is almost universally admitted that cells, or the units of the body, propagate themselves by self-division or proliferation, retaining the same nature, and ultimately becoming converted into the various tissues and substances of the body. but besides this means of increase i assume that cells, before their conversion into completely passive or "formed material," throw off minute granules or atoms, which circulate freely throughout the system, and when supplied with proper nutriment multiply by self-division, subsequently becoming developed into cells like those from which they were derived. these granules for the sake of distinctness may be called cell-gemmules, or, as the cellular theory is not fully established, simply gemmules. they are supposed to be transmitted from the parents to the offspring, and are generally developed in the generation which immediately succeeds, but are often transmitted in a dormant state during many generations and are then developed. their development is supposed to depend on their union with other partially developed cells or gemmules which precede them in the regular course of growth. why i use the term union, will be seen when we discuss the direct action of pollen on the tissues of the mother-plant. gemmules are supposed to be thrown off by every cell or unit, not only during the adult state, but during all the stages of development. lastly, i assume that the gemmules in their dormant state have a mutual affinity for each other, leading to their aggregation either into buds or into the sexual elements. hence, speaking strictly, it is not the reproductive elements, nor the buds, which generate new organisms, but the cells themselves throughout the body. these assumptions constitute the provisional hypothesis which i have called pangenesis. nearly { } similar views have been propounded, as i find, by other authors, more especially by mr. herbert spencer;[ ] but they are here modified and amplified. { } before proceeding to show, firstly, how far these assumptions are in themselves probable, and secondly, how far they connect and explain the various groups of facts with which we are concerned, it may be useful to give an illustration of the hypothesis. if one of the simplest protozoa be formed, as appears under the microscope, of a small mass of homogeneous gelatinous matter, a minute atom thrown off from any part and nourished under favourable circumstances would naturally reproduce the whole; but if the upper and lower surfaces were to differ in texture from the central portion, then all three parts would have to throw off atoms or gemmules, which when aggregated by mutual affinity would form either buds or the sexual elements. precisely the same view may be extended to one of the higher animals; although in this case many thousand gemmules must be thrown off from the various parts of the body. now, when the leg, for instance, of a salamander is cut off, a slight crust forms over the wound, and beneath this crust the uninjured cells or units of bone, muscle, nerves, &c., are supposed to unite with the diffused gemmules of those cells which in the perfect leg come next in order; and these as they become slightly developed unite with others, and so on until a papilla of soft cellular tissue, the "budding leg," is formed, and in time a perfect leg.[ ] thus, that portion of the leg which had { } been cut off, neither more nor less, would be reproduced. if the tail or leg of a young animal had been cut off, a young tail or leg would have been reproduced, as actually occurs with the amputated tail of the tadpole; for gemmules of all the units which compose the tail are diffused throughout the body at all ages. but during the adult state the gemmules of the larval tail would remain dormant, for they would not meet with pre-existing cells in a proper state of development with which to unite. if from changed conditions or any other cause any part of the body should become permanently modified, the gemmules, which are merely minute portions of the contents of the cells forming the part, would naturally reproduce the same modification. but gemmules previously derived from the same part before it had undergone any change, would still be diffused throughout the organisation, and would be transmitted from generation to generation, so that under favourable circumstances they might be redeveloped, and then the new modification would be for a time or for ever lost. the aggregation of gemmules derived from every part of the body, through their mutual affinity, would form buds, and their aggregation in some special manner, apparently in small quantity, together probably with the presence of gemmules of certain primordial cells, would constitute the sexual elements. by means of these illustrations the hypothesis of pangenesis has, i hope, been rendered intelligible. * * * * * physiologists maintain, as we have seen, that each cell, though to a large extent dependent on others, is likewise, to a certain extent, independent or autonomous. i go one small step further, and assume that each cell casts off a free gemmule, which is capable of reproducing a similar cell. there is some analogy between this view and what we see in compound animals and in the flower-buds on the same tree; for these are distinct individuals capable of true or seminal reproduction, yet have parts in common and are dependent on each other; thus { } the tree has its bark and trunk, and certain corals, as the virgularia, have not only parts, but movements in common. the existence of free gemmules is a gratuitous assumption, yet can hardly be considered as very improbable, seeing that cells have the power of multiplication through the self-division of their contents. gemmules differ from true ovules or buds inasmuch as they are supposed to be capable of multiplication in their undeveloped state. no one probably will object to this capacity as improbable. the blastema within the egg has been known to divide and give birth to two embryos; and thuret[ ] has seen the zoospore of an alga divide itself, and both halves germinate. an atom of small-pox matter, so minute as to be borne by the wind, must multiply itself many thousand-fold in a person thus inoculated.[ ] it has recently been ascertained[ ] that a minute portion of the mucous discharge from an animal affected with rinderpest, if placed in the blood of a healthy ox, increases so fast that in a short space of time "the whole mass of blood, weighing many pounds, is infected, and every small particle of that blood contains enough poison to give, within less than forty-eight hours, the disease to another animal." the retention of free and undeveloped gemmules in the same body from early youth to old age may appear improbable, but we should remember how long seeds lie dormant in the earth and buds in the bark of a tree. their transmission from generation to generation may appear still more improbable; but here again we should remember that many rudimentary and useless organs are transmitted and have been transmitted during an indefinite number of generations. we shall presently see how well the long-continued transmission of undeveloped gemmules explains many facts. as each unit, or group of similar units throughout the body, casts off its gemmules, and as all are contained within the smallest egg or seed, and within each spermatozoon or pollen-grain, their number and minuteness must be something { } inconceivable. i shall hereafter recur to this objection, which at first appears so formidable; but it may here be remarked that a cod-fish has been found to produce , , eggs, a single ascaris about , , eggs, and a single orchidaceous plant probably as many million seeds.[ ] in these several cases, the spermatozoa and pollen-grains must exist in considerably larger numbers. now, when we have to deal with numbers such as these, which the human intellect cannot grasp, there is no good reason for rejecting our present hypothesis on account of the assumed existence of cell-gemmules a few thousand times more numerous. the gemmules in each organism must be thoroughly diffused; nor does this seem improbable considering their minuteness, and the steady circulation of fluids throughout the body. so it must be with the gemmules of plants, for with certain kinds even a minute fragment of a leaf will reproduce the whole. but a difficulty here occurs; it would appear that with plants, and probably with compound animals, such as corals, the gemmules do not spread from bud to bud, but only through the tissues developed from each separate bud. we are led to this conclusion from the stock being rarely affected by the insertion of a bud or graft from a distinct variety. this non-diffusion of the gemmules is still more plainly shown in the case of ferns; for mr. bridgman[ ] has proved that, when spores (which it should be remembered are of the nature of buds) are taken from a monstrous part of a frond, and others from an ordinary part, { } each reproduces the form of the part whence derived. but this non-diffusion of the gemmules from bud to bud may be only apparent, depending, as we shall hereafter see, on the nature of the first-formed cells in the buds. the assumed elective affinity of each gemmule for that particular cell which precedes it in the order of development is supported by many analogies. in all ordinary cases of sexual reproduction the male and female elements have a mutual affinity for each other: thus, it is believed that about ten thousand species of compositæ exist, and there can be no doubt that if the pollen of all these species could be, simultaneously or successively, placed on the stigma of any one species, this one would elect with unerring certainty its own pollen. this elective capacity is all the more wonderful, as it must have been acquired since the many species of this great group of plants branched off from a common progenitor. on any view of the nature of sexual reproduction, the protoplasm contained within the ovules and within the sperm-cells (or the "spermatic force" of the latter, if so vague a term be preferred) must act on each other by some law of special affinity, either during or subsequently to impregnation, so that corresponding parts alone affect each other; thus, a calf produced from a short-horned cow by a long-horned bull has its horns and not its horny hoofs affected by the union of the two forms, and the offspring from two birds with differently coloured tails have their tails and not their whole plumage affected. the various tissues of the body plainly show, as many physiologists have insisted,[ ] an affinity for special organic substances, whether natural or foreign to the body. we see this in the cells of the kidneys attracting urea from the blood; in the worrara poison affecting the nerves; upas and digitalis the muscles; the lytta vesicatoria the kidneys; and in the poisonous matter of many diseases, as small-pox, scarlet-fever, hooping-cough, glanders, cancer, and hydrophobia, affecting certain definite parts of the body or certain tissues or glands. the affinity of various parts of the body for each other during { } their early development was shown in the last chapter, when discussing the tendency to fusion in homologous parts. this affinity displays itself in the normal fusion of organs which are separate at an early embryonic age, and still more plainly in those marvellous cases of double monsters in which each bone, muscle, vessel, and nerve in the one embryo, blends with the corresponding part in the other. the affinity between homologous organs may come into action with single parts, or with the entire individual, as in the case of flowers or fruits which are symmetrically blended together with all their parts doubled, but without any other trace of fusion. it has also been assumed that the development of each gemmule depends on its union with another cell or unit which has just commenced its development, and which, from preceding it in order of growth, is of a somewhat different nature. nor is it a very improbable assumption that the development of a gemmule is determined by its union with a cell slightly different in nature, for abundant evidence was given in the seventeenth chapter, showing that a slight degree of differentiation in the male and female sexual elements favours in a marked manner their union and subsequent development. but what determines the development of the gemmules of the first-formed or primordial cell in the unimpregnated ovule, is beyond conjecture. it must also be admitted that analogy fails to guide us towards any determination on several other points: for instance, whether cells, derived from the same parent-cell, may, in the regular course of growth, become developed into different structures, from absorbing peculiar kinds of nutriment, independently of their union with distinct gemmules. we shall appreciate this difficulty if we call to mind, what complex yet symmetrical growths the cells of plants yield when they are inoculated by the poison of a gall-insect. with animals various polypoid excrescences and tumours are now generally admitted[ ] to be the direct product, through proliferation, of normal cells which have become abnormal. in the regular growth and repair of bones, the tissues undergo, as virchow remarks,[ ] a whole series of permutations and substitutions. "the cartilage-cells may be { } converted by a direct transformation into marrow-cells, and continue as such; or they may first be converted into osseous and then into medullary tissue; or lastly, they may first be converted into marrow and then into bone. so variable are the permutations of these tissues, in themselves so nearly allied, and yet in their external appearance so completely distinct." but as these tissues thus change their nature at any age, without any obvious change in their nutrition, we must suppose in accordance with our hypothesis that gemmules derived from one kind of tissue combine with the cells of another kind, and cause the successive modifications. it is useless to speculate at what period of development each organic unit casts off its gemmules; for the whole subject of the development of the various elemental tissues is as yet involved in much doubt. some physiologists, for instance, maintain that muscle or nerve-fibres are developed from cells, which are afterwards nourished by their own proper powers of absorption; whilst other physiologists deny their cellular origin; and beale maintains that such fibres are renovated exclusively by the conversion of fresh germinal matter (that is the so-called nuclei) into "formed material." however this may be, it appears probable that all external agencies, such as changed nutrition, increased use or disuse, &c., which induced any permanent modification in a structure, would at the same time or previously act on the cells, nuclei, germinal or formative matter, from which the structures in question were developed, and consequently would act on the gemmules or cast-off atoms. there is another point on which it is useless to speculate, namely, whether all gemmules are free and separate, or whether some are from the first united into small aggregates. a feather, for instance, is a complex structure, and, as each separate part is liable to inherited variations, i conclude that each feather certainly generates a large number of gemmules; but it is possible that these may be aggregated into a compound gemmule. the same remark applies to the petals of a flower, which in some cases are highly complex, with each ridge and hollow contrived for special purposes, so that each part must have been separately modified, and the modifications transmitted; consequently, separate gemmules, according to our hypothesis, { } must have been thrown off from each cell or part. but, as we sometimes see half an anther or a small portion of a filament becoming petaliform, or parts or mere stripes of the calyx assuming the colour and texture of the corolla, it is probable that with petals the gemmules of each cell are not aggregated together into a compound gemmule, but are freely and separately diffused. * * * * * having now endeavoured to show that the several foregoing assumptions are to a certain extent supported by analogous facts, and having discussed some of the most doubtful points, we will consider how far the hypothesis brings under a single point of view the various cases enumerated in the first part. all the forms of reproduction graduate into each other and agree in their product; for it is impossible to distinguish between organisms produced from buds, from self-division, or from fertilised germs; such organisms are liable to variations of the same nature and to reversion of character; and as we now see that all the forms of reproduction depend on the aggregation of gemmules derived from the whole body, we can understand this general agreement. it is satisfactory to find that sexual and asexual generation, by both of which widely different processes the same living creature is habitually produced, are fundamentally the same. parthenogenesis is no longer wonderful; in fact, the wonder is that it should not oftener occur. we see that the reproductive organs do not actually create the sexual elements; they merely determine or permit the aggregation of the gemmules in a special manner. these organs, together with their accessory parts, have, however, high functions to perform; they give to both elements a special affinity for each other, independently of the contents of the male and female cells, as is shown in the case of plants by the mutual reaction of the stigma and pollen-grains; they adapt one or both elements for independent temporary existence, and for mutual union. the contrivances for these purposes are sometimes wonderfully complex, as with the spermatophores of the cephalopoda. the male element sometimes possesses attributes which, if observed in an independent animal, would be put down to instinct guided by sense-organs, as when the { } spermatozoon of an insect finds its way into the minute micropyle of the egg, or as when the antherozoids of certain algæ swim by the aid of their ciliæ to the female plant, and force themselves into a minute orifice. in these latter cases, however, we must believe that the male element has acquired its powers, on the same principle with the larvæ of animals, namely by successive modifications developed at corresponding periods of life: we can hardly avoid in these cases looking at the male element as a sort of premature larva, which unites, or, like one of the lower algæ, conjugates, with the female element. what determines the aggregation of the gemmules within the sexual organs we do not in the least know; nor do we know why buds are formed in certain definite places, leading to the symmetrical growth of trees and corals, nor why adventitious buds may be formed almost anywhere, even on a petal, and frequently upon healed wounds.[ ] as soon as the gemmules have aggregated themselves, development apparently commences, but in the case of buds is often afterwards suspended, and in the case of the sexual elements soon ceases, unless the elements of the opposite sexes combine; even after this has occurred, the fertilised germ, as with seeds buried in the ground, may remain during a lengthened period in a dormant state. the antagonism which has long been observed,[ ] though exceptions occur,[ ] between active growth and the power of sexual reproduction--between the repair of injuries and gemmation--and with plants, between rapid increase by buds, rhizomes, &c., and the production of seed, is partly explained by the gemmules not existing in sufficient numbers for both processes. { } but this explanation hardly applies to those plants which naturally produce a multitude of seeds, but which, through a comparatively small increase in the number of the buds on their rhizomes or offsets, yield few or no seed. as, however, we shall presently see that buds probably include tissue which has already been to a certain extent developed or differentiated, some additional organised matter will thus have been expended. from one of the forms of reproduction, namely, spontaneous self-division, we are led by insensible steps to the repair of the slightest injury; and the existence of gemmules, derived from every cell or unit throughout the body and everywhere diffused, explains all such cases,--even the wonderful fact that, when the limbs of the salamander were cut off many times successively by spallanzani and bonnet, they were exactly and completely reproduced. i have heard this process compared with the recrystallisation which occurs when the angles of a broken crystal are repaired; and the two processes have this much in common, that in the one case the polarity of the molecules is the efficient cause, and in the other the affinity of the gemmules for particular nascent cells. pangenesis does not throw much light on hybridism, but agrees well with most of the ascertained facts. we may conclude from the fact of a single spermatozoon or pollen-grain being insufficient for impregnation, that a certain number of gemmules derived from each cell or unit are required for the development of each part. from the occurrence of parthenogenesis, more especially in the case of the silk-moth, in which the embryo is often partially formed, we may also infer that the female element includes nearly sufficient gemmules of all kinds for independent development, so that when united with the male element the gemmules must be superabundant. now, as a general rule, when two species or races are crossed reciprocally, the offspring do not differ, and this shows that both sexual elements agree in power, in accordance with the view that they include the same gemmules. hybrids and mongrels are generally intermediate in character between the two parent-forms, yet occasionally they closely resemble one parent in one part and the other parent in another part, or even in their whole structure: nor is this difficult to understand on { } the admission that the gemmules in the fertilised germ are superabundant in number, and that those derived from one parent have some advantage in number, affinity, or vigour over those derived from the other parent. crossed forms sometimes exhibit the colour or other characters of either parent in stripes or blotches; and this may occur in the first generation, or through reversion in succeeding bud and seminal generations, as in the several instances given in the eleventh chapter. in these cases we must follow naudin,[ ] and admit that the "essence" or "element" of the two species, which terms i should translate into the gemmules, have an affinity for their own kind, and thus separate themselves into distinct stripes or blotches; and reasons were given, when discussing in the fifteenth chapter the incompatibility of certain characters to unite, for believing in such mutual affinity. when two forms are crossed, one is not rarely found to be prepotent in the transmission of character over the other; and this we can explain only by again assuming that the one form has some advantage in the number, vigour, or affinity of its gemmules, except in those cases, where certain characters are present in the one form and latent in the other. for instance, there is a latent tendency in all pigeons to become blue, and, when a blue pigeon is crossed with one of any other colour, the blue tint is generally prepotent. when we consider latent characters, the explanation of this form of prepotency will be obvious. when one species is crossed with another it is notorious that they do not yield the full or proper number of offspring; and we can only say on this head that, as the development of each organism depends on such nicely-balanced affinities between a host of gemmules and developing cells or units, we need not feel at all surprised that the commixture of gemmules derived from two distinct species should lead to a partial or complete failure of development. with respect to the sterility of hybrids produced from the union of two distinct species, it was shown in the nineteenth chapter that this depends exclusively on the reproductive organs being specially affected; but why these organs should be thus affected we do not know, any more than { } why unnatural conditions of life, though compatible with health, should cause sterility; or why continued close interbreeding, or the illegitimate unions of dimorphic and trimorphic plants, induce the same result. the conclusion that the reproductive organs alone are affected, and not the whole organisation, agrees perfectly with the unimpaired or even increased capacity in hybrid plants for propagation by buds; for this implies, according to our hypothesis, that the cells of the hybrids throw off hybridised cell-gemmules, which become aggregated into buds, but fail to become aggregated within the reproductive organs, so as to form the sexual elements. in a similar manner many plants, when placed under unnatural conditions, fail to produce seed, but can readily be propagated by buds. we shall presently see that pangenesis agrees well with the strong tendency to reversion exhibited by all crossed animals and plants. it was shown in the discussion on graft-hybrids that there is some reason to believe that portions of cellular tissue taken from distinct plants become so intimately united, as afterwards occasionally to produce crossed or hybridised buds. if this fact were fully established, it would, by the aid of our hypothesis, connect gemmation and sexual reproduction in the closest manner. abundant evidence has been advanced proving that pollen taken from one species or variety and applied to the stigma of another sometimes directly affects the tissues of the mother-plant. it is probable that this occurs with many plants during fertilisation, but can only be detected when distinct forms are crossed. on any ordinary theory of reproduction this is a most anomalous circumstance, for the pollen-grains are manifestly adapted to act on the ovule, but in these cases they act on the colour, texture, and form of the coats of the seeds, on the ovarium itself, which is a modified leaf, and even on the calyx and upper part of the flower-peduncle. in accordance with the hypothesis of pangenesis pollen includes gemmules, derived from every part of the organisation, which diffuse themselves and multiply by self-division; hence it is not surprising that gemmules within the pollen, which are derived from the parts near the reproductive organs, should sometimes be able to affect the same parts, whilst still undergoing development, in the mother-plant. { } as, during all the stages of development, the tissues of plants consist of cells, and as new cells are not known to be formed between, or independently of, pre-existing cells, we must conclude that the gemmules derived from the foreign pollen do not become developed merely in contact with pre-existing cells, but actually penetrate the nascent cells of the mother-plant. this process may be compared with the ordinary act of fertilisation, during which the contents of the pollen-tubes penetrate the closed embryonic sack within the ovule, and determine the development of the embryo. according to this view, the cells of the mother-plant may almost literally be said to be fertilised by the gemmules derived from the foreign pollen. with all organisms, as we shall presently see, the cells or organic units of the embryo during the successive stages of development may in like manner be said to be fertilised by the gemmules of the cells, which come next in the order of formation. animals, when capable of sexual reproduction, are fully developed, and it is scarcely possible that the male element should affect the tissues of the mother in the same direct manner as with plants; nevertheless it is certain that her ovaria are sometimes affected by a previous impregnation, so that the ovules subsequently fertilised by a distinct male are plainly influenced in character; and this, as in the case of foreign pollen, is intelligible through the diffusion, retention, and action of the gemmules included within the spermatozoa of the previous male. each organism reaches maturity through a longer or shorter course of development. the changes may be small and insensibly slow, as when a child grows into a man, or many, abrupt, and slight, as in the metamorphoses of certain ephemerous insects, or again few and strongly marked, as with most other insects. each part may be moulded within a previously existing and corresponding part, and in this case it will appear, falsely as i believe, to be formed from the old part; or it may be developed within a wholly distinct part of the body, as in the extreme cases of metagenesis. an eye, for instance, may be developed at a spot where no eye previously existed. we have also seen that allied organic beings in the course of their metamorphoses sometimes attain nearly the same structure after passing { } through widely different forms; or conversely, after passing through nearly the same early forms, arrive at a widely different termination. in these cases it is very difficult to believe that the early cells or units possess the inherent power, independently of any external agent, of producing new structures wholly different in form, position, and function. but these cases become plain on the hypothesis of pangenesis. the organic units, during each stage of development, throw off gemmules, which, multiplying, are transmitted to the offspring. in the offspring, as soon as any particular cell or unit in the proper order of development becomes partially developed, it unites with (or to speak metaphorically is fertilised by) the gemmule of the next succeeding cell, and so onwards. now, supposing that at any stage of development, certain cells or aggregates of cells had been slightly modified by the action of some disturbing cause, the cast-off gemmules or atoms of the cell-contents could hardly fail to be similarly affected, and consequently would reproduce the same modification. this process might be repeated until the structure of the part at this particular stage of development became greatly changed, but this would not necessarily affect other parts whether previously or subsequently developed. in this manner we can understand the remarkable independence of structure in the successive metamorphoses, and especially in the successive metageneses of many animals. the term growth ought strictly to be confined to mere increase of size, and development to change of structure.[ ] now, a child is said to grow into a man, and a foal into a horse, but, as in these cases there is much change of structure, the process properly belongs to the order of development. we have indirect evidence of this in many variations and diseases supervening during so-called growth at a particular period, and being inherited at a corresponding period. in the case, however, of diseases which supervene during old age, subsequently to the ordinary period of procreation, and which nevertheless are sometimes inherited, as occurs with brain and heart complaints, we { } must suppose that the organs were in fact affected at an earlier age and threw off at this period affected gemmules; but that the affection became visible or injurious only after the prolonged growth of the part in the strict sense of the word. in all the changes of structure which regularly supervene during old age, we see the effects of deteriorated growth, and not of true development. in the so-called process of _alternate generation_ many individuals are generated asexually during very early or later stages of development. these individuals may closely resemble the preceding larval form, but generally are wonderfully dissimilar. to understand this process we must suppose that at a certain stage of development the gemmules are multiplied at an unusual rate, and become aggregated by mutual affinity at many centres of attraction, or buds. these buds, it may be remarked, must include gemmules not only of all the succeeding but likewise of all the preceding stages of development; for when mature they have the power of transmitting by sexual generation gemmules of all the stages, however numerous these may be. it was shown in the first part, at least in regard to animals, that the new beings which are thus at any period asexually generated do not retrograde in development--that is, they do not pass through those earlier stages, through which the fertilised germ of the same animal has to pass; and an explanation of this fact was attempted as far as the final or teleological cause is concerned. we can likewise understand the proximate cause, if we assume, and the assumption is far from improbable, that buds, like chopped-up fragments of a hydra, are formed of tissue which has already passed through several of the earlier stages of development; for in this case their component cells or units would not unite with the gemmules derived from the earlier-formed cells, but only with those which came next in the order of development. on the other hand, we must believe that, in the sexual elements, or probably in the female alone, gemmules of certain primordial cells are present; and these, as soon as their development commences, unite in due succession with the gemmules of every part of the body, from the first to the last period of life. the principle of the independent formation of each part, in { } so far as its development depends on the union of the proper gemmules with certain nascent cells, together with the superabundance of the gemmules derived from both parents and self-multiplied, throws light on a widely different group of facts, which on any ordinary view of development appears very strange. i allude to organs which are abnormally multiplied or transposed. thus gold-fish often have supernumerary fins placed on various parts of their bodies. we have seen that, when the tail of a lizard is broken off, a double tail is sometimes reproduced, and when the foot of the salamander is divided longitudinally, additional digits are occasionally formed. when frogs, toads, &c., are born with their limbs doubled, as sometimes occurs, the doubling, as gervais remarks,[ ] cannot be due to the complete fusion of two embryos, with the exception of the limbs, for the larvæ are limbless. the same argument is applicable[ ] to certain insects produced with multiple legs or antennæ, for these are metamorphosed from apodal or antennæless larvæ. alphonse milne-edwards[ ] has described the curious case of a crustacean in which one eye-peduncle supported, instead of a complete eye, only an imperfect cornea, out of the centre of which a portion of an antenna was developed. a case has been recorded[ ] of a man who had during both dentitions a double tooth in place of the left second incisor, and he inherited this peculiarity from his paternal grandfather. several cases are known[ ] of additional teeth having been developed in the palate, more especially with horses, and in the orbit of the eye. certain breeds of sheep bear a whole crowd of horns on their foreheads. hairs occasionally appear in strange situations, as within the ears of the siamese hairy family; and hairs "quite natural in structure" have been observed "within the substance of the brain."[ ] as many as five spurs have been seen on both legs in certain game-fowls. in the polish fowl the male is ornamented with a topknot of hackles { } like those on his neck, whilst the female has one of common feathers. in feather-footed pigeons and fowls, feathers like those on the wing arise from the outer side of the legs and toes. even the elemental parts of the same feather may be transposed; for in the sebastopol goose, barbules are developed on the divided filaments of the shaft. analogous cases are of such frequent occurrence with plants that they do not strike us with sufficient surprise. supernumerary petals, stamens, and pistils, are often produced. i have seen a leaflet low down in the compound leaf of _vicia sativa_ converted into a tendril, and a tendril possesses many peculiar properties, such as spontaneous movement and irritability. the calyx sometimes assumes, either wholly or by stripes, the colour and texture of the corolla. stamens are so frequently converted, more or less completely, into petals, that such cases are passed over as not deserving notice; but as petals have special functions to perform, namely, to protect the included organs, to attract insects, and in not a few cases to guide their entrance by well-adapted contrivances, we can hardly account for the conversion of stamens into petals merely by unnatural or superfluous nourishment. again, the edge of a petal may occasionally be found including one of the highest products of the plant, namely the pollen; for instance, i have seen in an ophrys a pollen-mass with its curious structure of little packets, united together and to the caudicle by elastic threads, formed between the edges of an upper petal. the segments of the calyx of the common pea have been observed partially converted into carpels, including ovules, and with their tips converted into stigmas. numerous analogous facts could be given.[ ] i do not know how physiologists look at such facts as the foregoing. according to the doctrine of pangenesis, the free and superabundant gemmules of the transposed organs are developed in the wrong place, from uniting with wrong cells or aggregates of cells during their nascent state; and this would follow from a slight modification in the elective affinity of such cells, or possibly of certain gemmules. nor ought we to feel much surprise at the affinities of cells and gemmules varying { } under domestication, when we remember the many curious cases given, in the seventeenth chapter, of cultivated plants which absolutely refuse to be fertilised by their own pollen or by that of the same species, but are abundantly fertile with pollen of a distinct species; for this implies that their sexual elective affinities--and this is the term used by gärtner--have been modified. as the cells of adjoining or homologous parts will have nearly the same nature, they will be liable to acquire by variation each other's elective affinities; and we can thus to a certain extent understand such cases as a crowd of horns on the heads in certain sheep, of several spurs on the leg, and of hackles on the head of the fowl, and with the pigeon the occurrence of wing-feathers on their legs and of membrane between their toes; for the leg is the homologue of the wing. as all the organs of plants are homologous and spring from a common axis, it is natural that they should be eminently liable to transposition. it ought to be observed that when any compound part, such as an additional limb or an antenna, springs from a false position, it is only necessary that the few first gemmules should be wrongly attached; for these whilst developing would attract others in due succession, as in the regrowth of an amputated limb. when parts which are homologous and similar in structure, as the vertebræ in snakes or the stamens in polyandrous flowers, &c., are repeated many times in the same organism, closely allied gemmules must be extremely numerous, as well as the points to which they ought to become united; and, in accordance with the foregoing views, we can to a certain extent understand isid. geoffroy st. hilaire's law, namely, that parts, which are already multiple, are extremely liable to vary in number. the same general principles apply to the fusion of homologous parts; and with respect to mere cohesion there is probably always some degree of fusion, at least near the surface. when two embryos during their early development come into close contact, as both include corresponding gemmules, which must be in all respects almost identical in nature, it is not surprising that some derived from one embryo and some from the other should unite at the point of contact with a single nascent cell or aggregate of cells, and thus give rise to a single part or organ. for instance, two embryos might thus come to have on their { } adjoining sides a single symmetrical arm, which in one sense will have been formed by the fusion of the bones, muscles, &c., belonging to the arms of both embryos. in the case of the fish described by lereboullet, in which a double head was seen gradually to fuse into a single one, the same process must have taken place, together with the absorption of all the parts which had been already formed. these cases are exactly the reverse of those in which a part is doubled either spontaneously or after an injury; for in the case of doubling, the superabundant gemmules of the same part are separately developed in union with adjoining points; whilst in the case of fusion the gemmules derived from two homologous parts become mingled and form a single part; or it may be that the gemmules from one of two adjoining embryos alone become developed. * * * * * variability often depends, as i have attempted to show, on the reproductive organs being injuriously affected by changed conditions; and in this case the gemmules derived from the various parts of the body are probably aggregated in an irregular manner, some superfluous and others deficient. whether a superabundance of gemmules, together with fusion during development, would lead to the increased size of any part cannot be told; but we can see that their partial deficiency, without necessarily leading to the entire abortion of the part, might cause considerable modifications; for in the same manner as a plant, if its own pollen be excluded, is easily hybridised, so, in the case of a cell, if the properly succeeding gemmules were absent, it would probably combine easily with other and allied gemmules. we see this in the case of imperfect nails growing on the stumps of amputated fingers,[ ] for the gemmules of the nails have manifestly been developed at the nearest point. in variations caused by the direct action of changed conditions, whether of a definite or indefinite nature, as with the fleeces of sheep in hot countries, with maize grown in cold countries, with inherited gout, &c., the tissues of the body, according to the doctrine of pangenesis, are directly affected by the new conditions, and consequently throw off modified gemmules, which are transmitted with their newly acquired peculiarities to the offspring. on any ordinary view it is unintelligible how changed { } conditions, whether acting on the embryo, the young or adult animal, can cause inherited modifications. it is equally or even more unintelligible on any ordinary view, how the effects of the long-continued use or disuse of any part, or of changed habits of body or mind, can be inherited. a more perplexing problem can hardly be proposed; but on our view we have only to suppose that certain cells become at last not only functionally but structurally modified; and that these throw off similarly modified gemmules. this may occur at any period of development, and the modification will be inherited at a corresponding period; for the modified gemmules will unite in all ordinary cases with the proper preceding cells, and they will consequently be developed at the same period at which the modification first arose. with respect to mental habits or instincts, we are so profoundly ignorant on the relation between the brain and the power of thought that we do not know whether an inveterate habit or trick induces any change in the nervous system; but when any habit or other mental attribute, or insanity, is inherited, we must believe that some actual modification is transmitted;[ ] and this implies, according to our hypothesis, that gemmules derived from modified nerve-cells are transmitted to the offspring. it is generally, perhaps always, necessary that an organism should be exposed during several generations to changed conditions or habits, in order that any modification in the structure of the offspring should ensue. this may be partly due to the changes not being at first marked enough to catch the attention, but this explanation is insufficient; and i can account for the fact, only by the assumption, which we shall see under the head of reversion is strongly supported, that gemmules derived from each cell before it had undergone the least modification are transmitted in large numbers to successive generations, but that the gemmules derived from the same cells after modification, naturally go on increasing under the same favouring conditions, until at last they become sufficiently numerous to overpower and supplant the old gemmules. another difficulty may be here noticed; we have seen that { } there is an important difference in the frequency, though not in the nature, of the variations in plants propagated by sexual and asexual generation. as far as variability depends on the imperfect action of the reproductive organs under changed conditions, we can at once see why seedlings should be far more variable than plants propagated by buds. we know that extremely slight causes,--for instance, whether a tree has been grafted or grows on its own stock, the position of the seeds within the capsule, and of the flowers on the spike,--sometimes suffice to determine the variation of a plant, when raised from seed. now, it is probable, as explained when discussing alternate generation, that a bud is formed of a portion of already differentiated tissue; consequently an organism thus formed does not pass through the earlier phases of development, and cannot be so freely exposed, at the age when its structure would be most readily modified, to the various causes inducing variability; but it is very doubtful whether this is a sufficient explanation of the difficulty. with respect to the tendency to reversion, there is a similar difference between plants propagated from buds and seed. many varieties, whether originally produced from seed or buds, can be securely propagated by buds, but generally or invariably revert by seed. so, also, hybridised plants can be multiplied to any extent by buds, but are continually liable to reversion by seed,--that is, to the loss of their hybrid or intermediate character. i can offer no satisfactory explanation of this fact. here is a still more perplexing case: certain plants with variegated leaves, phloxes with striped flowers, barberries with seedless fruit, can all be securely propagated by the buds on cuttings; but the buds developed from the roots of these cuttings almost invariably lose their character and revert to their former condition. finally, we can see on the hypothesis of pangenesis that variability depends on at least two distinct groups of causes. firstly, on the deficiency, superabundance, fusion, and transposition of gemmules, and on the redevelopment of those which have long been dormant. in these cases the gemmules themselves have undergone no modification; but the mutations in the above respects will amply account for much fluctuating { } variability. secondly, in the cases in which the organisation has been modified by changed conditions, the increased use or disuse of parts, or any other cause, the gemmules cast off from the modified units of the body will be themselves modified, and, when sufficiently multiplied, will be developed into new and changed structures. * * * * * turning now to inheritance: if we suppose a homogeneous gelatinous protozoon to vary and assume a reddish colour, a minute separated atom we aid naturally, as it grew to full size, retain the same colour; and we should have the simplest form of inheritance.[ ] precisely the same view may be extended to the infinitely numerous and diversified units of which the whole body in one of the higher animals is composed; and the separated atoms are our gemmules. we have already sufficiently discussed the inheritance of the direct effects of changed conditions, and of increased use or disuse of parts, and, by implication, the important principle of inheritance at corresponding ages. these groups of facts are to a large extent intelligible on the hypothesis of pangenesis, and on no other hypothesis as yet advanced. a few words must be added on the complete abortion or suppression of organs. when a part becomes diminished by disuse prolonged during many generations, the principle of economy of growth, as previously explained, will tend to reduce it still further; but this will not account for the complete or almost complete obliteration of, for instance, a minute papilla of cellular tissue representing a pistil, or of a microscopically minute nodule of bone representing a tooth. in certain cases of suppression not yet completed, in which a rudiment occasionally reappears through reversion, diffused gemmules derived from this part must, according to our view, still exist; hence we must suppose that the cells, in union with which the rudiment was formerly developed, in these cases fail in their affinity for such gemmules. but in the cases of complete and final abortion the gemmules themselves no doubt have perished; nor is this { } in any way improbable, for, though a vast number of active and long-dormant gemmules are diffused and nourished in each living creature, yet there must be some limit to their number; and it appears natural that gemmules derived from an enfeebled and useless rudiment would be more liable to perish than those derived from other parts which are still in full functional activity. with respect to mutilations, it is certain that a part may be removed or injured during many generations, and no inherited result follow; and this is an apparent objection to the hypothesis which will occur to every one. but, in the first place, a being can hardly be intentionally mutilated during its early stages of growth whilst in the womb or egg; and such mutilations, when naturally caused, would appear like congenital deficiencies, which are occasionally inherited. in the second place, according to our hypothesis, gemmules multiply by self-division and are transmitted from generation to generation; so that during a long period they would be present and ready to reproduce a part which was repeatedly amputated. nevertheless it appears, from the facts given in the twelfth chapter, that in some rare cases mutilations have been inherited, but in most of these the mutilated surface became diseased. in this case it may be conjectured that the gemmules of the lost part were gradually all attracted by the partially diseased surface, and thus perished. although this would occur in the injured individual alone, and therefore in only one parent, yet this might suffice for the inheritance of a mutilation, on the same principle that a hornless animal of either sex, when crossed with a perfect animal of the opposite sex, often transmits its deficiency. the last subject that need here be discussed, namely reversion, rests on the principle that transmission and development, though generally acting in conjunction, are distinct powers; and the transmission of gemmules and their subsequent development show us how the existence of these two distinct powers is possible. we plainly see this distinction in the many cases in which a grandfather transmits to his grandson, through his daughter, characters which she does not, or cannot, possess. why the development of certain characters, not necessarily in any way connected with the reproductive organs, should be confined to one sex alone--that is, why certain cells in one sex { } should unite with and cause the development of certain gemmules--we do not in the least know; but it is the common attribute of most organic beings in which the sexes are separate. the distinction between transmission and development is likewise seen in all ordinary cases of reversion; but before discussing this subject it may be advisable to say a few words on those characters which i have called latent, and which would not be classed under reversion in its usual sense. most, or perhaps all, the secondary characters, which appertain to one sex, lie dormant in the other sex; that is, gemmules capable of development into the secondary male sexual characters are included within the female; and conversely female characters in the male. why in the female, when her ovaria become diseased or fail to act, certain masculine gemmules become developed, we do not clearly know, any more than why when a young bull is castrated his horns continue growing until they almost resemble those of a cow; or why, when a stag is castrated, the gemmules derived from the antlers of his progenitors quite fail to be developed. but in many cases, with variable organic beings, the mutual affinities of the cells and gemmules become modified, so that parts are transposed or multiplied; and it would appear that a slight change in the constitution of an animal, in connection with the state of the reproductive organs, leads to changed affinities in the tissues of various parts of the body. thus, when male animals first arrive at puberty, and subsequently during each recurrent season, certain cells or parts acquire an affinity for certain gemmules, which become developed into the secondary masculine characters; but if the reproductive organs be destroyed, or even temporarily disturbed by changed conditions, these affinities are not excited. nevertheless, the male, before he arrives at puberty, and during the season when the species does not breed, must include the proper gemmules in a latent state. the curious case formerly given of a hen which assumed the masculine characters, not of her own breed but of a remote progenitor, illustrates the connexion between latent sexual characters and ordinary reversion. with those animals and plants which habitually produce several forms, as with certain butterflies described by mr. wallace, in which three female forms and { } the male exist, or as with the trimorphic species of lythrum and oxalis, gemmules capable of reproducing several widely-different forms must be latent in each individual. the same principle of the latency of certain characters, combined with the transposition of organs, may be applied to those singular cases of butterflies and other insects, in which exactly one half or one quarter of the body resembles the male, and the other half or three quarters the female; and when this occurs the opposite sides of the body, separated from each other by a distinct line, sometimes differ in the most conspicuous manner. again, these same principles apply to the cases given in the thirteenth chapter, in which the right and left sides of the body differ to an extraordinary degree, as in the spiral winding of certain shells, and as in the genus verruca among cirripedes; for in these cases it is known that either side indifferently may undergo the same remarkable change of development. reversion, in the ordinary sense of the word, comes into action so incessantly, that it evidently forms an essential part of the general law of inheritance. it occurs with beings, however propagated, whether by buds or seminal generation, and sometimes may even be observed in the same individual as it advances in age. the tendency to reversion is often induced by a change of conditions, and in the plainest manner by the act of crossing. crossed forms are generally at first nearly intermediate in character between their two parents; but in the next generation the offspring generally revert to one or both of their grandparents, and occasionally to more remote ancestors. how can we account for these facts? each organic unit in a hybrid must throw off, according to the doctrine of pangenesis, an abundance of hybridised gemmules, for crossed plants can be readily and largely propagated by buds; but by the same hypothesis there will likewise be present dormant gemmules derived from both pure parent-forms; and as these latter retain their normal condition, they would, it is probable, be enabled to multiply largely during the lifetime of each hybrid. consequently the sexual elements of a hybrid will include both pure and hybridised gemmules; and when two hybrids pair, the combination of pure gemmules derived from the one hybrid with the pure gemmules of the same parts derived from the other would { } necessarily lead to complete reversion of character; and it is, perhaps, not too bold a supposition that unmodified and undeteriorated gemmules of the same nature would be especially apt to combine. pure gemmules in combination with hybridised gemmules would lead to partial reversion. and lastly, hybridised gemmules derived from both parent-hybrids would simply reproduce the original hybrid form.[ ] all these cases and degrees of reversion incessantly occur. it was shown in the fifteenth chapter that certain characters are antagonistic to each other or do not readily blend together; hence, when two animals with antagonistic characters are crossed, it might well happen that a sufficiency of gemmules in the male alone for the reproduction of his peculiar characters, and in the female alone for the reproduction of her peculiar characters, would not be present; and in this case dormant gemmules derived from some remote progenitor might easily gain the ascendency, and cause the reappearance of long-lost characters. for instance, when black and white pigeons, or black and white fowls, are crossed,--colours which do not readily blend,--blue plumage in the one case, evidently derived from the rock-pigeon, and red plumage in the other case, derived from the wild jungle-cock, occasionally reappear. with uncrossed breeds the same result would follow, under conditions which favoured the multiplication and development of certain dormant gemmules, as when animals become feral and revert to their pristine character. a certain number of gemmules being requisite for the development of each character, as is known to be the case from several spermatozoa or pollen-grains being necessary for fertilisation, and time favouring their multiplication, will together account for the curious cases, insisted on by mr. sedgwick, of certain diseases regularly appearing in alternate generations. this likewise holds good, more or less strictly, with other weakly inherited modifications. hence, as i have heard it remarked, certain diseases appear actually to gain strength by the intermission of a generation. the transmission of dormant gemmules during many successive generations is hardly in itself more improbable, as { } previously remarked, than the retention during many ages of rudimentary organs, or even only of a tendency to the production of a rudiment; but there is no reason to suppose that all dormant gemmules would be transmitted and propagated for ever. excessively minute and numerous as they are believed to be, an infinite number derived, during a long course of modification and descent, from each cell of each progenitor, could not be supported or nourished by the organism. on the other hand, it does not seem improbable that certain gemmules, under favourable conditions, should be retained and go on multiplying for a longer period than others. finally, on the views here given, we certainly gain some clear insight into the wonderful fact that the child may depart from the type of both its parents, and resemble its grandparents, or ancestors removed by many generations. _conclusion._ the hypothesis of pangenesis, as applied to the several great classes of facts just discussed, no doubt is extremely complex; but so assuredly are the facts. the assumptions, however, on which the hypothesis rests cannot be considered as complex in any extreme degree--namely, that all organic units, besides having the power, as is generally admitted, of growing by self-division, throw off free and minute atoms of their contents, that is gemmules. these multiply and aggregate themselves into buds and the sexual elements; their development depends on their union with other nascent cells or units; and they are capable of transmission in a dormant state to successive generations. in a highly organised and complex animal, the gemmules thrown off from each different cell or unit throughout the body must be inconceivably numerous and minute. each unit of each part, as it changes during development, and we know that some insects undergo at least twenty metamorphoses, must throw off its gemmules. all organic beings, moreover, include many dormant gemmules derived from their grandparents and more remote progenitors, but not from all their progenitors. these almost infinitely numerous and minute gemmules must be included in each bud, ovule, spermatozoon, and pollen-grain. such an admission will be declared impossible; but, as previously { } remarked, number and size are only relative difficulties, and the eggs or seeds produced by certain animals or plants are so numerous that they cannot be grasped by the intellect. the organic particles with which the wind is tainted over miles of space by certain offensive animals must be infinitely minute and numerous; yet they strongly affect the olfactory nerves. an analogy more appropriate is afforded by the contagious particles of certain diseases, which are so minute that they float in the atmosphere and adhere to smooth paper; yet we know how largely they increase within the human body, and how powerfully they act. independent organisms exist which are barely visible under the highest powers of our recently-improved microscopes, and which probably are fully as large as the cells or units in one of the higher animals; yet these organisms no doubt reproduce themselves by germs of extreme minuteness, relatively to their own minute size. hence the difficulty, which at first appears insurmountable, of believing in the existence of gemmules so numerous and so small as they must be according to our hypothesis, has really little weight. the cells or units of the body are generally admitted by physiologists to be autonomous, like the buds on a tree, but in a less degree. i go one step further and assume that they throw off reproductive gemmules. thus an animal does not, as a whole, generate its kind through the sole agency of the reproductive system, but each separate cell generates its kind. it has often been said by naturalists that each cell of a plant has the actual or potential capacity of reproducing the whole plant; but it has this power only in virtue of containing gemmules derived from every part. if our hypothesis be provisionally accepted, we must look at all the forms of asexual reproduction, whether occurring at maturity or as in the case of alternate generation during youth, as fundamentally the same, and dependent on the mutual aggregation and multiplication of the gemmules. the regrowth of an amputated limb or the healing of a wound is the same process partially carried out. sexual generation differs in some important respects, chiefly, as it would appear, in an insufficient number of gemmules being aggregated within the separate sexual elements, and probably in the presence of certain primordial cells. the development of each being, including all the { } forms of metamorphosis and metagenesis, as well as the so-called growth of the higher animals, in which structure changes though not in a striking manner, depends on the presence of gemmules thrown off at each period of life, and on their development, at a corresponding period, in union with preceding cells. such cells may be said to be fertilised by the gemmules which come next in the order of development. thus the ordinary act of impregnation and the development of each being are closely analogous processes. the child, strictly speaking, does not grow into the man, but includes germs which slowly and successively become developed and form the man. in the child, as well as in the adult, each part generates the same part for the next generation. inheritance must be looked at as merely a form of growth, like the self-division of a lowly-organised unicellular plant. reversion depends on the transmission from the forefather to his descendants of dormant gemmules, which occasionally become developed under certain known or unknown conditions. each animal and plant may be compared to a bed of mould full of seeds, most of which soon germinate, some lie for a period dormant, whilst others perish. when we hear it said that a man carries in his constitution the seeds of an inherited disease, there is much literal truth in the expression. finally, the power of propagation possessed by each separate cell, using the term in its largest sense, determines the reproduction, the variability, the development and renovation of each living organism. no other attempt, as far as i am aware, has been made, imperfect as this confessedly is, to connect under one point of view these several grand classes of facts. we cannot fathom the marvellous complexity of an organic being; but on the hypothesis here advanced this complexity is much increased. each living creature must be looked at as a microcosm--a little universe, formed of a host of self-propagating organisms, inconceivably minute and as numerous as the stars in heaven. * * * * * { } chapter xxviii. concluding remarks. domestication--nature and causes of variability--selection--divergence and distinctness of character--extinction of races--circumstances favourable to selection by man--antiquity of certain races--the question whether each particular variation has been specially preordained. as summaries have been added to nearly all the chapters, and as, in the chapter on pangenesis, various subjects, such as the forms of reproduction, inheritance, reversion, the causes and laws of variability, &c., have been recently discussed, i will here only make a few general remarks on the more important conclusions which may be deduced from the multifarious details given throughout this work. savages in all parts of the world easily succeed in taming wild animals; and those inhabiting any country or island, when first invaded by man, would probably have been still more easily tamed. complete subjugation generally depends on an animal being social in its habits, and on receiving man as the chief of the herd or family. domestication implies almost complete fertility under new and changed conditions of life, and this is far from being invariably the case. an animal would not have been worth the labour of domestication, at least during early times, unless of service to man. from these circumstances the number of domesticated animals has never been large. with respect to plants, i have shown in the ninth chapter how their varied uses were probably first discovered, and the early steps in their cultivation. man could not have known, when he first domesticated an animal or plant, whether it would flourish and multiply when transported to other countries, therefore he could not have been thus influenced in his choice. we see that the close adaptation of the reindeer and camel to extremely cold and hot countries has not prevented their domestication. still less { } could man have foreseen whether his animals and plants would vary in succeeding generations and thus give birth to new races; and the small capacity of variability in the goose and ass has not prevented their domestication from the remotest epoch. with extremely few exceptions, all animals and plants which have been long domesticated, have varied greatly. it matters not under what climate, or for what purpose, they are kept, whether as food for man or beast, for draught or hunting, for clothing or mere pleasure,--under all these circumstances domesticated animals and plants have varied to a much greater extent than the forms which in a state of nature are ranked as one species. why certain animals and plants have varied more under domestication than others we do not know, any more than why some are rendered more sterile than others under changed conditions of life. but we frequently judge of the amount of variation by the production of numerous and diversified races, and we can clearly see why in many cases this has not occurred, namely, because slight successive variations have not been steadily accumulated; and such variations will never be accumulated when an animal or plant is not closely observed, or much valued, or kept in large numbers. the fluctuating, and, as far as we can judge, never-ending variability of our domesticated productions,--the plasticity of their whole organisation,--is one of the most important facts which we learn from the numerous details given in the earlier chapters of this work. yet domesticated animals and plants can hardly have been exposed to greater changes in their conditions than have many natural species during the incessant geological, geographical, and climatal changes of the whole world. the former will, however, commonly have been exposed to more sudden changes and to less continuously uniform conditions. as man has domesticated so many animals and plants belonging to widely different classes, and as he certainly did not with prophetic instinct choose those species which would vary most, we may infer that all natural species, if subjected to analogous conditions, would, on an average, vary to the same degree. few men at the present day will maintain that animals and plants were created with a tendency to vary, which long remained dormant, in order that fanciers in after ages might { } rear, for instance, curious breeds of the fowl, pigeon, or canary-bird. from several causes it is difficult to judge of the amount of modification which our domestic productions have undergone. in some cases the primitive parent-stock has become extinct, or cannot be recognised with certainty owing to its supposed descendants having been so much modified. in other cases two or more closely allied forms, after being domesticated, have crossed; and then it is difficult to estimate how much of the change ought to be attributed to variation. but the degree to which our domestic breeds have been modified by the crossing of distinct natural forms has probably been exaggerated by some authors. a few individuals of one form would seldom permanently affect another form existing in much greater numbers; for, without careful selection, the stain of the foreign blood would soon be obliterated, and during early and barbarous times, when our animals were first domesticated, such care would seldom have been taken. there is good reason to believe that several of the breeds of the dog, ox, pig, and of some other animals, are respectively descended from distinct wild prototypes; nevertheless the belief in the multiple origin of our domesticated animals has been extended by some few naturalists and by many breeders to an unauthorised extent. breeders refuse to look at the whole subject under a single point of view; i have heard one, who maintained that our fowls were the descendants of at least half-a-dozen aboriginal species, protest that he was in no way concerned with the origin of pigeons, ducks, rabbits, horses, or any other animal. they overlook the improbability of many species having been domesticated at an early and barbarous period. they do not consider the improbability of species having existed in a state of nature which, if like our present domestic breeds, would have been highly abnormal in comparison with all their congeners. they maintain that certain species, which formerly existed, have become extinct or unknown, although the world is now so much better known. the assumption of so much recent extinction is no difficulty in their eyes; for they do not judge of its probability by the facility or difficulty of the extinction of other closely allied wild forms. lastly, { } they often ignore the whole subject of geographical distribution as completely as if its laws were the result of chance. although from the reasons just assigned it is often difficult to judge accurately of the amount of change which our domesticated productions have undergone, yet this can be ascertained in the cases in which we know that all the breeds are descended from a single species, as with the pigeon, duck, rabbit, and almost certainly with the fowl; and by the aid of analogy this is to a certain extent possible in the case of animals descended from several wild stocks. it is impossible to read the details given in the earlier chapters, and in many published works, or to visit our various exhibitions, without being deeply impressed with the extreme variability of our domesticated animals and cultivated plants. i have in many instances purposely given details on new and strange peculiarities which have arisen. no part of the organisation escapes the tendency to vary. the variations generally affect parts of small vital or physiological importance, but so it is with the differences which exist between closely allied species. in these unimportant characters there is often a greater difference between the breeds of the same species than between the natural species of the same genus, as isidore geoffroy has shown to be the case with size, and as is often the case with the colour, texture, form, &c., of the hair, feathers, horns, and other dermal appendages. it has often been asserted that important parts never vary under domestication, but this is a complete error. look at the skull of the pig in any one of the highly improved breeds, with the occipital condyles and other parts greatly modified; or look at that of the niata ox. or again, in the several breeds of the rabbit, observe the elongated skull, with the differently shaped occipital foramen, atlas, and other cervical vertebræ. the whole shape of the brain, together with the skull, has been modified in polish fowls; in other breeds of the fowl the number of the vertebræ and the forms of the cervical vertebræ have been changed. in certain pigeons the shape of the lower jaw, the relative length of the tongue, the size of the nostrils and eyelids, the number and shape of the ribs, the form and size of the oesophagus, have all varied. in certain quadrupeds the length of the intestines has been much increased or { } diminished. with plants we see wonderful differences in the stones of various fruits. in the cucurbitaceæ several highly important characters have varied, such as the sessile position of the stigmas on the ovarium, the position of the carpels within the ovarium, and its projection out of the receptacle. but it would be useless to run through the many facts given in the earlier chapters. it is notorious how greatly the mental disposition, tastes, habits, consensual movements, loquacity or silence, and the tone of voice have varied and been inherited with our domesticated animals. the dog offers the most striking instance of changed mental attributes, and these differences cannot be accounted for by descent from distinct wild types. new mental characters have certainly often been acquired, and natural ones lost, under domestication. new characters may appear and disappear at any stage of growth, and be inherited at a corresponding period. we see this in the difference between the eggs of various breeds of the fowl, and in the down on chickens; and still more plainly in the differences between the caterpillars and cocoons of various breeds of the silk-moth. these facts, simple as they appear, throw light on the characters which distinguish the larval and adult states of natural species, and on the whole great subject of embryology. new characters are liable to become attached exclusively to that sex in which they first appeared, or they may be developed in a much higher degree in the one than the other sex; or again, after having become attached to one sex, they may be partially transferred to the opposite sex. these facts, and more especially the circumstance that new characters seem to be particularly liable, from some unknown cause, to become attached to the male sex, have an important bearing on the acquirement by animals in a state of nature of secondary sexual characters. it has sometimes been said that our domestic productions do not differ in constitutional peculiarities, but this cannot be maintained. in our improved cattle, pigs, &c., the period of maturity, including that of the second dentition, has been much hastened. the period of gestation varies much, but has been modified in a fixed manner in only one or two cases. in { } our poultry and pigeons the acquirement of down and of the first plumage by the young, and of the secondary sexual characters by the males, differ. the number of moults through which the larvæ of silk-moths pass, varies. the tendency to fatten, to yield much milk, to produce many young or eggs at a birth or during life, differs in different breeds. we find different degrees of adaptation to climate, and different tendencies to certain diseases, to the attacks of parasites, and to the action of certain vegetable poisons. with plants, adaptation to certain soils, as with some kinds of plums, the power of resisting frost, the period of flowering and fruiting, the duration of life, the period of shedding the leaves and of retaining them throughout the winter, the proportion and nature of certain chemical compounds in the tissues or seeds, all vary. there is, however, one important constitutional difference between domestic races and species; i refer to the sterility which almost invariably follows, in a greater or less degree, when species are crossed, and to the perfect fertility of the most distinct domestic races, with the exception of a very few plants, when similarly crossed. it certainly appears a remarkable fact that many closely allied species which in appearance differ extremely little should yield when united only a few, more or less sterile offspring, or none at all; whilst domestic races which differ conspicuously from each other, are when united remarkably fertile, and yield perfectly fertile offspring. but this fact is not in reality so inexplicable as it at first appears. in the first place, it was clearly shown in the nineteenth chapter that the sterility of crossed species does not closely depend on differences in their external structure or general constitution, but results exclusively from differences in the reproductive system, analogous with those which cause the lessened fertility of the illegitimate unions and illegitimate offspring of dimorphic and trimorphic plants. in the second place, the pallasian doctrine, that species after having been long domesticated lose their natural tendency to sterility when crossed, has been shown to be highly probable; we can scarcely avoid this conclusion when we reflect on the parentage and present fertility of the several breeds of the dog, of indian and european cattle, sheep, and pigs. hence it would be unreasonable to expect that races formed under domestication { } should acquire sterility when crossed, whilst at the same time we admit that domestication eliminates the normal sterility of crossed species. why with closely allied species their reproductive systems should almost invariably have been modified in so peculiar a manner as to be mutually incapable of acting on each other--though in unequal degrees in the two sexes, as shown by the difference in fertility between reciprocal crosses in the same species--we do not know, but may with much probability infer the cause to be as follows. most natural species have been habituated to nearly uniform conditions of life for an incomparably longer period of time than have domestic races; and we positively know that changed conditions exert an especial and powerful influence on the reproductive system. hence this difference in habituation may well account for the different action of the reproductive organs when domestic races and when species are crossed. it is a nearly analogous fact, that most domestic races may be suddenly transported from one climate to another, or be placed under widely different conditions, and yet retain their fertility unimpaired; whilst a multitude of species subjected to lesser changes are rendered incapable of breeding. with the exception of fertility, domestic varieties resemble species when crossed in transmitting their characters in the same unequal manner to their offspring, in being subject to the prepotency of one form over the other, and in their liability to reversion. by repeated crosses a variety or a species may be made completely to absorb another. varieties, as we shall see when we treat of their antiquity, sometimes inherit their new characters almost, or even quite, as firmly as species. with both, the conditions leading to variability and the laws governing its nature appear to be the same. domestic varieties can be classed in groups under groups, like species under genera, and these under families and orders; and the classification may be either artificial,--that is, founded on any arbitrary character,--or natural. with varieties a natural classification is certainly founded, and with species is apparently founded, on community of descent, together with the amount of modification which the forms have undergone. the characters by which domestic varieties differ from each other are more { } variable than those distinguishing species, though hardly more so than with certain protean species; but this greater degree of variability is not surprising, as varieties have generally been exposed within recent times to fluctuating conditions of life, are much more liable to have been crossed, and are still in many cases undergoing, or have recently undergone, modification by man's methodical or unconscious selection. domestic varieties as a general rule certainly differ from each other in less important parts of their organisation than do species; and when important differences occur, they are seldom firmly fixed; but this fact is intelligible if we consider man's method of selection. in the living animal or plant he cannot observe internal modifications in the more important organs; nor does he regard them as long as they are compatible with health and life. what does the breeder care about any slight change in the molar teeth of his pigs, or for an additional molar tooth in the dog; or for any change in the intestinal canal or other internal organ? the breeder cares for the flesh of his cattle being well marbled with fat, and for an accumulation of fat within the abdomen of his sheep, and this he has effected. what would the floriculturist care for any change in the structure of the ovarium or of the ovules? as important internal organs are certainly liable to numerous slight variations, and as these would probably be inherited, for many strange monstrosities are transmitted, man could undoubtedly effect a certain amount of change in these organs. when he has produced any modification in an important part, it has generally been unintentionally in correlation with some other conspicuous part, as when he has given ridges and protuberances to the skulls of fowls, by attending to the form of the comb, and in the case of the polish fowl to the plume of feathers on the head. by attending to the external form of the pouter-pigeon, he has enormously increased the size of the oesophagus, and has added to the number of the ribs, and given them greater breadth. with the carrier-pigeon, by increasing, through steady selection, the wattles on the upper mandible, he has greatly modified the form of the lower mandible; and so in many other cases. natural species, on the other hand, have been modified exclusively for their own good, to fit them for infinitely { } diversified conditions of life, to avoid enemies of all kinds, and to struggle against a host of competitors. hence, under such complex conditions, it would often happen that modifications of the most varied kinds, in important as well as in unimportant parts, would be advantageous or even necessary; and they would slowly but surely be acquired through the survival of the fittest. various indirect modifications would likewise arise through the law of correlated variation. domestic breeds often have an abnormal or semi-monstrous character, as the italian greyhound, bulldog, blenheim spaniel, and bloodhound amongst dogs,--some breeds of cattle and pigs, several breeds of the fowl, and the chief breeds of the pigeon. the differences between such abnormal breeds occur in parts which in closely-allied natural species differ but slightly or not at all. this may be accounted for by man's often selecting, especially at first, conspicuous and semi-monstrous deviations of structure. we should, however, be cautious in deciding what deviations ought to be called monstrous: there can hardly be a doubt that, if the brush of horse-like hair on the breast of the turkey-cock had first appeared on the domesticated bird, it would have been considered a monstrosity; the great plume of feathers on the head of the polish cock has been thus designated, though plumes are common with many kinds of birds; we might call the wattle or corrugated skin round the base of the beak of the english carrier-pigeon a monstrosity, but we do not thus speak of the globular fleshy excrescence at the base of the beak of the male _carpophaga oceanica_. some authors have drawn a wide distinction between artificial and natural breeds; although in extreme cases the distinction is plain, in many other cases an arbitrary line has to be drawn. the difference depends chiefly on the kind of selection which has been applied. artificial breeds are those which have been intentionally improved by man; they frequently have an unnatural appearance, and are especially liable to loss of excellence through reversion and continued variability. the so-called natural breeds, on the other hand, are those which are now found in semi-civilised countries, and which formerly inhabited separate districts in nearly all the european kingdoms. they have been rarely acted on by man's { } intentional selection; more frequently, it is probable, by unconscious selection, and partly by natural selection, for animals kept in semi-civilised countries have to provide largely for their own wants. such natural breeds will also, it may be presumed, have been directly acted on to some extent by the differences, though slight, in the surrounding physical conditions. it is a much more important distinction that some breeds have been from their first origin modified in so slow and insensible a manner, that if we could see their early progenitors we should hardly be able to say when or how the breed first arose; whilst other breeds have originated from a strongly-marked or semi-monstrous deviation of structure, which, however, may subsequently have been augmented by selection. from what we know of the history of the racehorse, greyhound, gamecock, &c., and from their general appearance, we may feel nearly confident that they were formed by a slow process of improvement: and with the carrier-pigeon, as well as with some other pigeons, we know that this has been the case. on the other hand, it is certain that the ancon and mauchamp breeds of sheep, and almost certain that the niata cattle, turnspit and pug-dogs, jumper and frizzled fowls, short-faced tumbler pigeons, hook-billed ducks, &c., and with plants a multitude of varieties, suddenly appeared in nearly the same state as we now see them. the frequency of these cases is likely to lead to the false belief that natural species have often originated in the same abrupt manner. but we have no evidence of the appearance, or at least of the continued procreation, under nature, of abrupt modifications of structure; and various general reasons could be assigned against such a belief: for instance, without separation a single monstrous variation would almost certainly be soon obliterated by crossing. on the other hand, we have abundant evidence of the constant occurrence under nature of slight individual differences of the most diversified kinds; and thus we are led to conclude that species have generally originated by the natural selection, not of abrupt modifications, but of extremely slight differences. this process may be strictly compared with the slow and gradual improvement of the racehorse, greyhound, and gamecock. as every detail of structure in each species is closely adapted to its general { } habits of life, it will rarely happen that one part alone will be modified; but the co-adapted modifications, as formerly shown, need not be absolutely simultaneous. many variations, however, are from the first connected by the law of correlation. hence it follows that even closely-allied species rarely or never differ from each other by some one character alone; and this same remark applies to a certain extent to domestic races; for these, if they differ much, generally differ in many respects. some naturalists boldly insist[ ] that species are absolutely distinct productions, never passing by intermediate links into each other; whilst they maintain that domestic varieties can always be connected either with each other or with their parent-forms. but if we could always find the links between the several breeds of the dog, horse, cattle, sheep, pigs, &c., the incessant doubts whether they are descended from one or several species would not have arisen. the greyhound genus, if such a term may be used, cannot be closely connected with any other breed, unless, perhaps, we go back to the ancient egyptian monuments. our english bulldog also forms a very distinct breed. in all these cases crossed breeds must of course be excluded, for the most distinct natural species can thus be connected. by what links can the cochin fowl be closely united with others? by searching for breeds still preserved in distant lands, and by going back to historical records, tumbler-pigeons, carriers, and barbs can be closely connected with the parent rock-pigeon; but we cannot thus connect the turbit or the pouter. the degree of distinctness between the various domestic breeds depends on the amount of modification which they have undergone, and especially on the neglect and final extinction of the linking, intermediate, and less valued forms. it has often been argued that no light is thrown, from the admitted changes of domestic races, on the changes which natural species are believed to undergo, as the former are said to be mere temporary productions, always reverting, as soon as they become feral, to their pristine form. this argument has been well combated by mr. wallace;[ ] and full details were given in the thirteenth chapter, showing that the tendency to reversion in feral { } animals and plants has been greatly exaggerated, though no doubt to a certain extent it exists. it would be opposed to all the principles inculcated in this work, if domestic animals, when exposed to new conditions and compelled to struggle for their own wants against a host of foreign competitors, were not in the course of time in some manner modified. it should also be remembered that many characters lie latent in all organic beings ready to be evolved under fitting conditions; and in breeds modified within recent times the tendency to reversion is particularly strong. but the antiquity of various breeds clearly proves that they remain nearly constant as long as their conditions of life remain the same. it has been boldly maintained by some authors that the amount of variation to which our domestic productions are liable is strictly limited; but this is an assertion resting on little evidence. whether or not the amount in any particular direction is fixed, the tendency to general variability seems unlimited. cattle, sheep, and pigs have been domesticated and have varied from the remotest period, as shown by the researches of rütimeyer and others, yet these animals have, within quite recent times, been improved in an unparalleled degree; and this implies continued variability of structure. wheat, as we know from the remains found in the swiss lake-habitations, is one of the most anciently cultivated plants, yet at the present day new and better varieties occasionally arise. it may be that an ox will never be produced of larger size or finer proportions than our present animals, or a race-horse fleeter than eclipse, or a gooseberry larger than the london variety; but he would be a bold man who would assert that the extreme limit in these respects has been finally attained. with flowers and fruit it has repeatedly been asserted that perfection has been reached, but the standard has soon been excelled. a breed of pigeons may never be produced with a beak shorter than that of the present short-faced tumbler, or with one longer than that of the english carrier, for these birds have weak constitutions and are bad breeders; but the shortness and length of the beak are the points which have been steadily improved during at least the last years; and some of the best judges deny that the goal has yet been reached. we may, also, reasonably suspect, from what { } we see in natural species of the variability of extremely modified parts, that any structure, after remaining constant during a long series of generations, would, under new and changed conditions of life, recommence its course of variability, and might again be acted on by selection. nevertheless, as mr. wallace[ ] has recently remarked with much force and truth, there must be both with natural and domestic productions a limit to change in certain directions; for instance, there must be a limit to the fleetness of any terrestrial animal, as this will be determined by the friction to be overcome, the weight to be carried, and the power of contraction in the muscular fibres. the english racehorse may have reached this limit; but it already surpasses in fleetness its own wild progenitor, and all other equine species. it is not surprising, seeing the great difference between many domestic breeds, that some few naturalists have concluded that all are descended from distinct aboriginal stocks, more especially as the principle of selection has been ignored, and the high antiquity of man, as a breeder of animals, has only recently become known. most naturalists, however, freely admit that various extremely dissimilar breeds are descended from a single stock, although they do not know much about the art of breeding, cannot show the connecting links, nor say where and when the breeds arose. yet these same naturalists will declare, with an air of philosophical caution, that they can never admit that one natural species has given birth to another until they behold all the transitional steps. but fanciers have used exactly the same language with respect to domestic breeds; thus an author of an excellent treatise says he will never allow that carrier and fantail pigeons are the descendants of the wild rock-pigeon, until the transitions have "actually been observed, and can be repeated whenever man chooses to set about the task." no doubt it is difficult to realise that slight changes added up during long centuries can produce such results; but he who wishes to understand the origin of domestic breeds or natural species must overcome this difficulty. the causes inducing and the laws governing variability have been so lately discussed, that i need here only enumerate the leading points. as domesticated organisms are much more { } liable to slight deviations of structure and to monstrosities, than species living under their natural conditions, and as widely-ranging species vary more than those which inhabit restricted areas, we may infer that variability mainly depends on changed conditions of life. we must not overlook the effects of the unequal combination of the characters derived from both parents, nor reversion to former progenitors. changed conditions have an especial tendency to render the reproductive organs more or less impotent, as shown in the chapter devoted to this subject; and these organs consequently often fail to transmit faithfully the parental characters. changed conditions also act directly and definitely on the organisation, so that all or nearly all the individuals of the same species thus exposed become modified in the same manner; but why this or that part is especially affected we can seldom or never say. in most cases, however, of the direct action of changed conditions, independently of the indirect variability caused by the reproductive organs being affected, indefinite modifications are the result; in nearly the same manner as exposure to cold or the absorption of the same poison affects different individuals in various ways. we have reason to suspect that an habitual excess of highly nutritious food, or an excess relatively to the wear and tear of the organisation from exercise, is a powerful exciting cause of variability. when we see the symmetrical and complex outgrowths, caused by a minute atom of the poison of a gall-insect, we may believe that slight changes in the chemical nature of the sap or blood would lead to extraordinary modifications of structure. the increased use of a muscle with its various attached parts, and the increased activity of a gland or other organ, lead to their increased development. disuse has a contrary effect. with domesticated productions organs sometimes become rudimentary through abortion; but we have no reason to suppose that this has ever followed from mere disuse. with natural species, on the contrary, many organs appear to have been rendered rudimentary through disuse, aided by the principle of the economy of growth, and by the hypothetical principle discussed in the last chapter, namely, the final destruction of the germs or gemmules of such useless parts. this difference may be partly { } accounted for by disuse having acted on domestic forms for an insufficient length of time, and partly from their exemption from any severe struggle for existence, entailing rigid economy in the development of each part, to which all species under nature are subjected. nevertheless the law of compensation or balancement apparently affects, to a certain extent, our domesticated productions. we must not exaggerate the importance of the definite action of changed conditions in modifying all the individuals of the same species in the same manner, or of use and disuse. as every part of the organisation is highly variable, and as variations are so easily selected, both consciously and unconsciously, it is very difficult to distinguish between the effects of the selection of indefinite variations, and the direct action of the conditions of life. for instance, it is possible that the feet of our water-dogs, and of the american dogs which have to travel much over the snow, may have become partially webbed from the stimulus of widely extending their toes; but it is far more probable that the webbing, like the membrane between the toes of certain pigeons, spontaneously appeared and was afterwards increased by the best swimmers and the best snow-travellers being preserved during many generations. a fancier who wished to decrease the size of his bantams or tumbler-pigeons would never think of starving them, but would select the smallest individuals which spontaneously appeared. quadrupeds are sometimes born destitute of hair, and hairless breeds have been formed, but there is no reason to believe that this is caused by a hot climate. within the tropics heat often causes sheep to lose their fleeces, and on the other hand wet and cold act as a direct stimulus to the growth of hair; it is, however, possible that these changes may merely be an exaggeration of the regular yearly change of coat; and who will pretend to decide how far this yearly change, or the thick fur of arctic animals, or as i may add their white colour, is due to the direct action of a severe climate, and how far to the preservation of the best protected individuals during a long succession of generations? of all the laws governing variability, that of correlation is the most important. in many cases of slight deviations of structure as well as of grave monstrosities, we cannot even { } conjecture what is the nature of the bond of connexion. but between homologous parts--between the fore and hind limbs--between the hair, hoofs, horns, and teeth--we can see that parts which are closely similar during their early development, and which are exposed to similar conditions, would be liable to be modified in the same manner. homologous parts, from having the same nature, are apt to blend together and, when many exist, to vary in number. although every variation is either directly or indirectly caused by some change in the surrounding conditions, we must never forget that the nature of the organisation which is acted on essentially governs the result. distinct organisms, when placed under similar conditions, vary in different manners, whilst closely-allied organisms under dissimilar conditions often vary in nearly the same manner. we see this in the same modification frequently reappearing at long intervals of time in the same variety, and likewise in the several striking cases given of analogous or parallel varieties. although some of these latter cases are simply due to reversion, others cannot thus be accounted for. from the indirect action of changed conditions on the organisation, through the impaired state of the reproductive organs--from the direct action of such conditions (and this will cause the individuals of the same species either to vary in the same manner, or differently in accordance with slight differences in their constitution)--from the effects of the increased or decreased use of parts,--and from correlation,--the variability of our domesticated productions is complicated in an extreme degree. the whole organisation becomes slightly plastic. although each modification must have its proper exciting cause, and though each is subjected to law, yet we can so rarely trace the precise relation between cause and effect, that we are tempted to speak of variations as if they spontaneously arose. we may even call them accidental, but this must be only in the sense in which we say that a fragment of rock dropped from a height owes its shape to accident. * * * * * it may be worth while briefly to consider the results of the exposure to unnatural conditions of a large number of animals of the same species, allowed to cross freely, with no selection of any { } kind; and afterwards to consider the results when selection is brought into play. let us suppose that wild rock-pigeons were confined in their native land in an aviary, and fed in the same manner as pigeons usually are; and that they were not allowed to increase in number. as pigeons propagate so rapidly, i suppose that a thousand or fifteen hundred birds would have to be annually killed by mere chance. after several generations had been thus reared, we may feel sure that some of the young birds would vary, and the variations would tend to be inherited; for at the present day slight deviations of structure often occur, but, as most breeds are already well established, these modifications are rejected as blemishes. it would be tedious even to enumerate the multitude of points which still go on varying or have recently varied. many variations would occur in correlation, as the length of the wing and tail feathers--the number of the primary wing-feathers, as well as the number and breadth of the ribs, in correlation with the size and form of the body--the number of the scutellæ, with the size of the feet--the length of the tongue, with the length of the beak--the size of the nostrils and eyelids and the form of lower jaw in correlation with the development of wattle--the nakedness of the young with the future colour of the plumage--the size of the feet and beak, and other such points. lastly, as our birds are supposed to be confined in an aviary, they would use their wings and legs but little, and certain parts of the skeleton, such as the sternum and scapulæ and the feet, would in consequence become slightly reduced in size. as in our assumed case many birds have to be indiscriminately killed every year, the chances are against any new variety surviving long enough to breed. and as the variations which arise are of an extremely diversified nature, the chances are very great against two birds pairing which have varied in the same manner; nevertheless, a varying bird even when not thus paired would occasionally transmit its character to its young; and these would not only be exposed to the same conditions which first caused the variation in question to appear, but would in addition inherit from their one modified parent a tendency again to vary in the same manner. so that, if the conditions decidedly tended to induce some particular variation, all the birds might { } in the course of time become similarly modified. but a far commoner result would be, that one bird would vary in one way and another bird in another way; one would be born with a little longer beak, and another with a shorter beak; one would gain some black feathers, another some white or red feathers. and as these birds would be continually intercrossing, the final result would be a body of individuals differing from each other slightly in many ways, yet far more than did the original rock-pigeons. but there would not be the least tendency to the formation of distinct breeds. if two separate lots of pigeons were to be treated in the manner just described, one in england and the other in a tropical country, the two lots being supplied with different food, would they, after many generations had passed, differ? when we reflect on the cases given in the twenty-third chapter, and on such facts as the difference in former times between the breeds of cattle, sheep, &c., in almost every district of europe, we are strongly inclined to admit that the two lots would be differently modified through the influence of climate and food. but the evidence on the definite action of changed conditions is in most cases insufficient; and, with respect to pigeons, i have had the opportunity of examining a large collection of domesticated birds, sent to me by sir w. elliot from india, and they varied in a remarkably similar manner with our european birds. if two distinct breeds were to be confined together in equal numbers, there is reason to suspect that they would to a certain extent prefer pairing with their own kind; but they would likewise intercross. from the greater vigour and fertility of the crossed offspring, the whole body would by this means become interblended sooner than would otherwise have occurred. from certain breeds being prepotent over others, it does not follow that the interblended progeny would be strictly intermediate in character. i have, also, proved that the act of crossing in itself gives a strong tendency to reversion, so that the crossed offspring would tend to revert to the state of the aboriginal rock-pigeon. in the course of time they would probably be not much more heterogeneous in character than in our first case, when birds of the same breed were confined together. { } i have just said that the crossed offspring would gain in vigour and fertility. from the facts given in the seventeenth chapter there can be no doubt of this; and there can be little doubt, though the evidence on this head is not so easily acquired, that long-continued close interbreeding leads to evil results. with hermaphrodites of all kinds, if the sexual elements of the same individual habitually acted on each other, the closest possible interbreeding would be perpetual. therefore we should bear in mind that with all hermaphrodite animals, as far as i can learn, their structure permits and frequently necessitates a cross with a distinct individual. with hermaphrodite plants we incessantly meet with elaborate and perfect contrivances for this same end. it is no exaggeration to assert that, if the use of the talons and tusks of a carnivorous animal, or the use of the viscid threads of a spider's web, or of the plumes and hooks on a seed may be safely inferred from their structure, we may with equal safety infer that many flowers are constructed for the express purpose of ensuring a cross with a distinct plant. from these various considerations, the conclusion arrived at in the chapter just referred to--namely, that great good of some kind is derived from the sexual concourse of distinct individuals--must be admitted. to return to our illustration: we have hitherto assumed that the birds were kept down to the same number by indiscriminate slaughter; but if the least choice be permitted in their preservation and slaughter, the whole result will be changed. should the owner observe any slight variation in one of his birds, and wish to obtain a breed thus characterised, he would succeed in a surprisingly short time by carefully selecting and pairing the young. as any part which has once varied generally goes on varying in the same direction, it is easy, by continually preserving the most strongly marked individuals, to increase the amount of difference up to a high, predetermined standard of excellence. this is methodical selection. if the owner of the aviary, without any thought of making a new breed, simply admired, for instance, short-beaked more than long-beaked birds, he would, when he had to reduce the number, generally kill the latter; and there can be no doubt that he would thus in the course of time sensibly modify his { } stock. it is improbable, if two men were to keep pigeons and act in this manner, that they would prefer exactly the same characters; they would, as we know, often prefer directly opposite characters, and the two lots would ultimately come to differ. this has actually occurred with strains or families of cattle, sheep, and pigeons, which have been long kept and carefully attended to by different breeders without any wish on their part to form new and distinct sub-breeds. this unconscious kind of selection will more especially come into action with animals which are highly serviceable to man; for every one tries to get the best dog, horse, cow, or sheep, and these animals will transmit more or less surely their good qualities to their offspring. hardly any one is so careless as to breed from his worst animals. even savages, when compelled from extreme want to kill some of their animals, would destroy the worst and preserve the best. with animals kept for use and not for mere amusement, different fashions prevail in different districts, leading to the preservation, and consequently to the transmission, of all sorts of trifling peculiarities of character. the same process will have been pursued with our fruit-trees and vegetables, for the best will always have been the most largely cultivated, and will occasionally have yielded seedlings better than their parents. the different strains, just alluded to, which have been raised by different breeders without any wish for such a result, and the unintentional modification of foreign breeds in their new homes, both afford excellent evidence of the power of unconscious selection. this form of selection has probably led to far more important results than methodical selection, and is likewise more important under a theoretical point of view from closely resembling natural selection. for during this process the best or most valued individuals are not separated and prevented crossing with others of the same breed, but are simply preferred and preserved; but this inevitably leads during a long succession of generations to their increase in number and to their gradual improvement; so that finally they prevail to the exclusion of the old parent-form. with our domesticated animals natural selection checks the production of races with any injurious deviation of { } structure. in the case of animals kept by savages and semi-civilised people, which have to provide largely for their own wants under different circumstances, natural selection will probably play a more important part. hence such animals often closely resemble natural species. as there is no limit to man's desire to possess animals and plants more and more useful in any respect, and as the fancier always wishes, from fashion running into extremes, to produce each character more and more strongly pronounced, there is a constant tendency in every breed, through the prolonged action of methodical and unconscious selection, to become more and more different from its parent-stock; and when several breeds have been produced and are valued for different qualities, to differ more and more from each other. this leads to divergence of character. as improved sub-varieties and races are slowly formed, the older and less improved breeds are neglected and decrease in number. when few individuals of any breed exist within the same locality, close interbreeding, by lessening their vigour and fertility, aids in their final extinction. thus the intermediate links are lost, and breeds which have already diverged gain distinctness of character. in the chapters on the pigeon, it was proved by historical details and by the existence of connecting sub-varieties in distant lands that several breeds have steadily diverged in character, and that many old and intermediate sub-breeds have become extinct. other cases could be adduced of the extinction of domestic breeds, as of the irish wolf-dog, the old english hound, and of two breeds in france, one of which was formerly highly valued.[ ] mr. pickering remarks[ ] that "the sheep figured on the most ancient egyptian monuments is unknown at the present day; and at least one variety of the bullock, formerly known in egypt, has in like manner become extinct." so it has been with some animals, and with several plants cultivated by the ancient inhabitants of europe during the neolithic period. in peru, von tschudi[ ] found in certain tombs, apparently prior to the dynasty of the incas, two kinds of maize not now known in the country. with our flowers and culinary vegetables, { } the production of new varieties and their extinction has incessantly recurred. at the present time improved breeds sometimes displace at an extraordinarily rapid rate older breeds; as has recently occurred throughout england with pigs. the long-horn cattle in their native home were "suddenly swept away as if by some murderous pestilence," by the introduction of short-horns.[ ] what grand results have followed from the long-continued action of methodical and unconscious selection, checked and regulated to a certain extent by natural selection, is seen on every side of us. compare the many animals and plants which are displayed at our exhibitions with their parent-forms when these are known, or consult old historical records with respect to their former state. almost all our domesticated animals have given rise to numerous and distinct races, excepting those which cannot be easily subjected to selection--such as cats, the cochineal insect, and the hive-bee,--and excepting those animals which are not much valued. in accordance with what we know of the process of selection, the formation of our many races has been slow and gradual. the man who first observed and preserved a pigeon with its oesophagus a little enlarged, its beak a little longer, or its tail a little more expanded than usual, never dreamed that he had made the first step in the creation of the pouter, carrier, and fantail-pigeon. man can create not only anomalous breeds, but others with their whole structure admirably co-ordinated for certain purposes, such as the race-horse and dray-horse, or the greyhound. it is by no means necessary that each small change of structure throughout the body, leading towards excellence, should simultaneously arise and be selected. although man seldom attends to differences in organs which are important under a physiological point of view, yet he has so profoundly modified some breeds, that assuredly, if found wild, they would be ranked under distinct genera. the best proof of what selection has effected is perhaps afforded by the fact that whatever part or quality in any animal, and more especially in any plant, is most valued by man, that part or quality differs most in the several races. this result is well seen by comparing the amount of difference { } between the fruits produced by the varieties of the same fruit-tree, between the flowers of the varieties in our flower-garden, between the seeds, roots, or leaves of our culinary and agricultural plants, in comparison with the other and not valued parts of the same plants. striking evidence of a different kind is afforded by the fact ascertained by oswald heer,[ ] namely, that the seeds of a large number of plants,--wheat, barley, oats, peas, beans, lentils, poppies,--cultivated for their seed by the ancient lake-inhabitants of switzerland, were all smaller than the seeds of our existing varieties. rütimeyer has shown that the sheep and cattle which were kept by the earlier lake-inhabitants were likewise smaller than our present breeds. in the middens of denmark, the earliest dog of which the remains have been found was the weakest; this was succeeded during the bronze age by a stronger kind, and this again during the iron age by one still stronger. the sheep of denmark during the bronze period had extraordinarily slender limbs, and the horse was smaller than our present animal.[ ] no doubt in these cases the new and larger breeds were generally introduced from foreign lands by the immigration of new hordes of men. but it is not probable that each larger breed, which in the course of time supplanted a previous and smaller breed, was the descendant of a distinct and larger species; it is far more probable that the domestic races of our various animals were gradually improved in different parts of the great europæo-asiatic continent, and thence spread to other countries. this fact of the gradual increase in size of our domestic animals is all the more striking as certain wild or half-wild animals, such as red-deer, aurochs, park-cattle, and boars,[ ] have within nearly the same period decreased in size. the conditions favourable to selection by man are,--the closest attention being paid to every character,--long-continued perseverance,--facility in matching or separating animals,--and especially a large number being kept, so that the inferior individuals may be freely rejected or destroyed, and the better ones preserved. when many are kept there will also be a { } greater chance of the occurrence of well-marked deviations of structure. length of time is all-important; for as each character, in order to become strongly pronounced, has to be augmented by the selection of successive variations of the same nature, this can only be effected during a long series of generations. length of time will, also, allow any new feature to become fixed by the continued rejection of those individuals which revert or vary, and the preservation of those which inherit the new character. hence, although some few animals have varied rapidly in certain respects under new conditions of life, as dogs in india and sheep in the west indies, yet all the animals and plants which have produced strongly marked races were domesticated at an extremely remote epoch, often before the dawn of history. as a consequence of this, no record has been preserved of the origin of our chief domestic breeds. even at the present day new strains or sub-breeds are formed so slowly that their first appearance passes unnoticed. a man attends to some particular character, or merely matches his animals with unusual care, and after a time a slight difference is perceived by his neighbours;--the difference goes on being augmented by unconscious and methodical selection, until at last a new sub-breed is formed, receives a local name, and spreads; but, by this time, its history is almost forgotten. when the new breed has spread widely, it gives rise to new strains and sub-breeds, and the best of these succeed and spread, supplanting other and older breeds; and so always onwards in the march of improvement. when a well-marked breed has once been established, if not supplanted by still improving sub-breeds, and if not exposed to greatly changed conditions of life, inducing further variability or reversion to long-lost characters, it may apparently last for an enormous period. we may infer that this is the case from the high antiquity of certain races; but some caution is necessary on this head, for the same variation may appear independently after long intervals of time, or in distant places. we may safely assume that this has occurred with the turnspit-dog which is figured on the ancient egyptian monuments, with the solid-hoofed swine[ ] mentioned by aristotle, with five-toed fowls { } described by columella, and certainly with the nectarine. the dogs represented on the egyptian monuments, about b.c., show us that some of the chief breeds then existed, but it is extremely doubtful whether any are identically the same with our present breeds. a great mastiff sculptured on an assyrian tomb, b.c., is said to be the same with the dog still imported into the same region from thibet. the true greyhound existed during the roman classical period. coming down to a later period, we have seen that, though most of the chief breeds of the pigeon existed between two and three centuries ago, they have not all retained to the present day exactly the same character; but this has occurred in certain cases in which improvement was not desired, for instance in the case of the spot or the indian ground-tumbler. de candolle[ ] has fully discussed the antiquity of various races of plants; he states that the black-seeded poppy was known in the time of homer, the white-seeded sesamum by the ancient egyptians, and almonds with sweet and bitter kernels by the hebrews; but it does not seem improbable that some of these varieties may have been lost and reappeared. one variety of barley and apparently one of wheat, both of which were cultivated at an immensely remote period by the lake-inhabitants of switzerland, still exist. it is said[ ] that "specimens of a small variety of gourd which is still common in the market of lima were exhumed from an ancient cemetery in peru." de candolle remarks that, in the books and drawings of the sixteenth century, the principal races of the cabbage, turnip, and gourd can be recognised; this might have been expected at so late a period, but whether any of these plants are absolutely identical with our present sub-varieties is not certain. it is, however, said that the brussels sprout, a variety which in some places is liable to degeneration, has remained genuine for more than four centuries in the district where it is believed to have originated.[ ] * * * * * in accordance with the views maintained by me in this work and elsewhere, not only the various domestic races, but the { } most distinct genera and orders within the same great class,--for instance, whales, mice, birds, and fishes--are all the descendants of one common progenitor, and we must admit that the whole vast amount of difference between these forms of life has primarily arisen from simple variability. to consider the subject under this point of view is enough to strike one dumb with amazement. but our amazement ought to be lessened when we reflect that beings, almost infinite in number, during an almost infinite lapse of time, have often had their whole organisation rendered in some degree plastic, and that each slight modification of structure which was in any way beneficial under excessively complex conditions of life, will have been preserved, whilst each which was in any way injurious will have been rigorously destroyed. and the long-continued accumulation of beneficial variations will infallibly lead to structures as diversified, as beautifully adapted for various purposes, and as excellently co-ordinated, as we see in the animals and plants all around us. hence i have spoken of selection as the paramount power, whether applied by man to the formation of domestic breeds, or by nature to the production of species. i may recur to the metaphor given in a former chapter: if an architect were to rear a noble and commodious edifice, without the use of cut stone, by selecting from the fragments at the base of a precipice wedge-formed stones for his arches, elongated stones for his lintels, and flat stones for his roof, we should admire his skill and regard him as the paramount power. now, the fragments of stone, though indispensable to the architect, bear to the edifice built by him the same relation which the fluctuating variations of each organic being bear to the varied and admirable structures ultimately acquired by its modified descendants. some authors have declared that natural selection explains nothing, unless the precise cause of each slight individual difference be made clear. now, if it were explained to a savage utterly ignorant of the art of building, how the edifice had been raised stone upon stone, and why wedge-formed fragments were used for the arches, flat stones for the roof, &c.; and if the use of each part and of the whole building were pointed out, it would be unreasonable if he declared that nothing had been { } made clear to him, because the precise cause of the shape of each fragment could not be given. but this is a nearly parallel case with the objection that selection explains nothing, because we know not the cause of each individual difference in the structure of each being. the shape of the fragments of stone at the base of our precipice may be called accidental, but this is not strictly correct; for the shape of each depends on a long sequence of events, all obeying natural laws; on the nature of the rock, on the lines of deposition or cleavage, on the form of the mountain which depends on its upheaval and subsequent denudation, and lastly on the storm or earthquake which threw down the fragments. but in regard to the use to which the fragments may be put, their shape may be strictly said to be accidental. and here we are led to face a great difficulty, in alluding to which i am aware that i am travelling beyond my proper province. an omniscient creator must have foreseen every consequence which results from the laws imposed by him. but can it be reasonably maintained that the creator intentionally ordered, if we use the words in any ordinary sense, that certain fragments of rock should assume certain shapes so that the builder might erect his edifice? if the various laws which have determined the shape of each fragment were not predetermined for the builder's sake, can it with any greater probability be maintained that he specially ordained for the sake of the breeder each of the innumerable variations in our domestic animals and plants;--many of these variations being of no service to man, and not beneficial, far more often injurious, to the creatures themselves? did he ordain that the crop and tail-feathers of the pigeon should vary in order that the fancier might make his grotesque pouter and fantail breeds? did he cause the frame and mental qualities of the dog to vary in order that a breed might be formed of indomitable ferocity, with jaws fitted to pin down the bull for man's brutal sport? but if we give up the principle in one case,--if we do not admit that the variations of the primeval dog were intentionally guided in order that the greyhound, for instance, that perfect image of symmetry and vigour, might be formed,--no shadow of reason can be assigned for the belief that variations, alike in nature and the result { } of the same general laws, which have been the groundwork through natural selection of the formation of the most perfectly adapted animals in the world, man included, were intentionally and specially guided. however much we may wish it, we can hardly follow professor asa gray in his belief "that variation has been led along certain beneficial lines," like a stream "along definite and useful lines of irrigation." if we assume that each particular variation was from the beginning of all time preordained, the plasticity of organisation, which leads to many injurious deviations of structure, as well as that redundant power of reproduction which inevitably leads to a struggle for existence, and, as a consequence, to the natural selection or survival of the fittest, must appear to us superfluous laws of nature. on the other hand, an omnipotent and omniscient creator ordains everything and foresees everything. thus we are brought face to face with a difficulty as insoluble as is that of free will and predestination. * * * * * { } index. abbas pacha, a fancier of fantailed pigeons, i. . abbey, mr., on grafting, ii. ; on mignonette, ii. . abbott, mr. keith, on the persian tumbler pigeon, i. . abbreviation of the facial bones, i. . abortion of organs, ii. - , . absorption of minority in crossed races, ii. - , . acclimatisation, ii. - ; of maize, i. . acerbi, on the fertility of domestic animals in lapland, ii. . _achatinella_, ii. . _achillea millefolium_, bud variation in, i. . _aconitum napellus_, roots of, innocuous in cold climates, ii. . _acorus calamus_, sterility of, ii. . acosta, on fowls in south america at its discovery, i. . _acropera_, number of seeds in, ii. . adam, mr., origin of _cytisus adami_, i. . adam, w., on consanguineous marriages, ii. . adams, mr., on hereditary diseases, ii. . advancement in scale of organisation, i. . _Ægilops triticoides_, observations of fabre and godron on, i. ; increasing fertility of hybrids of, with wheat, ii. . _Æsculus flava_ and _rubicunda_, i. . _Æsculus pavia_, tendency of, to become double, ii. . _Æthusa cynapium_, ii. . affinity, sexual elective, ii. . africa, white bull from, i. ; feral cattle in, i. ; food-plants of savages of, i. - ; south, diversity of breeds of cattle in, i. ; west, change in fleece of sheep in, i. . _agave vivipara_, seeding of, in poor soil, ii. . age, changes in trees, dependent on, i. . agouti, fertility of, in captivity, ii. . agriculture, antiquity of, ii. . _agrostis_, seeds of, used as food, i. . aguara, i. . ainsworth, mr., on the change in the hair of animals at angora, ii. . akbar khan, his fondness for pigeons, i. ; ii. . _alauda arvensis_, ii. . albin, on "golden hamburgh" fowls, i. ; figure of the hook-billed duck, i. . albinism, i. , ii. . albino, negro, attacked by insects, ii. . albinoes, heredity of, ii. . albinus, thickness of the epidermis on the palms of the hands in man, ii. . alco, i. , ii. . aldrovandi, on rabbits, i. ; description of the nun pigeon, i. ; on the fondness of the dutch for pigeons in the seventeenth century, i. ; notice of several varieties of pigeons, i. - ; on the breeds of fowls, i. ; on the origin of the domestic duck, i. . alefield, dr., on the varieties of peas and their specific unity, i. ; on the varieties of beans, i. . alexander the great, his selection of indian cattle, ii. . algÆ, retrogressive metamorphosis in, ii. ; division of zoospores of, ii. . allen, w., on feral fowls, i. ; ii. . allman, professor, on a monstrous _saxifraga geum_, ii. ; on the development of the hydroida, ii. . almond, i. ; antiquity of, ii. ; bitter, not eaten by mice, ii. . _alnus glutinosa_ and _incana_, hybrids of, ii. . alpaca, selection of, ii. . _althæa rosea_, i. , ii. . _amaryllis_, ii. . _amaryllis vittata_, effect of foreign pollen on, i. . amaurosis, hereditary, ii. . america, limits within which no useful plants have been furnished by, i. ; colours of feral horses in, i. - ; north, native cultivated plants of, i. ; skin of feral pig from, i. ; south, variations in cattle of, i. , . _amygdalus persica_, i. - , . { } ammon, on the persistency of colour in horses, ii. . _anagallis arvensis_, ii. . analogous variation, i. , ii. - ; in horses, i. ; in the horse and ass, i. ; in fowls, i. - . _anas boschas_, i. , ii. ; skull of, figured, i. . _anas moschata_, ii. . "ancon" sheep of massachusetts, i. , ii. . andalusian fowls, i. . andalusian rabbits, i. . anderson, j., on the origin of british sheep, i. ; on the selection of qualities in cattle, ii. ; on a one-eared breed of rabbits, i. ; on the inheritance of characters from a one-eared rabbit and three-legged bitch, ii. ; on the persistency of varieties of peas, i. ; on the production of early peas by selection, ii. ; on the varieties of the potato, i. - ; on crossing varieties of the melon, i. ; on reversion in the barberry, i. . anderson, mr., on the reproduction of the weeping ash by seed, ii. ; on the cultivation of the tree pæony in china, ii. . andersson, mr., on the damara, bechuana, and namaqua cattle, i. ; on the cows of the damaras, ii. ; selection practised by the damaras and namaquas, ii. ; on the use of grass-seeds and the roots of reeds as food in south africa, i. . _anemone coronaria_, doubled by selection, ii. . angina pectoris, hereditary, occurring at a certain age, ii. . anglesea, cattle of, i. . angola sheep, i. . angora, change in hair of animals at, ii. ; cats of, i. , ; rabbits of, i. , . animals, domestication of, facilitated by fearlessness of man, i. ; refusal of wild, to breed in captivity, ii. ; compound, individual peculiarities of, reproduced by budding, i. ; variation by selection in useful qualities of, ii. . annual plants, rarity of bud-variation in, i. . anomalies in the osteology of the horse, i. . anomalous breeds of pigs, i. ; of cattle, i. . _anser albifrons_, characters of, reproduced in domestic geese, i. . _anser ægyptiacus_, i. ; ii. . _anser canadensis_, ii. . _anser cygnoides_, i. . _anser ferus_, the original of the domestic goose, i. ; fertility of cross of, with domestic goose, i. . anson, on feral fowls in the ladrones, i. . antagonism between growth and reproduction, ii. . _anthemis nobilis_, bud-variation in flowers of, i. ; becomes single in poor soil, ii. . antherozoids, apparent independence of, in algæ, ii. . anthers, contabescence of, ii. - . antigua, cats of, i. ; changed fleece of sheep in, i. . _antirrhinum majus_, peloric, i. ; ii. , , ; double-flowered, ii. ; bud-variation in, i. . ants, individual recognition of, ii. . apes, anthropomorphous, ii. . aphides, attacking pear-trees, ii. ; development of, ii. - . apoplexy, hereditary, occurring at a certain age, ii. . apple, i. - ; fruit of, in swiss lake-dwellings, i. ; rendered fastigate by heat in india, i. ; bud-variation in the, i. ; with dimidiate fruit, i. - ; with two kinds of fruit on the same branch, i. ; artificial fecundation of, i. ; st. valéry, i. ; ii. ; reversion in seedlings of, ii. ; crossing of varieties of, ii. ; growth of the, in ceylon, ii. ; winter majetin, not attacked by _coccus_, ii. ; flower-buds of, attacked by bullfinches, ii. ; american, change of when grown in england, ii. . apricot, i. - ; glands on the leaves of, ii. ; analogous variation in the, ii. . _aquila fusca_, copulating in captivity, ii. . _aquilegia vulgaris_, i. ; ii. . arab boarhound, described by harcourt, i. . _arabis blepharophylla_ and _a. soyeri,_ effects of crossing, i. . _aralia trifoliata_, bud-variation in leaves of, i. . araucarias, young, variable resistance of, to frost, ii. . archangel pigeon, ii. . arctic regions, variability of plants and shells of, ii. . _aria vestita_, grafted on thorns, i. . aristophanes, fowls mentioned by, i. . aristotle, on solid-hoofed pigs, i. ; domestic duck unknown to, i. ; on the assumption of male characters by old hens, ii. . { } arni, domestication of the, i. . arrest of development, ii. - . arteries, increase of anastomosing branches of, when tied, ii. . aru islands, wild pig of, i. . arum, polynesian varieties of, ii. . _ascaris_, number of eggs of, ii. . ash, varieties of the, i. ; weeping, i. ; simple-leaved, i. ; bud-variation in, i. ; effects of graft upon the stock in the, i. ; production of the blotched breadalbane, _ibid._; weeping, capricious reproduction of, by seed, ii. . _asinus burchellii_, i. . _asinus hemionus_, ii. . _asinus indicus_, ii. - , . _asinus quagga_, i. . _asinus tæniopus_, ii. ; the original of the domestic ass, i. . asparagus, increased fertility of cultivated, ii. . ass, early domestication of the, i. ; breeds of, _ibid._; small size of, in india, _ibid._; stripes of, i. - ; ii. ; dislike of to cross water, i. ; reversion in, ii. - , ; hybrid of the, with mare and zebra, ii. ; prepotency of the, over the horse, ii. - ; crossed with wild ass, ii. ; variation and selection of the, ii. . assyrian sculpture of a mastiff, i. . asters, ii. , . asthma, hereditary, ii. , . atavism. _see_ reversion. athelstan, his care of horses, ii. . atkinson, mr., on the sterility of the tarroo silk-moth in confinement, ii. . aubergine, ii. . audubon, on feral hybrid ducks, i. ; ii. ; on the domestication of wild ducks on the mississippi, i. ; on the wild cock turkey visiting domestic hens, i. ; fertility of _fringilla ciris_ in captivity, ii. ; fertility of _columba migratoria_ and _leucocephala_ in captivity, ii. ; breeding of _anser canadensis_ in captivity, ii. . audubon and bachman, on the change of coat in _ovis montana_, i. ; sterility of _sciurus cinerea_ in confinement, ii. . auricula, effect of seasonal conditions on the, ii. ; blooming of, ii. . australia, no generally useful plants derived from, i. ; useful plants of, enumerated by hooker, i. . austria, heredity of character in emperors of, ii. . autenrieth, on persistency of colour in horses, ii. . ava, horses of, i. . _avena fatua_, cultivability of, i. . ayeen akbery, pigeons mentioned in the, i. , , , , , . ayres, w. p., on bud-variation in pelargoniums, i. . _azalea indica_, bud-variation in, i. . azara, on the feral dogs of la plata, i. ; on the crossing of domestic with wild cats in paraguay, i. ; on hornlike processes in horses, i. ; on curled hair in horses, i. ; ii. , ; on the colours of feral horses, i. , ; ii. ; on the cattle of paraguay and la plata, i. , , ; ii. ; on a hornless bull, ii. ; on the increase of cattle in south america, ii. ; on the growth of horns in the hornless cattle of corrientes, ii. ; on the "niata" cattle, i. ; on naked quadrupeds, ii. ; on a race of black-skinned fowls in south america, i. ; ii. ; on a variety of maize, i. . babington, c. c., on the origin of the plum, i. ; british species of the genus _rosa_, i. ; distinctness of _viola lutea_ and _tricolor_, i. . bachmann, mr., on the turkey, ii. . _see also_ audubon. badger, breeding in confinement, ii. . "bagadotten-taube," i. . baily, mr., on the effect of selection on fowls, ii. ; on dorking fowls, ii. . baird, s., on the origin of the turkey, i. . baker, mr., on heredity in the horse, ii. ; on the degeneration of the horse by neglect, ii. ; orders of henrys vii. and viii. for the destruction of undersized mares, ii. . bakewell, change in the sheep effected by, ii. . balancement, ii. - ; of growth, law of, i. . baldhead, pigeon, i. . baldness, in man, inherited, ii. - ; with deficiency in teeth, ii. - . ballance, mr., on the effects of interbreeding on fowls, ii. ; on variation in the eggs of fowls, i. . _ballota nigra_, transmission of variegated leaves in, i. . bamboo, varieties of the, ii. . banana, variation of the, i. ; ii. , ; bud-variation in the, i. ; sterility of the, ii. . bantam fowls, i. ; sebright, origin of, ii. ; sterility of, ii. . barb (pigeon), i. - , ; ii. ; { } figure of, i. ; figure of lower jaw of, i. . barbs, of wheat, i. . barberry, dark or red-leaved variety, i. ; ii. ; reversion in suckers of seedless variety, i. . barbut, j., on the dogs of guinea, i. ; on the domestic pigeons in guinea, i. ; fowls not native in guinea, i. . barking, acquisition of the habit of, by various dogs, i. . barley, wild, i. ; of the lake-dwellings, i. - ; ancient variety of, ii. . barnes, mr., production of early peas by selection, ii. . barnet, mr., on the intercrossing of strawberries, i. ; dioeciousness of the hautbois strawberry, i. ; on the scarlet american strawberry, ii. . barth, dr., use of grass-seeds as food in central africa, i. . bartlett, a. d., on the origin of "himalayan" rabbits by intercrossing, i. ; on the feral rabbits of porto santo, i. ; on geese with reversed feathers on the head and neck, i. ; on the young of the black-shouldered peacock, i. ; on the breeding of the felidæ in captivity, ii. . bartram, on the black wolf-dog of florida, i. . bates, h. w., refusal of wild animals to breed in captivity, ii. , ; sterility of american monkeys in captivity, ii. ; sterility of tamed guans, ii. . batrachia, regeneration of lost parts in, ii. . beach, raised, in peru, containing heads of maize, i. . beak, variability of, in fowls, i. ; individual differences of, in pigeons, i. ; correlation of, with the feet in pigeons, i. - . beale, lionel, on the contents of cells, ii. ; on the multiplication of infectious atoms, ii. ; on the origin of fibres, ii. . beans, i. ; of swiss lake-dwellings, i. ; varieties of, produced by selection, ii. ; french and scarlet, variable resistance of to frost, ii. , ; superiority of native seed of, ii. ; a symmetrical variation of scarlet, ii. ; experiments on kidney, i. ; with monstrous stipules and abortive leaflets, ii. . beard, pigeon, i. . bears, breeding in captivity, ii. . beasley, j., reversion in crossed cattle, ii. . beaton, d., effect of soil upon strawberries, i. ; on varieties of pelargonium, i. , ii. , ; bud-variation in _gladiolus colvillii_, i. ; cross between scotch kail and cabbage, ii. ; hybrid gladiolus, ii. ; constant occurrence of new forms among seedlings, ii. ; on the doubling of the compositæ, ii. . bechuana cattle, i. . beck, mr., constitutional differences in pelargoniums, i. . beckmann, on changes in the odours of plants, ii. . beckstein, on the burrowing of wolves, i. ; "spitz" dog, i. ; origin of the newfoundland dog, i. ; crossing of domestic and wild swine, i. ; on the jacobin pigeon, i. , ; notice of swallow-pigeons, i. ; on a fork-tailed pigeon, i. ; variations in the colour of the croup in pigeons, i. ; on the german dove-cot pigeon, i. ; fertility of mongrel pigeons, i. ; on hybrid turtle-doves, i. ; on crossing the pigeon with _columba oenas_, _c. palumbus_, _turtur risoria_, and _t. vulgaris_, i. ; development of spurs in the silk-hen, i. ; on polish fowls, i. , ; on crested birds, i. ; on the canary-bird, i. , ii. , ; german superstition about the turkey, i. ; occurrence of horns in hornless breeds of sheep, ii. ; hybrids of the horse and ass, ii. ; crosses of tailless fowls, ii. ; difficulty of pairing dove-cot and fancy pigeons, ii. ; fertility of tame ferrets and rabbits, ii. ; fertility of wild sow, _ibid._; difficulty of breeding caged birds, ii. ; comparative fertility of _psittacus erithacus_ in captivity, ii. ; on changes of plumage in captivity, ii. ; liability of light-coloured cattle to the attacks of flies, ii. ; want of exercise a cause of variability, ii. ; effect of privation of light upon the plumage of birds, ii. ; on a sub-variety of the monk-pigeon, ii. . beddoe, dr., correlation of complexion with consumption, ii. . bedeguar gall, ii. . bee, persistency of character of, ii. , ; intercrossing, ii. ; conveyance, of pollen of peas by, i. . bee-ophrys, self-fertilisation of, ii. . beech, dark-leaved, i. , ii. ; fern-leaved, reversion of, i. ; weeping, non-production of by seed, ii. . beechey, horses of loochoo islands, i. . beet, i. ; increase of sugar in, by selection, ii. . { } _begonia frigida_, singular variety of, i. ; sterility of, ii. . belgian rabbit, i. . bell, t., statement that white cattle have coloured ears, i. . bell, w., bud-variation in _cistus tricuspis_, i. . bellingeri, observations on gestation in the dog, i. ; on the fertility of dogs and cats, ii. . belon, on high-flying pigeons in paphlagonia, i. ; varieties of the goose, i. . benguela, cattle of, i. . bennett, dr. g., pigs of the pacific islands, i. , ; dogs of the pacific islands, i. ; varieties of cultivated plants in tahiti, ii. . bennett, mr., on the fallow deer, ii. . bentham, g., number and origin of cultivated plants, i. ; cereals all cultivated varieties, i. ; species of the orange group, i. - ; distinctions of almond and peach, i. ; british species of _rosa_, i. ; identity of _viola lutea_ and _tricolor_, i. . _berberis vulgaris_, i. , ii. . _berberis wallichii_, indifference of, to climate, ii. . berjean, on the history of the dog, i. , . berkeley, g. f., production of hen-cocks in a strain of game-fowls, i. . berkeley, m. j., crossing of varieties of the pea, i. ; effect of foreign pollen on grapes, i. ; on hybrid plants, ii. ; analogy between pollen of highly-cultivated plants and hybrids, ii. ; on hungarian kidney-beans, ii. ; failure of indian wheat in england, ii. ; bud developed on the petal of a _clarkia_, ii. . bernard, inheritance of disease in the horse, ii. . bernard, c., independence of the organs of the body, ii. - ; special affinities of the tissues, ii. . bernhardi, varieties of plants with laciniated leaves, ii. . _bernicla antarctica_, i. . bertero, on feral pigeons in juan fernandez, i. . _betula alba_, ii. . bewick, on the british wild cattle, i. . bible, reference to breeding studs of horses in, i. ; references to domestic pigeons in the, i. ; indications of selection of sheep in the, ii. ; notice of mules in the, ii. . bidwell, mr., on self-impotence in _amaryllis_, ii. . birch, weeping, i. , ii. . birch, dr. s., on the ancient domestication of the pigeon in egypt, i. ; notice of bantam fowls in a japanese encyclopædia, i. , . birch, wyrley, on silver-grey rabbits, i. - . birds, sterility caused in, by change of conditions, ii. - . bladder-nut, tendency of the, to become double, ii. . blaine, mr., on wry-legged terriers, ii. . blainville, origin and history of the dog, i. - ; variations in the number of teeth in dogs, i. ; variations in the number of toes in dogs, i. ; on mummies of cats, i. ; on the osteology of solid-hoofed pigs, i. ; on feral patagonian and n. american pigs, i. . "blass-taube," i. . bleeding, hereditary, ii. , ; sexual limitation of excessive, ii. . blending of crossed races, time occupied by the, ii. . blindness, hereditary, ii. ; at a certain age, ii. ; associated with colour of hair, ii. . bloodhounds, degeneration of, caused by interbreeding, ii. . blumenbach, on the protuberance of the skull in polish fowls, i. ; on the effect of circumcision, ii. ; inheritance of a crooked finger, ii. ; on badger-dogs and other varieties of the dog, ii. ; on _hydra_, ii. ; on the "nisus formativus," ii. . blyth, e., on the pariah dog, i. ; hybrids of dog and jackal, i. ; early domestication of cats in india, i. ; origin of domestic cat, _ib._; crossing of domestic and wild cats, i. ; on indian cats resembling _felis chaus_, i. ; on striped burmese ponies, i. ; on the stripes of the ass, i. ; on indian wild pigs, i. ; on humped cattle, i. , ; occurrence of _bos frontosus_ in irish crannoges, i. ; fertile crossing of zebus and common cattle, i. ; on the species of sheep, i. ; on the fat-tailed indian sheep, i. ; origin of the goat, i. ; on rabbits breeding in india, i. ; number of tail-feathers in fantails, i. ; lotan tumbler pigeons, i. ; number of tail-feathers in _ectopistes_, i. ; on _columba affinis_, i. ; pigeons roosting in trees, i. ; on _columba leuconota_, i. ; on _columba intermedia_ of strickland, i. ; variation in colour of croup in pigeons, i. - , ; voluntary domestication of rock-pigeons in india, i. ; feral pigeons on the hudson, i. ; { } occurrence of sub-species of pigeons, i. ; notice of pigeon-fanciers in delhi, &c., i. ; hybrids of _gallus sonneratii_ and the domestic hen, i. ; supposed hybridity of _gallus temminckii_, i. ; variations and domestication of _gallus bankiva_, i. - , ; crossing of wild and tame fowls in burmah, i. ; restricted range of the larger gallinaceous birds, i. ; feral fowls in the nicobar islands, i. ; black-skinned fowls occurring near calcutta, i. ; weight of _gallus bankiva_, i. ; degeneration of the turkey in india, i. , ii. ; on the colour of gold-fish, i. ; on the ghor-khur (_asinus indicus_), ii. ; on _asinus hemionus_, ii. ; number of eggs of _gallus bankiva_, ii. ; on the breeding of birds in captivity, ii. ; co-existence of large and small breeds in the same country, ii. ; on the drooping ears of the elephant, ii. ; homology of leg and wing feathers, ii. . boethius on scotch wild cattle, i. . boitard and corbié, on the breeds of pigeons, i. ; lille pouter pigeon, i. ; notice of a gliding pigeon, i. ; variety of the pouter pigeon, i. ; dove-cot pigeon, i. ; crossing pigeons, i. - , ii. , ; sterility of hybrids of turtle-doves, i. ; reversion of crossed pigeons, i. , ii. ; on the fantail, i. , ii. ; on the trumpeter, ii. ; prepotency of transmission in silky fantail, ii. , ; secondary sexual characters in pigeons, ii. ; crossing of white and coloured turtle-doves, ii. ; fertility of pigeons, ii. . bombycidÆ, wingless females of, ii. . _bombyx hesperus_, ii. . _bombyx huttoni_, i. . _bombyx mori_, i. - . bonafous, on maize, i. , . bonaparte, number of species of columbidæ, i. ; number of tail-feathers in pigeons, i. ; size of the feet in columbidæ, i. ; on _columba guinea_, i. ; _columba turricola_, _rupestris_, and _schimperi_, i. . _bonatea speciosa_, development of ovary of, i. . bonavia, dr., growth of cauliflowers in india, ii. . bones, removal of portions of, ii. ; regeneration of, ii. ; growth and repair of, ii. - . bonnet, on the salamander, ii. , , , ; theory of reproduction, ii. . borchmeyer, experiments with the seeds of the weeping ash, ii. . borecole, i. . borelli, on polish fowls, i. . borneo, fowls of, with tail-bands, i. . bornet, e., condition of the ovary in hybrid _cisti_, i. ; self-impotence of hybrid _cisti_, ii. . borrow, g., on pointers, i. . bory de saint-vincent, on gold-fish, i. . _bos_, probable origin of european domestic cattle from three species of, i. . _bos frontosus_, i. , - . _bos indicus_, i. . _bos longifrons_, i. , . _bos primigenius_, i. - , . _bos sondaicus_, ii. . _bos taurus_, i. . _bos trochoceros_, i. . bosc, heredity in foliage-varieties of the elm, i. . bosse, production of double flowers from old seed, ii. . bossi, on breeding dark-coloured silkworms, i. . bouchardat, on the vine disease, i. . boudin, on local diseases, ii. ; resistance to cold of dark-complexioned men, ii. . "boulans," i. . "bouton d'alep," ii. . bowen, prof., doubts as to the importance of inheritance, ii. . bowman, mr., hereditary peculiarities in the human eye, ii. - ; hereditary cataract, ii. . brace, mr., on hungarian cattle, i. . _brachycome iberidifolia_, ii. . bracts, unusual development of, in gooseberries, i. . bradley, mr., effect of grafts upon the stock in the ash, i. ; effect of foreign pollen upon apples, i. ; on change of soil, ii. . "brahma pootras," a new breed of fowls, i. . brain, proportion of, in hares and rabbits, i. - . brandt, origin of the goat, i. . _brassica_, varieties of, with enlarged stems, ii. . _brassica asperifolia_, ii. . _brassica napus_, i. . _brassica oleracea_, i. . _brassica rapa_, i. , ii. . braun, a., bud-variation in the vine, i. ; in the currant, i. ; in _mirabilis jalapa_, i. ; in _cytisus adami_, i. ; on reversion in the foliage of trees, i. ; spontaneous production of _cytisus purpureo-elongatus_, i. ; reversion of flowers by stripes and blotches, ii. ; excess of nourishment a source of variability, ii. . { } brazil, cattle of, i. . bread-fruit, varieties of, ii. ; sterility and variability of, ii. . bree, w. t., bud-variation in _geranium pratense_ and _centaurea cyanus_, i. ; by tubers in the dahlia, i. ; on the deafness of white cats with blue eyes, ii. . breeding, high, dependent on inheritance, ii. - . breeds, domestic, persistency of, ii. , - ; artificial and natural, ii. - ; extinction of, ii. ; of domestic cats, i. - ; of pigs produced by crossing, i. ; of cattle, i. - , - ; of goats, i. . brehm, on _columba amaliæ_, i. . brent, b. p., number of mammæ in rabbits, i. ; habits of the tumbler pigeon, i. ; laugher pigeon, i. ; colouring of the kite tumbler, i. ; crossing of the pigeon with _columba oenas_, i. ; mongrels of the trumpeter pigeon, ii. ; close interbreeding of pigeons, ii. ; opinion on aldrovandi's fowls, i. ; on stripes in chickens, i. - ; on the combs of fowls, i. ; double-spurred dorking fowls, i. ; effect of crossing on colour of plumage in fowls, i. ; incubatory instinct of mongrels between non-sitting varieties of fowls, ii. ; origin of the domestic duck, i. ; fertility of the hook-billed duck, _ibid._; occurrence of the plumage of the wild duck in domestic breeds, i. ; voice of ducks, i. ; occurrence of a short upper mandible in crosses of hook-billed and common ducks, i. ; reversion in ducks produced by crossing, ii. ; variation of the canary-bird, i. ; fashion in the canary, ii. ; hybrids of canary and finches, ii. . brickell, on raising nectarines from seed, i. ; on the horses of north carolina, ii. . bridges, mr., on the dogs of tierra del fuego, i. ; on the selection of dogs by the fuegians, ii. . bridgman, w. k., reproduction of abnormal ferns, i. , ii. . briggs, j. j., regeneration of portions of the fins of fishes, ii. . broca, p., on the intercrossing of dogs, i. - ; on hybrids of hare and rabbit, i. ; on the rumpless fowl, i. ; on the character of half-castes, ii. ; degree of fertility of mongrels, ii. ; sterility of descendants of wild animals bred in captivity, ii. . broccoli, i. ; rudimentary flowers in, ii. ; tenderness of, ii. . bromehead, w., doubling of the canterbury bell by selection, ii. . bromfield, dr., sterility of the ivy and _acorus calamus_, ii. . _bromus secalinus_, i. . bronn, h. g., bud-variation in _anthemis_, i. ; effects of cross-breeding on the female, i. ; on heredity in a one-horned cow, ii. , ; propagation of a pendulous peach by seed, ii. ; absorption of the minority in crossed races, ii. ; on the crossing of horses, ii. ; fertility of tame rabbits and sheep, ii. ; changes of plumage in captivity, ii. ; on the dahlia, ii. . bronze period, dog of, i. . brown, g., variations in the dentition of the horse, i. . brown-sÉquard, dr., inheritance of artificially-produced epilepsy in the guinea-pig, ii. . _brunswigia_, ii. . brussels sprouts, i. , ii. . _bubo maximus_, ii. . buckland, f., on oysters, ii. ; number of eggs in a codfish, ii. . buckle, mr., doubts as to the importance of inheritance, ii. . buckley, miss, carrier-pigeons roosting in trees, i. . buckman, prof., cultivation of _avena fatua_, i. ; cultivation of the wild parsnip, i. , ii. , ; reversion in the parsnip, ii. . buckwheat, injurious to white pigs, when in flower, ii. . bud and seed, close analogy of, i. . bud-reversion, ii. . buds, adventitious, ii. . bud-variation, i. - , ii. , - , ; contrasted with seminal reproduction, i. ; peculiar to plants, i. ; in the peach, i. , ; in plums, i. ; in the cherry, _ibid._; in grapes, _ibid._; in the gooseberry, currant, pear, and apple, i. ; in the banana, camellia, hawthorn, _azalea indica_, and _cistus tricuspis_, i. ; in the hollyhock and pelargonium, i. ; in _geranium pratense_ and the chrysanthemum, i. ; in roses, i. , - ; in sweet williams, carnations, pinks, stocks, and snapdragons, i. ; in wall-flowers, cyclamen, _oenothera biennis_, _gladiolus colvillii_, fuchsias, and _mirabilis jalapa_, i. ; in foliage of various trees, i. - ; in cryptogamic plants, i. ; by suckers in _phlox_ and barberry, i. ; by tubers in the potato, _ibid._; in the dahlia, i. ; by bulbs in hyacinths, _imatophyllum miniatum_, and tulips, i. ; in _tigridia conchiflora_, i. ; { } in _hemerocallis_, _ibid._; doubtful cases, i. - ; in _cytisus adami_, i. - ; probable in _Æsculus rubicunda_, i. ; summary of observations on, . buffon, on crossing the wolf and dog, i. ; increase of fertility by domestication, ii. ; improvement of plants by unconscious selection, ii. ; theory of reproduction, ii. . _bulimus_, ii. . bull, apparent influence of, on offspring, ii. . bullace, i. . bulldog, recent modifications of, i. . bullfinch, breeding in captivity, ii. ; attacking flower-buds, ii. . bult, mr., selection of pouter pigeons, ii. . "bÜndtnerschwein," i. . bunting, reed, in captivity, ii. . burdach, crossing of domestic and wild animals, i. ; aversion of the wild boar to barley, ii. . burke, mr., inheritance in the horse, ii. . _burlingtonia_, ii. . burmah, cats of, i. . burmese ponies, striped, i. , . burnes, sir a., on the karakool sheep, i. , ii. ; varieties of the vine in cabool, i. ; hawks, trained in scinde, ii. ; pomegranates producing seed, ii. . burton constable, wild cattle at, i. . "burzel-tauben," i. . bussorah carrier, i. . _buteo vulgaris_, copulation of, in captivity, ii. . butterflies, polymorphic, ii. - . buzareingues, girou de, inheritance of tricks, ii. . cabanis, pears grafted on the quince, ii. . cabbage, i. - ; varieties of, i. ; unity of character in flowers and seeds of, i. - ; cultivated by ancient celts, i. ; classification of varieties of, _ibid._; ready crossing of, _ibid._, ii. , , , ; origin of, i. ; increased fertility of, when cultivated, ii. ; growth of, in tropical countries, ii. . cabool, vines of, i. . cabral, on early cultivation in brazil, i. . cactus, growth of cochineal on, in india, ii. . cÆsar, _bos primigenius_ wild in europe in the time of, i. ; notice of fowls in britain, i. ; notice of the importation of horses by the celts, ii. . caffre fowls, i. . caffres, different kinds of cattle possessed by the, i. . "cÁgias," a breed of sheep, i. . calceolarias, i. ; ii. ; effects of seasonal conditions on, ii. ; peloric flowers in, ii. . "calongos," a columbian breed of cattle, i. . calver, mr., on a seedling peach producing both peaches and nectarines, i. . calyx, segments of the, converted into carpels, ii. . camel, its dislike to crossing water, i. . _camellia_, bud-variations in, i. ; recognition of varieties of, ii. ; variety in, hardiness of, ii. . cameron, d., on the cultivation of alpine plants, ii. . cameronn, baron, value of english blood in race-horses, ii. . _campanula medium_, ii. . canary-bird, i. ; conditions of inheritance in, ii. ; hybrids of, ii. ; period of perfect plumage in, ii. ; diminished fertility of, ii. ; standard of perfection in, ii. ; analogous variation in, ii. . cancer, heredity of, ii. , , . canine teeth, development of the, in mares, ii. . _canis alopex_, i. . _canis antarcticus_, i. . _canis argentatus_, ii. . _canis aureus_, i. . _canis cancrivorus_, domesticated and crossed in guiana, i. . _canis cinereo-variegatus_, i. . _canis fulvus_, i. . _canis ingæ_, the naked peruvian dog, i. . _canis latrans_, resemblance of, to the hare indian dog, i. ; one of the original stocks, i. . _canis lupaster_, i. . _canis lupus_, var. _occidentalis_, resemblance of, to north american dogs, i. ; crossed with dogs, i. ; one of the original stocks, i. . _canis mesomelas_, i. , . _canis primævus_, tamed by mr. hodgson, i. . _canis sabbar_, i. . _canis simensis_, possible original of greyhounds, i. . _canis thaleb_, i. . _canis variegatus_, i. . canterbury bell, doubled by selection, ii. . cape of good hope, different kinds of cattle at the, i. ; { } no useful plants derived from the, i. . capercailzie, breeding in captivity, ii. . _capra ægagrus_ and _c. falconeri_, probable parents of domestic goat, i. . capsicum, i. . cardan, on a variety of the walnut, i. ; on grafted walnuts, ii. - . cardoon, ii. . _carex rigida_, local sterility of the, ii. . carlier, early selection of sheep, ii. . carlisle, sir a., inheritance of peculiarities, ii. , ; of polydactylism, ii. . "carme" pigeon, i. . carnation, bud-variation in, i. ; variability of, i. ; striped, produced by crossing red and white, i. ; effect of conditions of life on the, ii. . carnivora, general fertility of, in captivity, ii. . caroline archipelago, cats of, i. . carp, ii. . carpels, variation of, in cultivated cucurbitaceæ, i. . carpenter, w. b., regeneration of bone, ii. ; production of double monsters, ii. ; number of eggs in an _ascaris_, ii. . _carpinus betulus_, i. . _carpophaga littoralis_ and _luctuosa_, i. . carrier pigeon, i. - ; english, i. - ; figured, i. ; skull figured, i. ; history of the, i. ; persian, i. ; bussorah, _ibid._; bagadotten, skull figured, i. ; lower jaw figured, i. . carriÈre, cultivation of the wild carrot, i. ; intermediate form between the almond and the peach, i. ; glands of peach-leaves, i. ; bud-variation in the vine, i. ; grafts of _aria vestita_ upon thorns, i. ; variability of hybrids of _erythrina_, ii. . carrot, wild, effects of cultivation on the, i. ; reversion in the, ii. ; run wild, ii. ; increased fertility of cultivated, ii. ; experiments on the, ii. ; acclimatisation of the, in india, ii. . _carthamus_, abortion of the pappus in, ii. . cartier, cultivation of native plants in canada, i. . caryophyllaceÆ, frequency of contabescence in the, ii. . caspary, bud-variation in the moss-rose, i. ; on the ovules and pollen of _cytisus_, i. - ; crossing of _cytisus purpureus_ and _c. laburnum_, i. ; trifacial orange, i. ; differently-coloured flowers in the wild _viola lutea,_ i. ; sterility of the horse-radish, ii. . castelnau, on brazilian cattle, i. . castration, assumption of female characters caused by, ii. - . _casuarius bennettii_, ii. . cat, domestic, i. - ; early domestication and probable origin of the, i. - ; intercrossing of with wild species, i. - ; variations of, i. - ; feral, i. , ii. ; anomalous, i. ; polydactylism in, ii. ; black, indications of stripes in young, ii. ; tortoiseshell, ii. ; effects of crossing in, ii. ; fertility of, ii. ; difficulty of selection in, ii. , ; length of intestines in, ii. ; white with blue eyes, deafness of, ii. ; with tufted ears, ii. . cataract, hereditary, ii. , . caterpillars, effect of changed food on, ii. . catlin, g., colour of feral horses in north america, i. . cattle, european, their probable origin from three original species, i. - ; humped, or zebus, i. - ; intercrossing of, i. , - ; wild, of chillingham, hamilton, chartley, burton constable, and gisburne, i. , ii. ; colour of feral, i. - , ii. ; british breeds of, i. - ; south african breeds of, i. ; south american breeds of, i. , ii. ; niata, i. - , ii. , , ; effects of food and climate on, i. - ; effects of selection on, i. - ; dutch-buttocked, ii. ; hornless, production of horns in, ii. - , ; reversion in, when crossed, ii. ; wildness of hybrid, ii. ; short-horned, prepotency of, ii. ; wild, influence of crossing and segregation on, ii. ; crosses of, ii. , , ; of falkland islands, ii. ; mutual fertility of all varieties of, ii. ; effects of interbreeding on, ii. - ; effects of careful selection on, ii. , ; naked, of columbia, ii. ; crossed with wild banteng in java, ii. ; with reversed hair in banda oriental, ii. ; selection of trifling characters in, ii. ; fashion in, ii. ; similarity of best races of, ii. ; unconscious selection in, ii. ; effects of natural selection on anomalous breeds of, ii. - ; light-coloured, attacked by flies, ii. , ; jersey, rapid improvement of, ii. ; effects of disuse of parts in, ii. ; rudimentary horns in, ii. ; supposed influence of humidity on the hair of, ii. ; { } white spots of, liable to disease, ii. ; supposed analogous variation in, ii. ; displacement of long-horned by short-horned, ii. . cauliflower, i. ; free-seeding of, in india, ii. ; rudimentary flowers in, ii. . cavalier pigeon, ii. . _cavia aperea_, ii. . cay (_cebus azaræ_), sterility of, in confinement, ii. . _cebus azaræ_, ii. . _cecidomyia_, larval development of, ii. , , ; and _misocampus_, i. . cedars of lebanon and atlas, i. . celery, turnip-rooted, i. ; run wild, ii. . cell-theory, ii. . _celosia cristata_, i. . celsus, on the selection of seed-corn, i. , ii. . celts, early cultivation of the cabbage by the, i. ; selection of cattle and horses by the, ii. - . _cenchrus_, seeds of a, used as food, i. . _centaurea cyanus_, bud-variation in, i. . cephalopoda, spermatophores of, ii. . _cerasus padus_, yellow-fruited, ii. . _cercoleptes_, sterility of, in captivity, ii. . _cercopithecus_, breeding of a species of, in captivity, ii. . cereals, i. - ; of the neolithic period in switzerland, i. ; adaptation of, to soils, ii. . _cereus_, ii. . _cereus speciosissimus_ and _phyllanthus_, reversion in hybrids of, i. . _cervus canadensis_, ii. . _cervus dama_, ii. . cetacea, correlation of dermal system and teeth in the, ii. . ceylon, cats of, i. ; pigeon-fancying in, i. . _chamærops humilis_, crossed with date palm, i. . chamisso, on seeding bread-fruit, ii. . channel islands, breeds of cattle in, i. . chapman, professor, peach-trees producing nectarines, i. . chapuis, f., sexual peculiarities in pigeons, i. , ii. ; effect produced by first male upon the subsequent progeny of the female, i. ; sterility of the union of some pigeons, ii. . characters, fixity of, ii. ; latent, ii. - , - ; continued divergence of, ii. ; antagonistic, ii. . chardin, abundance of pigeons in persia, i. . charlemagne, orders as to the selection of stallions, ii. . chartley, wild cattle of, i. . chatÉ, reversion of the upper seeds in the pods of stocks, ii. - . chatin, on _ranunculus ficaria_, ii. . chaundy, mr., crossed varieties of cabbage, ii. . cheetah, general sterility of, in captivity, ii. . _cheiranthus cheiri_, i. . cherries, i. - ; bud-variation in, i. ; white tartarian, ii. ; variety of, with curled petals, ii. ; period of vegetation of, changed by forcing, ii. . chevreul, on crossing fruit-trees, ii. . chickens, differences in characters of, i. - ; white, liable to gapes, ii. , . chigoe, ii. . chile, sheep of, i. . chillingham cattle, identical with _bos primigenius_, i. ; characters of, i. - . chiloe, half-castes of, ii. . china, cats of, with drooping ears, i. ; horses of, i. ; striped ponies of, i. ; asses of, i. ; notice of rabbits in, by confucius, i. ; breeds of pigeons reared in, i. ; breeds of fowls of, in fifteenth century, i. , ; goose of, i. . chinchilla, fertility of, in captivity, ii. . chinese, selection practised by the, ii. - ; preference of the, for hornless rams, ii. ; recognition of the value of native breeds by the, ii. . chinese, or himalayan rabbit, i. . "chivos," a breed of cattle in paraguay, i. . choux-raves, i. . christ, h., on the plants of the swiss lake-dwellings, i. , ; intermediate forms between _pinus sylvestris_ and _montana_, i. . chrysanthemum, i. . _chrysotis festiva_, ii. . cineraria, effects of selection on the, ii. . circassia, horses of, ii. . circumcision, ii. . cirripedes, metagenesis in, ii. . _cistus_, intercrossing and hybrids of, i. , , ii. . _cistus tricuspis_, bud-variation in, i. . citrons, i. - . "_citrus aurantium fructu variabili_," i. . _citrus decumana_, i. . _citrus lemonum_, i. . { } _citrus medica_, i. - . cleft palate, inheritance of, ii. . clemente, on wild vines in spain, i. . clermont-tonnerre, on the st. valery apple, i. . clapham, a., bud-variation in the hawthorn, i. . "claquant," i. . "claquers" (pigeons), i. . clark, g., on the wild dogs of juan de nova, i. ; on striped burmese and javanese ponies, i. ; breeds of goats imported into the mauritius, i. ; variations in the mammæ of goats, i. ; bilobed scrotum of muscat goat, _ibid._ clark, h. j., on fission and gemmation, ii. . clarke, r. t., intercrossing of strawberries, i. . clarke, t., hybridisation of stocks, i. , ii. . clarkson, mr., prize-cultivation of the gooseberry, i. . classification, explained by the theory of natural selection, i. . climate, effect of, upon breeds of dogs, i. ; on horses, i. , ; on cattle, i. , ; on the fleece of sheep, i. , ; on seeds of wheat, i. ; on cultivated cabbages, i. ; adaptation of maize to, i. . climate and pasture, adaptation of breeds of sheep to, i. - . climate and soil, effects of, upon strawberries, i. . cline, mr., on the skull in horned and hornless rams, ii. . clos, on sterility in _ranunculus ficaria_, ii. . clotzsch, hybrids of various trees, ii. . clover, pelorism in, ii. . coate, mr., on interbreeding pigs, ii. . coccus of apple trees, ii. . cochin fowls, i. , , , - ; occipital foramen of, figured, i. ; section of skull of, figured, i. ; cervical vertebra of, figured, i. . cochineal, persistence of, ii. ; preference of, for a particular cactus, ii. . _cochlearia armoracia_, ii. . cock, game, natural selection in, ii. ; spur of, grafted on the comb, ii. ; spur of, inserted into the eye of an ox, ii. ; effect of castration upon the, ii. - . cock's-comb, varieties of the, i. . cocoons, of silkworms, variations in, i. - . codfish, bulldog, i. ; number of eggs in the, ii. . _coelogenys paca_, ii. . colin, prepotency of the ass over the horse, ii. - ; on cross-breeding, ii. ; on change of diet, ii. . collinson, peter, peach-tree producing a nectarine, i. . coloration, in pigeons, an evidence of unity of descent, i. - . colour, correlation of, in dogs, i. - ; persistence of, in horses, i. ; inheritance and diversity of, in horses, i. ; variations of, in the ass, i. - ; of wild or feral cattle, i. ; transmission of, in rabbits, i. ; peculiarities of, in himalayan rabbits, i. ; influence of, ii. - ; correlation of, in head and limbs, ii. ; correlated with constitutional peculiarities, ii. - . colour and odour, correlation of, ii. . colour-blindness, hereditary, ii. ; more common in men than in women, ii. - ; associated with inability to distinguish musical sounds, ii. . colours, sometimes not blended by crossing, ii. . _columba affinis_, blyth, a variety of _c. livia_, i. . _columba amaliæ_, brehm, a variety of _c. livia_, i. . _columba guinea_, i. . _columba gymnocyclus_, gray, a form of _c. livia_, i. . _columba gymnophthalmos_, hybrids of, with _c. oenas_, i. ; with _c. maculosa_, i. . _columba intermedia_, strickland, a variety of _c. livia_, i. . _columba leucocephala_, ii. . _columba leuconota_, i. , . _columba littoralis_, i. . _columba livia_, ii. , ; the parent of domestic breeds of pigeons, i. ; measurements of, i. ; figured, i. ; skull figured, i. ; lower jaw figured, i. , ; scapula figured, i. . _columba luctuosa_, i. . _columba migratoria_ and _leucocephala_, diminished fertility of, in captivity, ii. . _columba oenas_, i. ; crossed with common pigeon and _c. gymnophthalmos_, i. . _columba palumbus_, i. , ii. . _columba rupestris_, i. , , . _columba schimperi_, i. . _columba torquatrix_, ii. . _columba turricola_, i. . columbia, cattle of, i. . columbine, double, i. , ii. . { } columbus, on west indian dogs, i. . columella, on italian shepherd's dogs, i. ; on domestic fowls, i. , , ii. , ; on the keeping of ducks, i. ; on the selection of seed-corn, i. ; on the benefits of change of soil to plants, ii. ; on the value of native breeds, ii. . colza, i. . comb, in fowls, variations of, i. - ; sometimes rudimentary, ii. . compensation, law of, i. . compensation of growth, ii. - . complexion, connexion of, with constitution, ii. . compositÆ, double flowers of, i. , ii. , . conception, earlier in alderney and zetland cows than in other breeds, i. . conditions of life, changed, effect of, ii. - ; on horses, i. ; upon variation in pigeons, i. - ; upon wheat, i. - ; upon trees, i. ; in producing bud-variation, i. ; advantages of, ii. - , - ; sterility caused by, ii. - ; conducive to variability, ii. - , ; accumulative action of, ii. - ; direct action of, ii. - . condor, breeding in captivity, ii. . confinement, effect of, upon the cock, ii. . confucius, on the breeding of rabbits in china, i. . conolly, mr., on angora goats, ii. . constitutional differences in sheep, i. - ; in varieties of apples, i. - ; in pelargoniums, i. ; in dahlias, i. . constitutional peculiarities in strawberries, i. ; in roses, i. . consumption, hereditary, ii. ; period of appearance of, ii. ; correlated with complexion, ii. . contabescence, ii. - . _convolvulus batatas_, ii. , . _convolvulus tricolor_, bud-variation in, i. . cooper, mr., improvement of vegetables by selection, ii. . cooper, white, hereditary peculiarities of vision, ii. ; association of affections of the eyes with those of other systems, ii. . corals, bud-variation in, i. ; non-diffusion of cell-gemmules in, ii. . corbiÉ. _see_ boitard. cornea, opacity of, inherited, ii. . _cornus mascula_, yellow-fruited, ii. . correlation, ii. ; of neighbouring parts, ii. ; of change in the whole body and in some of its parts, ii. ; of homologous parts, ii. - ; inexplicable, ii. - ; commingling of, with the effects of other agencies, ii. - . correlation of skull and limbs in swine, i. ; of tusks and bristles in swine, i. ; of multiplicity of horns and coarseness of wool in sheep, i. ; of beak and feet in pigeons, i. - ; between nestling down and colour of plumage in pigeons, i. ; of changes in silkworms, i. ; in plants, ii. ; in maize, i. ; in pigeons, i. - , ; in fowls, i. - . corresponding periods, inheritance at, ii. - . corrientes, dwarf cattle of, i. . corringham, mr., influence of selection on pigs, ii. . corsica, ponies of, i. . "cortbeck" (pigeon) of aldrovandi, i. . _corvus corone_ and _c. cornix_, hybrids of, ii. . _corydalis_, flower of, ii. . _corydalis cava_, ii. - . _corydalis solida_, sterile when peloric, ii. . _corydalis tuberosa_, peloric by reversion, ii. - . _corylus avellana_, i. . costa, a., on shells transferred from england to the mediterranean, ii. . "couve tronchuda," i. . cow, inheritance of loss of one horn in the, ii. , ; amount of milk furnished by the, ii. ; development of six mammæ in, ii. . cowslip, ii. , . cracidÆ, sterility of the, in captivity, ii. . cranes, fertility of, in captivity, ii. . _cratægus oxyacantha_, i. , ii. , , , . _cratægus monogyna_, i. . _cratægus sibirica_, i. . crawfurd, j., malasian cats, i. ; horses of the malay archipelago, i. ; horses of japan, i. ; occurrence of stripes in young wild pigs of malacca, i. ; on a burmese hairy family with deficient teeth, ii. , ; japanese origin of the bantam, i. ; game fowls of the philippine islands, i. ; hybrids of _gallus varius_ and domestic fowl, i. ; domestication of _gallus bankiva_, i. ; feral fowls in the pellew islands, i. ; history of the fowl, i. ; history of the domestic duck, i. ; domestication of the goose, i. ; cultivated plants of new zealand, i. ; { } breeding of tame elephants in ava, ii. ; sterility of _goura coronata_ in confinement, ii. ; geese of the philippine islands, ii. . creepers, a breed of fowls, i. . crested fowl, i. ; figured, i. . "crÈve-coeur," a french sub-breed of fowls, i. . crisp, dr., on the brains of the hare and rabbit, i. . crocker, c. w., singular form of _begonia frigida_, i. - , ii. ; sterility in _ranunculus ficaria_, ii. . crocus, ii. . cross-breeding, permanent effect of, on the female, i. . crossing, ii. - , - ; a cause of uniformity, ii. - , ; occurs in all organised beings, ii. - ; some characters not blended by, ii. - , ; modifications and new races produced by, ii. - ; causes which check, ii. - ; domestication and cultivation favourable to, ii. - , ; beneficial effects of, ii. - , - ; necessary in some plants, ii. - , - , ; summary of subject of, ii. - ; of dogs with wolves in north america, i. - ; with _canis cancrivorus_ in guiana, i. ; of dog with wolf, described by pliny and others, i. ; characters furnished by, brought out by reversion in the progeny, ii. - ; a direct cause of reversion, ii. - , ; a cause of variability, ii. - . crustacea, macrourous, differences in the development of the, ii. . crustacean with an antenna-like development of the eye-peduncle, ii. . cryptogamic plants, bud-variation in, i. . cuba, wild dogs of, i. . "cuckoo," sub-breeds of fowls, i. . cucumber, variation in number of carpels of, i. ; supposed crossing of varieties of the, i. . _cucumis momordica_, i. . _cucumis sativa_, i. . _cucurbita_, dwarf, correlation of leaves in, ii. . _cucurbita maxima_, i. , . _cucurbita moschata_, i. , . _cucurbita pepo_, i. , ii. ; varieties of, i. ; relation in size and number of fruit of, ii. . cucurbitaceÆ, i. - ; supposed crossing of, i. ; naudin's observations on hybrids of, ii. ; acclimatisation of, ii. . "culbutants" (pigeons), i. . cultivation of plants, origin of, among savages, i. - ; fertility increased by, ii. - . cunier, on hereditary night-blindness, ii. . currants, of tierra del fuego, i. ; bud-variation in, i. . curtis, mr., bud-variation in the rose, i. . cuvier, on the gestation of the wolf, i. ; the odour of the jackal, an obstacle to domestication, i. ; differences of the skull in dogs, i. ; external characters of dogs, i. ; elongation of the intestines in domestic pigs, i. , ii. ; fertility of the hook-billed duck, i. ; number of digits, ii. ; hybrid of ass and zebra, ii. ; breeding of animals in the jardin des plantes, ii. ; sterility of predaceous birds in captivity, ii. ; facility of hybridisation in confinement, ii. . cyanosis, affection of fingers in, ii. . cyclamen, bud-variation in, i. . _cynara cardunculus_, ii. . _cynips fecundatrix_, ii. . _cynocephalus hamadryas_, ii. . _cyprinus auratus_, i. - . _cyrtanthus_, ii. . _cyrtopodium_, ii. . _cytisus adami_, ii. ; its bud-variation, i. - , , ii. ; seedlings from, i. ; different views of its origin, i. - ; experiments in crossing _c. purpureus_ and _laburnum_ to produce, i. ; its production by m. adam, i. ; discussion of origin of, i. . _cytisus alpino-laburnum_, ovules and pollen of, i. ; origin of, i. . _cytisus alpinus_, i. . _cytisus laburnum_, i. , , , . _cytisus purpureo-elongatus_, ovules and pollen of, i. ; production of, i. . _cytisus purpureus_, i. , , , , . dahlbom, effects of food on hymenoptera, ii. . dahlia, i. - , ii. ; bud-variation by tubers in the, i. ; improvement of, by selection, ii. ; steps in cultivation of, ii. ; effect of conditions of life on, ii. ; correlation of form and colour in, ii. . daisy, hen and chicken, i. ; swan river, ii. . dalbret, varieties of wheat, i. . dalibert, changes in the odours of plants, ii. . dally, dr., on consanguineous marriages, ii. . daltonism, hereditary, ii. . damaras, cattle of, i. , ii. - . { } damson, i. . dandolo, count, on silkworms, i. . daniell, fertility of english dogs in sierra leone, ii. . danish middens, remains of dogs in, i. . dappling in horses, asses, and hybrids, i. . dareste. c., on the skull of the polish fowl, i. ; on the production of monstrous chickens, ii. ; co-existence of anomalies, ii. ; production of double monsters, ii. . darvill, mr., heredity of good qualities in horses, ii. . darwin, c., on _lepus magellanicus_, i. ; on the wild potato, i. ; dimorphism in the polyanthus and primrose, ii. . darwin, dr., improvement of vegetables by selection, ii. . darwin, sir f., wildness of crossed pigs, ii. . d'asso, monogynous condition of the hawthorn in spain, i. . _dasyprocta aguti_, ii. . date-palm, varieties of the, ii. ; effect of pollen of, upon the fruit of _chamærops_, i. . _datura_, ii. ; variability in, ii. . _datura lævis_ and _stramonium_, reversion in hybrids of, i. . _datura stramonium_, ii. . daubenton, variations in the number of mammæ in dogs, i. ; proportions of intestines in wild and domestic cats, i. , ii. . daudin, on white rabbits, ii. . davy, dr., on sheep in the west indies, i. . dawkins and sandford, early domestication of _bos longifrons_ in britain, i. . deaf-mutes, non-heredity of, ii. . deafness, inheritance of, ii. . deby, wild hybrids of common and musk ducks, ii. . de candolle, alph., number and origin of cultivated plants, i. - , ; regions which have furnished no useful plants, i. ; wild wheat, i. - ; wild rye and oats, i. ; antiquity of varieties of wheat, i. ; apparent inefficacy of selection in wheat, i. ; origin and cultivation of maize, i. , ii. ; colours of seeds of maize, i. ; varieties and origin of the cabbage, i. - ; origin of the garden-pea, i. ; on the vine, i. , ii. ; cultivated species of the orange group, i. ; probable chinese origin of the peach, i. ; on the peach and nectarine, i. , ; varieties of the peach, i. ; origin of the apricot, i. ; origin and varieties of the plum, i. ; origin of the cherry, i. ; varieties of the gooseberry, i. ; selection practised with forest-trees, i. ; wild fastigate oak, i. ; dark-leaved varieties of trees, i. ; conversion of stamens into pistils in the poppy, i. ; variegated foliage, i. ; heredity of white hyacinths, i. , ii. ; changes in oaks dependent on age, i. ; inheritance of anomalous characters, ii. ; variation of plants in their native countries, ii. ; deciduous bushes becoming evergreen in hot climates, ii. ; antiquity of races of plants, ii. . de candolle, p., non-variability of monotypic genera, ii. ; relative development of root and seed in _raphanus sativus_, ii. . decaisne, on the cultivation of the wild carrot, i. ; varieties of the pear, i. ; inter-crossing of strawberries, i. ; fruit of the apple, i. ; sterility of _lysimachia nummularia_, ii. ; tender variety of the peach, ii. . deer, assumption of horns by female, ii. ; imperfect development of horns in a, on a voyage, ii. . deer, fallow, ii. . deerhound. scotch, difference in size of the sexes of, ii. ; deterioration of, ii. . degeneration of high-bred races, under neglect, ii. . de jonghe, j., on strawberries, i. , ii. ; soft-barked pears, ii. ; on accumulative variation, ii. ; resistance of blossoms to frost, ii. . delamer, e. s., on rabbits, i. , . _delphinium ajacis_, ii. . _delphinium consolida_, ii. - . _dendrocygna viduata_, i. , ii. . dentition, variations of, in the horse, i. . deodar, i. . desmarest, distribution of white on dogs, i. ; cat from the cape of good hope, i. ; cats of madagascar, i. ; occurrence of striped young in turkish pigs, i. ; french breeds of cattle, i. ; horns of goats, i. ; on hornless goats, ii. . desor, e., on the anglo-saxon race in america, ii. . desportes, number of varieties of roses, i. . devay, dr., singular case of albinism, ii. ; on the marriage of cousins, ii. ; on the effects of close interbreeding, ii. , . development and metamorphosis, ii. - . development, arrests of, ii. - . development, embryonic, ii. - . { } d'hervey-saint-denys, l., on the ya-mi, or imperial rice of the chinese, ii. . dhole, fertility of the, in captivity, ii. . diabetes, occurrence of, in three brothers, ii. . _dianthus_, contabescent plants of, ii. - ; hybrid varieties of, ii. . _dianthus armeria_ and _deltoides_, hybrids of, ii. . _dianthus barbatus_, i. . _dianthus caryophyllus_, i. . _dianthus japonicus_, contabescence of female organs in, ii. . dichogamous plants, ii. . dickson, mr., on "running" in carnations, i. ; on the colours of tulips, i. . _dicotyles torquatus_ and _labiatus_, ii. . dieffenbach, dog of new zealand, i. ; feral cats in new zealand, i. ; polydactylism in polynesia, ii. . _dielytra_, ii. . diet, change of, ii. - . _digitalis_, properties of, affected by culture, ii. ; poison of, ii. . digits, supernumerary, ii. ; analogy of, with embryonic conditions, ii. ; fusion of, ii. . dimorphic plants, ii. ; conditions of reproduction in, ii. - . dimorphism, reciprocal, ii. . dingo, i. ; variation of, in colour, i. ; half-bred, attempting to burrow, i. ; attraction of foxes by a female, i. ; variations of, in confinement, ii. . dioeciousness of strawberries, i. . diseases, inheritance of, ii. - ; family uniformity of, ii. ; inherited at corresponding periods of life, ii. - ; peculiar to localities and climates, ii. ; obscure correlations in, ii. - ; affecting certain parts of the body, ii. ; occurring in alternate generations, ii. . distemper, fatal to white terriers, ii. . disuse and use of parts, effects of, ii. - , - , - ; in the skeleton of rabbits, i. - ; in pigeons, i. - ; in fowls, i. - ; in ducks, i. - ; in the silk-moth, i. - . divergence, influence of, in producing breeds of pigeons, i. . dixon, e. s., on the musk duck, i. ; on feral ducks, i. ; on feral pigeons in norfolk island, i. ; crossing of pigeons, i. ; origin of domestic fowls, i. ; crossing of _gallus sonneratii_ and common fowl, i. ; occurrence of white in the young chicks of black fowls, i. ; paduan fowl of aldrovandi, i. ; peculiarities of the eggs of fowls, i. ; chickens, i. - ; late development of the tail in cochin cocks, i. ; comb of lark-crested fowls, i. ; development of webs in polish fowls, i. ; on the voice of fowls, i. ; origin of the duck, i. ; ducks kept by the romans, i. ; domestication of the goose, i. ; gander frequently white, i. ; breeds of turkeys, i. ; incubatory instinct of mongrels of non-sitting races of fowls, ii. ; aversion of the dove-cot pigeon to pair with fancy birds, ii. ; fertility of the goose, ii. ; general sterility of the guans in captivity, ii. ; fertility of geese in captivity, ii. ; white peafowl, ii. . dobell, h., inheritance of anomalies of the extremities, ii. ; non-reversion to a malformation, ii. . dobrizhoffer, abhorrence of incest by the abipones, ii. . dogs, origin of, i. ; ancient breeds of, i. , ii. ; of neolithic, bronze and iron periods in europe, i. - , ii. ; resemblance of to various species of canidæ, i. ; of north america compared with wolves, i. - ; of the west indies, south america, and mexico, i. , ; of guiana, i. ; naked dogs of paraguay and peru, _ibid._ and ; dumb, on juan fernandez, i. ; of juan de nova, i. ; of la plata, i. ; of cuba, i. ; of st. domingo, i. ; correlation of colour in, i. - ; gestation of, i. - ; hairless turkish, i. , ii. ; inter-crossing of different breeds of, i. ; characters of different breeds of, discussed, i. - ; degeneration of european, in warm climates, i. , ; ii. , ; liability to certain diseases in different breeds of, i. and _note_; causes of differences of breeds discussed, i. - ; catching fish and crabs in new guinea and tierra del fuego, i. ; webbing of the feet in, i. ; influence of selection in producing different breeds of, i. , ; retention of original habits by, i. ; inheritance of polydactylism in, ii. ; feral, ii. ; reversion in fourth generation of, ii. ; of the pacific islands, ii. , , ; mongrel, ii. - ; comparative facility of crossing different breeds of, ii. ; fertility of, ii. , ; inter-breeding of, ii. - ; selection of, among the greeks, ii. , ; among savages, ii. - ; unconscious selection of, ii. - ; valued by the fuegians, ii. ; climatal changes in hair of, ii. ; production of drooping ears in, ii. ; { } rejection of bones of game by, ii. ; inheritance of rudiments of limbs in, ii. ; development of fifth toe in, ii. ; hairless, deficiency of teeth in, ii. ; short-faced, teeth of, ii. ; probable analogous variation in, ii. ; extinction of breeds of, ii. . dombrain, h. h., on the auricula, ii. - . domestication, essential points in, ii. - ; favourable to crossing, ii. - ; fertility increased by, ii. - , . domesticated animals, origin of, ii. - ; occasional sterility of, under changed conditions, ii. - . donders, dr., hereditary hypermetropia, ii. . dorking fowl, i. , ; furcula of, figured, i. . dormouse, ii. . double flowers, ii. - , - ; produced by selection, ii. . doubleday, h., cultivation of the filbert pine strawberry, i. . douglas, j., crossing of white and black game-fowls, ii. . downing, mr., wild varieties of the hickory, i. ; peaches and nectarines from seed, i. - ; origin of the boston nectarine, i. ; american varieties of the peach, i. ; north american apricot, i. ; varieties of the plum, i. ; origin and varieties of the cherry, i. - ; "twin cluster pippins," i. ; varieties of the apple, i. ; on strawberries, i. , ; fruit of the wild gooseberry, i. ; effects of grafting upon the seed, ii. ; diseases of plum and peach trees, ii. - ; injury done to stone fruit in america by the "weevil," ii. ; grafts of the plum and peach, ii. ; wild varieties of pears, ii. ; varieties of fruit-trees suitable to different climates, ii. . _draba sylvestris_, ii. . dragon, pigeon, i. , . "draijer" (pigeon), i. . drinking, effects of, in different climates, ii. . dromedary, selection of, ii. - . druce, mr., inter-breeding of pigs, ii. . du chaillu, fruit-trees in west africa, i. . duchesne on _fragaria vesca_, i. , , . dufour, léon, on _cecidomyia_ and _misocampus_, i. . duck, musk, retention of perching habit by the, i. ; feral hybrid of, i. . duck, penguin, hybrid of, with egyptian goose, ii. . duck, wild, difficulty of rearing, ii. ; effects of domestication on, ii. . ducks, breeds of, i. - ; origin of, i. ; history of, _ibid._; wild, easily tamed, i. - ; fertility of breeds of, when crossed, i. ; with the plumage of _anas boschas_, i. ; malayan penguin, identical in plumage with english, i. ; characters of the breeds of, i. - ; eggs of, i. ; effects of use and disuse in, i. - , ii. ; feral, in norfolk, i. ; aylesbury, inheritance of early hatching by, ii. ; reversion in, produced by crossing, ii. ; wildness of half-bred wild, ii. ; hybrids of, with the musk duck, ii. - ; assumption of male plumage by, ii. ; crossing of labrador and penguin, ii. ; increased fertility of, by domestication, ii. ; general fertility of, in confinement, ii. ; increase of size of, by care in breeding, ii. ; change produced by domestication in, ii. . dumÉril, aug., breeding of _siredon_ in the branchiferous stage, ii. . dun-coloured horses, origin of, i. . dureau de la malle, feral pigs in louisiana, ii. ; feral fowls in africa, _ibid._; bud-variation in the pear, i. ; production of mules among the romans, ii. . _dusicyon sylvestris_, i. . dutch rabbit, i. . dutch roller pigeon, i. . dutrochet, pelorism in the laburnum, ii. . duval, growth of pears in woods in france, ii. . duval-jouve, on _leersia oryzoides_, ii. . duvernoy, self-impotence in _lilium candidum_, ii. . dzierzon, variability in the characters and habits of bees, i. . earle, dr., on colour-blindness, ii. , . ears, of fancy rabbits, i. ; deficiency of, in breeds of rabbits, i. ; rudimentary, in chinese sheep, ii. ; drooping, ii. ; fusion of, ii. . eaton, j. m., on fancy pigeons, i. , ; variability of characters in breeds of pigeons, i. ; reversion of crossed pigeons to coloration of _columba livia_, i. ; on pigeon-fancying, i. , - ; on tumbler-pigeons, i. , ii. ; carrier-pigeon, i. ; effects of interbreeding on pigeons, ii. ; properties of pigeons, ii. - ; death of short-faced tumblers in the egg, ii. ; { } archangel pigeon, ii. . echinodermata, metagenesis in, ii. . _ectopistes_, specific difference in number of tail-feathers in, i. . _ectopistes migratorius_, sterile hybrids of, with _turtur vulgaris_, i. . edentata, correlation of dermal system and teeth in the, ii. . edgeworth, mr., use of grass-seeds as food in the punjab, i. . edmonston, dr., on the stomach in _larus argentatus_ and the raven, ii. . edwards and colin, on english wheat in france, ii. . edwards, w. f., absorption of the minority in crossed races, ii. . edwards, w. w., occurrence of stripes in a nearly thoroughbred horse, i. ; in foals of racehorses, i. . eggs, of fowls, characters of, i. ; variations of, in ducks, i. ; of the silkmoth, i. . egypt, ancient dogs of, i. - ; ancient domestication of the pigeon in, i. ; absence of the fowl in ancient, i. . egyptian goose, hybrids of, with penguin duck, i. . ehrenberg, prof., multiple origin of the dog, i. ; dogs of lower egypt, i. ; mummies of _felis maniculata_, i. . element, male, compared to a premature larva, ii. . elements of the body, functional independence of the, ii. - . elephant, its sterility in captivity, ii. . elk, irish, correlations in the, ii. - . elliot, sir walter, on striped horses, i. ; indian domestic and wild swine, i. ; pigeons from cairo and constantinople, i. ; fantail pigeons, i. ; lotan tumbler pigeons, i. ; a pigeon uttering the sound _yahu_, i. ; _gallus bankiva_ in pegu, i. . ellis, mr., varieties of cultivated plants in tahiti, ii. . elm, nearly evergreen cornish variety of the, i. , ii. ; foliage-varieties of the, i. . elm, weeping, i. ; not reproduced by seed, ii. . _emberiza passerina_, ii. . embryos, similarity of, i. ; fusion of, ii. . engel, on _laurus sassafras_, ii. . england, domestication of _bos longifrons_ in, i. ; selection of horses in, in mediæval times, ii. ; laws against the early slaughter of rams in, ii. . ephemeridÆ, development of the, ii. . _epidendrum cinnabarinum_ and _e. zebra_, ii. . epilepsy, hereditary, ii. , . erdt, disease of the white parts of cattle, ii. . ericaceÆ, frequency of contabescence in the, ii. . erichthonius, an improver of horses by selection, ii. . erman, on the fat-tailed kirghisian sheep, i. , ii. ; on the dogs of the ostyaks, ii. . _erodium_, ii. . _erythrina crista-galli_ and _e. herbacea_, hybrids of, ii. . esquilant, mr., on the naked young of dun-coloured pigeons, i. . esquimaux dogs, their resemblance to wolves, i. ; selection of, ii. . eudes-deslongchamps, on appendages under the jaw of pigs, i. - . _euonymus japonicus_, i. . european cultivated plants, still wild in europe, i. . evans, mr., on the lotan tumbler pigeon, i. . evelyn, pansies grown in his garden, i. . everest, r., on the newfoundland dog in india, i. , ii. ; degeneration of setters in india, i. ; indian wild boars, i. . ewes, hornless, ii. . extinction of domestic races, i. . eyes, hereditary peculiarities of the, ii. - ; loss of, causing microphthalmia in children, ii. ; modification of the structure of, by natural selection, ii. - ; fusion of, ii. . eyebrows, hereditary elongation of hairs in, ii. . eyelids, inherited peculiarities of the, ii. . eyton, mr., on gestation in the dog, i. ; variability in number of vertebræ in the pig, i. ; individual sterility, ii. . _faba vulgaris_, i. . fabre, observations on _Ægilops triticoides,_ i. . _fagus sylvatica_, ii. . fairweather, mr., production of double flowers from old seed, ii. . _falco albidus_, resumption of young plumage by, in captivity, ii. . _falco ossifragus_, ii. . _falco subbuteo_, copulating in captivity, ii. . _falco tinnunculus_, breeding in captivity, ii. . { } falconer, dr., sterility of english bulldogs in india, i, ; resemblance between _sivatherium_ and niata cattle, i. ; selection of the silkworm in india, i. ; fastigate apple-trees in calcutta, i. ; reproduction of a supernumerary thumb after amputation, ii. ; fertility of the dhole in captivity, ii. ; fertility of english dogs in india, ii. ; sterility of the tiger in captivity, ii. ; turkeys at delhi, ii. ; on indian cultivated plants, ii. ; thibet mastiff and goat, ii. . falcons, sterility of, in captivity, ii. . falkland islands, horses of the, i. - , ; feral pigs of the, i. ; feral cattle of the, i. , ; feral rabbits of the, i. . fallow deer, ii. , . fantail pigeons, i. - , ii. ; figured, i. ; furcula of, figured, i. ; history of, i. ; absence of oil-gland in, ii. . faroe islands, pigeons of the, i. . fashion, influence of, in breeding, ii. . fastigate trees, ii. , . faunas, geographical differences, of, i. . "favourite" bull, ii. , . feathers, homologous variation in, ii. . feet, of pigeons, individual differences of, i. ; correlations of external characters in, i. - . feet and beak, correlation of, in pigeons, i. - . felidÆ, fertility of, in captivity, ii. . _felis bubastes_, i. . _felis caffra_, i. . _felis caligulata_, i. . _felis chaus_, i. - . _felis jubata_, ii. . _felis lybica_, i. . _felis maniculata_, i. . _felis manul_, i. . _felis ornata_, i. . _felis sylvestris_, i. . _felis torquata_, i. . female, affected by male element, ii. , - . female flowers, in male panicle of maize, i. . fennel, italian variety of, i. . feral cats, i. ; cattle, i. ; rabbits, i. - ; guinea fowl, i. ; animals and plants, reversion in, ii. - , . ferguson, mr., supposed plurality of origin of domestic fowls, i. ; chickens of black game-fowls, i. ; relative size of eggs of fowls, i. ; yolk of eggs of game-fowls, i. ; early pugnacity of game-cocks, i. ; voice of the malay fowl, i. ; effects of interbreeding on fowls, ii. ; selection in cochin china fowls, ii. ; on fashion in poultry, ii. . fernandez, on mexican dogs, i. . ferns, reproduction of abnormal forms of, by spores, i. ; non-diffusion of cell-gemmules in, ii. . ferrets, ii. , , . fertilisation, artificial, of the st. valery apple, i. . fertility, various degrees of, in sheep, i. ; unlimited mutual, of breeds of pigeons, i. - ; comparative of mongrels and hybrids, ii. - , - ; influence of nourishment on, ii. ; diminished by close interbreeding, ii. , ; reduced, of chillingham wild cattle, ii. ; of domesticated varieties when crossed, ii. . _festuca_, species of, propagated by bulblets, ii. . filberts, spared by tomtits, ii. . filippi, on the breeding of branchiferous tritons, ii. . finches, general sterility of, in captivity, ii. . finnikin (pigeon), i. . finnochio, i. . fir, scotch, acclimatisation of, ii. . fish, mr., advantage of change of soil to plants, ii. . fishes, regeneration of portions of fins of, ii. ; variability of, when kept in tanks, ii. ; marine, living in fresh water, ii. ; double monsters of, ii. . fission and gemmation, ii. . fitch, mr., persistency of a variety of the pea, i. . fittest, survival of the, i. . fitzinger, origin of sheep, i. ; african maned sheep, i. . fixedness of character, conditions of, discussed, ii. - . flax, found in the swiss lake-dwellings, i. ; climatal difference in products of, ii. . fleece, fineness of, in austrian merinos, ii. . fleischmann, on german sheep crossed with merinos, ii. - . "florentiner-taube," i. - . flounder, ii. . flourens, crossing of wolf and dog, i. ; prepotency of the jackal over the dog, ii. ; hybrids of the horse and ass, ii. ; breeding of monkeys in europe, ii. . { } flower-garden, earliest known, in europe, ii. . flowers, capricious transmission of colour-varieties in, ii. - ; tendency to uniformity in striped, ii. ; scorching of, dependent on colour, ii. ; change in, caused by conditions of life, ii. ; rudimentary, ii. ; relative position of, to the axis, ii. . foetation, abdominal, ii. . foley, mr., wild varieties of pears, ii. . foliage, inherited peculiarities of, i. ; variegation, of, i. ; bud-variation in, i. - . food, influence of, on the pig, i. ; on cattle, i. ; excess of, a cause of variability, ii. . forbes, d., on chilian sheep, i. ; on the horses of spain, chili, and the pampas, i. . _formica rufa_, ii. . fortune, r., sterility of the sweet potato in china, ii. ; development of axillary bulbs in the yam, _ibid._ fowl, common, breeds of, i. - ; supposed plurality of origin, i. ; early history of, i. - ; causes of production of breeds of, i. ; origin of from _gallus bankiva_, i. - , ; feral, notices of, i. - ; reversion and analogous variation in, i. - , ii. , , , , , ; "cuckoo" sub-breeds of, i. ; history of, i. - ; structural characters of, i. - ; sexual peculiarities of, i. - , ii. ; external differences of, i. - ; differences of breeds of, from _g. bankiva_, i. ; osteological characters of, i. - ; effects of disuse of parts in, i. - , ii. ; feral, i. , ii. ; polydactylism in, ii. ; fertility of, increased by domestication, ii. , ; sterility of, under certain conditions, ii. ; influence of selection on, ii. , , , ; evils of close interbreeding of, ii. - ; crossing of, ii. , , ; prepotency of transmission in, ii. ; rudimentary organs in, ii. ; crossing of non-sitting varieties of, ii. - ; homology of wing and leg feathers in, ii. ; hybrids of, with pheasants and _gallus sonneratii_, ii. ; black-skinned, ii. - ; black, preyed upon by the osprey in iceland, ii. ; five-toed, mentioned by columella, ii. ; rumpless, tailed chickens produced by, ii. ; dorking, crosses of, ii. ; form of comb and colour of plumage in, ii. ; game, crossing of white and black, ii. ; five-spurred, ii. ; spanish, liable to suffer from frost, ii. ; polish, peculiarities of skull of, ii. - . fox, sterility of, in captivity, ii. . fox, s. bevan, races of bees, i. . fox, w. darwin, gestation of the dog, i. ; "negro" cat, i. ; reversion of sheep in colour, ii. ; period of gestation in the pig, i. ; young of the himalayan rabbit, i. ; crossing of wild and domestic turkeys, i. ; reversion in crossed musk ducks, ii. ; spontaneous segregation of varieties of geese, ii. ; effects of close interbreeding upon bloodhounds, ii. ; deafness of white cats with blue eyes, ii. . foxhounds, i. , ii. . _fragaria chiloensis_, i. . _fragaria collina_, i. . _fragaria dioica_ of duchesne, i. . _fragaria elatior_, i. . _fragaria grandiflora_, i. . _fragaria vesca_, i. . _fragaria virginiana_, i. . _fraxinus excelsior_, i. , , ii. . _fraxinus lentiscifolia_, ii. . friesland cattle, probably descended from _bos primigenius_, i. . frillback (pigeon), i. ; indian, i. . _fringilla ciris_, ii. . _fringilla spinus_, ii. . frizzled fowls, i. ; horses, i. . frog, polydactylism in the, ii. . fruit, seedless, ii. . fruit-trees, varieties of, occurring wild, i. . fry, mr., on fertile hybrid cats, i. ; on feral fowls in ascension, i. . fuchsias, origin of, i. ; bud-variation in, i. . _fuchsia coccinea_ and _fulgens_, twin seed produced by crossing, i. . fuegians, their superstition about killing young water-fowl, i. ; selection of dogs by the, ii. ; their comparative estimation of dogs and old women, ii. ; their power of distant vision, ii. . fungi, parasitic, ii. - . furcula, characters and variations of the, in pigeons, i. ; alteration of, by disuse, in pigeons, i. ; characters of, in fowls, i. . fusion of homologous parts, ii. . gait, inheritance of peculiarities of, ii. . galapagos archipelago, its peculiar fauna and flora, i. . _galeobdolon luteum_, pelorism in, ii. , . { } galls, ii. - . gall-gnats, ii. . gall-like excrescences not inherited, ii. . gallinaceous birds, restricted range of large, i. ; general fertility of in captivity, ii. . _gallinula chloropus_, ii. . _gallinula nesiotis_, i. . galton, mr., fondness of savages for taming animals, i. , ii. ; cattle of benguela, i. ; on hereditary talent, ii. . gallesio, species of oranges, i. , , ; hybridisation of oranges, i. ; persistency of races in the peach, i. ; supposed specific distinctions of peach and nectarine, i. ; bizzaria orange, i. ; crossing of red and white carnations, i. ; crossing of the orange and lemon, i. , ii. ; effect of foreign pollen on maize, i. ; spontaneous crossing of oranges, ii. ; monstrosities a cause of sterility in plants, ii. ; seeding of ordinarily seedless fruits, ii. ; sterility of the sugar cane, ii. ; tendency of male flowers to become double, ii. ; effects of selection in enlarging fruit, &c., ii. ; variation of the orange tree in north italy, ii. ; naturalisation of the orange in italy, ii. . _gallus æneus_, a hybrid of _g. varius_ and the domestic fowl, i. . _gallus bankiva_, probable original of domestic fowls, i. , - , ; game-fowl, nearest to, i. ; crossed with _g. sonneratii_, i. ; its character and habits, i. - , ii. ; differences of various breeds of fowls from, i. ; occipital foramen of, figured, i. ; skull of, figured, i. ; cervical vertebra of, figured, i. ; furcula of, figured, i. ; reversion to, in crossed fowls, ii. - ; hybrid of, with _g. varius_, i. , ii. ; number of eggs of, ii. . _gallus ferrugineus_, i. . _gallus furcatus_, i. . _gallus giganteus_, i. . _gallus sonneratii_, characters and habits of, i. ; hybrids of, i. , ii. . _gallus stanleyi_, hybrids of, i. . _gallus temminckii_, probably a hybrid, i. . _gallus varius_, character and habits of, i. ; hybrids and probable hybrids of, i. - . gambier, lord, his early cultivation of the pansy, i. . game-fowl, i. , , , . gapes, ii. . garcilazo de la vega, annual hunts of the peruvian incas, ii. . garnett, mr., migratory propensities of hybrid ducks, ii. . garrod, dr., on hereditary gout, ii. . gasparini, a genus of pumpkins, founded on stigmatic characters, i. . gaudichaud, bud-variation in the pear, i. ; apple tree with two kinds of fruit on branch, i. . gaudry, anomalous structure in the feet of horses, i. . gay, on _fragaria grandiflora_, i. ; on _viola lutea_ and _tricolor_, i. ; on the nectary of _viola grandiflora_, i. . gayal, domestication of the, i. . gayot, _see_ moll. gÄrtner, on the sterility of hybrids, i. , ii. ; acquired sterility of varieties of plants when crossed, i. ; sterility in transplanted plants, and in the lilac in germany, ii. ; mutual sterility of blue and red flowers of the pimpernel, ii. ; supposed rules of transmission in crossing plants, ii. ; on crossing plants, ii. , , , ; on repeated crossing, ii. ; absorption of one species by another, when crossed, ii. ; crossing of varieties of the pea, i. ; crossing maize, ii. ; crossing of species of _verbascum_, ii. , ; reversion in hybrids, ii. , , ; of _cereus_, i. ; of _tropæolum majus_ and _minus_, i. ; variability of hybrids, ii. ; variable hybrids from one variable parent, ii. ; graft hybrid produced by inosculation in the vine, i. ; effect produced by grafts on the stock, i. , ii. ; tendency of hybrid plants to produce double flowers, ii. ; production of perfect fruit by sterile hybrids, ii. ; sexual elective affinity, ii. ; self-impotence in _lobelia_, _verbascum_, _lilium_, and _passiflora_, ii. - ; on the action of pollen, ii. ; fertilisation of _malva_, i. - , ii. ; prepotency of pollen, ii. ; prepotency of transmission in species of _nicotiana_, ii. ; bud-variation in _pelargonium zonale_, i. ; in _oenothera biennis_, i. ; in _achillæa millefolium_, i. ; effect of manure on the fertility of plants, ii. ; on contabescence, ii. - ; inheritance of plasticity, ii. ; villosity of plants, ii. . geese (_anseres_) general fertility of, in captivity, ii. . gegenbaur, on the number of digits, ii. . gemmation and fission, ii. . { } gemmules, or cell-gemmules, ii. , - , . genet, fertility of the, in captivity, ii. . generation, alternate, ii. , , . generation, sexual, ii. - . genius, inheritance of, ii. . _gentiana amarella_, ii. . geoffroy saint-hilaire, production of monstrous chickens, ii. ; "_loi de l'affinité de soi pour soi_," ii. ; compensation of growth, ii. . geoffroy saint-hilaire, isid., origin of the dog, i. ; barking of a jackal, i. ; period of gestation and odour of the jackal, i. ; anomalies in the teeth of dogs, i. ; variations in the proportions of dogs, i. ; webbed feet of newfoundland dogs, i. ; crossing of domestic and wild cats, i. ; domestication of the arni, i. ; supposed introduction of cattle into europe from the east, _ibid._; absence of interdigital pits in sheep, i. ; origin of the goat, i. ; feral geese, i. ; ancient history of the fowl, i. ; skull of the polish fowl, i. ; preference of the romans for the liver of white geese, i. ; polydactylism, ii. ; assumption of male characters by female birds, ii. ; supernumerary mammæ in women, ii. ; development of a proboscis in the pig, _ibid._; transmission and blending of characters in hybrids, ii. ; refusal of animals to breed in captivity, ii. ; on the guinea pig, ii. ; silkworms producing white cocoons, ii. ; on the carp, ii. ; on _helix lactea_, ii. ; on monstrosities, ii. ; injury to the embryo a cause of monstrosity, ii. ; alteration in the coat of horses in coal mines, ii. ; length of the intestines in wild and tame animals, ii. - ; inheritance of rudimentary limbs in the dog, ii. ; correlation in monstrosities, ii. ; supernumerary digits in man, ii. ; co-existence of anomalies, ii. ; fusion of homologous parts, ii. - ; presence of hairs and teeth in ovarian tumours, ii. ; development of teeth on the palate in the horse, ii. . geographical differences of faunas, i. . geological succession of organisms, i. . _geranium_, ii. . _geranium phæum_ and _pyrenaicum_, ii. . _geranium pratense_, i. . gerard, asserted climatal change in burgundian bees, i. . gerarde, on varieties of the hyacinth, i. . gerstÄcker, on hive-bees, i. . gervais, prof., origin of the dog, i. ; resemblance of dogs and jackals, i. ; taming of the jackal, i. ; number of teeth in dogs, i. ; breeds of dogs, i. ; on tertiary horses, i. ; biblical notices of horses, i. ; species of _ovis_, i. ; wild and domestic rabbits, i. ; rabbits from mount sinai and algeria, i. ; earless rabbits, i. ; batrachia with doubled limbs, ii. . gestation, period of, in the dog, wolf, &c, i. - ; in the pig, i. ; in cattle, i. , ii. ; in sheep, i. . gestures, inheritance of peculiarities in, ii. . "ghoondooks" a sub-breed of fowls, i. . ghor-khur, ii. . giles, mr., effect of cross-breeding in the pig, i. . giraffe, co-ordination of structure of, ii. . girard, period of appearance of permanent teeth in dogs, i. . girou de buzareingues, inheritance in the horse, ii. ; reversion by age in cattle, ii. ; prepotency of transmission of character in sheep and cattle, ii. ; on crossing gourds, ii. . gisburne, wild cattle at, i. . _gladiolus_, i. ; self-impotence of hybrids of, ii. . _gladiolus colvillii_, bud-variation in, i. . glands, compensatory development of, ii. . glastonbury thorn, i. . glenny, mr., on the _cineraria_, ii. . gloede, f., on strawberries, i. . gloger, on the wings of ducks, ii. . "glouglou" (pigeon), i. . _gloxiniæ_, peloric, i. , ii. . gmelin, on red cats, at tobolsk, i. . goat, i. - , ii. ; polydactylism in the, ii. ; sexual differences in horns of, ii. ; valued by south africans, ii. ; thibet, ii. ; amount of milk and development of udders in the, ii. ; hornless, rudimentary bony cores in, ii. ; angora, ii. . godron, odour of the hairless turkish dog, i. ; differences in the skull of dogs, i. ; increase of breeds of horses, i. ; crossing of domestic and wild swine, i. ; on goats, i. - ; colour of the skin in fowls, i. ; bees of north and south of france, i. ; introduction of the silkworm into europe, i. ; variability in the silkworm, i. ; supposed species of wheat, i. - ; on _Ægilops triticoides_, i. ; variable presence of barbs in grasses, i. ; { } colours of the seeds of maize, i. ; unity of character in cabbages, i. ; correlation of colour and odour, i. ; effect of heat and moisture on the cabbage, i. ; on the cultivated species of _brassica_, i. ; on the rouncival and sugar peas, i. ; variation in the numbers of peas in the same pod, i. ; wild vines in spain, i. ; on raising peaches from seed, i. ; supposed specific distinctness of peach and nectarine, i. ; nectarine producing peaches, i. ; on the flower of _corydalis_, i. ; origin and variations of the plum, i. ; origin of the cherry, i. ; reversion of single-leaved strawberries, i. ; five-leaved variety of _fragaria collina_, i. ; supposed immutability of specific characters, i. - ; varieties of _robinia_, i. ; permanency of the simple-leaved ash, i. ; non-inheritance of certain mutilations, ii. ; wild turnips, carrots, and celery, ii. ; pre-potency of a goat-like ram, ii. ; benefit of change of soil to plants, ii. ; fertility of peloric flowers of _corydalis solida_, ii. ; seeding of ordinarily seedless fruit, ii. ; sexual sterility of plants propagated by buds, &c., ii. ; increase of sugar in beet-root, ii. ; effects of selection in enlarging particular parts of plants, ii. ; growth of the cabbage in the tropics, ii. ; rejection of bitter almonds by mice, ii. ; influence of marshy pasture on the fleece of sheep, ii. ; on the ears of ancient egyptian pigs, ii. ; primitive distinctness of species, ii. ; solid hoofed swine, ii. . goethe, on compensation of growth, ii. . goldfish, i. - , ii. . gomara, on south american cats, i. . gongora, number of seeds in the, ii. . goose, ancient domestication of, i. ; sacred to juno in rome, _ibid._; inflexibility of organisation of, i. ; skull perforated in tufted, i. ; characters of breeds and sub-breeds of, i. - ; variety of, from sebastopol, i. , ii. ; feral in la plata, i. ; egyptian, hybrid of, with penguin duck, ii. ; spontaneous segregation of varieties of, ii. ; fertility of, increased by domestication, ii. ; decreased fertility of, in bogota, ii. ; sterility of, in the philippine islands, ii. ; selection of, ii. ; white, preference of the romans for the liver of, ii. ; persistency of character in, ii. ; egyptian, change in breeding season of, ii. . gooseberry, i. - ; bud-variation in the, i. ; whitesmith's, ii. . gÖppert, on monstrous poppies, ii. . gosse, p. h., feral dogs in jamaica, i. ; feral pigs of jamaica, i. - ; feral rabbits of jamaica, i. ; on _columba leucocephala_, i. ; feral guinea fowl in jamaica, i. ; reproduction of individual peculiarities by gemmation in a coral, i. ; frequency of striped legs in mules, ii. . gould, dr., on hereditary hæmorrhage, ii. . gould, john, origin of the turkey, i. . _goura coronata_ and _victoriæ_, hybrids of, i. , ii. . gourds, i. ; crossing of varieties of, ii. ; ancient peruvian variety of, ii. . gout, inheritance of, ii. ; period of appearance of, ii. . graba, on the pigeon of the faroe islands, i. . grafting, ii. ; effects of, ii. , ; upon the stock, i. - ; upon the variability of trees, ii. ; changes analogous to bud-variation produced by, i. , . graft-hybrids, i. - , - , ii. - . grapes, bud-variation in, i. ; cross of white and purple, i. ; green, liable to disease, ii. ; effect of foreign pollen on, i. . grasses, seeds of, used as food by savages, i. - . gray, asa, superior wild varieties of fruit-trees, i. ; cultivated native plants of north america, i. , ; non-variation of weeds, i. ; supposed spontaneous crossing of pumpkins, i. ; pre-ordination of variation, ii. ; progeny of husked form of maize, i. ; wild intermediate forms of strawberries, i. . gray, g. r., on _columba gymnocyclus_, i. . gray, j. e., on _sus pliciceps_, i. ; on a variety of the gold-fish, i. ; hybrids of the ass and zebra, ii. - ; on the breeding of animals at knowsley, ii. ; on the breeding of birds in captivity, ii. . greene, j. reay, on the development of the echinodermata, ii. . greenhow, mr., on a canadian web-footed dog, i. . greening, mr., experiments on _abraxas grossulariata_, ii. . gregson, mr., experiments on _abraxas grossulariata_, ii. . grey, sir george, preservation of seed-bearing plants by the australian savages, i. ; { } detestation of incest by australian savages, ii. . greyhounds, sculptured on egyptian monuments, and in the villa of antoninus, i. ; modern breed of, i. ; crossed with the bulldog, by lord orford, ii. ; co-ordination of structure of, due to selection, ii. - ; italian, ii. . greyness, inherited at corresponding periods of life, ii. . grieve, mr., on early-flowering dahlias, i. . grigor, mr., acclimatisation of the scotch fir, ii. . groom-napier, c. o., on the webbed feet of the otter-hound, i. . "grosses-gorges" (pigeons), i. . ground-tumbler, indian, i. . grouse, fertility of, in captivity, ii. . grÖnland, hybrids of _Ægilops_ and wheat, ii. . _grus montigresia_, _cinerea_, and _antigone_, ii. . guanacos, selection of, ii. . guans, general fertility of, in captivity, ii. . guelder-rose, ii. . guelderland fowls, i. . guiana, selection of dogs by the indians of, ii. . guinea fowl, i. ; feral in ascension, and jamaica, i. , ii. ; indifference of to change of climate, ii. . guinea pig, ii. , . gÜldenstadt, on the jackal, i. . gull, herring, breeding in confinement, ii. . gulls, general sterility of, in captivity, ii. . _gulo_, sterility of, in captivity, ii. . gÜnther, a., on tufted ducks and geese, i. ; on the regeneration of lost parts in batrachia, ii. . gurney, mr., owls breeding in captivity, ii. ; appearance of "black-shouldered" among ordinary peacocks, i. . habit, influence of, in acclimatisation, ii. - . habits, inheritance of, ii. . hÄckel, on cells, ii. ; on the double reproduction of medusæ, ii. ; on inheritance, ii. . hackles, peculiarities of, in fowls, i. . hair, on the face, inheritance of, in man, ii. ; peculiar lock of, inherited, ii. ; growth of, under stimulation of skin, ii. ; homologous variation of, ii. ; development of, within the ears and in the brain, ii. . hair and teeth, correlation of, ii. - . hairy family, corresponding period of inheritance in, ii. . half-castes, character of, ii. . half-lop rabbits, figured and described, i. - ; skull of, i. . _haliætus leucocephalus_, copulating in captivity, ii. . hallam, col., on a two-legged race of pigs, ii. . hamburgh fowl, i. , ; figured, i. . hamilton, wild cattle of, i. . hamilton, dr., on the assumption of male plumage by the hen pheasant, ii. . hamilton, f. buchanan, on the shaddock, i. ; varieties of indian cultivated plants, ii. . hancock, mr., sterility of tamed birds, ii. - . handwriting, inheritance of peculiarities in, ii. . hanmer, sir j., on selection of flower seeds, ii. . hansell, mr., inheritance of dark yolks in duck's eggs, i. . harcourt, e. v., on the arab boar-hound, i. ; aversion of the arabs to dun-coloured horses, i. . hardy, mr., effect of excess of nourishment on plants, ii. . hare, hybrids of, with rabbit, i. ; sterility of the, in confinement, ii. ; preference of, for particular plants, ii. . hare-lip, inheritance of, ii. . harlan, dr., on hereditary diseases, ii. . harmer, mr., on the number of eggs in a codfish, ii. . harvey, mr., monstrous red and white african bull, i. . harvey, prof., singular form of _begonia frigida_, i. - ; effects of cross-breeding on the female, i. ; monstrous saxifrage, ii. . hasora wheat, i. . hautbois strawberry, i. . hawker, col., on call or decoy ducks, i. . hawthorn, varieties of, i. - ; pyramidal, i. ; pendulous hybridised, ii. ; changes of, by age, i. , ; bud-variation in the, i. ; flower buds of, attacked by bullfinches, ii. . hayes, dr., character of esquimaux dogs, i. - . haywood, w., on the feral rabbits of porto santo, i. . hazel, purple-leaved, i. , , ii. . head of wild boar and yorkshire pig, figured, i. . { } head and limbs, correlated variability of, ii. . headache, inheritance of, ii. . heartsease, i. - ; change produced in the, by transplantation, i. ; reversion in, ii. , ; effects of selection on, ii. ; scorching of, ii. ; effects of seasonal conditions on the, ii. ; annual varieties of the, ii. . heat, effect of, upon the fleece of sheep, i. . heber, bishop, on the breeding of the rhinoceros in captivity, ii. . hebrides, cattle of the, i. ; pigeons of the, i. . heer, o., on the plants of the swiss lake-dwellings, i. , ii. , ; on the cereals, i. - ; on the peas, i. ; on the vine growing in italy in the bronze age, i. . _helix lactea_, ii. . _hemerocallis fulva_ and _flava_, interchanging by bud-variation, i. . hemlock yields no conicine in scotland, ii. . hemp, differences of, in various parts of india, ii. ; climatal difference in products of, ii. . hempseed, effect of, upon the colour of birds, ii. . hermaphrodite flowers, occurrence of, in maize, i. . hen, assumption of male characters by the, ii. , ; development of spurs in the, ii. . "hennies," or hen-like male fowls, i. . henry, t. a., a variety of the ash produced by grafting, i. ; crossing of species of _rhododendron_ and _arabis_, i. . henslow, prof., individual variation in wheat, i. ; bud-variation in the austrian bramble rose, i. ; partial reproduction of the weeping ash by seed, ii. . hepatica, changed by transplantation, i. . herbert, dr., variations of _viola grandiflora_, i. ; bud-variation in camellias, i. ; seedlings from reverted _cytisus adami_, i. ; crosses of swedish and other turnips, ii. ; on hollyhocks, ii. ; breeding of hybrids, ii. ; self-impotence in hybrid hippeastrums, ii. - ; hybrid _gladiolus_, ii. ; on _zephyranthes candida_, ii. ; fertility of the crocus, ii. ; on contabescence, ii. ; hybrid _rhododendron_, ii. . herculaneum, figure of a pig found in, i. . heron, sir r., appearance of "black-shouldered" among ordinary peacocks, i. - ; non-inheritance of monstrous characters by goldfish, i. ; crossing of white and coloured angora rabbits, ii. ; crosses of solid-hoofed pigs, ii. . _herpestes fasciatus_ and _griseus_, ii. . heusinger, on the sheep of the tarentino, ii. ; on correlated constitutional peculiarities, ii. . hewitt, mr., reversion in bantam cocks, i. ; degeneration of silk fowls, i. ; partial sterility of hen-like male fowls, i. ; production of tailed chickens by rumpless fowls, i. ; on taming and rearing wild ducks, i. - , ii. , - ; conditions of inheritance in laced sebright bantams, ii. ; reversion in rumpless fowls, ii. ; reversion in fowls by age, ii. ; hybrids of pheasant and fowl, ii. , ; assumption of male characters by female pheasants, ii. ; development of latent characters in a barren bantam hen, ii. ; mongrels from the silk-fowl, ii. ; effects of close interbreeding on fowls, ii. - ; on feathered-legged bantams, ii. . hibbert, mr., on the pigs of the shetland islands, i. . highland cattle, descended from _bos longifrons_, i. . hildebrand, dr., on the fertilisation of _orchideæ_, i. - ; occasional necessary crossing of plants, ii. ; on _primula sinensis_ and _oxalis rosea_, ii. ; on _corydalis cava_, ii. - . hill, r., on the alco, i. ; feral rabbits in jamaica, i. ; feral peacocks in jamaica, i. ; variation of the guinea fowl in jamaica, i. ; sterility of tamed birds in jamaica, ii. , . himalaya, range of gallinaceous birds in the, i. . himalayan rabbit, i. , - ; skull of, i. . himalayan sheep, i. . hindmarsh, mr., on chillingham cattle, i. . "hinkel-taube," i. - . hinny and mule, difference of, ii. - . _hipparion_, anomalous resemblance to in horses, i. . _hippeastrum_, hybrids of, ii. - . hive-bees, ancient domestication of, i. ; breeds of, i. ; smaller when produced in old combs, i. ; variability in, i. ; crossing of ligurian and common, i. . "hocker-taube," i. . hobbs, fisher, on interbreeding pigs, ii. . hodgkin, dr., on the attraction of foxes by a female dingo, i. ; { } origin of the newfoundland dog, i. ; transmission of a peculiar lock of hair, ii. . hodgson, mr., domestication of _canis primævus_, i. ; development of a fifth digit in thibet mastiffs, i. ; number of ribs in humped cattle, i. ; on the sheep of the himalaya, i. ; presence of four mammæ in sheep, _ibid._; arched nose in sheep, i. ; measurements of the intestines of goats, i. ; presence of interdigital pits in goats, _ibid._; disuse a cause of drooping ears, ii. . hofacker, persistency of colour in horses, i. , ii. ; production of dun horses from parents of different colours, i. ; inheritance of peculiarities in handwriting, ii. ; heredity in a one-horned stag, ii. ; on consanguineous marriages, ii. . hog, red river, ii. . hogg, mr., retardation of breeding in cows by hard living, ii. . holland, sir h., necessity of inheritance, ii. ; on hereditary diseases, ii. ; hereditary peculiarity in the eyelid, ii. ; morbid uniformity in the same family, ii. ; transmission of hydrocele through the female, ii. ; inheritance of habits and tricks, ii. . holly, varieties of the, i. , ; bud-reversion in, i. ; yellow-berried, ii. , . hollyhock, bud-variation in, i. ; non-crossing of double varieties of, ii. ; tender variety of the, ii. . homer, notice of geese, i. ; breeding of the horses of Æneas, ii. . homologous parts, correlated variability of, ii. - , - ; fusion of, ii. ; affinity of, ii. - . hoofs, correlated with hair in variation, ii. . hook-billed duck, skull figured, i. . hooker, dr. j. d., forked shoulder-stripe in syrian asses, i. ; voice of the cock in sikkim, i. ; use of arum-roots as food, i. ; native useful plants of australia, i. ; wild walnut of the himalayas, i. ; variety of the plane tree, i. ; production of _thuja orientalis_ from seeds of _t. pendula_, i. ; singular form of _begonia frigida_, i. ; reversion in plants run wild, ii. ; on the sugar-cane, ii. ; on arctic plants, ii. ; on the oak grown at the cape of good hope, ii. ; on _rhododendron ciliatum_, ii. ; stock and mignonette, perennial in tasmania, ii. . hopkirk, mr., bud-variation in the rose, i. ; in _mirabilis jalapa_, i. ; in _convolvulus tricolor_, i. . hornbeam, heterophyllous, i. . horned fowl, i. ; skull figured, i. . hornless cattle in paraguay, i. . horns of sheep, i. ; correlation of, with fleece in sheep, ii. ; correlation of, with the skull, ii. ; rudimentary in young polled cattle, ii. ; of goats, i. . horses, in swiss lake-dwellings, i. ; different breeds of, in malay archipelago, i. ; anomalies in osteology and dentition of, i. ; mutual fertility of different breeds, i. ; feral, i. ; habit of scraping away snow, i. ; mode of production of breeds of, i. ; inheritance and diversity of colour in, i. ; dark stripes in, i. - , ii. ; dun-coloured, origin of, i. ; colours of feral, i. - ; effect of fecundation by a quagga on the subsequent progeny of, i. - ; inheritance of peculiarities in, ii. - ; polydactylism in, ii. ; inheritance of colour in, ii. ; inheritance of exostoses in legs of, ii. ; reversion in, ii. , ; hybrids of, with ass and zebra, ii. ; prepotency of transmission in the sexes of, ii. ; segregation of, in paraguay, ii. ; wild species of, breeding in captivity, ii. ; curly, in paraguay, ii. , ; selection of, for trifling characters, ii. ; unconscious selection of, ii. - ; natural selection in circassia, ii. ; alteration of coat of, in coal-mines, ii. ; degeneration of, in the falkland islands, ii. ; diseases of, caused by shoeing, ii. ; feeding on meat, ii. ; white and white-spotted, poisoned by mildewed vetches, ii. ; analogous variations in the colour of, ii. ; teeth developed on palate of, ii. ; of bronze period in denmark, ii. . horse-chesnut, early, at the tuileries, i. ; tendency to doubleness in, ii. . horse-radish, general sterility of the, ii. . "houdan," a french sub-breed of fowls, i. . howard, c., on an egyptian monument, i. ; on crossing sheep, ii. , . huc, on the emperor khang-hi, ii. ; chinese varieties of the bamboo, ii. . humboldt, a., character of the zambos, ii. ; parrot speaking the language of an extinct tribe, ii. ; on _pulex penetrans_, ii. . humidity, injurious effect of, upon horses, i. . humphreys, col., on ancon sheep, i. . hungarian cattle, i. . { } hunter, john, period of gestation in the dog, i. ; on secondary sexual characters, i. ; fertile crossing of _anser ferus_ and the domestic goose, i. ; inheritance of peculiarities in gestures, voice, &c., ii. ; assumption of male characters by the human female, ii. ; period of appearance of hereditary diseases, ii. ; graft of the spur of a cock upon its comb, ii. ; on the stomach of _larus tridentatus_, ii. ; double-tailed lizards, ii. . hunter, w., evidence against the influence of imagination upon the offspring, ii. . hutton, capt., on the variability of the silk moth, i. ; on the number of species of silkworms, i. ; markings of silkworms, i. ; domestication of the rock-pigeon in india, i. ; domestication and crossing of _gallus bankiva_, i. . hutchinson, col., liability of dogs to distemper, i. . huxley, prof., on the transmission of polydactylism, ii. ; on unconscious selection, ii. ; on correlation in the mollusca, ii. ; on gemmation and fission, ii. ; development of star-fishes, ii. . hyacinths, i. - ; bud-variation in, i. ; graft-hybrid by union of half bulbs of, i. ; white, reproduced by seed, ii. ; red, ii. , ; varieties of, recognisable by the bulb, ii. . hyacinth, feather, ii. , . _hyacinthus orientalis_, i. . _hybiscus syriacus_, ii. . hybrids, of hare and rabbit, i. ; of various species of _gallus_, i. - ; of almond, peach, and nectarine, i. ; naturally produced, of species of _cytisus_, i. ; from twin-seed of _fuchsia coccinea_ and _fulgens_, i. ; reversion of, i. - , ii. , - ; from mare, ass, and zebra, ii. ; of tame animals, wildness of, ii. - ; female instincts of sterile male, ii. ; transmission and blending of characters in, ii. - ; breed better with parent species than with each other, ii. ; self-impotence in, ii. - ; readily produced in captivity, ii. . hybridisation, singular effects of, in oranges, i. ; of cherries, i. ; difficulty of, in _cucurbitæ_, i. ; of roses, i. . hybridism, ii. - ; the cause of a tendency to double flowers, ii. ; in relation to pangenesis, ii. . hybridity in cats, i. - ; supposed of peach and nectarine, i. . _hydra_, i. , ii. , . hydrangea, colour of flowers of, influenced by alum, ii. . hydrocele, ii. . hydrocephalus, ii. . _hypericum calycinum_, ii. . _hypericum crispum_, ii. , . hypermetamorphosis, ii. . hypermetropia, hereditary, ii. . ichthyopterygia, number of digits in the, ii. . _ilex aquifolium_, ii. . imagination, supposed effect of, on offspring, ii. . _imatophyllum miniatum_, bud-variation in, i. . incest, abhorred by savages, ii. - . incubation, by crossed fowls of non-sitting varieties, ii. - . india, striped horses of, i. ; pigs of, i. , , ; breeding of rabbits in, i. ; cultivation of pigeons in, i. - . individual variability in pigeons, i. - . ingledew, mr., cultivation of european vegetables in india, ii. . "indische taube," ii. . inheritance, ii. - , - , , - ; doubts entertained of by some writers, ii. ; importance of to breeders, - ; evidence of, derived from statistics of chances, ; of peculiarities in man, - , - ; of disease, - , ; of peculiarities in the eye, - ; of deviations from symmetry, ; of polydactylism, - ; capriciousness of, - , ; of mutilations, - ; of congenital monstrosities, ; causes of absence of, - ; by reversion or atavism, - ; its connexion with fixedness of character, - ; affected by prepotency of transmission of character, - ; limited by sex, - ; at corresponding periods of life, - ; summary of the subject of, - ; laws of, the same in seminal and bud varieties, i. ; of characters in the horse, i. - ; in cattle, i. ; in rabbits, i. ; in the peach, i. ; in the nectarine, i. ; in plums, i. ; in apples, i. ; in pears, i. ; in the pansy, i. ; of primary characters of _columba livia_ in crossed pigeons, i. ; of peculiarities of plumage in pigeons, i. - ; of peculiarities of foliage in trees, i. ; effects of, in varieties of the cabbage, i. . insanity, inheritance of, ii. , . insects, regeneration of lost parts in, ii. , ; agency of, in fecundation of larkspurs, ii. ; effect of changed conditions upon, ii. ; sterile neuter, ii. - ; { } monstrosities in, ii. , . instincts, defective, of silkworms, i. . interbreeding, close, ill effects of, ii. - , . intercrossing, of species, as a cause of variation, i. ; natural, of plants, i. ; of species of canidæ and breeds of dogs, i. - ; of domestic and wild cats, i. - ; of breeds of pigs, i. , ; of cattle, i. ; of varieties of cabbage, i. ; of peas, i. , - ; of varieties of orange, i. ; of species of strawberries, i. - ; of _cucurbitæ_, i. - ; of flowering plants, i. ; of pansies, i. . interdigital pits, in goats, i. . intermarriages, close, ii. - . intestines, elongation of, in pigs, i. ; relative measurements of parts of, in goats, i. ; effects of changed diet on, ii. . _ipomoea purpurea_, ii. . ireland, remains of _bos frontosus_ and _longifrons_ found in, i. . iris, hereditary absence of the, ii. ; hereditary peculiarities of colour of the, ii. - . irish, ancient, selection practised by the, ii. . iron period, in europe, dog of, i. . islands, oceanic, scarcity of useful plants on, i. . islay, pigeons of, i. . isolation, effect of, in favour of selection, ii. - . italy, vine growing in, during the bronze period, i. . ivy, sterility of, in the north of europe, ii. . jack, mr., effect of foreign pollen on grapes, i. . jackal, i. , , ; hybrids of, with the dog, i. ; prepotency of, over the dog, ii. . jacobin pigeon, i. , . jacquemet-bonnefort, on the mulberry, i. . jaguar, with crooked legs, i. . jamaica, feral dogs of, i. ; feral pigs of, i. ; feral rabbits of, i. . japan, horses of, i. . japanese pig (figured), i. . jardine, sir w., crossing of domestic and wild cats, i. . jarves, j., silkworm in the sandwich islands, i. . java, fantail pigeon in, i. . javanese ponies, i. , . jemmy button, i. . jenyns, l., whiteness of ganders, i. ; sunfish-like variety of the goldfish, i. . jerdon, j. c., number of eggs laid by the pea-hen, ii. ; origin of domestic fowl, i. . jersey, arborescent cabbages of, i. . jessamine, i. . jeitteles, hungarian sheep-dogs, i. ; crossing of domestic and wild cats, i. . john, king, importation of stallions from flanders by, ii. . johnson, d., occurrence of stripes on young wild pigs in india, i. . jordan, a., on vibert's experiments on the vine, i. ; origin of varieties of the apple, i. ; varieties of pears found wild in woods, ii. . jourdan, parthenogenesis in the silk moth, ii. . juan de nova, wild dogs on, i. . juan fernandez, dumb dogs on, i. . _juglans regia_, i. - . jukes, prof., origin of the newfoundland dog, i. . julien, stanislas, early domestication of pigs in china, i. ; antiquity of the domestication of the silk-worm in china, i. . jumpers, a breed of fowls, i. . juniper, variations of the, i. , . _juniperus suecica_, i. . _jussiæa grandiflora_, ii. . jussieu, a. de, structure of the pappus in _carthamus_, ii. . kail, scotch, reversion in, ii. . "kala-par" pigeon, i. . kales, i. . kalm, p., on maize, i. , ii. ; introduction of wheat into canada, i. ; sterility of trees growing in marshes and dense woods, ii. . "kalmi lotan," tumbler pigeon, i. . kane, dr., on esquimaux dogs, i. . karakool sheep, i. . karkeek, on inheritance in the horse, ii. . "karmeliten taube," i. . karsten on _pulex penetrans_, ii. . kattywar horses, i. . keeley, r., pelorism in _galeobdolon luteum_, ii. . kerner on the culture of alpine plants, ii. . kestrel, breeding in captivity, ii. . "khandÉsi," i. . khang-hi, selection of a variety of rice by, ii. . kiang, ii. . kidd, on the canary bird, i. , ii. . kidney bean, i. ; varieties of, ii. , . { } kidneys, compensatory development of the, ii. ; fusion of the, ii. ; shape of, in birds, influenced by the form of the pelvis, ii. . king, col., domestication of rock doves from the orkneys, i. , . king, p. s., on the dingo, i. , . kirby and spence, on the growth of galls, ii. . kirghisian sheep, i. . kite, breeding in captivity, ii. . kleine, variability of bees, i. . knight, andrew, on crossing horses of different breeds, i. ; crossing varieties of peas, i. , ii. ; persistency of varieties of peas, i. ; origin of the peach, i. ; hybridisation of the morello by the elton cherry, i. ; on seedling cherries, _ibid._; variety of the apple not attacked by coccus, i. ; intercrossing of strawberries, i, , ; broad variety of the cock's comb, i. ; bud variation in the cherry and plum, i. ; crossing of white and purple grapes, i. ; experiments in crossing apples, i. , ii. ; hereditary disease in plants, ii. ; on interbreeding, ii. ; crossed varieties of wheat, ii. ; necessity of intercrossing in plants, ii. ; on variation, ii. , ; effects of grafting, i. , ii. ; bud-variation in a plum, ii. ; compulsory flowering of early potatoes, ii. ; correlated variation of head and limbs, ii. . knox, mr., breeding of the eagle owl in captivity, ii. . koch, degeneracy in the turnip, i. . kohlrabi, i. . kÖlreuter, reversion in hybrids, i. , ii. ; acquired sterility of crossed varieties of plants, i. , ii. ; absorption of _mirabilis vulgaris_ by _m. longiflora_, ii. ; crosses of species of _verbascum_, ii. , ; on the hollyhock, ii. ; crossing varieties of tobacco, ii. ; benefits of crossing plants, ii. , , - ; self-impotence in _verbascum_, ii. , ; effects of conditions of growth upon fertility in _mirabilis_, ii. ; great development of tubers in hybrid plants, ii. ; inheritance of plasticity, ii. ; variability of hybrids of _mirabilis_, ii. ; repeated crossing a cause of variation, ii. - ; number of pollen-grains necessary for fertilization, ii. . "krauseschwein," i. . krohn, on the double reproduction of medusæ, ii. . "kropf-tauben," i. . labat, on the tusks of feral bears in the west indies, i. ; on french wheat grown in the west indies, ii. ; on the culture of the vine in the west indies, ii. . laburnum, adam's, see _cytisus adami_; oak-leaved, reversion of, i. ; pelorism in the, ii. ; waterer's, i. . lachmann, on gemmation and fission, ii. . _lachnanthes tinctoria_, ii. , . lactation, imperfect, hereditary, ii. ; deficient, of wild animals in captivity, ii. . ladrone islands, cattle of, i. . laing, mr., resemblance of norwegian and devonshire cattle, i. . lake-dwellings, sheep of, i. , ii. ; cattle of, ii. ; absence of the fowl in, i. ; cultivated plants of, i. , ii. , ; cereals of, i. - ; peas found in, i. ; beans found in, i. . lamare-piquot, observations on half-bred north american wolves, i. . lambert, a. b., on _thuja pendula_ or _filiformis_, i. . lambert family, ii. , . lambertye on strawberries, i. , ; five-leaved variety of _fragaria collina_, i. . landt, l., on sheep in the faroe islands, ii. . la plata, wild dogs of, i. ; feral cat from, i. . larch, ii. . larkspurs, insect agency necessary for the full fecundation of, ii. . _larus argentatus_, ii. . _larus tridactylus_, ii. . lasterye, merino sheep in different countries, i. . latent characters, ii. - . latham, on the fowl not breeding in the extreme north, ii. . _lathyrus_, ii. . _lathyrus aphaca_, ii. . _lathyrus odoratus_, ii. , , , , . la touche, j. d., on a canadian apple with dimidiate fruit, i. - . "latz-taube," i. . laugher pigeon, i. , . _laurus sassafras_, ii. . lawrence, j., production of a new breed of fox-hounds, i. ; occurrence of canines in mares, i. ; on three-parts-bred horses, i. ; on inheritance in the horse, ii. - . lawson, mr., varieties of the potato, i. . laxton, mr., bud-variation in the gooseberry, i. ; crossing of varieties of the pea, i. - ; { } double-flowered peas, ii. . layard, e. l., resemblance of a caffre dog to the esquimaux breed, i. , ii. ; crossing of the domestic cat with _felis caffra_, i. ; feral pigeons in ascension, i. ; domestic pigeons of ceylon, i. ; on _gallus stanleyi_, i. ; on black-skinned ceylonese fowls, i. . le compte family, blindness inherited in, ii. . lecoq, bud-variation in _mirabilis jalapa_, i. ; hybrids of _mirabilis_, i. , ii. , ; crossing in plants, ii. ; fecundation of _passiflora_, ii. ; hybrid _gladiolus_, ii. ; sterility of _ranunculus ficaria_, ii. ; villosity in plants, ii. ; double asters, ii. . le couteur, j., varieties of wheat, i. - ; acclimatisation of exotic wheat in europe, i. ; adaptation of wheat to soil and climate, i. ; selection of seed-corn, i. ; on change of soil, ii. ; selection of wheat, ii. ; natural selection in wheat, ii. ; cattle of jersey, ii. . ledger, mr., on the llama and alpaca, ii. . lee, mr., his early culture of the pansy, i. . _leersia oryzoides_, ii. . lefour, period of gestation in cattle, i. . legs, of fowls, effects of disuse on, i. - ; characters and variations of, in ducks, i. - ; fusion of, ii. . leguat, cattle of the cape of good hope, i. . lehmann, occurrence of wild double-flowered plants near a hot spring, ii. . leighton, w. a., propagation of a weeping yew by seed, ii. . leitner, effects of the removal of anthers, ii. . lemming, ii. . lemoine, variegated _symphytum_ and _phlox_, i. . lemon, i. , ; orange fecundated by pollen of the, i. . lemurs, hybrid, ii. . leporides, ii. - , . lepsius, figures of ancient egyptian dogs, i. ; domestication of pigeons in ancient egypt, i. . _leptotes_, ii. . _lepus glacialis_, i. . _lepus magellanicus_, i. . _lepus nigripes_, i. . _lepus tibetanus_, i. . _lepus variabilis_, i. . lereboullet, double monsters of fishes, ii. . leslie, on scotch wild cattle, i. . lesson, on _lepus magellanicus_, i. . leuckart on the larva of cecidomyidæ, ii. . lewis, g., cattle of the west indies, ii. . lherbette and quatrefages, on the horses of circassia, ii. , . liebig, differences in human blood, according to complexion, ii. . liebreich, occurrence of pigmentary retinitis in deaf-mutes, ii. . lichens, sterility in, ii. . lichtenstein, resemblance of bosjesman's dogs to _canis mesomelas_, i. ; newfoundland dog at the cape of good hope, i. . lilacs, ii. . liliaceÆ, contabescence in, ii. . _lilium candidum_, ii. . limbs, regeneration of, ii. - . limbs and head, correlated variation of, ii. . lime, effect of, upon shells of the mollusca, ii. . lime tree, changes of by age, i. , . limitation, sexual, ii. - . limitation, supposed, of variation, ii. . _linaria_, pelorism in, ii. , , ; peloric, crossed with the normal form, ii. ; sterility of, ii. . _linaria vulgaris_ and _purpurea_, hybrids of, ii. . lindley, john, classification of varieties of cabbages, i. ; origin of the peach, i. ; influence of soil on peaches and nectarines, i. ; varieties of the peach and nectarine, i. ; on the new town pippin, i. ; freedom of the winter majetin apple from coccus, i. ; production of monoecious hautbois strawberries by bud-selection, i. ; origin of the large tawny nectarine, i. ; bud-variation in the gooseberry, i. ; hereditary disease in plants, ii. ; on double flowers, ii. ; seeding of ordinarily seedless fruits, ii. ; sterility of _acorus calamus_, ii. ; resistance of individual plants to cold, ii. . linnÆus, summer and winter wheat regarded as distinct species by, i. ; on the single-leaved strawberry, i. ; sterility of alpine plants in gardens, ii. ; recognition of individual reindeer by the laplanders, ii. ; growth of tobacco in sweden, ii. . linnet, ii. . _linota cannabina_, ii. . { } linum, ii. . lion, fertility of, in captivity, ii. , . lipari, feral rabbits of, i. . livingstone, dr., striped young pigs on the zambesi, i. ; domestic rabbits at loanda, i. ; use of grass-seeds as food in africa, i. ; planting of fruit-trees by the batokas, i. ; character of half-castes, ii. ; taming of animals among the barotse, ii. ; selection practised in south africa, ii. , . livingstone, mr., disuse a cause of drooping ears, ii. . lizards, reproduction of tail in, ii. ; with a double tail, ii. . llama, selection of, ii. . lloyd, mr., taming of the wolf, i. ; english dogs in northern europe, i. ; fertility of the goose increased by domestication, i. ; number of eggs laid by the wild goose, ii. ; breeding of the capercailzie in captivity, ii. . loanda, domestic rabbits at, i. . _loasa_, hybrid of two species of, ii. . _lobelia_, reversion in hybrids of, ii. ; contabescence in, ii. . _lobelia fulgens_, _cardinalis_, and _syphilitica_, ii. . lockhart, dr., on chinese pigeons, i. . locust-tree, ii. . loiseleur-deslongchamps, originals of cultivated plants, i. ; mongolian varieties of wheat, i. ; characters of the ear in wheat, i. ; acclimatisation of exotic wheat in europe, i. ; effect of change of climate on wheat, i. ; on the supposed necessity of the coincident variation of weeds and cultivated plants, i. ; advantage of change of soil to plants, ii. . _lolium temulentum_, variable presence of barbs in, i. . long-tailed sheep, i. , . loochoo islands, horses of, i. . lord, j. k., on canis latrans, i. . "lori rajah," how produced, ii. . _lorius garrulus_, ii. . "lotan," tumbler pigeon, i. . loudon, j. w., varieties of the carrot, i. ; short duration of varieties of peas, i. ; on the glands of peach-leaves, i. ; presence of bloom on russian apples, i. ; origin of varieties of the apple, i. ; varieties of the gooseberry, i. ; on the nut tree, i. ; varieties of the ash, i. ; fastigate juniper (_j. suecica_), i. ; on _ilex aquifolium ferox_, i. ; varieties of the scotch fir, i. ; varieties of the hawthorn, _ibid._; variation in the persistency of leaves on the elm and turkish oak, i. ; importance of cultivated varieties, _ibid._; varieties of _rosa spinosissima_, i. ; variation of dahlias from the same seed, i. ; production of provence roses from seeds of the moss rose, i. ; effect of grafting the purple-leaved upon the common hazel, i. ; nearly evergreen cornish variety of the elm, ii. . low, g., on the pigs of the orkney islands, i. . low, prof., pedigrees of greyhounds, ii. ; origin of the dog, i. ; burrowing instinct of a half-bred dingo, i. ; inheritance of qualities in horses, i. ; comparative powers of english race-horses, arabs, &c., i. ; british breeds of cattle, i. ; wild cattle of chartley, i. ; effect of abundance of food on the size of cattle, i. ; effects of climate on the skin of cattle, i. , ii. ; on interbreeding, ii. ; selection in hereford cattle, ii. ; formation of new breeds, ii. ; on "sheeted" cattle, ii. . lowe, mr., on hive bees, i. . lowe, rev. mr., on the range of _pyrus malus_ and _p. acerba_, i. . "lowtan" tumbler pigeon, i. . _loxia pyrrhula_, ii. . lubbock, sir j., developments of the ephemeridæ, ii. . lucas, p., effects of cross-breeding on the female, i. ; hereditary diseases, ii. , - ; hereditary affections of the eye, ii. - ; inheritance of anomalies in the human eye and in that of the horse, ii. , ; inheritance of polydactylism, ii. ; morbid uniformity in the same family, ii. ; inheritance of mutilations, ii. ; persistency of cross-reversion, ii. ; persistency of character in breeds of animals in wild countries, ii. ; prepotency of transmission, ii. , ; supposed rules of transmission in crossing animals, ii. ; sexual limitations of transmission of peculiarities, ii. - ; absorption of the minority in crossed races, ii. ; crosses without blending of certain characters, ii. ; on interbreeding, ii. ; variability dependent on reproduction, ii. ; period of action of variability, ii. ; inheritance of deafness in cats, ii. ; complexion and constitution, ii. . lucaze-duthiers, structure and growth of galls, ii. - . luizet, grafting of a peach-almond on a peach, i. . { } lÜtke, cats of the caroline archipelago, i. . luxuriance, of vegetative organs, a cause of sterility in plants, ii. - . lyonnet, on the scission of _nais_, ii. . _lysimachia nummularia_, sterility of, ii. . _lythrum_, trimorphic species of, ii. . _lythrum salicaria_, ii. ; contabescence in, ii. . _lytta vesicatoria_, affecting the kidneys, ii. . _macacus_, species of, bred in captivity, ii. . macaulay, lord, improvement of the english horse, ii. . mcclelland, dr., variability of fresh-water fishes in india, ii. . mccoy, prof., on the dingo, i. . macfayden, influence of soil in producing sweet or bitter oranges from the same seed, i. . macgillivray, domestication of the rock-dove, i. ; feral pigeons in scotland, i. ; number of vertebræ in birds, i. ; on wild geese, i. ; number of eggs of wild and tame ducks, ii. . mackenzie, sir g., peculiar variety of the potato, i. . mackenzie, p., bud-variation in the currant, i. . mackinnon, mr., horses of the falkland islands, i. ; feral cattle of the falkland islands, i. . macknight, c., on interbreeding cattle, ii. . macnab, mr., on seedling weeping birches, ii. ; non-production of the weeping beech by seed, ii. . madagascar, cats of, i. . madden, h., on interbreeding cattle, ii. . madeira, rock pigeon of, i. . _magnolia grandiflora_, ii. . maize, its unity of origin, i. ; antiquity of, _ibid._; with husked grains said to grow wild, _ibid._; variation of, i. ; irregularities in the flowers of, i. ; persistence of varieties, _ibid._; adaptation of to climate, i. , ii. ; acclimatisation of, ii. , ; crossing of, i. , ii. - ; extinct peruvian varieties of, ii. . malay fowl, i. . malay archipelago, horses of, i. ; short-tailed cats of, i. ; striped young wild pigs of, i. ; ducks of, i. . male, influence of, on the fecundated female, i. - ; supposed influence of, on offspring, ii. . male flowers, appearance of, among female flowers in maize, i. . malformations, hereditary, ii. . _malva_, fertilisation of, i. , ii. . _mamestra suasa_, ii. . mammÆ, variable in number in the pig, i. ; rudimentary, occasional full development of, in cows, i. , ii. ; four present in some sheep, i. ; variable in number in rabbits, i. ; latent functions of, in male animals, ii. , ; supernumerary and inguinal, in women, ii. . mangles, mr., annual varieties of the heartsease, ii. . mantell, mr., taming of birds by the new zealanders, ii. . manu, domestic fowl noticed in the institutes of, i. . manure, effect of, on the fertility of plants, ii. . manx cats, i. , ii. . marcel de serres, fertility of the ostrich, ii. . marianne islands, varieties of _pandanus_ in, ii. . markham, gervase, on rabbits, i. , ii. . markhor, probably one of the parents of the goat, i. . marquand, cattle of the channel islands, i. . marrimpoey, inheritance in the horse, ii. . marrow, vegetable, i. . marryatt, capt., breeding of asses in kentucky, ii. . marsden, notice of _gallus giganteus_, i. . marshall, mr., voluntary selection of pasture by sheep, i. ; adaptation of wheats to soil and climate, i. ; "dutch-buttocked" cattle, ii. ; segregation of herds of sheep, ii. ; advantage of change of soil to wheat and potatoes, ii. ; fashionable change in the horns of cattle, ii. ; sheep in yorkshire, ii. . marshall, prof., growth of the brain in microcephalous idiots, ii. . martens, e. von, on _achatinella_, ii. . martin, w. c. l., origin of the dog, i. ; egyptian dogs, i. ; barking of a mackenzie river dog, i. ; african hounds in the tower menagerie, i. ; on dun horses and dappled asses, i. ; breeds of the horse, i. ; wild horses, i. ; syrian breeds of asses, i. ; asses without stripes, i. ; effects of cross-breeding on the female in dogs, i. ; striped legs of mules, ii. . martins, defective instincts of silkworms, i. . martins, c., fruit trees of stockholm, ii. . { } mason, w., bud-variation in the ash, i. . masters, dr., reversion in the spiral-leaved weeping willow, i. ; on peloric flowers, ii. ; pelorism in a clover, ii. ; position as a cause of pelorism, ii. , . masters, mr., persistence of varieties of peas, i. ; reproduction of colour in hyacinths, ii. ; on hollyhocks, ii. ; selection of peas for seed, ii. - ; on _opuntia leucotricha_, ii. ; reversion by the terminal pea in the pod, ii. . mastiff, sculptured on an assyrian monument, i. , ii. ; tibetan, i. - , ii. . matthews, patrick, on forest trees, ii. . _matthiola annua_, i. , ii. . _matthiola incana_, i. , . mauchamp, merino sheep, i. . mauduyt, crossing of wolves and dogs in the pyrenees, i. . maund, mr. crossed varieties of wheat, ii. . maupertuis, axiom of "least action," i. . mauritius, importation of goats into, i. . maw, g., correlation of contracted leaves and flowers in pelargoniums, ii. , . mawz, fertility of _brassica rapa_, ii. . _maxillaria_, self-fertilised capsules of, ii. ; number of seeds in, ii. . _maxillaria atro-rubens_, fertilisation of, by _m. squalens_, ii. . mayes, m., self-impotence in _amaryllis_, ii. . meckel, on the number of digits, ii. ; correlation of abnormal muscles in the leg and arm, ii. . medusÆ, development of, ii. , . meehan, mr., comparison of european and american trees, ii. . _meleagris mexicana_, i. . _meles taxus_, ii. . melons, i. - ; mongrel, supposed to be produced from a twin-seed, i. ; crossing of varieties of, i. , ii. , ; inferiority of, in roman times, ii. ; changes in, by culture and climate, ii. ; serpent, correlation of variations in, ii. ; analogous variations in, ii. . membranes, false, ii. - . mÉnÉtries, on the stomach of _strix grallaria_, ii. . meningitis, tubercular, inherited, ii. . metagenesis, ii. . metamorphosis, ii. . metamorphosis and development, ii. , . metzger, on the supposed species of wheat, i. - ; tendency of wheat to vary, i. ; variation of maize, i. - ; cultivation of american maize in europe, i. , ii. ; on cabbages, i. - ; acclimatisation of spanish wheat in germany, ii. ; advantage of change of soil to plants, ii. ; on rye, ii. ; cultivation of different kinds of wheat, ii. . mexico, dog from, with tan spots on the eyes, i. ; colours of feral horses in, i. . meyen, on sending of bananas, ii. . mice, grey and white, colours of, not blended by crossing, ii. ; rejection of bitter almonds by, ii. ; naked, ii. . michaux, f., roan-coloured feral horses of mexico, i. ; origin of domestic turkey, i. ; on raising peaches from seed, i. . michel, f., selection of horses in mediæval times, ii. ; horses preferred on account of slight characters, ii. . michely, effects of food on caterpillars, ii. ; on _bombyx hesperus_, ii. . microphthalmia, associated with defective teeth, ii. . middens, danish, remains of dogs in, i. , ii. . mignonette, ii. , . millet, i. . mills, j., diminished fertility of mares when first turned out to grass, ii. . milne-edwards, on the development of the crustacea, ii. . milne-edwards, a., on a crustacean with a monstrous eye-peduncle, ii. . _milvus niger_, ii. . _mimulus luteus_, ii. . minor, w. c., gemmation and fission in the annelida, ii. . _mirabilis_, fertilisation of, ii. ; hybrids of, ii. , , . _mirabilis jalapa_, i. , . _mirabilis longiflora_, ii. . _mirabilis vulgaris_, ii. . _misocampus_ and _cecidomyia_, i. . mitchell, dr., effects of the poison of the rattlesnake, ii. . mitford, mr., notice of the breeding of horses by erichthonius, ii. . moccas court, weeping oak at, ii. . mogford, horses poisoned by fool's parsley, ii. . mÖller, l., effects of food on insects, ii. . moquin-tandon, original form of maize, i. ; variety of the double columbine, i. ; { } peloric flowers, ii. - , ; position as a cause of pelorism in flowers, ii. ; tendency of peloric flowers to become irregular, ii. ; on monstrosities, ii. ; correlation in the axis and appendages of plants, ii. ; fusion of homologous parts in plants, ii. , - ; on a bean with monstrous stipules and abortive leaflets, ii. ; conversion of parts of flowers, ii. . mole, white, ii. . moll and gayot, on cattle, i. , ii. , . mollusca, change in shells of, ii. . monke, lady, culture of the pansy by, i. . monkeys, rarely fertile in captivity, ii. . monnier, identity of summer and winter wheat, i. . monster, cyclopean, ii. . monsters, double, ii. - . monstrosities, occurrence of, in domesticated animals and cultivated plants, i. , ii. ; due to persistence of embryonic conditions, ii. ; occurring by reversion, ii. - ; a cause of sterility, ii. - ; caused by injury to the embryo, ii. . montegazza, growth of a cock's-spur inserted into the eye of an ox, ii. . montgomery, e., formation of cells, ii. . moor, j. h., deterioration of the horse in malasia, i. . moorcroft, mr., on hasora wheat, i. ; selection of white-tailed yaks, ii. ; melon of kaschmir, ii. ; varieties of the apricot cultivated in ladakh, i. ; varieties of the walnut cultivated in kaschmir, i. . moore, mr., on breeds of pigeons, i. , , , , . mooruk, fertility of, in captivity, ii. . morlot, dogs of the danish middens, i. ; sheep and horse of the bronze period, ii. . _mormodes ignea_, ii. . morocco, estimation of pigeons in, i. . morren, c., on pelorism, ii. ; in _calceolaria_, ii. ; non-coincidence of double flowers and variegated leaves, ii. . morris, mr., breeding of the kestrel in captivity, ii. . morton, lord, effect of fecundation by a quagga on an arab mare, i. - . morton, dr., origin of the dog, i. ; hybrid of zebra and mare, ii. . _morus alba_, i. . moscow, rabbits of, i. , ; effects of cold on pear-trees at, ii. . mosses, sterility in, ii. ; retrogressive metamorphosis in, ii. . moss-rose, probable origin of, from _rosa centifolia_, i. ; provence roses produced from seeds of, i. . mosto, cada, on the introduction of rabbits into porto santo, i. . mottling of fruits and flowers, i. . moufflon, i. . mountain-ash, ii. . mouse, barbary, ii. . "mÖven-taube," i. . mowbray, mr., on the eggs of game fowls, i. ; early pugnacity of game cocks, i. ; diminished fecundity of the pheasant in captivity, ii. . mowbray, mr., reciprocal fecundation of _passiflora alata_ and _racemosa_, ii. . mulattos, character of, ii. . mulberry, i. , ii. . mule and hinny, differences in the, ii. - . mules, striped colouring of, ii. ; obstinacy of, ii. ; production of, among the romans, ii. ; noticed in the bible, ii. . mÜller, fritz, reproduction of orchids, ii. - ; development of crustacea, ii. ; number of seeds in a _maxillaria_, ii. . mÜller, h., on the face and teeth in dogs, i. , , ii. . mÜller, j., production of imperfect nails after partial amputation of the fingers, ii. ; tendency to variation, ii. ; atrophy of the optic nerve consequent on destruction of the eye, ii. ; on janus-like monsters, ii. ; on gemmation and fission, ii. ; identity of ovules and buds, ii. ; special affinities of the tissues, ii. . mÜller, max, antiquity of agriculture, ii. . multiplicity of origin of pigeons, hypotheses of, discussed, i. - . muniz, f., on niata cattle, i. . munro, r., on the fertilisation of orchids, ii. ; reproduction of _passiflora alata_, ii. . "murassa" pigeon, i. . murphy, j. j., the structure of the eye not producible by selection, ii. . _mus alexandrinus_, ii. - . _musa sapientum_, _chinensis_ and _cavendishii_, i. . _muscari comosum_, ii. , . muscles, effects of use on, ii. . musk duck, feral hybrid of, with the common duck, i. . { } musmon, female, sometimes hornless, i. . mutilations, inheritance or non-inheritance of, ii. - , . myatt, on a five-leaved variety of the strawberry, i. . myopia, hereditary, ii. . myriapoda, regeneration of lost parts in, ii. , . nails, growing on stumps of fingers, ii. . nais, scission of, ii. . namaquas, cattle of the, i. , ii. . narcissus, double, becoming single in poor soil, ii. . narvaez, on the cultivation of native plants in florida, i. . _nasua_, sterility of, in captivity, ii. . "natas," or niatas, a south american breed of cattle, i. - . nathusius, h. von, on the pigs of the swiss lake-dwellings, i. ; on the races of pigs, i. - ; convergence of character in highly-bred pigs, i. , ii. ; causes of changes in the form of the pig's skull, i. - ; changes in breeds of pigs by crossing, i. ; change of form in the pig, ii. ; effects of disuse of parts in the pig, ii. ; period of gestation in the pig, i. ; appendages to the jaw in pigs, i. ; on _sus pliciceps_, i. ; period of gestation in sheep, i. ; on niata cattle, i. ; on short-horn cattle, ii. ; on interbreeding, ii. ; in the sheep, ii. ; in pigs, ii. ; unconscious selection in cattle and pigs, ii. ; variability of highly selected races, ii. . nato, p., on the bizzaria orange, i. . natural selection, its general principles, i. - . nature, sense in which the term is employed, i. . naudin, supposed rules of transmission in crossing plants, ii. ; on the nature of hybrids, ii. - ; essences of the species in hybrids, ii. , ; reversion of hybrids, ii. , - ; reversion in flowers by stripes and blotches, ii. ; hybrids of _linaria vulgaris_ and _purpurea_, ii. ; pelorism in _linaria_, ii. , ; crossing of peloric _linaria_ with the normal form, ii. ; variability in _datura_, ii. ; hybrids of _datura lævis_ and _stramonium_, i. ; prepotency of transmission of _datura stramonium_ when crossed, ii. ; on the pollen of _mirabilis_ and of hybrids, i. ; fertilisation of _mirabilis_, ii. ; crossing of _chamærops humilis_ and the date palm, i. ; cultivated cucurbitaceæ, i. - , ii. ; rudimentary tendrils in gourds, ii. ; dwarf _cucurbitæ_, ii. ; relation between the size and number of the fruit in _cucurbita pepo_, ii. ; analogous variation in _cucurbitæ_, ii. ; acclimatisation of cucurbitaceæ, ii. ; production of fruit by sterile hybrid cucurbitaceæ, ii. ; on the melon, i. , ii. , ; incapacity of the cucumber to cross with other species, i. . nectarine, i. - ; derived from the peach, i. , - ; hybrids of, i. ; persistency of characters in seedling, i. ; origin of, _ibid._; produced on peach trees, i. - ; producing peaches, i. ; variation in, i. - ; bud-variation in, i. ; glands in the leaves of the, ii. ; analogous variation in, ii. . nectary, variations of, in pansies, i. . nees, on changes in the odour of plants, ii. . "negro" cat, i. . negroes, polydactylism in, ii. ; selection of cattle practised by, ii. . neolithic period, domestication of _bos longifrons_ and _primigenius_ in the, i. ; cattle of the, distinct from the original species, i. ; domestic goat in the, i. ; cereals of the, i. . nerve, optic, atrophy of the, ii. . neumeister, on the dutch and german pouter pigeons, i. ; on the jacobin pigeon, i. ; duplication of the middle flight feather in pigeons, i. ; on a peculiarly coloured breed of pigeons, "staarhalsige taube," i. ; fertility of hybrid pigeons, i. ; mongrels of the trumpeter pigeon, ii. ; period of perfect plumage in pigeons, ii. ; advantage of crossing pigeons, ii. . neuralgia, hereditary, ii. . new zealand, feral cats of, i. ; cultivated plants of, i. . newfoundland dog, modification of, in england, i. . newman, e., sterility of sphingidæ under certain conditions, ii. . newport, g., non-copulation of _vanessæ_ in confinement, ii. ; regeneration of limbs in myriapoda, ii. ; fertilisation of the ovule in batrachia, ii. . newt, polydactylism in the, ii. . newton, a., absence of sexual distinctions in the columbidæ, i. ; production of a "black-shouldered" pea-hen among the ordinary kind, i. ; on hybrid ducks, ii. . ngami, lake, cattle of, i. . "niata" cattle, i. - ; resemblance of to _sivatherium_, i. ; { } prepotency of transmission of character by, ii. . "nicard" rabbit, i. . nicholson, dr., on the cats of antigua, i. ; on the sheep of antigua, i. . _nicotiana_, crossing of varieties and species of, ii. ; prepotency of transmission of characters in species of, ii. ; contabescence of female organs in, ii. . _nicotiana glutinosa_, ii. . niebuhr, on the heredity of mental characteristics in some roman families, ii. . night-blindness, non-reversion to, ii. . nilsson, prof., on the barking of a young wolf, i. ; parentage of european breeds of cattle, i. , ; on _bos frontosus_ in scania, i. . nind, mr., on the dingo, i. . "nisus formativus," i. , , . nitzsch, on the absence of the oil-gland in certain columbæ, i. . non-inheritance, causes of, ii. - . "nonnain" pigeon, i. . nordmann, dogs of awhasie, i. . normandy, pigs of, with appendages under the jaw, i. . norway, striped ponies of, i. . nott and gliddon, on the origin of the dog, i. ; mastiff represented on an assyrian tomb, i. ; on egyptian dogs, i. ; on the hare-indian dog, i. . _notylia_, ii. . nourishment, excess of, a cause of variability, ii. . number, importance of, in selection, ii. . _numida ptilorhyncha_, the original of the guinea-fowl, i. . nun pigeon, i. ; known to aldrovandi, i. . nutmeg tree, ii. . oak, weeping, i. , ii. , ; pyramidal, i. ; hessian, i. ; late-leaved, i. ; variation in persistency of leaves of, i. ; valueless as timber at the cape of good hope, ii. ; changes in, dependent on age, i. ; galls of the, ii. . oats, wild, i. ; in the swiss lake-dwellings, i. . oberlin, change of soil beneficial to the potato, ii. . odart, count, varieties of the vine, i. , ii. ; bud-variation in the vine, i. . odour and colour, correlation of, ii. . _oecidium_, ii. . _oenothera biennis_, bud-variation in, i. . ogle, w., resemblance of twins, ii. . oil-gland, absence of, in fantail pigeons, i. , . oldfield, mr., estimation of european dogs among the natives of australia, ii. . oleander, stock affected by grafting in the, i. . ollier, dr., insertion of the periosteum of a dog beneath the skin of a rabbit, ii. . _oncidium_, reproduction of, ii. - , . onions, crossing of, ii. ; white, liable to the attacks of fungi and disease, ii. , . _ophrys apifera_, self-fertilisation of, ii. ; formation of pollen by a petal in, ii. . _opuntia leucotricha_, ii. . orange, i. - ; crossing of, ii. ; with the lemon, i. , ii. ; naturalisation of, in italy, ii. ; variation of, in north italy, ii. ; peculiar variety of, ii. ; bizzaria, i. ; trifacial, _ibid._ orchids, reproduction of, i. , ; ii. - . orford, lord, crossing greyhounds with the bulldog, i. . organisms, origin of, i. . organisation, advancement in, i. . organs, rudimentary and aborted, ii. - ; multiplication of abnormal, ii. . oriole, assumption of hen-plumage by a male in confinement, ii. . orkney islands, pigs of, i. ; pigeons of, i. . orthoptera, regeneration of hind legs in the, ii. . _orthosia munda_, ii. . orton, r., on the effects of cross-breeding on the female, i. ; on the manx cat, ii. ; on mongrels from the silk-fowl, ii. . osborne, dr., inherited mottling of the iris, ii. . osprey, preying on black-fowls, ii. . osten-sacken, baron, on american oak galls, ii. . osteological characters of pigs, i. , , - ; of rabbits, i. - ; of pigeons, i. - ; of ducks, i. - . ostrich, diminished fertility of the, in captivity, ii. . ostyaks, selection of dogs by the, ii. . otter, ii. . "otter" sheep of massachusetts, i. . oude, feral humped cattle in, i. . ouistiti, breed in europe, ii. . { } ovary, variation of, in _cucurbita moschata_, i. ; development of, independently of pollen, i. . _ovis montana_, i. . ovules and buds, identity of nature of, ii. . owen, capt., on stiff-haired cats at mombas, i. . owen, prof. r., palæontological evidence as to the origin of dogs, i. ; on _bos longifrons_, i. ; on the skull of the "niata" cattle, i. , ; on fossil remains of rabbits, i. ; on the significance of the brain, i. ; on the number of digits in the ichthyopterygia, ii. ; on metagenesis, ii. ; theory of reproduction and parthenogenesis, ii. . owl, eagle, breeding in captivity, ii. . owl pigeon, i. ; african, figured, i. ; known in , i. . _oxalis_, trimorphic species of, ii. . _oxalis rosea_, ii. . oxley, mr., on the nutmeg tree, ii. . oysters, differences in the shells of, ii. . paca, sterility of the, in confinement, ii. . pacific islands, pigs of the, i. . padua, earliest known flower garden at, ii. . paduan fowl of aldrovandi, i. . _pæonia moutan_, ii. . pÆony, tree, ancient cultivation of, in china, ii. . pampas, feral cattle on the, i. . _pandanus_, ii. . pangenesis, hypothesis of, ii. - . _panicum_, seeds of, used as food, i. ; found in the swiss lake-dwellings, i. . pansy, i. - . pappus, abortion of the, in _carthamus_, ii. . paget, on the hungarian sheep dog, i. . paget, inheritance of cancer, ii. ; hereditary elongation of hairs in the eyebrow, ii. ; period of inheritance of cancer, ii. - ; on _hydra_, ii. ; on the healing of wounds, ii. ; on the reparation of bones, _ibid._; growth of hair near inflamed surfaces or fractures, ii. ; on false membranes, _ibid._; compensatory development of the kidney, ii. ; bronzed skin in disease of supra-renal capsules, ii. ; unity of growth and gemmation, ii. ; independence of the elements of the body, ii. ; affinity of the tissues for special organic substances, ii. . pallas, on the influence of domestication upon the sterility of intercrossed species, i. , , , ii. ; hypothesis that variability is wholly due to crossing, i. , , ii. , ; on the origin of the dog, i. ; variation in dogs, i. ; crossing of dog and jackal, i. ; origin of domestic cats, i. ; origin of angora cat, i. ; on wild horses, i. , ; on persian sheep, i. ; on siberian fat-tailed sheep, ii. ; on chinese sheep, ii. ; on crimean varieties of the vine, i. ; on a grape with rudimentary seeds, ii. ; on feral musk-ducks, ii. ; sterility of alpine plants in gardens, ii. ; selection of white-tailed yaks, ii. . _paradoxurus_, sterility of species of, in captivity, ii. . paraguay, cats of, i. ; cattle of, i. ; horses of, ii. ; dogs of, ii. ; black-skinned domestic fowl of, i. . parallel variation, ii. - . paramos, woolly pigs of, i. . parasites, liability to attacks of, dependent on colour, ii. . pariah dog, with crooked legs, i. ; resembling the indian wolf, i. . pariset, inheritance of handwriting, ii. . parker, w. k., number of vertebræ in fowls, i. . parkinson, mr., varieties of the hyacinth, i. . parkyns, mansfield, on _columba guinea_, i. . parmentier, differences in the nidification of pigeons, i. ; on white pigeons, ii. . parrots, general sterility of, in confinement, ii. ; alteration of plumage of, ii. . parsnip, reversion in, ii. ; influence of selection on, ii. ; experiments on, ii. ; wild, enlargement of roots of, by cultivation, i. . parthenogenesis, ii. , . partridge, sterility of, in captivity, ii. . parturition, difficult, hereditary, ii. . _parus major_, ii. . _passiflora_, self-impotence in species of, ii. - ; contabescence of female organs in, ii. . _passiflora alata_, fertility of, when grafted, ii. . pasture and climate, adaptation of breeds of sheep to, i. , . pastrana, julia, peculiarities in the hair and teeth of, ii. . patagonia, crania of pigs from, i. . patagonian rabbit, i. . { } paterson, r., on the arrindy silk moth, ii. . paul, w., on the hyacinth, i. ; varieties of pelargoniums, i. ; improvement of pelargoniums, ii. . _pavo cristatus_ and _muticus_, hybrids of, i. . _pavo nigripennis_, i. - . "pavodotten-taube," i. . peach, i. - ; derived from the almond, i. ; stones of, figured, _ibid._; contrasted with almonds, i. ; double-flowering, i. - , ; hybrids of, i. ; persistency of races of, _ibid._; trees producing nectarines, i. - ; variation in, i. - , ii. ; bud-variation in, i. ; pendulous, ii. ; variation by selection in, ii. ; peculiar disease of the, ii. ; glands on the leaves of the, ii. ; antiquity of the, ii. ; increased hardiness of the, _ibid._; varieties of, adapted for forcing, ii. ; yellow-fleshed, liable to certain diseases, ii. . peach-almond, i. . peafowl, origin of, i. ; japanned or black-shouldered, i. - ; feral, in jamaica, i. ; comparative fertility of, in wild and tame states, ii. , ; white, ii. . pears, i. ; bud-variation in, i. ; reversion in seedling, ii. ; inferiority of, in pliny's time, ii. ; winter nelis, attacked by aphides, ii. ; soft-barked varieties of, attacked by wood-boring beetles, ii. ; origination of good varieties of, in woods, ii. ; forelle, resistance of, to frost, ii. . peas, i. - ; origin of, ; varieties of, - ; found in swiss lake-dwellings, i. , , - ; fruit and seeds figured, i. ; persistency of varieties, i. ; intercrossing of varieties, i. , , ii. ; effect of crossing on the female organs in, i. ; double-flowered, ii. ; maturity of, accelerated by selection, ii. ; varieties of, produced by selection, ii. ; thin-shelled, liable to the attacks of birds, ii. ; reversion of, by the terminal seed in the pod, ii. . peccary, breeding of the, in captivity, ii. . pedigrees of horses, cattle, greyhounds, game-cocks, and pigs, ii. . pegu, cats of, i. ; horses of, i. . pelargoniums, multiple origin of, i. ; zones of, i. ; bud-variation in, i. ; variegation in, accompanied by dwarfing, i. ; pelorism in, ii. , ; by reversion, ii. ; advantage of change of soil to, ii. ; improvement of, by selection, ii. ; scorching of, ii. ; numbers of, raised from seed, ii. ; effects of conditions of life on, ii. ; stove-variety of, ii. ; correlation of contracted leaves and flowers in, ii. - . _pelargonium fulgidum_, conditions of fertility in, ii. . "pelones," a columbian breed of cattle, i. . peloric flowers, tendency of, to acquire the normal form, ii. ; fertility or sterility of, ii. - . peloric races of _gloxinia speciosa_ and _antirrhinum majus_, i. . pelorism, ii. - , - . pelvis, characters of, in rabbits, i. - ; in pigeons, i. ; in fowls, i. ; in ducks, i. . pembroke cattle, i. . pendulous trees, i. , ii. ; uncertainty of transmission of, ii. - . penguin ducks, i. , ; hybrid of the, with the egyptian goose, i. . pennant, production of wolf-like curs at fochabers, i. ; on the duke of queensberry's wild cattle, i. . _pennisetum_, seeds of, used as food in the punjab, i. . _pennisetum distichum_, seeds of, used as food in central africa, i. . percival, mr., on inheritance in horses, ii. ; on horn-like processes in horses, i. . _perdix rubra_, occasional fertility of, in captivity, ii. . period of action of causes of variability, ii. . periosteum of a dog, producing bone in a rabbit, ii. . periwinkle, sterility of, in england, ii. . persia, estimation of pigeons in, i. ; carrier pigeon of, i. ; tumbler pigeon of, i. ; cats of, i. - ; sheep of, i. . _persica intermedia_, i. . persistence of colour in horses, i. ; of generic peculiarities, i. . peru, antiquity of maize in, i. ; peculiar potato from, i. ; selection of wild animals practised by the incas of, ii. - . "perÜcken-taube," i. . petals, rudimentary, in cultivated plants, ii. ; producing pollen, ii. . petunias, multiple origin of, i. ; double-flowered, ii. . "pfauen-taube," i. . _phacochoerus africanus_, i. . _phalænopsis_, pelorism in, ii. . phalanges, deficiency of, ii. . { } _phaps chalcoptera_, ii. . _phaseolus multiflorus_, ii. , . _phaseolus vulgaris_, ii. . _phasianus pictus_, i. . _phasianus amherstiæ_, i. . pheasant, assumption of male plumage by the hen, ii. ; wildness of hybrids of, with the common fowl, ii. ; prepotency of the, over the fowl, ii. ; diminished fecundity of the, in captivity, ii. . pheasants, golden and lady amherst's, i. . pheasant-fowls, i. . philipeaux, regeneration of limbs in the salamander, ii. . philippar, on the varieties of wheat, i. . philippine islands, named breeds of game fowl in the, i. . phillips, mr., on bud-variation in the potato, i. . _phlox_, bud-variation by suckers in, i. . phthisis, affection of the fingers in, ii. . pickering, mr., on the grunting voice of humped cattle, i. ; occurrence of the head of a fowl in an ancient egyptian procession, i. ; seeding of ordinarily seedless fruits, ii. ; extinction of ancient egyptian breeds of sheep and oxen, ii. ; on an ancient peruvian gourd, ii. . picotees, effect of conditions of life on, ii. . pictet, a., oriental names of the pigeon, i. . pictet, prof., origin of the dog, i. ; on fossil oxen, i. . piebalds, probably due to reversion, ii. . pigeaux, hybrids of the hare and rabbit, ii. , . pigeon à cravate, i. . pigeon bagadais, i. , . pigeon coquille, i. . pigeon cygne, i. . pigeon heurté, i. . pigeon patu plongeur, i. . pigeon polonais, i. . pigeon romain, i. , . pigeon tambour, i. . pigeon turc, i. . pigeons, origin of, i. - , - ; classified table of breeds of, i. ; pouter, i. - ; carrier, i. - ; runt, i. - ; barbs, i. - ; fantail, i. - ; turbit and owl, i. - ; tumbler, i. - ; indian frill-back, i. ; jacobin, i. ; trumpeter, i. ; other breeds of, i. - ; differences of, equal to generic, i. - ; individual variations of, i. - ; variability of peculiarities characteristic of breeds in, i. ; sexual variability in, i. - ; osteology of, i. - ; correlation of growth in, i. - , ii. ; young of some varieties naked when hatched, i. , ii. ; effects of disuse in, i. - ; settling and roosting in trees, i. ; floating in the nile to drink, i. ; dovecot, i. - ; arguments for unity of origin of, i. - ; feral in various places, i. , ii. ; unity of coloration in, i. - ; reversion of mongrel, to coloration of, _c. livia_, i. - ; history of the cultivation of, i. - ; history of the principal races of, i. - ; mode of production of races of, i. - ; reversion in, ii. , ; by age, ii. ; produced by crossing in, ii. , ; prepotency of transmission of character in breeds of, ii. - ; sexual differences in some varieties of, ii. ; period of perfect plumage in, ii. ; effect of segregation on, ii. ; preferent pairing of, within the same breed, ii. ; fertility of, increased by domestication, ii. , ; effects of interbreeding and necessity of crossing, ii. - ; indifference of, to change of climate, ii. ; selection of, ii. , , ; among the romans, ii. ; unconscious selection of, ii. , ; facility of selection of, ii. ; white, liable to the attacks of hawks, ii. ; effects of disuse of parts in, ii. ; fed upon meat, ii. ; effect of first male upon the subsequent progeny of the female, i. ; homology of the leg and wing feathers in, ii. ; union of two outer toes in feather-legged, _ibid._; correlation of beak, limbs, tongue, and nostrils in, ii. ; analogous variation in, ii. - ; permanence of breeds of, ii. . pigs, of swiss lake-dwellings, i. - ; types of, derived from _sus scrofa_ and _sus indica_, i. - ; japanese (_sus pliciceps_, gray), figured, i. ; of pacific islands, i. , ii. ; modifications, of skull in, i. - ; length of intestines in, i. , ii. ; period of gestation of, i. ; number of vertebræ and ribs in, i. ; anomalous forms, i. - ; development of tusks and bristles in, i. ; striped young of, i. - ; reversion of feral, to wild type, i. - , ii. , ; production and changes of breeds of, by intercrossing, i. ; effects produced by the first male upon the subsequent progeny of the female, i. ; two-legged race of, ii. ; { } polydactylism in, ii. ; cross-reversion in, ii. ; hybrid, wildness of, ii. ; monstrous development of a proboscis in, ii. ; disappearance of tusks in male under domestication, ii, ; solid hoofed, ii. ; crosses of, ii. , ; mutual fertility of all varieties of, ii. ; increased fertility by domestication, ii. ; ill effects of close interbreeding in, ii. - ; influence of selection on, ii. ; prejudice against certain colours in, ii. , , ; unconscious selection of, ii. ; black virginian, ii. , ; similarity of the best breeds of, ii. ; change of form in, ii. ; effects of disuse of parts in, ii. ; ears of, ii. ; correlations in, ii. ; white, buck-wheat injurious to, ii. ; tail of, grafted upon the back, ii. ; extinction of the older races of, ii. . pimenta, ii. . pimpernel, ii. . pine-apple, sterility and variability of the, ii. . pink, chinese. . pinks, bud-variation in, i. ; improvement of, ii. . _pinus pumilio_, _mughus_, and _nana_, varieties of _p. sylvestris_, i. . _pinus sylvestris_, i. , ii. ; hybrids of, with _p. nigricans_, ii. . piorry, on hereditary disease, ii. , . _pistacia lentiscus_, ii. . pistils, rudimentary, in cultivated plants, ii. . pistor, sterility of some mongrel pigeons, i. ; fertility of pigeons, ii. . _pisum arvense_ and _sativum_, i. . pityriasis versicolor, inheritance of, ii. . planchon, g., on a fossil vine, i. ; sterility of _jussiæa grandifiora_ in france, ii. . plane tree, variety of the, i. . plantigrade carnivora, general sterility of the, in captivity, ii. . plants, progress of cultivation of, i. - ; cultivated, their geographical derivation, i. ; crossing of, ii. , , ; comparative fertility of wild and cultivated, ii. - ; self-impotent, ii. - ; dimorphic and trimorphic, ii. , ; sterility of, from changed conditions, ii. - ; from contabescence of anthers, ii. - ; from monstrosities, ii. - ; from doubling of the flowers, ii. - ; from seedless fruit, ii. ; from excessive development of vegetative organs, ii. - ; influence of selection on, ii. - ; variation by selection, in useful parts of, ii. - ; variability of, ii. ; variability of, induced by crossing, ii. ; direct action of change of climate on, ii. ; change of period of vegetation in, ii. - ; varieties of, suitable to different climates, ii. ; correlated variability of, ii. - ; antiquity of races of, ii. . plasticity, inheritance of, ii. . plateau, f., on the vision of amphibious animals, ii. . _platessa flesus_, ii. . plato, notice of selection in breeding dogs by, ii. . plica polonica, ii. . pliny, on the crossing of shepherd's dogs with the wolf, i. ; on pyrrhus' breed of cattle, ii. ; on the estimation of pigeons among the romans, i. ; pears described by, ii. . plum, i. - ; stones figured, i. ; varieties of the, i. - , ii. ; bud-variation in the, i. ; peculiar disease of the, ii. ; flower-buds of, destroyed by bullfinches, ii. ; purple-fruited, liable to certain diseases, ii. . plumage, inherited peculiarities of, in pigeons, i. - ; sexual peculiarities of, in fowls, i. - . plurality of races, pouchet's views on, i. . _poa_, seeds of, used as food, i. ; species of, propagated by bulblets, ii. . podolian cattle, i. . pointers, modification of, i. ; crossed with the foxhound, ii. . pois sans parchemin, ii. . poiteau, origin of _cytisus adami_, i. ; origin of cultivated varieties of fruit-trees, ii. . polish fowl, i. , , , - , ; skull figured, i. ; section of skull figured, i. ; development of protuberance of skull, i. ; furcula figured, i. . polish, or himalayan rabbit, i. . pollen, ii. - ; action of, ii. ; injurious action of, in some orchids, ii. - ; resistance of, to injurious treatment, ii. ; prepotency of, ii. . pollock, sir f., transmission of variegated leaves in _ballota nigra_, i. ; on local tendency to variegation, ii. . polyanthus, ii. . polydactylism, inheritance of, ii. - ; significance of, ii. - . _polyplectron_, i. . ponies, most frequent on islands and mountains, i. ; javanese, i. . poole, col., on striped indian horses, i. , ; { } on the young of _asinus indicus_, ii. . poplar, lombardy, i. . pÖppig, on cuban wild dogs, i. . poppy, found in the swiss lake-dwellings, i. , ; with the stamens converted into pistils, i. ; differences of the, in different parts of india, ii. ; monstrous, fertility of, ii. ; black-seeded, antiquity of, ii. . porcupine, breeding of, in captivity, ii. . porcupine family, ii. , . _porphyrio_, breeding of a species of, in captivity, ii. . portal, on a peculiar hereditary affection of the eye, ii. . porto santo, feral rabbits of, i. . _potamochoerus penicillatus_, ii. . potato, i. - ; bud-variation by tubers in the, i. - ; graft-hybrid of, by union of half-tubers, i. ; individual self-impotence in the, ii. ; sterility of, ii. ; advantage of change of soil to the, ii. ; relation of tubers and flowers in the, ii. . potato, sweet, sterility of the, in china, ii. ; varieties of the, suited to different climates, ii. . pouchet, m., his views on plurality of races, i. . pouter pigeons, i. - ; furcula figured, i. ; history of, i. . powis, lord, experiments in crossing humped and english cattle, i. , ii. . poynter, mr., on a graft-hybrid rose, i. . prairie wolf, i. . precocity of highly-improved breeds, ii. . prepotency of pollen, ii. . prepotency of transmission of character, ii. , ; in the austrian emperors and some roman families, ii. ; in cattle, ii. - ; in sheep, ii. ; in cats, _ibid._; in pigeons, ii. - ; in fowls, ii. ; in plants, _ibid._; in a variety of the pumpkin, i. ; in the jackal over the dog, ii. ; in the ass over the horse, _ibid._; in the pheasant over the fowl, ii. ; in the penguin duck over the egyptian goose, _ibid._; discussion of the phenomena of, ii. - . prescott, mr., on the earliest known european flower-garden, ii. . pressure, mechanical, a cause of modification, ii. - . prevost and dumas, on the employment of several spermatozoids to fertilise one ovule, ii. . price, mr., variations in the structure of the feet in horses, i. . prichard, dr., on polydactylism in the negro, ii. ; on the lambert family, ii. ; on an albino negro, ii. ; on plica polonica, ii. . primrose, ii. ; double, rendered single by transplantation, ii. . _primula_, intercrossing of species of, i. ; contabescence in, ii. ; hose and hose, i. ; with coloured calyces, sterility of, ii. . _primula sinensis_, reciprocally dimorphic, ii. . _primula veris_, ii. , , . _primula vulgaris_, ii. , . prince, mr., on the intercrossing of strawberries, i. . _procyon_, sterility of, in captivity, ii. . prolificacy, increased by domestication, ii. . propagation, rapidity of, favourable to selection, ii. . protozoa, reproduction of the, ii. . _prunus armeniaca_, i. - . _prunus avium_, i. . _prunus cerasus_, i. , . _prunus domestica_, i. . _prunus insititia_, i. - . _prunus spinosa_, i. . prussia, wild horses in, i. . _psittacus erithacus_, ii. . _psittacus macoa_, ii. . _psophia_, general sterility of, in captivity, ii. . ptarmigan fowls, i. . _pulex penetrans_, ii. . pumpkins, i. . puno ponies of the cordillera, i. . purser, mr. on _cytisus adami_, i. . pusey, mr., preference of hares and rabbits for common rye, ii. . putsche and vertuch, varieties of the potato, i. . puvis, effects of foreign pollen on apples, i. ; supposed non-variability of monotypic genera, ii. . _pyrrhula vulgaris_, ii. ; assumption of the hen-plumage by the male, in confinement, ii. . pyrrhus, his breed of cattle, ii. . _pyrus_, fastigate chinese species of, ii. . _pyrus acerba_, i. . _pyrus aucuparia_, ii. . _pyrus communis_, i. , . _pyrus malus_, i. , . _pyrus paradisiaca_, i. . _pyrus præcox_, i. . quagga, effect of fecundation by, on the subsequent progeny of a mare, i. - . quatrefages, a. de, on the burrowing of a bitch to litter, i. ; { } selection in the silkworm, i. ; development of the wings in the silkmoth, i. , ii. ; on varieties of the mulberry, i. ; special raising of eggs of the silkmoth, ii. ; on disease of the silkworm, ii. ; on monstrosities in insects, ii. , ; on the anglo-saxon race in america, ii. ; on a change in the breeding season of the egyptian goose, ii. ; fertilisation of the _teredo_, ii. ; tendency to similarity in the best races, ii. ; on his "_tourbillon vital_," ii. ; on the independent existence of the sexual elements, ii. . _quercus cerris_, i. . _quercus robur_ and _pedunculata_, hybrids of, ii. . quince, pears grafted on the, ii. . rabbits, domestic, their origin, i. - ; of mount sinai and algeria, i. ; breeds of, i. - ; himalayan, chinese, polish, or russian, i. - , ii. ; feral, i. - ; of jamaica, i. ; of the falkland islands, i. ; of porto santo, i. - , ii. , ; osteological characters of, i. - ; discussion of modifications in, i. - ; one-eared, transmission of peculiarity of, ii. ; reversion in feral, ii. ; in the himalayan, ii. ; crossing of white and coloured angora, ii. ; comparative fertility of wild and tame, ii. ; high-bred, often bad breeders, ii. ; selection of, ii. ; white, liable to destruction, ii. ; effects of disuse of parts in, ii. ; skull of, affected by drooping ears, ii. ; length of intestines in, ii. ; correlation of ears and skull in, ii. - ; variations in skull of, ii. ; periosteum of a dog producing bone in, ii. . race-horse, origin of, i. . races, modification and formation of, by crossing, ii. - ; natural and artificial, ii. ; pouchet's views on plurality of, i. ; of pigeons, i. - . radishes, i. ; crossing of, ii. ; varieties of, ii. - . radclyffe, w. f., effect of climate and soil on strawberries, i. ; constitutional differences in roses, i. . radlkofer, retrogressive metamorphosis in mosses and algæ, ii. . raffles, sir stamford, on the crossing of javanese cattle with _bos sondaicus_, ii. . ram, goat-like, from the cape of good hope, ii. . ranchin, heredity of diseases, ii. . range of gallinaceous birds on the himalaya, i. . _ranunculus ficaria_, ii. . _ranunculus repens_, ii. . rape, i. . _raphanus sativus_, ii. . raspberry, yellow-fruited, ii. . rattlesnake, experiments with poison of the, ii. . raven, stomach of, affected by vegetable diet, ii. . rawson, a., self-impotence in hybrids of _gladiolus_, ii. - . rÉ, le compte, on the assumption of a yellow colour by all varieties of maize, i. . rÉaumur, effect of confinement upon the cock, ii. ; fertility of fowls in most climates, ii. . reed, mr., atrophy of the limbs of rabbits, consequent on the destruction of their nerves, ii. . regeneration of amputated parts in man, ii. ; in the human embryo, ii. ; in the lower vertebrata, insects, and myriapoda, _ibid._ reindeer, individuals recognised by the laplanders, ii. . regnier, early cultivation of the cabbage by the celts, i. . reissek, experiments in crossing _cytisus purpureus_ and _laburnum_, i. ; modification of a _thesium_ by _oecidium_, ii. . relations, characters of, reproduced in children, ii. . rengger, occurrence of jaguars with crooked legs in paraguay, i. ; naked dogs of paraguay, i. , , ii. , ; feral dogs of la plata, i. ; on the aguara, i. ; cats of paraguay, i. , ii. , ; dogs of paraguay, ii. ; feral pigs of buenos ayres, i. ; on the refusal of wild animals to breed in captivity, ii. ; on _dicotyles labiatus_, ii. ; sterility of plantigrade carnivora in captivity, ii. ; on _cavia aperea_, ii. ; sterility of _cebus azaræ_ in captivity, ii. ; abortions produced by wild animals in captivity, ii. . reproduction, sexual and asexual, contrasted, ii. ; unity of forms of, ii. ; antagonism of, to growth, ii. . _reseda odorata_, ii. . retinitis, pigmentary, in deaf-mutes, ii. . reversion, ii. - , - , , - ; in pigeons, ii. ; in cattle, ii. - ; in sheep, ii. ; in fowls, ii. ; in the heartsease, _ibid._; in vegetables, _ibid._; in feral animals and plants, ii. - ; to characters derived from a previous cross in man, dogs, pigeons, pigs, and fowls, ii. - ; { } in hybrids, ii. ; by bud-propagation in plants, ii. - ; by age in fowls, cattle, &c., ii. - ; caused by crossing, ii. - ; explained by latent characters, ii. - ; producing monstrosities, ii. ; producing peloric flowers, ii. - ; of feral pigs to the wild type, i. - ; of supposed feral rabbits to the wild type, i. , , ; of pigeons, in coloration, when crossed, i. - ; in fowls, i. - ; in the silkworm, i. ; in the pansy, i. ; in a pelargonium, i. ; in chrysanthemums, i. ; of varieties of the china rose in st. domingo, i. ; by buds in pinks and carnations, i. ; of laciniated varieties of trees to the normal form, i. ; in variegated leaves of plants, i. - ; in tulips, i. ; of suckers of the seedless barberry to the common form, i. ; by buds in hybrids of _tropæolum_, i. ; in plants, i. ; of crossed peloric snapdragons, ii. ; analogous variations due to, ii. - . reynier, selection practised by the celts, ii. - . rhinoceros, breeding in captivity in india, ii. . _rhododendron_, hybrid, ii. . _rhododendron ciliatum_, ii. . _rhododendron dalhousiæ_, effect of pollen of _r. nuttallii_ upon, i. . rhubarb, not medicinal when grown in england, ii. . _ribes grossularia_, i. - , . _ribes rubrum_, i. . ribs, number and characters of, in fowls, i. ; characters of, in ducks, i. - . rice, imperial, of china, ii. ; indian varieties of, ii. ; variety of, not requiring water, ii. . richardson, h. d., on jaw-appendages in irish pigs, i. ; management of pigs in china, i. ; occurrence of striped young in westphalian pigs, i. ; on crossing pigs, ii. ; on interbreeding pigs, ii. ; on selection in pigs, ii. . richardson, sir john, observations on the resemblance between north american dogs and wolves, i. - ; on the burrowing of wolves, i. ; on the broad feet of dogs, wolves, and foxes in north america, i. ; on north american horses scraping away the snow, i. . _ricinus_, annual in england, ii. . riedel, on the "bagadotte" pigeon, i. ; on the jacobin pigeon, i. ; fertility of hybrid pigeons, i. . rinderpest, ii. . risso, on varieties of the orange, i. , ii. , . rivers, lord, on the selection of greyhounds, ii. . rivers, mr., persistency of characters in seedling potatoes, i. ; on the peach, i. , ; persistency of races in the peach and nectarine, i. , ; connexion between the peach and the nectarine, i. ; persistency of character in seedling apricots, i. ; origin of the plum, i. ; seedling varieties of the plum, i. ; persistency of character in seedling plums, i. ; bud-variation in the plum, i. ; plum, attacked by bullfinches, ii. ; seedling apples with surface-roots, i. ; variety of the apple found in a wood, ii. ; on roses, i. - ; bud-variation in roses, i. - ; production of provence roses from seeds of the moss-rose, i. ; effect produced by grafting on the stock in jessamine, i. ; in the ash, i. ; on grafted hazels, i. ; hybridisation of a weeping thorn, ii. ; experiments with the seed of the weeping elm and ash, ii. ; variety of the cherry with curled petals, ii. . riviÈre, reproduction of _oncidium cavendishianum_, ii. . roberts, mr., on inheritance in the horse, ii. . robertson, mr., on glandular-leaved peaches, i. . robinet, on the silkworm, i. - , ii. . _robinia_, ii. . robson, mr., deficiencies of half-bred horses, ii. . robson, mr., on the advantage of change of soil to plants, ii. - ; on the growth of the verbena, ii. ; on broccoli, ii. . rock pigeon, measurements of the, i. ; figured, i. . rodents, sterility of, in captivity, ii. . _rodriguezia_, ii. , . rodwell, j., poisoning of horses by mildewed tares, ii. . rohilcund, feral humped cattle in, i. . rolle, f., on the history of the peach, ii. . roller-pigeons, dutch, i. . rolleston, prof., incisor teeth affected in form in cases of pulmonary tubercle, ii. . romans, estimation of pigeons by, i. ; breeds of fowls possessed by, i. , . { } rooks, pied, ii. . _rosa_, cultivated species of, i. . _rosa devoniensis_, graft-hybrid produced by, on the white banksian rose, i. . _rosa indica_ and _centifolia_, fertile hybrids of, i. . _rosa spinosissima_, history of the culture of, i. . rosellini, on egyptian dogs, i. . roses, i. - ; origin of, i. ; bud-variation in, i. - ; scotch, doubled by selection, ii. ; continuous variation of, ii. ; effect of seasonal conditions on, ii. ; noisette, ii. ; galls of, ii. . rouennais rabbit, i. . roulin, on the dogs of juan fernandez, i. ; on south american cats, i. ; striped young pigs, i. ; feral pigs in south america, i. , ii. ; on columbian cattle, i. , ii. , ; effects of heat on the hides of cattle in south america, i. ; fleece of sheep in the hot valleys of the cordilleras, i. ; diminished fertility of these sheep, ii. ; on black-boned south american fowls, i. ; variation of the guinea-fowl in tropical america, i. ; frequency of striped legs in mules, ii. ; geese in bogota, ii. ; sterility of fowls introduced into bolivia, ii. . roy, m., on a variety of _magnolia grandiflora_, ii. . royle, dr., indian varieties of the mulberry, i. ; on _agave vivipara_, ii. ; variety of rice not requiring irrigation, ii. ; sheep from the cape in india, ii. . _rubus_, pollen of, ii. . rudimentary organs, i. , ii. - . rufz de lavison, extinction of breeds of dogs in france, ii. . ruminants, general fertility of, in captivity, ii. . rumpless fowls, i. . runts, i. - ; history of, i. ; lower jaws and skull figured, i. - . russian or himalayan rabbit, i. . rÜtimeyer, prof., dogs of the neolithic period, i. ; horses of swiss lake-dwellings, i. ; diversity of early domesticated horses i. ; pigs of the swiss lake-dwellings, i. , - ; on humped cattle, i. ; parentage of european breeds of cattle, i. , , ii. ; on "niata" cattle, i. ; sheep of the swiss lake-dwellings, i. , ii. ; goats of the swiss lake-dwellings, i. ; absence of fowls in the swiss lake-dwellings, i. ; on crossing cattle, ii. ; differences in the bones of wild and domesticated animals, ii. ; decrease in size of wild european animals, ii. . rye, wild, de candolle's observations on, i. ; found in the swiss lake-dwellings, i. ; common, preferred by hares and rabbits, ii. ; less variable than other cultivated plants, ii. . sabine, mr., on the cultivation of _rosa spinosissima_, i. ; on the cultivation of the dahlia, i. - , ii. ; effect of foreign pollen on the seed-vessel in _amaryllis vittata_, i. . st. ange, influence of the pelvis on the shape of the kidneys in birds, ii. . st. domingo, wild dogs of, i. ; bud-variation of dahlias in, i. . st. hilaire, aug., milk furnished by cows in south america, ii. ; husked form of maize, i. . st. john, c., feral cats in scotland, i. ; taming of wild ducks, i. . st. valery apple, singular structure of the, i. ; artificial fecundation of the, i. . st. vitus' dance, period of appearance of, ii. . sageret, origin and varieties of the cherry, i. - ; origin of varieties of the apple, i. ; incapacity of the cucumber for crossing with other species, i. ; varieties of the melon, i. ; supposed twin-mongrel melon, i. ; crossing melons, ii. , ; on gourds, ii. ; effects of selection in enlarging fruit, ii. ; on the tendency to depart from type, ii. ; variation of plants in particular soils, ii. . salamander, experiments on the, ii. , ; regeneration of lost parts in the, ii. , , . _salamandra cristata_, polydactylism in, ii. . salisbury, mr., on the production of nectarines by peach-trees, i. ; on the dahlia, i. - . _salix_, intercrossing of species of, i. . _salix humilis_, galls of, ii. , . sallÉ, feral guinea-fowl in st. domingo, i. . salmon, early breeding of male, ii. . salter, mr., on bud-variation in pelargoniums, i. ; in the chrysanthemum, i. ; transmission of variegated leaves by seed, i. ; bud-variation by suckers in _phlox_, i. ; application of selection to bud-varieties of plants, i. ; accumulative effect of changed conditions of life, ii. ; on the variegation of strawberry leaves, ii. . salter, s. j., hybrids of _gallus sonneratii_ and the common fowl, i. , ii. ; { } crossing of races or species of rats, ii. - . samesreuther, on inheritance in cattle, ii. . sandford. _see_ dawkins. sap, ascent of the, ii. . _saponaria calabrica_, ii. . sardinia, ponies of, i. . sars, on the development of the hydroida, ii. . satiation of the stigma, i. - . _saturnia pyri_, sterility of, in confinement, ii. . saul, on the management of prize gooseberries, i. . sauvigny, varieties of the goldfish, i. . savages, their indiscriminate use of plants as food, i. - ; fondness of, for taming animals, ii. . savi, effect of foreign pollen on maize, i. . _saxifraga geum_, ii. . sayzid mohammed musari, on carrier-pigeons, i. ; on a pigeon which utters the sound "yahu," i. . scanderoons (pigeons), i. , . scania, remains of _bos frontosus_ found in, i. . scapula, characters of, in rabbits, i. ; in fowls, i. ; in pigeons, i. ; alteration of, by disuse, in pigeons, i. . scarlet fever, ii. . schaaffhausen, on the horses represented in greek statues, ii. . schacht, h., on adventitious buds, ii. . schleiden, excess of nourishment a cause of variability, ii. . schomburgk, sir r., on the dogs of the indians of guiana, i. , , ii. ; on the musk duck, i. ; bud-variation in the banana, i. ; reversion of varieties of the china rose in st. domingo, i. ; sterility of tame parrots in guiana, ii. ; on _dendrocygna viduata_, ii. ; selection of fowls in guiana, ii. . schreibers, on _proteus_, ii. . _sciuropterus volucella_, ii. . _sciurus palmarum_ and _cinerea_, ii. . sclater, p. l., on _asinus tæniopus_, i. , ii. ; on _asinus indicus_, ii. ; striped character of young wild pigs, i. ; osteology of _gallinula nesiotis_, i. ; on the black-shouldered peacock, i. ; on the breeding of birds in captivity, ii. . schmerling, dr., varieties of the dog, found in a cave, i. . scotch fir, local variation of, i. . scotch kail and cabbage, cross between, ii. . scott, john, irregularities in the sex of the flowers of maize, i. ; bud-variation in _imatophyllum miniatum_, i. ; crossing of species of _verbascum_, ii. - ; experiments on crossing _primulæ_, ii. ; reproduction of orchids, ii. ; fertility of _oncidium divaricatum_, ii. ; acclimatisation of the sweet pea in india, ii. ; number of seeds in _acropera_ and _gongora_, ii. . scott, sir w., former range of wild cattle in britain, i. . scrope, on the scotch deerhound, ii. , . sebright, sir john, effects of close interbreeding in dogs, ii. ; care taken by, in selection of fowls, ii. . _secale cereale_, ii. . sedgwick, w., effects of crossing on the female, i. ; on the "porcupine-man," ii. ; on hereditary diseases, ii. ; hereditary affections of the eye, ii. , - ; inheritance of polydactylism and anomalies of the extremities, ii. - ; morbid uniformity in the same family, ii. ; on deaf-mutes, ii. ; inheritance of injury to the eye, ii. ; atavism in diseases and anomalies of structure, ii. ; non-reversion to night-blindness, ii. ; sexual limitation of the transmission of peculiarities in man, ii. - ; on the effects of hard-drinking, ii. ; inherited baldness with deficiency of teeth, ii. - ; occurrence of a molar tooth in place of an incisor, ii. ; diseases occurring in alternate generations, ii. . sedillot, on the removal of portions of bone, ii. . seeds, early selection of, ii. ; rudimentary, in grapes, ii. ; relative position of, in the capsule, ii. . seeds and buds, close analogies of, i. . seemann, b., crossing of the wolf and esquimaux dog, i. . selby, p. j., on the bud-destroying habits of the bullfinch, ii. . selection, ii. - ; methodical, i. , ii. - ; by the ancients and semi-civilised people, ii. - ; of trifling characters, ii. - ; unconscious, i. , , ii. , - ; effects of, shown by differences in most valued parts, ii. - ; produced by accumulation of variability, ii. - ; natural, as affecting domestic productions, ii. - , - ; as the origin of species, genera and other groups, ii. - ; circumstances favourable to, ii. - ; tendency of towards extremes, ii. - ; { } possible limit of, ii. ; influence of time on, ii. - ; summary of subject, ii. - ; effects of, in modifying breeds of cattle, i. , ; in preserving the purity of breeds of sheep, i. - ; in producing varieties of pigeons, i. - ; in breeding fowls, i. - ; in the goose, i. ; in the canary, i. ; in the goldfish, i. ; in the silkworm, i. - ; contrasted in cabbages and cereals, i. ; in the white mulberry, i. ; on gooseberries, i. ; applied to wheat, i. - ; exemplified in carrots, &c., i. ; in the potato, i. ; in the melon, i. ; in flowering plants, i. ; in the hyacinth, i. ; applied to bud-varieties of plants, i. ; illustrations of, ii. - . selection, sexual, ii. . self-impotence in plants, ii. - ; in individual plants, ii. - ; of hybrids, ii. . selwyn, mr., on the dingo, i. . selys-longchamps, on hybrid ducks, i. , ii. , ; hybrid of the hook-billed duck and egyptian goose, i. . seringe, on the st. valery apple, i. . serpent melon, i. . serres, olivier de, wild poultry in guiana, i. . sesamum, white-seeded, antiquity of the, ii. . _setaria_, found in the swiss lake-dwellings, i. . setters, degeneration of, in india, i. ; youatt's remarks on, i. . sex, secondary characters of, latent, ii. - ; of parents, influence of, on hybrids, ii. . sexual characters, sometimes lost in domestication, ii. . sexual limitation of characters, ii. - . sexual peculiarities, induced by domestication in sheep, i. ; in fowls, i. - ; transfer of, i. - . sexual variability in pigeons, i. - . sexual selection, ii. . shaddock, i. . shailer, mr., on the moss-rose, i. - . shanghai fowls, i. . shanghai sheep, their fecundity, i. . shan ponies, striped, i. . sheep, disputed origin of, i. ; early domestication of, i. ; large-tailed, i. , , , ii. ; variations in horns, mammæ and other characters of, i. ; sexual characters of, induced by domestication, i. , ; adaptation of, to climate and pasture, i. , ; periods of gestation of, i. ; effect of heat on the fleece of, i. - , ii. ; effect of selection on, i. - ; "ancon" or "otter" breeds of, i. , , ; "mauchamp-merino," i. - ; cross of german and merino, ii. - ; black, of the tarentino, ii. ; karakool, ii. ; jaffna, with callosities on the knees, ii. ; chinese, ii. ; danish, of the bronze period, ii. ; polydactylism in, ii. ; occasional production of horns in hornless breeds of, ii. ; reversion of colour in, ii. ; influence of male, on offspring, ii. ; sexual differences in, ii. ; influence of crossing or segregation on, ii. , - , - ; interbreeding of, ii. - ; effect of nourishment on the fertility of, ii. - ; diminished fertility of, under certain conditions, ii. ; unconscious selection of, ii. ; natural selection in breeds of, ii. , , ; reduction of bones in, ii. ; individual differences of, ii. ; local changes in the fleece of, in england, ii. ; partial degeneration of, in australia, ii. ; with numerous horns, ii. ; correlation of horns and fleece in, ii. ; feeding on flesh, ii. ; acclimatisation of, ii. - ; mountain, resistance of, to severe weather, ii. ; white, poisoned by _hypericum crispum_, ii. . sheep dogs resembling wolves, i. . shells, sinistral and dextral, ii. . sheriff, mr. new varieties of wheat, i. , ; on crossing wheat, ii. - ; continuous variation of wheat, ii. . siam, cats of, i. ; horses of, i. . shirley, e. p., on the fallow-deer, ii. , . short, d., hybrids of the domestic cat and _felis ornata_, i, . siberia, northern range of wild horses in, i. . sichel, j., on the deafness of white cats with blue eyes, ii. . sidney, s., on the pedigrees of pigs, ii. ; on cross-reversion in pigs, ii. ; period of gestation in the pig, i. ; production of breeds of pigs by intercrossing, i. , ; fertility of the pig, ii. ; effects of interbreeding on pigs, ii. - ; on the colours of pigs, ii. , . siebold, on the sweet potato, ii. . siebold, von carl, on parthenogenesis, ii. . _silene_, contabescence in, ii. . silk-fowls, i. , ii. , . { } silk-moth, arrindy, ii. , ; tarroo, ii. . silk-moths, i. - ; domesticated species of, i. ; history of, _ibid._; causes of modification in, i. - ; differences presented by, i. - ; crossing of, ii. ; disease in, ii. ; effects of disuse of parts in, ii. ; selection practised with, ii. , ; variation of, ii. ; parthenogenesis in, ii. . silkworms, variations of, i. - ; yielding white cocoons, less liable to disease, ii. . silver-grey rabbit, i. , , . simonds, j. b., period of maturity in various breeds of cattle, i. ; differences in the periods of dentition in sheep, i. ; on the teeth in cattle, sheep, &c., ii. ; on the breeding of superior rams, ii. . simon, on the raising of eggs of the silk-moth in china, ii. . simpson, sir j., regenerative power of the human embryo, ii. . _siredon_, breeding in the branchiferous stage, ii. . siskin, breeding in captivity, ii. . _sivatherium_, resemblance of the, to niata cattle, i. . size, difference of, an obstacle to crossing, ii. . skin, and its appendages, homologous, ii. ; hereditary affections of the, ii. . skirving, r. s., on pigeons settling on trees in egypt, i. . skull, characters of the, in breeds of dogs, i. ; in breeds of pigs, i. ; in rabbits, i. - , ; in breeds of pigeons, i. - ; in breeds of fowls, i. - ; in ducks, i. - . skull and horns, correlation of the, ii. . skylark, ii. . sleeman, on the cheetah, ii. . sloe, i. . small-pox, ii. . smiter (pigeon), i. . smith, sir a., on caffrarian cattle, i. ; on the use of numerous plants as food in south africa, i. . smith, colonel hamilton, on the odour of the jackal, i. ; on the origin of the dog, i. ; wild dogs in st. domingo, i. ; on the thibet mastiff and the alco, i. - ; development of the fifth toe in the hind feet of mastiffs, i. ; differences in the skull of dogs, i. ; history of the pointer, i. ; on the ears of the dog, ii. ; on the breeds of horses, i. ; origin of the horse, i. ; dappling of horses, i. ; striped horses in spain, i. ; original colour of the horse, i. ; on horses scraping away snow, i. ; on _asinus hemionus_, ii. ; feral pigs of jamaica, i. - . smith, sir j. e., production of nectarines and peaches by the same tree, i. ; on _viola amoena_, i. ; sterility of _vinca minor_ in england, ii. . smith, j., development of the ovary in _bonatea speciosa_, by irritation of the stigma, i. . smith, n. h., influence of the bull "favourite" on the breed of short-horn cattle, ii. . smith, w., on the inter-crossing of strawberries, i. . snake-rat, ii. , . snakes, form of the viscera in, ii. . snapdragon, bud-variation in, i. ; non-inheritance of colour in, ii. ; peloric, crossed with the normal form, ii. , ; asymmetrical variation of the, ii. . soil, adaptation of plums to, i. ; influence of, on the zones of pelargoniums, i. ; on roses, i. ; on the variegation of leaves, i. ; advantages of change of, ii. - . soil and climate, effects of, on strawberries, i. . _solanum_, non-intercrossing of species of, ii. . _solanum tuberosum_, i. - . solid-hoofed pigs, i. . solomon, his stud of horses, i. . somerville, lord, on the fleece of merino sheep, i. ; on crossing sheep, ii. ; on selection of sheep, ii. ; diminished fertility of merino sheep brought from spain, ii. . sooty fowls, i. , . soto, ferdinand de, on the cultivation of native plants in florida, i. . _sorghum_, i. . spain, hawthorn monogynous in, i. . spallanzani, on feral rabbits in lipari, i. ; experiments on salamanders, ii. , , ; experiments in feeding a pigeon with meat, ii. . spaniels, in india, i. ; king charles's, i. ; degeneration of, caused by interbreeding, ii. . spanish fowls, i. , , ; figured, i. ; early development of sexual characters in, i. , ; furcula of, figured, i. . species, difficulty of distinguishing from varieties, i. ; conversion of varieties into, i. ; origin of, by natural selection, ii. - ; by mutual sterility of varieties, ii. - . { } spencer, lord, on selection in breeding, ii. . spencer, herbert, on the "survival of the fittest," i. ; increase of fertility by domestication, ii. ; on life, ii. , ; changes produced by external conditions, ii. ; effects of use on organs, ii. , ; ascent of the sap in trees, ii. ; correlation exemplified in the irish elk, ii. - ; on "physiological units," ii. ; antagonism of growth and reproduction, ii. ; formation of ducts in plants, ii. . spermatophores of the cephalopoda, ii. . spermatozoids, ii. - ; apparent independence of, in insects, ii. . sphingidÆ, sterility of, in captivity, ii. . spinola, on the injurious effect produced by flowering buckwheat on white pigs, ii. . spitz dog, i. . spooner, w. c., cross-breeding of sheep, i. , ii. - , ; on the effects of crossing, ii. - ; on crossing cattle, ii. ; individual sterility, ii. . spores, reproduction of abnormal forms by, i. . sports, i. ; in pigeons, i. . spot pigeon, i. , . sprengel, c. k., on dichogamous plants, ii. ; on the hollyhock, ii. ; on the functions of flowers, ii. . sproule, mr., inheritance of cleft-palate and hare-lip, ii. . spurs, of fowls, i. ; development of, in hens, ii. . squashes, i. . squinting, hereditary, ii. . squirrels, generally sterile in captivity, ii. . squirrels, flying, breeding in confinement, ii. . "staarhalsige taube," i. . stag, one-horned, supposed heredity of character in, ii. ; degeneracy of, in the highlands, ii. . stamens, occurrence of rudimentary, ii. ; conversion of, into pistils, i. ; into petals, ii. . _staphylea_, ii. . steenstrup, prof., on the dog of the danish middens, i. ; on the obliquity of flounders, ii. . steinan, j., on hereditary diseases, ii. , . sterility, in dogs, consequent on close confinement, i. ; comparative, of crosses, ii. , ; from changed conditions of life, ii. - ; occurring in the descendants of wild animals bred in captivity, ii. ; individual, ii. ; resulting from propagation by buds, cuttings, bulbs, &c., ii. ; in hybrids, ii. - , , - ; in specific hybrids of pigeons, i. ; as connected with natural selection, ii. - . sternum, characters of the, in rabbits, i. ; in pigeons, i. , - ; in fowls, i. , ; effects of disuse on the, i. - , . stephens, j. f., on the habits of the bombycidæ, i. . stewart, h., on hereditary disease, ii. . stigma, variation of the, in cultivated cucurbitaceæ, i. ; satiation of the, i. - . stocks, bud-variation in, i. ; effect of crossing upon the colour of the seed of, i. - ; true by seed, ii. ; crosses of, ii. ; varieties of, produced by selection, ii. ; reversion by the upper seeds in the pods of, ii. - . stockholm, fruit-trees of, ii. . stokes, prof., calculation of the chance of transmission of abnormal peculiarities in man, ii. . stolons, variations in the production of, by strawberries, i. . stomach, structure of the, affected by food, ii. . stone in the bladder, hereditary, ii. , . strawberries, i. - ; remarkable varieties of, i. - ; hautbois, dioecious, i. ; selection in, ii. ; mildew of, ii. ; probable further modification of, ii. ; variegated, effects of soil on, ii. . strickland, a., on the domestication of _anser ferus_, i. ; on the colour of the bill and legs in geese, i. . _strictoenas_, i. . stripes on young of wild swine, i. ; of domestic pigs of turkey, westphalia, and the zambesi, i. - ; of feral swine of jamaica and new granada, i. ; of fruit and flowers, i. , ii. ; in horses, i. - ; in the ass, i. - ; production of, by crossing species of equidæ, ii. - . _strix grallaria_, ii. . _strix passerina_, ii. . "strupp-taube," i. . struthers, mr., osteology of the feet in solid-hoofed pigs, i. ; on polydactylism, ii. - . sturm, prepotency of transmission of characters in sheep and cattle, ii. ; absorption of the minority in crossed races, ii. ; correlation of twisted horns and curled wool in sheep, ii. . { } sub-species, wild, of _columba livia_ and other pigeons, i. . succession, geological, of organisms, i. . suckers, bud-variation by, i. . sugar cane, sterility of, in various countries, ii. ; white, liability of, to disease, ii. , . suicide, hereditary tendency to, ii. , . sulivan, admiral, on the horses of the falkland islands, i. ; wild pigs of the falkland islands, i. ; feral cattle of the falkland islands, i. , ; feral rabbits of the falkland islands, i. . sultan fowl, i. , . _sus indica_, i. , - , ii. . _sus pliciceps_, i. (figured). _sus scrofa_, i. , , ii. . _sus scrofa palustris_, i. . _sus vittatus_, i. . swallows, a breed of pigeons, i. . swayne, mr., on artificial crossing of varieties of the pea, i. . sweet peas, ii. ; crosses of, ii. , ; varieties of, coming true by seed, ii. ; acclimatisation of, in india, ii. . sweet william, bud-variation in, i. . swinhoe, r., on chinese pigeons, i. , ; on striped chinese horses, i. . switzerland, ancient dogs of, i. ; pigs of, in the neolithic period, i. - ; goats of, i. . sycamore, pale-leaved variety of the, ii. . sykes, colonel, on a pariah dog with crooked legs, i. ; on small indian asses, i. ; on _gallus sonneratii_, i. ; on the voice of the indian kulm cock, i. ; fertility of the fowl in most climates, ii. . symmetry, hereditary departures from, ii. . _symphytum_, variegated, i. . syphilis, hereditary, ii. . syria, asses of, i. . _syringa persica_, _chinensis_, and _vulgaris_, ii. . tacitus, on the care taken by the celts in breeding animals, ii. . _tagetes signata_, dwarf variety of, ii. . tahiti, varieties of cultivated plants in, ii. . tail, occasional development of, in man, ii. ; never curled in wild animals, ii. ; rudimentary in chinese sheep, ii. . tail-feathers, numbers of, in breeds of pigeons, i. - ; peculiarities of, in cocks, i. - ; variability of, in fowls, i. ; curled, in _anas boschas_, and tame drakes, i. . talent, hereditary, ii. . tankerville, earl of, on chillingham cattle, i. , ii. . tanner, prof., effects of disuse of parts in cattle, ii. . tapir, sterility of the, in captivity, ii. . targioni-tozzetti, on cultivated plants, i. ; on the vine, i. ; varieties of the peach, i. ; origin and varieties of the plum, i. ; origin of the cherry, i. ; origin of roses, i. . tarsus, variability of the, in fowls, i. ; reproduction of the, in a thrush, ii. . tartars, their preference for spiral-horned sheep, ii. . tavernier, abundance of pigeons in persia, i. . _taxus baccata_, ii. . teebay, mr., reversion in fowls, ii. . teeth, number and position of, in dogs, i. ; deficiency of, in naked turkish dogs, i. ; period of appearance of, in breeds of dogs, i. ; precocity of, in highly bred animals, ii. ; correlation of, with hair, ii. ; double row of, with redundant hair, in julia pastrana, ii. ; affected in form by hereditary syphilis and by pulmonary tubercle, ii. ; fusion of, ii. ; developed on the palate, ii. . tegetmeier, mr., on a cat with monstrous teeth, i. ; on a swift-like pigeon, i. ; naked young of some pigeons, i. ; fertility of hybrid pigeons, i. ; on white pigeons, ii. ; reversion in crossed breeds of fowls, i. - ; chicks of the white silk-fowl, i. ; development of the cranial protuberance in polish fowls, i. ; on the skull in the polish fowl, i. , ; on the intelligence of polish fowls, i. ; correlation of the cranial protuberance and crest in polish fowls, i. ; development of the web in the feet of polish fowls, i. ; early development of several peculiarities in spanish cocks, i. ; on the comb in spanish fowls, i. ; on the spanish fowl, ii. ; varieties of game-fowls, i. ; pedigrees of game-fowls, ii. ; assumption of female plumage by a game cock, i. ; natural selection in the game cock, ii. ; pugnacity of game hens, i. ; length of the middle toe in cochin fowls, i. ; origin of the sebright bantam, ii. ; differences in the size of fowls, i. ; effect of crossing in fowls, i. , ii. ; effects of interbreeding in fowls, ii. - ; incubation by mongrels of non-sitting races of fowls, ii. ; inverse correlation of crest and comb in fowls, i. ; { } occurrence of pencilled feathers in fowls, ii. ; on a variety of the goose from sebastopol, i. ; on the fertility of the peahen, ii. ; on the intercrossing of bees, ii. . temminck, origin of domestic cats, i. ; origin of domestic pigeons, i. ; on _columba guinea_, i. ; on _columba leucocephala_, i. ; asserted reluctance of some breeds of pigeons to cross, i. ; sterility of hybrid turtle-doves, i. ; variations of _gallus bankiva_, i. ; on a buff-coloured breed of turkeys, i. ; number of eggs laid by the peahen, ii. ; breeding of guans in captivity, ii. ; behaviour of grouse in captivity, _ibid._; sterility of the partridge in captivity, _ibid._ tendrils in cucurbitaceæ, i. , ii. . tennent, sir j. e., on the goose, i. ; on the growth of the apple in ceylon, ii. ; on the jaffna sheep, ii. . _teredo_, fertilisation in, ii. . terriers, wry-legged, ii. ; white, subject to distemper, ii. . teschemacher, on a husked form of maize, i. . tessier, on the period of gestation of the dog, i. ; of the pig, i. ; in cattle, i. ; experiments on change of soil, ii. . _tetrao_, breeding of species of, in captivity, ii. . _tetrapteryx paradisea_, ii. . _teucrium campanulatum_, pelorism in, ii. . texas, feral cattle in, i. . theognis, his notice of the domestic fowl, i. . theophrastus, his notice of the peach, ii. . _thesium_, ii. . thompson, mr., on the peach and nectarine, i. ; on the varieties of the apricot, i. ; classification of varieties of cherries, i. - ; on the "sister ribston-pippin," i. ; on the varieties of the gooseberry, i. , . thompson, william, on the pigeons of islay, i. ; feral pigeons in scotland, i. ; colour of the bill and legs in geese, i. ; breeding of _tetrao scotius_ in captivity, ii. ; destruction of black-fowls by the osprey, ii. . thompson, prof. w., on the obliquity of the flounder, ii. . thorns, reconversion of, into branches, in pear trees, ii. . thorn, grafting of early and late, i. ; glastonbury, i. . thrush, asserted reproduction of the tarsus in a, ii. . _thuja pendula_ or _filiformis_, a variety of _t. orientalis_, i. . thuret, on the division of the zoospores of an alga, ii. . thwaites, g. h., on the cats of ceylon, i. ; on a twin seed of _fuchsia coccinea_ and _fulgens_, i. . tiburtius, experiments in rearing wild ducks, i. . tiger, rarely fertile in captivity, ii. , . _tigridia conchiflora_, bud-variation in, i. . time, importance of, in the production of races, ii. . tinzmann, self-impotence in the potato, ii. . tissues, affinity of, for special organic substances, ii. . titmice, destructive to thin-shelled walnuts, i. ; attacking nuts, i. ; attacking peas, ii. . tobacco, crossing of varieties of, ii. ; cultivation of in sweden, ii. . tobolsk, red-coloured cats of, i. . toes, relative length of, in fowls, i. ; development of fifth in dogs, ii. . tollet, mr., his selection of cattle, ii. . tomato, ii. . tomtits. see _titmice_. tongue, relation of, to the beak in pigeons, i. . tooth, occurrence of a molar, in place of an incisor, ii. . "torfschwein," i. . trail, r., on the union of half-tubers of different kinds of potatoes, i. . trees, varieties of, suddenly produced, i. ; weeping or pendulous, i. ; fastigate or pyramidal, i. ; with variegated or changed foliage, i. ; early or late in leaf, i. - ; forest, non-application of selection to, ii. . "trembleur" (pigeons), i. . trembley, on reproduction in hydra, ii. . "trevoltini" silkworms, i. - . _trichosanthes anguina_, i. . tricks, inheritance of, ii. - , . _trifolium minus_ and _repens_, ii. . trimorphic plants, conditions of reproduction in, ii. - . tristram, h. b., selection of the dromedary, ii. - . _triticum dicoccum_, i. . _triticum monococcum_, i. . _triticum spelta_, i. . _triticum turgidum_, i. . _triticum vulgare_, wild in asia, i. . { } triton, breeding in the branchiferous stage, ii. . "trommel-taube," i. . "tronfo" pigeon, i. . _tropæolum_, ii. . _tropæolum minus_ and _majus_, reversion in hybrids of, i. . troubetzkoy, prince, experiments with pear-trees at moscow, ii. . trousseau, prof., pathological resemblance of twins, ii. . trumpeter pigeon, i. ; known in , i. . tscharner, h. a. de, graft-hybrid produced by inosculation in the vine, i. . tschudi, on the naked peruvian dog, i. ; extinct varieties of maize from peruvian tombs, i. , ii. . tubers, bud-variation by, i. - . tuckerman, mr., sterility of _carex rigida_, ii. . tufted ducks, i. . tulips, variability of, i. ; bud-variation in, i. - ; influence of soil in "breaking," i. . tumbler pigeon, i. - ; short-faced, figured, i. ; skull figured, i. ; lower jaw figured, i. ; scapula and furcula figured, i. ; early known in india, i. ; history of, i. ; sub-breeds of, i. ; young unable to break the egg-shell, ii. ; probable further modification of, ii. . "tÜmmler" (pigeons), i. . tumours, ovarian, occurrence of hairs and teeth in, ii. ; polypoid, origin of, ii. . "tÜrkische taube," i. . turbit (pigeon), i. . turkey, domestic, origin of, i. - ; crossing of with north american wild turkey, i. - ; breeds of, i. ; crested white cock, i. ; wild, characters of, i. - ; degeneration of, in india, i. , ii. ; failure of eggs of, in delhi, ii. ; feral on the parana, i. ; change produced in by domestication, ii. . turkey, striped young pigs in, i. . turner (pigeon), i. . turner, w., on compensation in arteries and veins, ii. ; on cells, ii. . turnips, origin of, i. ; reversion in, ii. ; run wild, ii. ; crosses of, ii. , ; swedish, preferred by hares, ii. ; acclimatisation of, in india, ii. . turnspit, on an egyptian monument, i. ; crosses of the, ii. . turtle-dove, white and coloured, crossing of, ii. . _turtur auritus_, hybrids of, with _t. cambayensis_ and _t. suratensis_, i. . _turtur risorius_, crossing of, with the common pigeon, i. ; hybrid of, with _t. vulgaris_, _ibid._ _turtur suratensis_, sterile hybrids of, with _t. vulgaris_, i. ; hybrids of, with _t. auritus_, i. . _turtur vulgaris_, crossing of, with the common pigeon, i. ; hybrid of, with _t. risorius_, _ibid._; sterile hybrids of, with _t. suratensis_ and _ectopistes migratorius_, _ibid._ tusks of wild and domesticated pigs, i. , . _tussilago farfara_, variegated, i. . twin-seed _fuchsia coccinea_ and _fulgens_, i. . tyerman, b., on the pigs of the pacific islands, i. , ii. ; on the dogs of the pacific islands, ii. . tylor, mr., on the prohibition of consanguineous marriages, ii. - . udders, development of the, ii. . _ulex_, double-flowered, ii. . _ulmus campestris_ and _effusa_, hybrids of, ii. . uniformity of character, maintained by crossing, ii. - . units of the body, functional independence of the, ii. - . unity or plurality of origin of organisms, i. . upas poison, ii. . urea, secretion of, ii. . use and disuse of parts, effects of, ii. - , - , - ; in rabbits, i. - ; in ducks, i. - . utility, considerations of, leading to uniformity, ii. . valentin, experimental production of double monsters by, ii. . _vallota_, ii. . van beck, barbara, a hairy-faced woman, ii. . van mons on wild fruit-trees, i. , ii. ; production of varieties of the vine, i. ; correlated variability in fruit-trees, ii. ; production of almond-like fruit by peach-seedlings, i. . _vanessa_, species of, not copulating in captivity, ii. . variability, i. , ii. - , - , - ; causes of, ii. - ; correlated, ii. - , - , - ; law of equable, ii. - ; necessity of, for selection, ii. ; of selected characters, ii. - ; of multiple homologous parts, ii. . { } variation, laws of, ii. - ; continuity of, ii. ; possible limitation of, ii. , - ; in domestic cats, i. - ; origin of breeds of cattle by, i. ; in osteological characters of rabbits, i. - ; of important organs, i. ; analogous or parallel, i. - ; in horses, i. ; in the horse and ass, i. ; in fowls, i. - ; in geese, i. ; exemplified in the production of fleshy stems in cabbages, &c., i. ; in the peach, nectarine, and apricot, i. , ; individual, in wheat, i. . variegation of foliage, i. , ii. - . varieties and species, resemblance of, i. , ii. - ; conversion of, into species, i. ; abnormal, ii. ; domestic, gradually produced, ii. . varro, on domestic ducks, i. ; on feral fowls, ii. ; crossing of the wild and domestic ass, ii. . vasey, mr., on the number of sacral vertebræ in ordinary and humped cattle, i. ; on hungarian cattle, i. . vaucher, sterility of _ranunculus ficaria_ and _acorus calamus_, ii. . vegetables, cultivated, reversion in, ii. - ; european, culture of, in india, ii. - . veith, mr., on breeds of horses, i. . _verbascum_, intercrossing of species of, i. , ii. , - ; reversion in hybrids of, i. ; contabescent, wild plants of, ii. ; villosity in, ii. . _verbascum austriacum_, ii. . _verbascum blattaria_, ii. - . _verbascum lychnitis_, ii. - , . _verbascum nigrum_, ii. . _verbascum phoeniceum_, ii. , ; variable duration of, ii. . _verbascum thapsus_, ii. . verbenas, origin of, i. ; white, liability of, to mildew, ii. , ; scorching of dark, ii. , ; effect of changed conditions of life on, ii. . verlot, on the darkleaved barberry, i. ; inheritance of peculiarities of foliage in trees, i. ; production of _rosa cannabifolia_ by bud-variation from _r. alba_, i. ; bud-variation in _aralia trifoliata_, i. ; variegation of leaves, i. ; colours of tulips, i. ; uncertainty of inheritance, ii. ; persistency of white flowers, ii. ; peloric flowers of _linaria_, ii. ; tendency of striped flowers to uniformity of colour, ii. ; non-intercrossing of certain allied plants, ii. ; sterility of _primulæ_ with coloured calyces, ii. ; on fertile proliferous flowers, _ibid._; on the irish yew, ii. ; differences in the _camellia_, ii. ; effect of soil on the variegated strawberry, ii. ; correlated variability in plants, ii. . _verruca_, ii. , . vertebrÆ, characters of, in rabbits, i. - ; in ducks, i. - ; number and variations of, in pigeons, i. - ; number and characters of, in fowls, i. - ; variability of number of, in the pig, i. . vertuch, see putsche. "verugas," ii. . vespucius, early cultivation in brazil, i. . vibert's experiments on the cultivation of the vine from seed, i. . _viburnum opulus_, ii. , . _vicia sativa_, leaflet converted into a tendril in, ii. . vicunas, selection of, ii. . villosity of plants, influenced by dryness, ii. . vilmorin, cultivation of the wild carrot, i. , ii. ; colours of tulips, i. ; uncertainty of inheritance in balsams and roses, ii. ; experiments with dwarf varieties of _saponaria calabrica_ and _tagetes signata_, ii. ; reversion of flowers by stripes and blotches, ii. ; on variability, ii. . _vinca minor_, sterility in, ii. . vine, i. - ; parsley-leaved, reversion of, i. ; graft-hybrid produced by inosculation in the, i. ; disease of, influenced by colour of grapes, ii. ; influence of climate, &c., on varieties of the, ii. ; diminished extent of cultivation of the, ii. ; acclimatisation of the, in the west indies, ii. . _viola_, species of, i. . _viola lutea_, different coloured flowers in, i. . _viola tricolor_, reversion in, ii. , . virchow, prof., blindness occurring in the offspring of consanguineous marriages, ii. ; on the growth of bones, ii. , ; on cellular prolification, ii. ; independence of the elements of the body, ii. ; on the cell-theory, ii. ; presence of hairs and teeth in ovarian tumours, ii. ; of hairs in the brain, ii. ; special affinities of the tissues, ii. ; origin of polypoid excrescences and tumours, ii. . virgil on the selection of seed-corn, i. , ii. ; of cattle and sheep, ii. . virginian islands, ponies of, i. . _virgularia_, ii. . vision, hereditary peculiarities of, ii. - ; { } in amphibious animals, ii. ; varieties of, ii. ; affections of organs of, correlated with other peculiarities, ii. . _vitis vinifera_, i. - , . _viverra_, sterility of species of, in captivity, ii. . vogel, varieties of the date palm, ii. . vogt, on the indications of stripes on black kittens, ii. . voice, differences of, in fowls, i. ; peculiarities of, in ducks, i. ; inheritance of peculiarities of, ii. . volz, on the history of the dog, i. ; ancient history of the fowl, i. ; domestic ducks unknown to aristotle, i. ; indian cattle sent to macedonia by alexander, ii. ; mention of mules in the bible, ii. ; history of the increase of breeds, ii. . von berg on _verbascum phoeniceum_, ii. . voorhelm, g., his knowledge of hyacinths, i. , ii. . vrolik, prof., on polydactylism, ii. ; on double monsters, ii. ; influence of the shape of the mother's pelvis on her child's head, ii. . waders, behaviour of, in confinement, ii. . wahlenberg, on the propagation of alpine plants by buds, runners, bulbs, &c., ii. . "wahlverwandtschaft" of gärtner, ii. . wales, white cattle of, in the th century, i. . walker, a., on intermarriage, i. ; on the inheritance of polydactylism, ii. . walker, d., advantage of change of soil to wheat, ii. . wallace, a. r., on a striped javanese horse, i. ; on the conditions of life of feral animals, ii. ; artificial alteration of the plumage of birds, ii. ; on polymorphic butterflies, ii. - ; on reversion, ii. ; on the limit of change, ii. . wallace, dr., on the sterility of sphingidæ hatched in autumn, ii. . wallachian sheep, sexual peculiarities in the horns of, i. . wallflower, bud-variation in, i. . wallich, dr., on _thuja pendula_ or _filiformis_, i. . walnuts, i. - ; thin-shelled, attacked by tomtits, ii. ; grafting of, ii. . walsh, b. d., on galls, ii. , ; his "law of equable variability," ii. - . walther, f. l., on the history of the dog, i. ; on the intercrossing of the zebu and ordinary cattle, i. . waring, mr., on individual sterility, ii. . wart hog, i. . waterer, mr., spontaneous production of _cytisus alpino-laburnum_, i. . water melon, i. . waterhouse, g. r., on the winter-colouring of _lepus variabilis_, i. . waterton, c., production of tailless foals, i. ; on taming wild ducks, i. ; on the wildness of half-bred wild ducks, ii. ; assumption of male characters by a hen, ii. . watson, h. c., on british wild fruit-trees, i. ; on the non-variation of weeds, i. ; origin of the plum, i. ; variation in _pyrus malus_, i. ; on _viola amoena_ and _tricolor_, i. ; on reversion in scotch kail, ii. ; fertility of _draba sylvestris_ when cultivated, ii. ; on generally distributed british plants, ii. . wattles, rudimentary, in some fowls, ii. . watts, miss, on sultan fowls, i. . webb, james, interbreeding of sheep, ii. . weber, effect of the shape of the mother's pelvis on her child's head, ii. . weeds, supposed necessity for their modification, coincidently with cultivated plants, i. . weeping varieties of trees, i. . weeping habit of trees, capricious inheritance of, ii. - . weevil, injury done to stone-fruit by, in north america, ii. . welsh cattle, descended from _bos longifrons_, i. . west indies, feral pigs of, i. ; effect of climate of, upon sheep, i. . western, lord, change effected by, in the sheep, ii. . westphalia, striped young pigs in, i. . westwood, j. o., on peloric flowers of _calceolaria_, ii. . whately, archbishop, on grafting early and late thorns, i. . wheat, specific unity or diversity of, i. - , - ; hasora, i. ; presence or absence of barbs in, i. ; godron on variations in, _ibid._; varieties of, i. - ; effects of soil and climate on, i. ; deterioration of, _ibid._; crossing of varieties of, _ibid._, ii. , - , ; in the swiss lake-dwellings, i. - ; selection applied to, i. , ii. ; increased fertility of hybrids of, with _Ægilops_, ii. ; advantage of change of soil to, ii. ; { } differences of, in various parts of india, ii. ; continuous variation in, ii. ; red, hardiness of, ii. , ; fenton, ii. ; natural selection in, ii. ; varieties of, found wild, ii. ; effects of change of climate on, ii. ; ancient variety of, ii. . whitby, mrs., on the markings of silkworms, i. ; on the silkmoth, i. . white, mr., reproduction of supernumerary digits after amputation, ii. ; time occupied in the blending of crossed races, ii. . white, gilbert, vegetable diet of dogs, ii. . white and white-spotted animals, liability of, to disease, ii. - . white flowers, most truly reproduced by seed, ii. . wichura, max, on hybrid willows, ii. , , ; analogy between the pollen of old-cultivated plants, and of hybrids, ii. . wicking, mr., inheritance of the primary characters of _columba livia_ in cross-bred pigeons, i. ; production of a white head in almond tumblers, ii. . wicksted, mr., on cases of individual sterility, ii. . wiegmann, spontaneous crossing of blue and white peas, i. ; crossing of varieties of cabbage, ii. ; on contabescence, ii. . wight, dr., sexual sterility of plants propagated by buds, &c., ii. . wilde, sir w. r., occurrence of _bos frontosus_ and _longifrons_ in irish crannoges, i. ; attention paid to breeds of animals by the ancient irish, ii. . wildman, on the dahlia, ii. , . wildness of the progeny of crossed tame animals, ii. - . wilkes, capt., on the taming of pigeons among the polynesians, ii. . wilkinson, j., on crossed cattle, ii. . williams, mr., change of plumage in a hamburgh hen, i. . williams, mr., intercrossing of strawberries, i. . williamson, capt., degeneration of dogs in india, i. ; on small indian asses, i. . williamson, rev. w., doubling of _anemone coronaria_ by selection, ii. . willows, weeping, i. ; reversion of spiral-leaved weeping, i. ; hybrids of, ii. ; galls of, ii. - . willoughby, f., notice of spot pigeons, i. ; on a fantail pigeon, i. ; on tumbler pigeons, i. ; on the turbit, i. ; on the barb and carrier pigeons, i. ; on the hook-billed duck, i. . wilmot, mr., on a crested white turkey cock, i. ; reversion of sheep in colour, ii. . wilson, b. o., fertility of hybrids of humped and ordinary cattle in tasmania, i. . wilson, dr., prepotency of the manx over the common cat, ii. . wilson, james, origin of dogs, i. . wilson, mr., on prepotency of transmission in sheep, ii. ; on the breeding of bulls, ii. . wings, proportionate length of, in different breeds of pigeons, i. - ; of fowls, effects of disuse on, i. - ; characters and variations of, in ducks, i. - ; diminution of, in birds of small islands, i. - . wing-feathers, number of, in pigeons, i. ; variability of, in fowls, i. . wolf, recent existence of, in ireland, i. ; barking of young, i. ; hybrids of, with the dog, i. . wolf-dog, black, of florida, i. . wolves, north american, their resemblance to dogs of the same region, i. - ; burrowing of, i. . woodbury, mr., crossing of the ligurian and common hive bees, i. , ii. ; variability of bees, i. . woodward, s. p., on arctic mollusca, ii. . wood, willoughby, on mr. bates' cattle, ii. . wooler, w. a., on the young of the himalayan rabbit, i. ; persistency of the coloured calyx in a crossed polyanthus, i. . worrara poison, ii. . wounds, healing of, ii. . wright, j., production of crippled calves by shorthorned cattle, ii. ; on selection in cattle, ii. ; effect of close interbreeding on pigs, ii. - ; deterioration of game cocks by close interbreeding, ii. . wright, strethill, on the development of the hydroida, ii. . wyman, dr., on niata cattle, and on a similar malformation in the codfish, i. ; on virginian pigs, ii. . xenophon, on the colours of hunting dogs, ii. . ximenes, cardinal, regulations for the selection of rams, ii. . "yahoo," the name of the pigeon in persia, i. . yaks, domestication of, i. ; selection of white-tailed, ii. , . { } yam, development of axillary bulbs in the, ii. . yarrell, mr., deficiency of teeth in hairless dogs, i. , ii. ; on ducks, i. , ii. ; characters of domestic goose, resembling those of _anser albifrons_, i. ; whiteness of ganders, i. ; variations in goldfish, i. - ; assumption of male plumage by the hen-pheasant, ii. ; effect of castration upon the cock, ii. - ; breeding of the skylark in captivity, ii. ; plumage of the male linnet in confinement, ii. ; on the dingo, ii. . yellow fever, in mexico, ii. . yew, fastigate, ii. . yew, irish, hardy in new york, ii. . yew, weeping, i. ; propagation of, by seed, ii. - . yolk, variations of, in the eggs of ducks, i. . youatt, mr., history of the dog, i. - ; variations of the pulse in breeds of dogs, i. ; liability to disease in dogs, i. , ii. ; inheritance of goître in dogs, ii. ; on the greyhound, i. , ; on king charles' spaniels, i. ; on the setter, i. ; on breeds of horses, i. ; variation in the number of ribs in the horse, i. ; inheritance of diseases in the horse, ii. , ; introduction of eastern blood into english horses, ii. - ; on white welsh cattle, i. , ii. ; improvement of british breeds of cattle, i. ; rudiments of horns in young hornless cattle, ii. , ; on crossed cattle, ii. , ; on bakewell's long-horned cattle, ii. ; selection of qualities in cattle, ii. ; degeneration of cattle by neglect, ii. ; on the skull in hornless cattle, ii. ; disease of white parts of cattle, ii. ; displacement of long-horned by short-horned cattle, ii. ; on angola sheep, i. ; on the fleece of sheep, i. ; correlation of horns and fleece in sheep, i. ; adaptation of breeds of sheep to climate and pasture, i. ; horns of wallachian sheep, i. ; exotic sheep in the zoological gardens, i. - , ii. ; occurrence of horns in hornless breeds of sheep, ii. ; on the colour of sheep, ii. ; on interbreeding sheep, ii. ; on merino rams in germany, ii. ; effect of unconscious selection on sheep, ii. ; reversion of leicester sheep on the lammermuir hills, ii. ; on many-horned sheep, ii. ; reduction of bone in sheep, ii. ; persistency of character in breeds of animals in mountainous countries, ii. ; on interbreeding, ii. ; on the power of selection, ii. - ; slowness of production of breeds, ii. ; passages in the bible relating to the breeding of animals, ii. - . young, j., on the belgian rabbit, i. . yule, capt., on a burmese hairy family, ii. , . zambesi, striped young pigs on the, i. . zambos, character of the, ii. . zano, j. g., introduction of rabbits into porto santo by, i. . _zea mays_, i. . zebu, i. ; domestication of the, i. ; fertile crossing of, with european cattle, i. , ii. . zebra, hybrids of, with the ass and mare, ii. . _zephyranthes candida_, ii. . _zinnia_, cultivation of, ii. . zollinger on malayan penguin ducks, i. . zoospore, division of, in algæ, ii. . "zopf-taube," i. . the end. london: printed by w. clowes and sons, duke street, stamford street, and charing cross. * * * * * notes [ ] 'medical notes and reflections,' rd edit., , p. . [ ] mr. buckle, in his grand work on 'civilisation,' expresses doubts on the subject owing to the want of statistics. _see_ also mr. bowen, professor of moral philosophy, in 'proc. american acad. of sciences,' vol. v. p. [ ] for greyhounds, _see_ low's 'domest. animals of the british islands,' , p. . for game-fowls, _see_ 'the poultry book,' by mr. tegetmeier, , p. . for pigs, _see_ mr. sidney's edit. of 'youatt on the pig,' , pp. , . [ ] 'the stud farm,' by cecil, p. . [ ] 'philosophical transactions,' , p. . i have seen only second-hand accounts of the two grandsons. mr. sedgwick, in a paper to which i shall hereafter often refer, states that _four_ generations were affected, and in each the males alone. [ ] barbara van beck, figured, as i am informed by the rev. w. d. fox, in woodburn's 'gallery of rare portraits,' , vol. ii. [ ] 'proc. zoolog. soc.,' , p. [ ] hofacker, 'ueber die eigenschaften,' &c., , s. . report by pariset in 'comptes rendus,' , p. . [ ] hunter, as quoted in harlan's 'med. researches,' p. . sir a. carlisle, 'phil. transact.,' , p. . [ ] girou de buzareignues, 'de la génération,' p. . [ ] 'macmillan's magazine,' july and august, . [ ] the works which i have read and found most useful are dr. prosper lucas's great work, 'traité de l'hérédité naturelle,' . mr. w. sedgwick, in 'british and foreign medico-chirurg. review,' april and july, , and april and july, : dr. garrod on gout is quoted in these articles. sir henry holland, 'medical notes and reflections,' rd edit., . piorry, 'de l'hérédité dans les maladies,' . adams, 'a philosophical treatise on hereditary peculiarities,' nd edit., . essay on 'hereditary diseases,' by dr. j. steinan, . _see_ paget, in 'medical times,' , p. , on the inheritance of cancer; dr. gould, in 'proc. of american acad. of sciences,' nov. , , gives a curious case of hereditary bleeding in four generations. harlan, 'medical researches,' p. . [ ] marshall, quoted by youatt in his work on cattle, p. . [ ] 'philosoph. transact.,' , p. . [ ] 'medical notes and reflections,' rd edit., p. . [ ] this affection, as i hear from mr. bowman, has been ably described and spoken of as hereditary by dr. dondera, of utrecht, whose work was published in english by the sydenham society in . [ ] quoted by mr. herbert spencer, 'principles of biology,' vol. i. p. . [ ] 'british and foreign medico-chirurg. review, 'april, , p. - ; 'l'héréd. nat.,' tom. i. pp. - . [ ] dr. osborne, pres. of royal college of phys. in ireland, published this case in the 'dublin medical journal' for . [ ] these various statements are taken from the following works and papers:--youatt on 'the horse,' pp. , . lawrence, 'the horse,' p. . karkeek, in an excellent paper in 'gard. chronicle,' , p. . mr. burke, in 'journal of r. agricul. soc. of england,' vol. v. p. . 'encyclop. of rural sports,' p. . girou de buzareignues, 'philosoph. phys.,' p. . _see_ following papers in 'the veterinary:' roberts, in vol. ii. p. ; m. marrimpoey, vol. ii. p. ; mr. karkeek, vol. iv. p. ; youatt on goître in dogs, vol. v. p. ; youatt, in vol. vi. pp. , , ; m. bernard, vol. xi. p. ; dr. samesreuther, on cattle, in vol. xii. p. ; percivall, in vol. xiii. p. . with respect to blindness in horses, _see_ also a whole row of authorities in dr. p. lucas's great work, tom. i. p. . mr. baker, in 'the veterinary,' vol. xiii. p. , gives a strong case of hereditary imperfect vision and of jibbing. [ ] knight on 'the culture of the apple and pear,' p. . lindley's 'horticulture,' p. . [ ] these statements are taken from the following works in order:--youatt on 'the horse,' p. ; mr. darvill, in 'the veterinary,' vol. viii. p. . with respect to robson, _see_ 'the veterinary,' vol. iii. p. ; mr. lawrence on 'the horse,' , p. ; 'the stud farm,' by cecil, ; baron cameronn, quoted in 'the veterinary,' vol x. p. . [ ] 'recreations in agriculture and nat. hist.,' vol. i. p. . [ ] 'ueber die eigenschaften,' &c., , s. . [ ] bronn's 'geschichte der natur,' band ii. s. . [ ] vrolik has discussed this point at full length in a work published in dutch, from which mr. paget has kindly translated for me passages. _see_, also, isidore geoffroy st. hilaire's 'hist. des anomalies,' , tom. i. p. . [ ] 'edinburgh new phil. journal,' july, . [ ] some great anatomists, as cuvier and meckel, believe that the tubercle one side of the hinder foot of the tailless batrachians represents a sixth digit. certainly, when the hinder foot of a toad, as soon as it first sprouts from the tadpole, is dissected, the partially ossified cartilage of this tubercle resembles under the microscope, in a remarkable manner, a digit. but the highest authority on such subjects, gegenbaur (untersuchung. zur vergleich. anat. der wirbelthiere: carpus et tarsus, , s. ), concludes that this resemblance is not real, only superficial. [ ] for these several statements, _see_ dr. struthers, in work cited, especially on intermissions in the line of descent. prof. huxley, 'lectures on our knowledge of organic nature,' , p. . with respect to inheritance, _see_ dr. prosper lucas, 'l'hérédité nat.,' tom. i. p. . isid. geoffroy, 'anom.,' tom. i. p. . sir a. carlisle, in 'phil. transact.,' , p. . a. walker, on 'intermarriage,' , p. , gives a case of five generations; as does mr. sedgwick, in 'brit. and foreign medico-chirurg. review,' april, , p. . on the inheritance of other anomalies in the extremities, _see_ dr. h. dobell, in vol. xlvi. of 'medico-chirurg. transactions,' ; also mr. sedgwick, in op. cit., april, , p. . with respect to additional digits in the negro, _see_ prichard, 'physical history of mankind.' dr. dieffenbach ('journ. royal geograph. soc.,' , p. ) says this anomaly is not uncommon with the polynesians of the chatham islands. [ ] 'the poultry chronicle,' , p. . [ ] the statements in this paragraph are taken from isidore geoffroy st. hilaire, 'hist. des anomalies,' tom. i. pp. - . [ ] as quoted by carpenter, 'princ. of comp. physiology,' , p. . [ ] müller's 'phys.,' eng. translat., vol. i. , p. . a thrush, however, was exhibited before the british association at hull, in , which had lost its tarsus, and this member, it was asserted, had been thrice reproduced: i presume it was lost each time by disease. [ ] 'monthly journal of medical science,' edinburgh, , new series, vol. ii. p. . [ ] 'an essay on animal reproduction,' trans. by dr. maty, , p. . [ ] bonnet, 'oeuvres d'hist. nat.,' tom. v., part i., to. edit., , pp. , , . [ ] so with insects, the larvæ reproduce lost limbs, but, except in one order, the mature insect has no such power. but the myriapoda, which apparently represent the larvæ of true insects, have, as newport has shown, this power until their last moult. _see_ an excellent discussion on this whole subject by dr. carpenter in his 'princ. comp. phys.,' , p. . [ ] dr. günther, in owen's 'anatomy of vertebrates,' vol. i., , p. . spallanzani has made similar observations. [ ] 'on the anatomy of vertebrates,' , p. : with respect to the pectoral fins of fishes, pp. - . [ ] 'medical notes and reflections,' , pp. , . _see_, also, dr. p. lucas, 'l'héréd. nat.,' tom. ii. p. . [ ] 'du danger des mariages consanguins,' nd edit., , p. . [ ] 'british and foreign medico-chirurg. review,' july, , pp. , . [ ] verlot, 'la production des variétés,' , p. . [ ] loudon's 'gard. mag.,' vol. xii., , p. . [ ] verlot, 'la product. des variétés,' , p. . [ ] bronn's 'geschichte der natur,' b. ii. s. . [ ] rev. w. a. leighton, 'flora of shropshire,' p. ; and charlesworth's 'mag. of nat. hist.,' vol. i, , p. . [ ] verlot, op. cit., p. . [ ] for these several statements, _see_ loudon's 'gard. magazine,' vol. x., , pp. , ; and vol. ix., , p. . [ ] these statements are taken from alph. de candolle, 'bot. géograph.,' p. . [ ] verlot, op. cit., p. . [ ] op. cit., p. . [ ] alph. de candolle, 'géograph. bot.,' p. . [ ] _see_ 'cottage gardener,' april , , p. , and sept. , , p. ; 'gard. chron.,' , p. . [ ] darwin, in 'journal of proc. linn. soc. bot.,' , p. . [ ] hofacker, 'ueber die eigenschaften,' &c., s. . [ ] bechstein, 'naturgesch. deutschlands,' b. iv. s. . mr. brent, a great breeder of canaries, informs me that he believes that these statements are correct. [ ] 'the poultry book,' by w. b. tegetmeier, , p. . [ ] 'british and foreign med.-chirurg. review,' july, , pp. - . mr. sedgwick has given such full details on this subject, with ample references, that i need refer to no other authorities. [ ] 'de l'espèce,' tom. ii., , p. . [ ] 'philosoph. magazine,' vol. iv., , p. . [ ] this last case is quoted by mr. sedgwick in 'british and foreign medico-chirurg. review,' april, , p. . for blumenbach, _see_ above-cited paper. _see_, also, dr. p. lucas, 'traité de l'héréd. nat.,' tom. ii. p. . also 'transact. lin. soc.,' vol. ix. p. . some curious cases are given by mr. baker in 'the veterinary,' vol. xiii. p. . another curious case is given in the 'annales des scienc. nat.,' st series, tom. xi. p. . [ ] 'proc. royal soc.,' vol. x. p. . [ ] mr. sproule, in 'british medical journal,' april , . [ ] downing, 'fruits of america,' p. ; sageret, 'pom. phys.,' pp. , . [ ] youatt on sheep, pp. , . the same fact of loose horns occasionally appearing in hornless breeds has been observed in germany: bechstein, 'naturgesch. deutschlands,' b. i. s. . [ ] youatt on cattle, pp. , . [ ] youatt on sheep, , pp. , . [ ] i have been informed of this fact through the rev. w. d. fox, on the excellent authority of mr. wilmot: _see_, also, remarks on this subject in an original article in the 'quarterly review,' , p. . [ ] youatt, pp. , . [ ] 'the poultry book,' by mr. tegetmeier, , p. . [ ] loudon's 'gard. mag.,' vol. x., , p. : a nurseryman, with much experience on this subject, has likewise assured me that this sometimes occurs. [ ] 'gardener's chron.,' , p. . [ ] ibid., , p. . [ ] _see_ some excellent remarks on this subject by mr. wallace, 'journal proc. linn. soc.,' , vol. iii. p. . [ ] dureau de la malle, in 'comptes rendus,' tom. xli., , p. . from the statements above given, the author concludes that the wild pigs of louisiana are not descended from the european _sus scrofa_. [ ] capt. w. allen, in his 'expedition to the niger,' states that fowls have run wild on the island of annobon, and have become modified in form and voice. the account is so meagre and vague that it did not appear to me worth copying; but i now find that dureau de la malle ('comptes rendus,' tom. xli., , p. ) advances this as a good instance of reversion to the primitive stock, and as confirmatory of a still more vague statement in classical times by varro. [ ] 'flora of australia,' , introduct., p. ix. [ ] 'de l'espèce,' tom. ii. pp. , , . [ ] mr. sedgwick gives many instances in the 'british and foreign med.-chirurg. review,' april and july, , pp. , . [ ] in his edit. of 'youatt on the pig,' , p. . [ ] dr. p. lucas, 'héréd. nat.,' tom. ii. pp. , : _see_ a good practical article on this subject in 'gard. chronicle,' , p. . i could add a vast number of references, but they would be superfluous. [ ] kölreuter gives cases in his 'dritte fortsetzung,' , s. , ; and in his well-known 'memoirs on lavatera and jalapa.' gärtner, 'bastarderzeugung,' s. , , &c. naudin, in his 'recherches sur l'hybridité, nouvelles archives du muséum,' tom. i. p. . [ ] quoted by mr. sedgwick in 'med.-chirurg. review,' april, , p. . dr. h. dobell, in 'med.-chirurg. transactions,' vol. xlvi., gives an analogous case, in which, in a large family, fingers with thickened joints were transmitted to several members during five generations; but when the blemish once disappeared it never reappeared. [ ] verlot, 'des variétés,' , p. . [ ] 'nouvelles archives du muséum,' tom. i. p. . alex. braun (in his 'rejuvenescence,' ray soc., , p. ) apparently holds a similar opinion. [ ] mr. teebay, in 'the poultry book,' by mr. tegetmeier, , p. . [ ] quoted by hofacker, 'ueber die eigenschaften,' &c., s. . [ ] 'essais hist. nat. du paraguay,' tom. ii. , p. . [ ] these facts are given on the high authority of mr. hewitt, in 'the poultry book,' by mr. tegetmeier, , p. . [ ] 'the poultry book,' by tegetmeier, , p. . [ ] 'gardener's chron. and agricultural gazette,' , p. . [ ] ibid., , p. . [ ] sclater, in 'proc. zoolog. soc.,' , p. . [ ] 'history of the horse,' p. . [ ] 'mém. présentés par divers savans à l'acad. royale,' tom. vi. , p. . [ ] 'letters from alabama,' , p. . [ ] 'hist. nat. des mammifères,' , tom. i. [ ] 'philosoph. transact.,' , p. . [ ] sclater, in 'proc. zoolog. soc.,' , p. : this species is the ghor-khur of n.w. india, and has often been called the hemionus of pallas. _see_, also, mr. blyth's excellent paper in 'journ. of asiatic soc. of bengal,' vol. xxviii., , p. . [ ] another species of wild ass, the true _a. hemionus_ or _kiang_, which ordinarily has no shoulder-stripes, is said occasionally to have them; and these, as with the horse and ass, are sometimes double: _see_ mr. blyth, in the paper just quoted, and in 'indian sporting review,' , p. ; and col. hamilton smith, in 'nat. library, horses,' p. ; and 'dict. class. d'hist. nat.,' tom. iii. p. . [ ] figured in the 'gleanings from the knowsley menageries,' by dr. j. e. gray. [ ] cases of both spanish and polish hens sitting are given in the 'poultry chronicle,' , vol. iii. p. . [ ] 'the poultry book,' by mr. tegetmeier, , pp. , . the author, who remarks on the two negatives ('journ. of hort.,' , p. ), states that two broods were raised from a spanish cock and silver-pencilled hamburgh hen, neither of which are incubators, and no less than seven out of eight hens in these two broods "showed a perfect obstinacy in sitting." the rev. e. s. dixon ('ornamental poultry,' , p. ) says that chickens reared from a cross between golden and black polish fowls, are "good and steady birds to sit." mr. b. p. brent informs me that he raised some good sitting hens by crossing pencilled hamburgh and polish breeds. a cross-bred bird from a spanish non-incubating cock and cochin incubating hen is mentioned in the 'poultry chronicle,' vol. iii. p. , as an "exemplary mother." on the other hand, an exceptional case is given in the 'cottage gardener,' , p. , of a hen raised from a spanish cock and black polish hen which did not incubate. [ ] 'the poultry book,' by tegetmeier, , pp. , . [ ] 'natural history review,' , april, p. . [ ] 'essays on natural history,' p. . [ ] as stated by mr. orton, in his 'physiology of breeding,' p. . [ ] m. e. de selys-longchamps refers ('bulletin acad. roy. de bruxelles,' tom. xii. no. ) to more than seven of these hybrids shot in switzerland and france. m. deby asserts ('zoologist,' vol. v., - , p. ) that several have been shot in various parts of belgium and northern france. audubon ('ornitholog. biography,' vol. iii. p. ), speaking of these hybrids, says that, in north america, they "now and then wander off and become quite wild." [ ] 'journal of researches,' , p. . [ ] 'expedition to the zambesi,' , pp. , . [ ] dr. p. broca, on 'hybridity in the genus homo,' eng. translat., , p. . [ ] 'nouvelles archives du muséum,' tom. i. p. . [ ] 'bastarderzeugung,' s. , , &c. [ ] 'die bastardbefruchtung ... der weiden,' , s. . for gärtner's remarks on this head, _see_ 'bastarderzeugung,' s. , . [ ] yarrell, 'phil. transact.,' , p. ; dr. hamilton, in 'proc. zoolog. soc.,' , p. . [ ] 'archiv. skand. beiträge zur naturgesch.,' viii. s. - . [ ] in his 'essays on nat. hist.,' . mr. hewitt gives analogous cases with hen-pheasants in 'journal of horticulture,' july , , p. . isidore geoffroy saint hilaire, in his 'essais de zoolog. gén.' (suites à buffon, , pp. - ), has collected such cases in ten different kinds of birds. it appears that aristotle was well aware of the change in mental disposition in old hens. the case of the female deer acquiring horns is given at p. . [ ] 'cottage gardener,' , p. . [ ] 'art de faire eclorre,' &c., , tom. ii. p. . [ ] sir h. holland, 'medical notes and reflections,' rd edit., , p. . [ ] prof. thomson on steenstrup's views on the obliquity of flounders: 'annals and mag. of nat. hist.,' may, , p. . [ ] dr. e. von martens, in 'annals and mag. of nat. hist.,' march, , p. . [ ] darwin, 'balanidæ,' ray soc., , p. : _see_ also the appended remarks on the apparently capricious development of the thoracic limbs on the right and left sides in the higher crustaceans. [ ] mormodes ignea: darwin, 'fertilization of orchids,' , p. . [ ] 'journal of horticulture,' july, , p. . i have had the opportunity of examining these remarkable feathers through the kindness of mr. tegetmeier. [ ] 'the poultry book,' by mr. tegetmeier, , p. . [ ] carl vogt, 'lectures on man,' eng. translat., , p. . [ ] on cattle, p. . [ ] isid. geoffroy st. hilaire, 'des anomalies,' tom. iii. p. . with respect to the mammæ in women, _see_ tom. i. p. . [ ] 'natural hist. review,' april, , p. . _see_ also his lecture, royal institution, march , . on same subject, _see_ moquin-tandon, 'eléments de tératologie,' , pp. , . [ ] verlot, 'des variétés,' , p. ; naudin, 'nouvelles archives du muséum,' tom. i. p. . [ ] in his discussion on some curious peloric calceolarias, quoted in 'journal of horticulture,' feb. , , p. . [ ] for other cases of six divisions in peloric flowers of the labiatæ and scrophulariaceæ, _see_ moquin-tandon, 'tératologie,' p. . [ ] moquin-tandon, 'tératologie,' p. . [ ] _see_ youatt on cattle, pp. , , , , : also youatt on sheep, p. . also dr. lucas, 'l'héréd. nat.,' tom. ii. p. . [ ] 'héréd. nat.,' tom. ii. pp. - . [ ] sir h. holland, 'chapters on mental physiology,' , p. . [ ] 'gardener's chronicle,' , p. . [ ] mr. n. h. smith, observations on breeding, quoted in 'encyclop. of rural sports,' p. . [ ] quoted by bronn, 'geschichte der natur,' b. ii. s. . _see_ sturm, 'ueber racen,' , s. - . for the niata cattle, _see_ my 'journal of researches,' , p. . [ ] lucas, 'l'hérédité nat.,' tom. ii. p. . [ ] mr. orton, 'physiology of breeding,' , p. . [ ] boitard and corbié, 'les pigeons,' , p. . [ ] 'les pigeons, pp. , . [ ] 'das ganze,' &c., , s. . [ ] 'the pigeon book,' p. . [ ] 'physiology of breeding,' p. ; mr. hewitt, in 'the poultry book,' by tegetmeier, , p. . [ ] boitard and corbié, 'les pigeons,' , p. . [ ] 'bastarderzeugung,' s. , , &c. naudin ('nouvelles archives du muséum,' tom. i. p. ) gives a striking instance of prepotency in _datura stramonium_ when crossed with two other species. [ ] flourens, 'longévité humaine,' p. , on crossed jackals. with respect to the difference between the mule and the hinny, i am aware that this has generally been attributed to the sire and dam transmitting their characters differently; but colin, who has given in his 'traité phys. comp.,' tom. ii. pp. - , the fullest description which i have met with of these reciprocal hybrids, is strongly of opinion that the ass preponderates in both crosses, but in an unequal degree. this is likewise the conclusion of flourens, and of bechstein in his 'naturgeschichte deutschlands,' b. i. s. . the tail of the hinny is much more like that of the horse than is the tail of the mule, and this is generally accounted for by the males of both species transmitting with greater power this part of their structure; but a compound hybrid which i saw in the zoological gardens, from a mare by a hybrid ass-zebra, closely resembled its mother in its tail. [ ] mr. hewitt, who has had such great experience in raising these hybrids, says ('poultry book,' by mr. tegetmeier, , pp. - ) that in all, the head was destitute of wattles, comb, and ear-lappets; and all closely resembled the pheasant in the shape of the tail and general contour of the body. these hybrids were raised from hens of several breeds by a cock-pheasant; but another hybrid, described by mr. hewitt, was raised from a hen-pheasant by a silver-laced bantam cock, and this possessed a rudimental comb and wattles. [ ] 'l'héréd. nat.,' tom. ii. book ii. ch. i. [ ] 'bastarderzeugung,' s. - . naudin ('nouvelles archives du muséum,' tom. i. p. ) has arrived at a similar conclusion. [ ] 'cottage gardener,' , pp. , . [ ] _see_ some remarks on this head with respect to sheep by mr. wilson, in 'gardener's chronicle,' , p. . [ ] verlot, 'des variétés,' , p. . [ ] moquin-tandon, 'tératologie,' p. . [ ] 'nouvelles archives du muséum,' tom. i. p. . [ ] 'l'héréd. nat.,' tom. ii. pp. - . _see_, also, mr. sedgwick's four memoirs, immediately to be referred to. [ ] on sexual limitation in hereditary diseases, 'brit. and for. med.-chirurg. review,' april, , p. ; july, p. ; april, , p. ; and july, p. . [ ] w. scrope, 'art of deer stalking,' p. . [ ] boitard and corbié, 'les pigeons,' p. ; dr. f. chapuis, 'le pigeon voyageur belge,' , p. . [ ] prichard, 'phys. hist. of mankind,' , vol. i. p. . [ ] 'embassy to the court of ava,' vol. i. p. . the third generation is described by capt. yule in his 'narrative of the mission to the court of ava,' , p. . [ ] 'das ganze der taubenzucht,' , s. , tab. i., fig. ; s. , tab. iv., fig. . [ ] kidd's 'treatise on the canary,' p. . [ ] charlesworth, 'mag. of nat. hist.,' vol. i., , p. . [ ] dr. prosper lucas, 'héréd. nat.,' tom. ii. p. . [ ] 'l'héréd. dans les maladies,' , p. . for hunter, _see_ harlan's 'med. researches,' p. . [ ] 'l'héréd. nat.,' tom. ii. p. . [ ] sedgwick, 'brit. and for. med.-chirurg. review,' april , p. . i have seen three accounts, all taken from the same original authority (which i have not been able to consult), and all differ in the details! but as they agree in the main facts, i have ventured to quote this case. [ ] prosper lucas, 'héréd. nat.,' tom. i. p. . [ ] sedgwick, idem, july, , p. . [ ] piorry, p. ; prosper lucas, tom. ii. p. . [ ] prosper lucas, tom. ii. p. . [ ] prosper lucas, tom. ii. pp. , , ; sedgwick, idem, april, , p. , and july, , p. ; dr. j. steinan, 'essay on hereditary disease,' , pp. , . [ ] these cases are given by mr. sedgwick, on the authority of dr. h. stewart, in 'med.-chirurg. review,' april, , pp. , . [ ] 'héréd. nat.,' tom. ii. p. . [ ] communications to the board of agriculture, vol. i. p. . [ ] 'review of reports, north of england,' , p. . [ ] 'säugethiere von paraguay,' , s. . [ ] rengger, 'säugethiere,' &c., s. . [ ] white, 'regular gradation in man,' p. . [ ] dr. w. f. edwards, in his 'charactères physiolog. des races humaines,' p. , first called attention to this subject, and ably discussed it. [ ] rev. d. tyerman, and bennett, 'journal of voyages,' - , vol. i. p. . [ ] mr. s. j. salter, 'journal linn. soc.,' vol. vi., , p. . [ ] sturm, 'ueber racen, &c.,' , s. . bronn, 'geschichte der natur.,' b. ii. s. , gives a table of the proportions of blood after successive crosses. dr. p. lucas, 'l'hérédité nat.,' tom. ii. p. . [ ] 'bastarderzeugung,' s. , . [ ] 'nova acta petrop.,' , p. : _see_ also previous volume. [ ] as quoted in the 'true principles of breeding,' by c. h. macknight and dr. h. madden, , p. . [ ] with respect to plants, an admirable essay on this subject (die geschlechter-vertheilung bei den pflanzen: ) has lately been published by dr. hildebrand, who arrives at the same general conclusions as i have done. [ ] 'teoria della riproduzione vegetal,' , p. . [ ] verlot, 'des variétés,' , p. . [ ] duval-jouve, 'bull. soc. bot. de france,' tom. x., , p. . [ ] extract of a letter from sir r. heron, , given me by mr. yarrell. with respect to mice, _see_ 'annal. des sc. nat.,' tom. i. p. ; and i have heard of other similar cases. for turtle-doves, boitard and corbié, 'les pigeons,' &c., p. . for the game fowl, 'the poultry book,' , p. . for crosses of tailless fowls, _see_ bechstein, 'naturges. deutsch.' b. iii. s. . bronn, 'geschichte der natur,' b. ii. s. , gives analogous facts with horses. on the hairless condition of crossed south american dogs, _see_ rengger, 'säugethiere von paraguay,' s. : but i saw in the zoological gardens mongrels, from a similar cross, which were hairless, quite hairy, or hairy in patches, that is, piebald with hair. for crosses of dorking and other fowls, _see_ 'poultry chronicle,' vol. ii. p. . about the crossed pigs, extract of letter from sir r. heron to mr. yarrell. for other cases, _see_ p. lucas, 'héréd. nat.,' tom. i. p. . [ ] 'internat. hort. and bot. congress of london,' . [ ] 'bastarderzeugung,' s. . kölreuter ('dritte fortsetszung,' s. , ), however, obtained intermediate tints from similar crosses in the genus verbascum. with respect to the turnips, _see_ herbert's 'amaryllidaceæ,' , p. . [ ] 'nouvelles archives du muséum,' tom. i. p. . [ ] richardson, 'pigs,' , pp. , ; s. sidney's edition of 'youatt on the pig,' , p. . [ ] _see_ mr. w. c. spooner's excellent paper on cross-breeding, 'journal royal agricult. soc.,' vol. xx., part ii.: _see_ also an equally good article by mr. ch. howard, in 'gardener's chronicle,' , p. . [ ] 'gardener's chronicle,' , pp. , . [ ] 'bulletin de la soc. d'acclimat.,' , tom. ix. p. . _see_ also, for other cases, mm. moll and gayot, 'du boeuf,' , p. xxxii. [ ] 'poultry chronicle,' vol. ii., , p. . [ ] 'the poultry book,' by w. b. tegetmeier, , p. . [ ] 'gardener's chronicle,' , p. . [ ] spooner, in 'journal royal agricult. soc.,' vol. xx., part ii. [ ] _see_ colin's 'traité de phys. comp. des animaux domestiques,' tom. ii. p. , where this subject is well treated. [ ] 'les pigeons,' p. . [ ] vol. i., , p. . [ ] 'cottage gardener,' , p. . [ ] 'bastarderzeugung,' s. . [ ] dr. pigeaux, in 'bull. soc. d'acclimat.,' tom. iii., july , as quoted in 'annals and mag. of nat. hist.,' , vol. xx. p. . [ ] 'journal de physiolog.,' tom. ii., , p. . [ ] dec. , p. . [ ] on the varieties of wheat, p. . [ ] rengger, 'säugethiere von paraguay,' s. . [ ] _see_ a memoir by mm. lherbette and de quatrefages, in 'bull. soc. d'acclimat.,' tom. viii., july, , p. . [ ] for the norfolk sheep, _see_ marshall's 'rural economy of norfolk,' vol. ii. p. . _see_ rev. l. landt's 'description of faroe,' p. . for the ancon sheep, _see_ 'phil. transact.,' , p. . [ ] white's 'nat. hist. of selbourne,' edited by bennett, p. . with respect to the origin of the dark-coloured deer, _see_ 'some account of english deer parks,' by e. p. shirley, esq. [ ] 'the dovecote,' by the rev. e. s. dixon, p. ; bechstein, 'naturgesch. deutschlands,' band iv., , s. . [ ] 'cattle,' p. . [ ] mr. j. wilkinson, in 'remarks addressed to sir j. sebright,' , p. . [ ] 'gardener's chronicle,' , p. . [ ] 'bastarderzeugung,' s. , . _see_ also the table at the end of volume. [ ] 'bastarderzeugung,' s. , . [ ] 'kenntniss der befruchtung,' s. ; 'bastarderzeugung,' s. , . on raising the two varieties from seed _see_ s. . [ ] 'bastarderzeugung,' s. . [ ] the following facts, given by kölreuter in his 'dritte fortsetzung,' s. , , appear at first sight strongly to confirm mr. scott's and gärtner's statements; and to a certain limited extent they do so. kölreuter asserts, from innumerable observations, that insects incessantly carry pollen from one species and variety of verbascum to another; and i can confirm this assertion; yet he found that the white and yellow varieties of _verbascum lychnitis_ often grew wild mingled together: moreover, he cultivated these two varieties in considerable numbers during four years in his garden, and they kept true by seed; but when he crossed them, they produced flowers of an intermediate tint. hence it might have thought that both varieties must have a stronger elective affinity for the pollen of their own variety than for that of the other; this elective affinity, i may add, of each species for its own pollen (kölreuter, 'dritte forts.,' s. , and gärtner, 'bastarderz.,' _passim_) being a perfectly well-ascertained power. but the force of the foregoing facts is much lessened by gärtner's numerous experiments, for, differently from kölreuter, he never once got ('bastarderz.,' s. ) an intermediate tint when he crossed the yellow and white flowered varieties of verbascum. so that the fact of the white and yellow varieties keeping true to their colour by seed does not prove that they were not mutually fertilised by the pollen carried by insects from one to the other. [ ] 'amaryllidaceæ,' , p. . gärtner has made a similar observation. [ ] kölreuter first observed this fact. 'mém. de l'acad. st. petersburg,' vol. iii. p. . _see_ also c. k. sprengel, 'das entdeckte geheimniss,' s. . [ ] namely, barbarines, pastissons, giraumous: 'annal. des sc. nat.,' tom. xxx., , pp. and . [ ] 'mémoire sur les cucurbitaceæ,' , pp. , . [ ] 'annales des se. nat.,' th series, tom. vi. m. naudin considers these forms as undoubtedly varieties of _cucurbita pepo_. [ ] 'mém. cucurb.,' p. . [ ] 'zweite forts.,' s. , namely, nicotiana major vulgaris; ( ) perennis; ( ) transylvanica; ( ) a sub-var. of the last; ( ) major latifol. fl. alb. [ ] kölreuter was so much struck with this fact that he suspected that a little pollen of _n. glutinosa_ in one of his experiments might have accidentally got mingled with that of _var. perennis_, and thus aided its fertilising power. but we now know conclusively from gärtner ('bastarderz.,' s. , ) that two kinds of pollen never act _conjointly_ on a third species; still less will the pollen of a distinct species, mingled with a plant's own pollen, if the latter be present in sufficient quantity, have any effect. the sole effect of mingling two kinds of pollen is to produce in the same capsule seeds which yield plants, some taking after the one and some after the other parent. [ ] mr. scott has made some observations on the absolute sterility of a purple and white primrose (_primula vulgaris_) when fertilised by pollen from the primrose ('journal of proc. of linn. soc.,' vol. viii., , p. ); but these observations require confirmation. i raised a number of purple-flowered long-styled seedlings from seed kindly sent me by mr. scott, and, though they were all some degree sterile, they were much more fertile with pollen taken from the common primrose than with their own pollen. mr. scott has likewise described a red equal-styled cowslip (_p. veris_, idem, p. ), which was found by him to be highly sterile when crossed with the common cowslip; but this was not the case with several equal-styled red seedlings raised by me from his plant. this variety of the cowslip presents the remarkable peculiarity of combining male organs in every respect like those of the short-styled form, with female organs resembling in function and partly in structure those of the long-styled form; so that we have the singular anomaly of the two forms combined in the same flower. hence it is not surprising that these flowers should be spontaneously self-infertile in a high degree. [ ] 'act. acad. st. petersburg,' , part ii., pp. , . [ ] 'annales des sc. nat.,' tom. xxi. ( st series), p. . [ ] 'bull. bot. soc. de france,' dec. th, , tom. viii. p. . [ ] quoted by isid. geoffroy st. hilaire, 'hist. naturelle générale,' tom. iii. p. . since this ms. has been sent to press a full discussion on the present subject has appeared in mr. herbert spencer's 'principles of biology,' vol. ii. , p. _et seq._ [ ] for cats and dogs, &c., _see_ bellingeri, in 'annal. des sc. nat.,' nd series, zoolog., tom. xii. p. . for ferrets, bechstein, 'naturgeschichte deutschlands,' band i., , s. , . for rabbits, ditto, s. , ; and bronn's 'geschichte der natur,' b. ii. s. . for mountain sheep, ditto, s. . for the fertility of the wild sow, _see_ bechstein's 'naturgesch. deutschlands,' b. i., , s. ; for the domestic pig, sidney's edit. of youatt on the pig, , p. . with respect to lapland, _see_ acerbi's 'travels to the north cape,' eng. translat., vol. ii. p. . about the highland cows, _see_ hogg on sheep, p. . [ ] for the eggs of _gallus bankiva_, _see_ blyth, in 'annals and mag. of nat. hist., nd series, vol. i., , p. . for wild and tame ducks, macgillivray, 'british birds,' vol. v. p. ; and 'die enten,' s. . for wild geese, l. lloyd, 'scandinavian adventures,' vol. ii. , p. ; and for tame geese, 'ornamental poultry,' by rev. e. s. dixon, p. . on the breeding of pigeons, pistor, 'das ganze der taubenzucht,' , s. ; and boitard and corbié, 'les pigeons,' p. . with respect to peacocks, according to temminck ('hist. nat. gén. des pigeons,' &c., , tom. ii. p. ), the hen lays in india even as many as twenty eggs; but according to jerdon and another writer (quoted in tegetmeier's 'poultry book,' , pp. , ), she there lays only from four to nine or ten eggs: in england she is said, in the 'poultry book,' to lay five or six, but another writer says from eight to twelve eggs. [ ] 'the art of improving the breed, &c.,' , p. . [ ] for andrew knight, _see_ a. walker, on 'intermarriage,' , p. . sir j. sebright's treatise has just been quoted. [ ] 'cattle,' p. . [ ] nathusius, 'ueber shorthorn rindvieh,' , s. : _see_ also 'gardener's chronicle,' , p. . many analogous cases are given in a pamphlet recently published by mr. c. macknight and dr. h. madden, 'on the true principles of breeding;' melbourne, australia, . [ ] mr. willoughby wood, in 'gardener's chronicle,' , p. ; and , p. . _see_ the very clear tables and pedigrees given in nathusius' 'rindvieh,' s. - . [ ] mr. wright, 'journal of royal agricult. soc.,' vol. vii., , p. . [ ] youatt on cattle, p. . [ ] report british assoc., zoolog. sect., . [ ] azara, 'quadrupèdes du paraguay,' tom. ii. pp. , . [ ] for the case of the messrs. brown, _see_ 'gard. chronicle,' , p. . for the foscote flock, 'gard. chron.,' , p. . for the naz flock, 'bull. de la soc. d'acclimat.,' , p. . [ ] nathusius, 'rindvieh,' s. ; youatt on sheep, p. . [ ] 'gard. chronicle,' , p. . [ ] lord somerville, 'facts on sheep and husbandry,' p. . mr. spooner, in 'journal of royal agricult. soc. of england,' vol. xx., part ii. _see_ also an excellent paper on the same subject in 'gard. chronicle,' , p. , by mr. charles howard. [ ] 'some account of english deer parks,' by evelyn p. shirley, . [ ] 'the art of improving the breed,' &c., p. . with respect to scotch deer-hounds, _see_ scrope's 'art of deer stalking,' pp. - . [ ] 'cottage gardener,' , p. . [ ] sidney's edit. of youatt on the pig, , p. ; p. , quotation from mr. druce; p. , on lord western's case. [ ] 'journal, royal agricult. soc. of england,' , vol. vii. p. . [ ] 'ueber rindvieh,' &c., s. . [ ] sidney on the pig, p. . _see_ also note, p. . also richardson on the pig, , p. . [ ] dr. dally has published an excellent article (translated in the 'anthropolog. review,' may, , p. ), criticising all writers who have maintained that evil follows from consanguineous marriages. no doubt on this side of the question many advocates have injured their cause by inaccuracies: thus it has been stated (devay, 'du danger des mariages,' &c., , p. ) that the marriages of cousins have been prohibited by the legislature of ohio; but i have been assured, in answer to inquiries made in the united states, that this statement is a mere fable. [ ] _see_ his most interesting work on the 'early history of man,' , chap. x. [ ] on consanguinity in marriage, in the 'fortnightly review,' , p. ; hofacker, 'ueber die eigenschaften,' &c. [ ] sir g. grey's 'journal of expeditions into australia,' vol. ii. p. ; and dobrizhoffer, 'on the abipones of south america.' [ ] 'the art of improving the breed,' p. . [ ] 'the poultry book,' by w. b. tegetmeier, , p. . [ ] 'journal royal agricult. soc.' , vol. vii. p. ; _see_ also ferguson on the fowl, pp. , ; _see_ also 'the poultry book,' by tegetmeier, , p. , with respect to the extent to which cock-fighters found that they could venture to breed in-and-in, viz., occasionally a hen with her own son; "but they were cautious not to repeat the in-and-in breeding." [ ] 'the poultry book,' by w. b. tegetmeier, , p. . [ ] 'the poultry chronicle,' , vol. i. p. . [ ] 'the poultry book,' by w. b. tegetmeier, , p. . [ ] 'the poultry chronicle,' vol. i. p. . [ ] 'the poultry book,' , p. . [ ] ibid, , p. ; and 'poultry chronicle,' vol. iii., , p. . [ ] 'a treatise on fancy pigeons,' by j. m. eaton, p. . [ ] 'the pigeon book,' p. . [ ] 'das ganze der taubenzucht,' , s. . [ ] 'les pigeons,' , p. . [ ] 'proc. entomolog. soc.,' aug. th, , p. . [ ] 'journal of horticulture,' , pp. , , ; and , p. . [ ] 'beiträge zur kenntniss der befruchtung,' , s. . [ ] 'amaryllidaceæ,' p. . [ ] 'de la fécondation,' nd edit., , p. . [ ] 'mémoire sur les cucurbitacées,' pp. , , . [ ] loudon's 'gard. mag.,' vol. viii., , p. . [ ] 'transact. hort. soc.,' vol. i. p. . [ ] 'annal. des sc. nat.,' rd series, bot., tom. vi. p. . [ ] 'philosophical transactions,' , p. . [ ] 'ueber die bastarderzeugung,' , s. , . for mr. chaundy's case, _see_ loudon's 'gard. mag.,' vol. vii., , p. . [ ] 'gardener's chron.,' , p. . [ ] 'philosoph. transact.,' , p. . [ ] quoted in 'bull. bot. soc. france,' vol. ii., , p. . [ ] gärtner, 'bastarderzeugung,' s. , , _et seq._ [ ] 'fortsetzung,' , s. ; 'dritte fortsetzung,' s. , ; 'act. acad. st. petersburg,' , part ii., p. ; 'nova acta,' , pp. , ; 'nova acta,' , pp. , . [ ] 'die bastardbefruchtung,' &c., , s. , , . [ ] max wichura fully accepts this view ('bastardbefruchtung,' s. ), as does the rev. m. j. berkeley, in 'journal of hort. soc.,' jan. , p. . [ ] 'bastarderzeugung,' s. , , . [ ] kölreuter,' nova acta,' , p. . [ ] gärtner, 'bastarderzeugung,' s. . [ ] 'botanische zeitung,' jan. , s. . [ ] 'monatsbericht akad. wissen,' berlin, , s. . [ ] international hort. congress, london, . [ ] 'proc. bot. soc. of edinburgh,' may, : these observations are given in abstract, and others are added, in the 'journal of proc. of linn. soc.,' vol. viii. bot., , p. . [ ] prof. lecoq, 'de la fécondation,' nd edit., , p. . [ ] 'bastarderzeugung,' s. , . [ ] idem, s. . [ ] 'zweite fortsetzung,' s. ; 'dritte fort.,' s. . [ ] duvernoy, quoted by gärtner, 'bastarderzeugung,' s. . [ ] 'gardner's chronicle,' , p. . [ ] 'transact. hort. soc.,' vol. vii., , p. . [ ] prof. lecoq, 'de la fécondation,' , p. ; gärtner, 'bastarderzeugung,' s. . [ ] 'gardener's chron.' , p. . [ ] 'journal of proc. of linn. soc.,' vol. viii., , p. . [ ] 'amaryllidaceæ,' , p. ; 'journal of hort. soc.,' vol. ii., , p. . [ ] loudon's 'gardener's magazine,' vol. xi., , p. . [ ] 'gardener's chronicle,' , p. . [ ] 'journal hort. soc., vol. v. p. . the seedlings thus raised were given to the hort. soc.; but i find, on inquiry, that they unfortunately died the following winter. [ ] mr. d. beaton, in 'journal of hort.,' , p. . lecoq, however ('de la fécond.,' , p. ), states that this hybrid is descended from _g. psittacinus_ and _cardinalis_; but this is opposed to herbert's experience, who found that the former species could not be crossed. [ ] this is the conclusion of prof. devay, 'du danger des mariages consang.,' , p. . virchow quotes, in the 'deutsche jahrbücher,' , s. , some curious evidence on half the cases of a peculiar form of blindness occurring in the offspring from near relations. [ ] for england, _see_ below. for germany, _see_ metzger, 'getreidearten,' , s. . for france, loiseleur-deslongchamps ('consid. sur les céreales,' , p. ) gives numerous references on this subject. for southern france, _see_ godron, 'florula juvenalis,' , p. . [ ] 'a general treatise of husbandry,' vol. iii. p. . [ ] 'gardener's chronicle and agricult. gazette,' , p. ; and for the second statement, idem, , p. . on this same subject, _see_ also rev. d. walker's 'prize essay of highland agricult. soc.,' vol. ii. p. . also marshall's 'minutes of agriculture,' november, . [ ] oberlin's 'memoirs,' eng. translat., p. . for lancashire, _see_ marshall's 'review of reports,' , p. . [ ] 'cottage gardener,' , p. . for mr. robson's subsequent statements, _see_ 'journal of horticulture,' feb. , , p. . for mr. abbey's remarks on grafting, &c., idem, july , , p. . [ ] 'mém. de l'acad. des sciences,' , p. . [ ] 'on the varieties of wheat,' p. . [ ] mr. spencer has fully and ably discussed this whole subject in his 'principles of biology,' , vol. ii. ch. x. in the first edition of my 'origin of species,' , p. , i spoke of the good effects from slight changes in the conditions of life and from cross-breeding, and of the evil effects from great changes in the conditions and from crossing widely distinct forms, as a series of facts "connected together by some common but unknown bond, which is essentially related to the principle of life." [ ] 'essais de zoologie générale,' , p. . [ ] du rut, 'annales du muséum,' , tom. ix. p. . [ ] 'säugethiere von paraguay,' , s. , , , , , , , , . [ ] 'the naturalist on the amazons,' , vol. i. pp. , ; vol. ii. p. . [ ] 'embassy to the court of ava,' vol. i. p. . [ ] 'journal,' vol. i. p. . [ ] 'säugethiere,' s. . [ ] on the breeding of the larger felidæ, 'proc. zoolog. soc.,' , p. . [ ] sleeman's 'rambles in india,' vol. ii. p. . [ ] wiegmann's 'archif für naturgesch.,' , s. . [ ] rengger, 'säugethiere,' &c., s. . on the parentage of the guinea-pig, _see_ also isid. geoffroy st. hilaire, 'hist. nat. gén.' [ ] although the existence of the _leporides_, as described by dr. broca ('journal de phys.,' tom. ii. p. ), is now positively denied, yet dr. pigeaux ('annals and mag. of nat. hist.,' vol. xx., , p. ) affirms that the hare and rabbit have produced hybrids. [ ] 'quadrupeds of north america,' by audubon and bachman, , p. . [ ] loudon's 'mag. of nat. hist.,' vol. ix., , p. ; audubon and bachman's 'quadrupeds of north america,' p. . [ ] flourens, 'de l'instinct,' &c., , p. . [ ] _see_ 'annual reports zoolog. soc.,' , , , ; 'times' newspaper, aug. th, ; flourens, 'de l'instinct,' p. . [ ] 'säugethiere,' &c., s. , . [ ] art. brazil, 'penny cyclop.,' p. . [ ] 'the naturalist on the river amazon,' vol. i. p. . [ ] 'encyclop. of rural sports,' p. . [ ] according to sir a. burnes ('cabool,' &c., p. ), eight species are used for hawking in scinde. [ ] loudon's 'mag. of nat. hist.,' vol. vi., , p. . [ ] f. cuvier, 'annal. du muséum,' tom. ix. p. . [ ] 'the zoologist,' vol. vii.-viii., - , p. . [ ] knox, 'ornithological rambles in sussex,' p. . [ ] 'the zoologist,' vol. vii.-viii., - , p. ; vol. ix.-x., - , p. . [ ] bechstein, 'naturgesch. der stubenvögel,' , s. . [ ] 'ornithological biography,' vol. v. p. . [ ] a case is recorded in 'the zoologist,' vol. i.-ii., - , p. . for the siskin breeding, vol. iii.-iv., - , p. . bechstein, 'stubenvögel,' s. , speaks of bullfinches making nests, but rarely producing young. [ ] yarrell's 'hist. british birds,' , vol. i. p. . [ ] loudon's 'mag. of nat. history,' vol. ix., , p. . [ ] 'mémoires du muséum d'hist. nat.,' tom. x. p. : five cases of parrots breeding in france are here recorded. _see_, also, 'report brit. assoc. zoolog.,' . [ ] 'stubenvögel,' s. , . [ ] dr. hancock remarks ('charlesworth's mag. of nat. hist.,' vol. ii., , p. ), "it is singular that, amongst the numerous useful birds that are indigenous to guiana, none are found to propagate among the indians; yet the common fowl is reared in abundance throughout the country." [ ] 'a week at port royal,' , p. . [ ] audubon, 'american ornithology,' vol. v. pp. , . [ ] moubray on poultry, th edit., p. . [ ] temminck, 'hist. nat. gén. des pigeons,' &c., , tom. iii. pp. , ; 'annals and mag. of nat. hist.,' vol. xii., , p. . other species of partridge have occasionally bred; as the red-legged (_p. rubra_), when kept in a large court in france (_see_ 'journal de physique,' tom. xxv. p. ), and in the zoological gardens in . [ ] rev. e. s. dixon, 'the dovecote,' , pp. - . [ ] temminck, 'hist. nat. gén. des pigeons,' &c., tom. ii. pp. , ; tom. iii. pp. , , . [ ] bates, 'the naturalist on the amazons,' vol. i. p. ; vol. ii. p. . [ ] temminck, 'hist. nat. gén.,' &c., tom. iii. p. . for _tetrao urogallus_, _see_ l. lloyd, 'field sports of north of europe,' vol. i. pp. , ; and 'bull. de la soc. d'acclimat.,' tom. vii., , p. . for _t. scoticus_, thompson, 'nat. hist. of ireland,' vol. ii., , p. . for _t. cupido_, 'boston journal of nat. hist.,' vol. iii. p. . [ ] marcel de serres, 'annales des sci. nat.,' nd series, zoolog., tom. xiii. p. . [ ] dr. hancock, in 'charlesworth's mag. of nat. hist.' vol. ii., , p. ; r. hill, 'a week at port royal,' p. ; 'guide to the zoological gardens,' by p. l. sclater, , pp. , ; 'the knowsley menagerie,' by dr. gray, , pl. xiv.; e. blyth, 'report asiatic soc. of bengal,' may, . [ ] prof. newton, in 'proc. zoolog. soc.,' , p. . [ ] 'the dovecote and aviary,' p. . [ ] 'ornithological biography,' vol. iii. p. . [ ] 'geograph. journal,' vol. xiii., , p. . [ ] loudon's 'mag. of nat. hist.,' vol. v., , p. . [ ] 'zoologist,' vols. v.-vi., - , p. . [ ] 'transact. entomolog. soc.,' vol. iv., , p. . [ ] 'transact. linn. soc.,' vol. vii. p. . [ ] _see_ an interesting paper by mr. newman, in the 'zoologist,' , p. ; and dr. wallace, in 'proc. entomolog. soc.,' june th, , p. . [ ] yarrell's 'british birds,' vol. i. p. ; bechstein, 'stubenvögel,' s. ; 'philosoph. transact.,' , p. . bronn ('geschichte der natur,' band ii. s. ) has collected a number of cases. for the case of the deer, _see_ 'penny cyclop.,' vol. viii. p. . [ ] 'journal de physiologie,' tom. ii. p. . [ ] for additional evidence on this subject, _see_ f. cuvier, in 'annales du muséum,' tom. xii. p. . [ ] numerous instances could be given. thus livingstone ('travels,' p. ) states that the king of the barotse, an inland tribe which never had any communication with white men, was extremely fond of taming animals, and every young antelope was brought to him. mr. galton informs me that the damaras are likewise fond of keeping pets. the indians of south america follow the same habit. capt. wilkes states that the polynesians of the samoan islands tamed pigeons; and the new zealanders, as mr. mantell informs me, kept various kinds of birds. [ ] for analogous cases with the fowl, _see_ réaumur, 'art de faire eclorre,' &c., , p. ; and col. sykes, in 'proc. zoolog. soc.,' , &c. with respect to the fowl not breeding in northern regions, _see_ latham's 'hist. of birds,' vol. viii., , p. . [ ] 'mém. par divers savans, acad. des sciences,' tom. vi., , p. . [ ] youatt on sheep, p. . [ ] j. mills, 'treatise on cattle,' , p. . [ ] bechstein, 'stubenvögel,' s. . [ ] crawfurd's 'descriptive dict. of the indian islands,' , p. . [ ] 'bull. de la soc. acclimat., tom. ix., , pp. , . [ ] for pigeons, _see_ dr. chapuis, 'le pigeon voyageur belge,' , p. . [ ] 'swedish acts,' vol. i., , p. . pallas makes the same remark in his travels (eng. translat.), vol. i. p. . [ ] a. kerner, 'die cultur der alpenflanzen,' , s. ; watson's 'cybele britannica,' vol. i. p. ; mr. d. cameron, also, has written on the culture of alpine plants in 'gard. chronicle,' , pp. , , and mentions a few which seed. [ ] 'beiträge zur kenntniss der befruchtung,' , s. . [ ] 'nova acta petrop.,' , p. . [ ] 'cottage gardener,' , pp. , . [ ] dr. herbert, 'amaryllidaceæ,' p. . [ ] gärtner, 'beiträge zur kenntniss,' &c., s. , . [ ] 'gardener's chronicle,' , p. ; , p. . [ ] 'beiträge zur kenntniss,' &c., s. , . [ ] 'journal of hort. soc.,' vol. ii. , p. . [ ] 'beiträge zur kenntniss,' &c., s. _et seq._; kölreuter, 'zweite fortsetzung,' s. , ; 'dritte fortsetzung,' s. . herbert, 'amaryllidaceæ,' p. . wiegmann, 'ueber die bastarderzeugung,' s. . [ ] 'bastarderzeugung,' s. . [ ] 'teoria della riproduzione,' , p. ; 'traité du citrus,' , p. . [ ] mr. c. w. crocker, in 'gard. chronicle,' , p. . [ ] verlot, 'des variétés,' , p. . [ ] verlot, idem, p. . [ ] prof. allman, brit. assoc., quoted in the 'phytologist,' vol. ii. p. . prof. harvey, on the authority of mr. andrews, who discovered the plant, informed me that this monstrosity could be propagated by seed. with respect to the poppy, _see_ prof. goeppert, as quoted in 'journal of horticulture,' july st, , p. . [ ] 'comptes rendus,' dec. th, , p. . [ ] 'gardener's chronicle,' , p. . [ ] 'theory of horticulture,' p. . [ ] mr. fairweather, in 'transact. hort. soc.,' vol. iii. p. ; bosse, quoted by bronn, 'geschichte der natur,' b. ii. s. . on the effects of the removal of the anthers, _see_ mr. leitner, in silliman's 'north american journ. of science,' vol. xxiii. p. ; and verlot, 'des variétés,' , p. . [ ] lindley's 'theory of horticulture,' p. . [ ] 'gardener's chronicle,' , p. ; , pp. , ; and verlot, 'des variétés,' p. . [ ] 'gardener's chronicle,' , p. . in this article i suggested the following theory on the doubleness of flowers. [ ] quoted by gärtner, 'bastarderzeugung,' s. . [ ] 'gardener's chronicle,' , p. . [ ] lindley, 'theory of horticulture,' p. - ; godron, 'de l'espèce,' tom. i. p. : pickering, 'races of man;' gallesio, 'teoria della riproduzione,' , p. - . meyen ('reise um erde,' th. ii. s. ) states that at manilla one variety of the banana is full of seeds; and chamisso (hooker's 'bot. misc.,' vol. i. p. ) describes a variety of the bread-fruit in the mariana islands with small fruit, containing seeds which are frequently perfect. burnes, in his 'travels in bokhara,' remarks on the pomegranate seeding in mazenderan, as a remarkable peculiarity. [ ] ingledew, in 'transact. of agricult. and hort. soc. of india,' vol. ii. [ ] 'de la fécondation,' , p. . [ ] hooker's 'bot. misc.,' vol. i. p. ; gallesio, 'teoria della riproduzione,' p. . [ ] 'transact. linn. soc.,' vol. xvii. p. . [ ] godron, 'de l'espèce,' tom. ii. p. ; herbert on crocus, in 'journal of hort. soc.,' vol. i., , p. .--dr. wight, from what he has seen in india, believes in this view; 'madras journal of lit. and science,' vol. iv., , p. . [ ] wahlenberg specifies eight species in this state on the lapland alps: _see_ appendix to linnæus' 'tour in lapland,' translated by sir j. e. smith, vol. ii. pp. - . [ ] 'travels in north america,' eng. translat., vol. iii. p. . [ ] with respect to the ivy and acorus, _see_ dr. bromfield in the 'phytologist,' vol. iii. p. . _see_ also lindley and vaucher on the acorus. [ ] 'annal. des sc. nat.,' rd series, zool., tom. iv. p. . prof. decaisne refers also to analogous cases with mosses and lichens near paris. [ ] mr. tuckerman, in silliman's 'american journal of science,' vol. xlv. p. . [ ] sir j. e. smith, 'english flora,' vol. i. p. . [ ] g. planchon, 'flora de montpellier,' , p. . [ ] on the non-production of seeds in england _see_ mr. crocker, in 'gardener's weekly magazine,' , p. ; vaucher, 'hist. phys. plantes d'europe,' tom. i. p. ; lecoq, 'géograph. bot. de l'europe,' tom. iv. p. ; dr. d. clos, in 'annal. des sc. nat.,' rd series, bot., tom. xvii., , p. : this latter author refers to other analogous cases. on the non-production of pollen by this ranunculus _see_ chatin, in 'comptes rendus,' june th, . [ ] 'bastarderzeugung,' s. . kölreuter ('dritte fortsetzung,' s. , , ) also shows that when two species, one single and the other double, are crossed, the hybrids are apt to be extremely double. [ ] 'teoria della riproduzione veg.,' , p. . [ ] 'bastarderzeugung,' s. . [ ] ibid., s. . [ ] 'transactions phil. soc.,' , p. . for kölreuter, _see_ 'mém. de l'acad. de st. pétersbourg,' tom. iii., (published ), p. . in reading c. k. sprengel's remarkable work, 'das entdeckte geheimniss,' &c., , it is curious to observe how often this wonderfully acute observer failed to understand the full meaning of the structure of the flowers which he has so well described, from not always having before his mind the key to the problem, namely, the good derived from the crossing of distinct individual plants. [ ] this abstract was published in the fourth edition ( ) of my 'origin of species;' but as this edition will be in the hands of but few persons, and as my original observations on this point have not as yet been published in detail, i have ventured here to reprint the abstract. [ ] the term _unconscious selection_ has been objected to as a contradiction: but _see_ some excellent observations on this head by prof. huxley ('nat. hist. review,' oct. , p. ), who remarks that when the wind heaps up sand-dunes it sifts and _unconsciously selects_ from the gravel on the beach grains of sand of equal size. [ ] sheep, , p. . [ ] mr. j. wright on shorthorn cattle, in 'journal of royal agricult. soc.,' vol. vii. pp. , . [ ] h. d. richardson on pigs, , p. . [ ] 'journal of r. agricult. soc.,' vol. i. p. . [ ] sheep, pp. , . [ ] loudon's 'mag. of nat. hist.,' vol. viii., , p. . [ ] 'a treatise on the art of breeding the almond tumbler,' , p. . [ ] 'recreations in agriculture,' vol. ii. p. . [ ] youatt on cattle, pp. , . [ ] ferguson, 'prize poultry,' , p. . [ ] wilson, in 'transact. highland agricult. soc.,' quoted in 'gard. chronicle,' , p. . [ ] simmonds, quoted in 'gard. chronicle,' , p. . and for the second quotation, _see_ youatt on sheep, p. . [ ] robinet, 'vers à soie,' , p. . [ ] quatrefages, 'les maladies du ver à soie,' , p. . [ ] m. simon, in 'bull. de la soc. d'acclimat.,' tom. ix., , p. . [ ] 'the poultry chronicle,' vol. i., , p. . [ ] j. m. eaton, 'a treatise on fancy pigeons,' , p. xiv., and 'a treatise on the almond tumbler,' , p. . [ ] 'journal royal agricultural soc.,' vol. vi. p. . [ ] 'poultry chronicle,' vol. ii., , p. . [ ] isid. geoffroy st. hilaire, 'hist. nat. gén.,' tom. iii. p. . [ ] 'gardener's chronicle,' , p. . [ ] 'transact. hort. soc.,' vol. vi. p. . [ ] 'journal of horticulture,' , p. . [ ] 'transact. hort. soc.,' vol. iv. p. . [ ] 'transact. hort. soc.,' vol. iv. p. . [ ] rev. w. bromehead, in 'gard. chronicle,' , p. . [ ] 'gard. chronicle,' , p. . [ ] dr. anderson, in 'the bee,' vol. vi. p. ; mr. barnes, in 'gard. chronicle,' , p. . [ ] godron, 'de l'espèce,' , tom. ii. p. ; 'gard. chronicle,' , p. . [ ] on sheep, p. . [ ] volz, 'beiträge zur kulturgeschichte,' , s. . [ ] mitford's 'history of greece,' vol. i. p. . [ ] dr. dally, translated in 'anthropological review,' may , p. . [ ] volz, 'beiträge,' &c., , s. . [ ] 'history of the world,' ch. . [ ] 'gardener's chronicle,' , p. . [ ] reynier, 'de l'economie des celtes,' , pp. , . [ ] le couteur on wheat, p. . [ ] michel, 'des haras,' , p. . [ ] sir w. wilde, an 'essay on unmanufactured animal remains,' &c., , p. . [ ] col. hamilton smith, 'nat. library,' vol. xii., horses, pp. , . [ ] michel, 'des haras,' p. . [ ] mr. baker, 'history of the horse,' veterinary, vol. xiii. p. . [ ] m. l'abbé carlier, in 'journal de physique,' vol. xxiv., , p. : this memoir contains much information on the ancient selection of sheep; and is my authority for rams not being killed young in england. [ ] 'gardener's chronicle,' , p. . [ ] communications to board of agriculture, quoted in dr. darwin's 'phytologia,' , p. . [ ] 'mémoire sur les chinois,' , tom. xi. p. ; tom. v. p. . [ ] 'recherches sur l'agriculture des chinois,' par l. d'hervey-saint-denys, , p. . with respect to khang-hi, _see_ huc's 'chinese empire,' p. . [ ] anderson, in 'linn. transact.,' vol. xii. p. . [ ] 'mém. de l'acad.' (divers savans), tom. vi., , p. . [ ] 'des quadrupèdes du paraguay,' , tom. ii. p. , . [ ] 'the great sahara,' by the rev. h. b. tristram, , p. . [ ] pallas, 'act. acad. st. petersburg,' , p. ; moorcroft and trebeck, 'travels in the himalayan provinces,' . [ ] quoted from raffles, in the 'indian field,' , p. ; for varro, _see_ pallas, _ut supra_. [ ] erman's 'travels in siberia,' eng. translat., vol. i. p. . [ ] _see_ also 'journal of r. geograph. soc.,' vol. xiii. part i. p. . [ ] livingstone's 'first travels,' pp. , , ; _see_ also 'expedition to the zambesi,' , p. , for an analogous case respecting a good breed of goats. [ ] andersson's 'travels in south africa,' pp. , , . [ ] dr. vavasseur, in 'bull. de la soc. d'acclimat.,' tom. viii., , p. . [ ] 'the natural history of dee side,' , p. . [ ] 'bull. de la soc. d'acclimat.,' tom. vii., , p. . [ ] 'cattle,' p. . [ ] livingstone's travels, p. ; andersson, 'lake ngami,' , p. . with respect to the sale in kaffraria, _see_ 'quarterly review,' , p. . [ ] 'mémoire sur les chinois' (by the jesuits), , tom. xi. p. . [ ] f. michel, 'des haras,' pp. , . [ ] col. hamilton smith, dogs, in 'nat. lib.,' vol. x. p. . [ ] azara, 'quadrupèdes du paraguay,' tom. ii. p. . [ ] sidney's edit. of youatt, , pp. , . [ ] 'rural economy of yorkshire,' vol. ii. p. . [ ] moll et gayot, 'du boeuf,' , p. . [ ] 'the india sporting review,' vol. ii. p. ; 'the stud farm,' by cecil, p. . [ ] 'the horse,' p. . [ ] 'history of england,' vol. i. p. . [ ] 'uber beständigkeit der arten.' [ ] youatt on sheep, p. . [ ] 'ueber shorthorn rindvieh,' , s. . [ ] low, 'domesticated animals,' , p. . [ ] 'quarterly review,' , p. . [ ] h. von nathusius, 'vorstudien ... schweineschædel,' , s. . [ ] _see_ also dr. christ, in 'rütimeyer's pfahlbauten,' , s. . [ ] the passage is given 'bull. soc. d'acclimat.,' , p. . [ ] 'journal of horticulture,' , p. . [ ] 'gardener's chronicle,' , p. . [ ] _see_ mr. wildman's address to the floricult. soc., in 'gardener's chronicle,' , p. . [ ] 'journal of horticulture,' oct. th, , p. . [ ] prescott's 'hist. of mexico,' vol. ii. p. . [ ] sageret, 'pomologie physiologique,' , p. ; gallesio, 'teoria della riproduzione,' , p. ; godron, 'de l'espèce,' , tom. ii. pp. , , . in my tenth and eleventh chapters i have given details on the potato; and i can confirm similar remarks with respect to the onion. i have also shown how far naudin concurs in regard to the varieties of the melon. [ ] godron, 'de l'espèce,' tom. ii. p. . [ ] 'the anthropological treatises of blumenbach,' , p. . [ ] mr. j. j. murphy in his opening address to the belfast nat. hist. soc., as given in the belfast northern whig, nov. , . mr. murphy here follows the line of argument against my views previously and more cautiously given by the rev. c. pritchard, pres. royal astronomical soc., in his sermon (appendix, p. ) preached before the british association at nottingham, . [ ] on the vision of fishes and amphibia, translated in 'annals and mag. of nat. hist.,' vol. xviii., , p. . [ ] fourth edition, , p. . [ ] quoted by youatt on sheep, p. . _see_ also youatt on cattle, pp. , . [ ] mm. lherbette and de quatrefages, in 'bull. soc. acclimat.,' tom. viii., , p. . [ ] 'the poultry book,' , p. . [ ] youatt on sheep, p. . [ ] 'treatise on the almond tumbler,' , p. . [ ] dr. heusinger, 'wochenschrift für die heilkunde,' berlin, , s. . [ ] youatt on the dog, p. . [ ] 'the fruit-trees of america,' , p. : for peaches, p. . [ ] 'proc. royal soc. of arts and sciences of mauritius,' , p. cxxxv. [ ] 'gardener's chronicle,' , p. . [ ] quatrefages, 'maladies actuelles du ver à soie,' , pp. , . [ ] 'gardener's chronicle,' , p. . [ ] 'journal of horticulture,' , p. . [ ] 'gardener's chronicle,' , pp. , . [ ] bechstein, 'naturgesch. deutschlands,' , b. i. s. . [ ] prichard, 'phys. hist. of mankind,' , vol. i. p. . [ ] g. lewis's 'journal of residence in west indies,' 'home and col. library,' p. . [ ] sidney's edit. of youatt on the pig, p. . [ ] 'journal of horticulture,' , pp. , ; , p. . with respect to the heartsease, 'gardener's chronicle,' , p. . [ ] 'des jacinthes, de leur culture,' , p. : on wheat, 'gardener's chronicle,' , p. . [ ] w. b. tegetmeier, 'the field,' feb. , . with respect to black fowls, _see_ a quotation in thompson's 'nat. hist. of ireland,' , vol. i. p. . [ ] 'bull. de la soc. d'acclimat.,' tom. vii. , p. . [ ] 'transact. hort. soc.,' vol. i. nd series, , p. . for raspberries, _see_ 'gard. chronicle,' , p. , and , p. . [ ] 'gardener's chronicle,' , p. . [ ] ibid., , p. . [ ] ibid., , p. . [ ] j. de jonghe, in 'gard. chronicle,' , p. . [ ] downing, 'fruit-trees of north america,' pp. , : in regard to the cherry, p. . [ ] 'gardener's chronicle,' , p. . [ ] 'journal of horticulture,' sept. th, , p. ; _see_ other references given in chap. x. [ ] mr. selby, in 'mag. of zoology and botany,' edinburgh, vol. ii., , p. . [ ] the reine claude de bavay, 'journal of horticulture,' dec. , , p. . [ ] mr. pusey, in 'journal of r. agricult. soc., vol. vi. p. . for swedish turnips, _see_ 'gard. chron.,' , p. . [ ] godron, 'de l'espèce,' tom. ii. p. . [ ] 'gardener's chron.,' , p. . [ ] 'gardener's chronicle,' , pp. , . [ ] 'on the varieties of wheat,' p. . [ ] mr. hewitt and others, in 'journal of hort.,' , p. . [ ] 'encyclop. of rural sports,' p. . [ ] col. le couteur, 'journal roy. agricult. soc.,' vol. iv. p. . [ ] 'gardener's chronicle,' , p. . [ ] 'journal of horticulture,' , p. . [ ] 'cottage gardener,' , p. . [ ] 'a review of reports,' , p. . [ ] 'gardener's chronicle,' , p. . [ ] isidore geoffroy st. hilaire, 'hist. nat. gén.,' tom. iii. p. . on the cochineal insect, p. . [ ] capt. marryat, quoted by blyth in 'journ. asiatic soc. of bengal,' vol. xxviii. p. . [ ] mr. oxley, 'journal of the indian archipelago,' vol. ii., , p. . [ ] mr. abbey, in 'journal of horticulture,' dec. , , p. . [ ] 'on naval timber,' , p. . [ ] mr. baily, in 'the poultry chronicle,' vol. ii., , p. . also vol. i. p. ; vol. iii. p. . [ ] 'cottage gardener,' , december, p. ; , january, pp. , . [ ] 'ueber shorthorn rindvieh,' , s. . [ ] 'the veterinary,' vol. xiii. p. . for the glamorganshire cattle, _see_ youatt on cattle, p. . [ ] j. m. eaton, 'a treatise on fancy pigeons,' p. ; ferguson, on 'rare and prize poultry,' p. ; mr. brent, in 'cottage gardener,' oct. . p. . [ ] 'die racen des schweines,' , s. . [ ] _see_ some good remarks on this head by m. de quatrefages, 'unité de l'espèce humaine,' , p. . [ ] verlot, 'des variétés,' , p. . [ ] mr. patrick sheriff, in 'gard. chronicle,' , p. . [ ] 'pomologie physiolog.,' , p. . [ ] youatt on sheep, p. . [ ] 'a treatise on the almond tumbler,' p. i. [ ] m. j. de jonghe, in 'gard. chron.,' , p. . [ ] max. müller, 'science of language,' , p. . [ ] youatt on cattle, pp. , . [ ] 'domesticated animals,' p. . [ ] volz, 'beiträge zur kulturgeschichte,' , s. _et passim_. [ ] blaine, 'encyclop. of rural sports,' p. . [ ] 'des jacinthes,' &c., amsterdam, , p. ; verlot, 'des variétés,' &c., p. . on the reindeer, _see_ linnæus, 'tour in lapland,' translated by sir j. e. smith, vol. i. p. . the statement in regard to german shepherds is given on the authority of dr. weinland. [ ] müller's 'physiology,' eng. translation, vol. ii. p. . with respect to the similarity of twins in constitution, dr. william ogle has given me the following extract from professor trousseau's lectures ('clinique médicale,' tom. i. p. ), in which a curious case is recorded:--"j'ai donné mes soins à deux frères jumeaux, tous deux si extraordinairement ressemblants qu'il m'était impossible de les reconnaître, à moins de les voir l'un à côté de l'autre. cette ressemblance physique s'étendait plus loin: ils avaient, permettez-moi l'expression, une similitude pathologique plus remarquable encore. ainsi l'un d'eux que je voyais aux néothermes à paris malade d'une ophthalmie rhumatismale me disait, 'en ce moment mon frère doit avoir une ophthalmie comme la mienne;' et comme je m'étais récrié, il me montrait quelques jours après une lettre qu'il venait de recevoir de ce frère alors à vienne, et qui lui écrivait en effet--'j'ai mon ophthalmie, tu dois avoir la tienne.' quelque singulier que ceci puisse paraître, le fait non est pas moins exact: on ne me l'a pas raconté, je l'ai vu, et j'en ai vu d'autres analogues dans ma pratique. ces deux jumeaux étaient aussi tous deux asthmatiques, et asthmatiques à un effroyable degré. originaires de marseille, ils n'ont jamais pu demeurer dans cette ville, où leurs intérêts les appelaient souvent, sans être pris de leurs accès; jamais ils n'en éprouvaient à paris. bien mieux, il leur suffisait de gagner toulon pour être guéris de leurs attaques de marseilles. voyageant sans cesse et dans tous pays pour leurs affaires, ils avaient remarqué que certaines localités leur étaient funestes, que dans d'autres ils étaient exempts de tout phénomène d'oppression." [ ] isid. geoffroy st. hilaire, 'hist. des anomalies,' tom. iii. p. ; moquin tandon, 'tératologie végétale,' , p. . [ ] metzger, 'die getreidearten,' , s. . [ ] on the date-palm, _see_ vogel, 'annals and mag. of nat. hist.,' , p. . on indian varieties, dr. f. hamilton, 'transact. linn. soc.,' vol. xiv. p. . on the varieties cultivated in tahiti, _see_ dr. bennett, in loudon's 'mag. of n. hist.,' vol. v., , p. . also ellis, 'polynesian researches,' vol. i. pp. , . on twenty varieties of the pandanus and other trees in the marianne island, _see_ 'hooker's miscellany,' vol. i. p. . on the bamboo in china, _see_ huc's 'chinese empire,' vol. ii. p. . [ ] 'treatise on the culture of the apple,' &c., p. . [ ] gallesio, 'teoria della riproduzione veg.,' p. . [ ] _see_ dr. hooker's memoir on arctic plants in 'linn. transact.,' vol. xxiii, part ii. mr. woodward, and a higher authority cannot be quoted, speaks of the arctic mollusca (in his 'rudimentary treatise,' , p. ) as remarkably subject to variation. [ ] bechstein, in his 'naturgeschichte der stubenvögel,' , s. , has some good remarks on this subject. he states that his canary-birds varied in colour, though kept on uniform food. [ ] 'the plant,' by schleiden, translated by henfrey, , p. . _see_ also alex. braun, in 'bot. memoirs,' ray. soc., , p. . [ ] messrs. hardy and son, of maldon, in 'gard. chronicle,' , p. . [ ] 'quadrupèdes du paraguay,' , tom. ii. p. . [ ] mcclelland on indian cyprinidæ, 'asiatic researches,' vol. xix. part ii., , pp. , , . [ ] quoted by sageret, 'pom. phys.,' , p. . [ ] 'the fruits of america,' , p. . [ ] m. cardan, in 'comptes rendus,' dec. , quoted in 'gard. chronicle,' , p. . [ ] m. alexis jordan mentions four excellent pears found in woods in france, and alludes to others ('mém. acad. de lyon,' tom. ii. , p. ). poiteau's remark is quoted in 'gardener's mag.,' vol. iv., , p. . _see_ 'gard. chronicle,' , p. , for another case of a new variety of the pear found in a hedge in france. also for another case, _see_ loudon's 'encyclop. of gardening,' p. . mr. rivers has given me similar information. [ ] duval, 'hist. du poirier,' , p. . [ ] i infer that this is the fact from van mons' statement ('arbres fruitiers,' , tom. i. p. ) that he finds in the woods seedlings resembling all the chief cultivated races of both the pear and apple. van mons, however, looked at these wild varieties as aboriginal species. [ ] downing, 'fruit-trees of north america,' p. ; foley, in 'transact. hort. soc.,' vol. vi. p. . [ ] 'gard. chronicle,' , p. . [ ] 'gardener's chronicle,' , p. ; , p. ; , p. . [ ] 'die getreidearten,' , s. , , . [ ] sabine, in 'hort. transact.,' vol. iii. p. ; bronn, 'geschichte der natur,' b. ii. s. . [ ] 'journal of horticulture,' , p. ; on zinnia, 'gardener's chronicle,' , p. . [ ] 'the chrysanthemum, its history, &c.,' , p. . [ ] 'gardener's chron.,' , p. ; 'journal of horticulture,' may , , p. . [ ] quoted by verlot, 'des variétés,' &c., , p. . [ ] 'examination of the characteristics of genera and species:' charleston, , p. . [ ] mr hewitt, 'journal of hort.,' , p. . [ ] devay, 'mariages consanguins,' pp. , . in conversation i have found two or three naturalists of the same opinion. [ ] müller has conclusively argued against this belief, 'elements of phys.,' eng. translat., vol. ii., , p. . [ ] 'act. acad. st. petersburg,' , part ii. p. , &c. [ ] 'bastarderzeugung,' s. , , . [ ] 'nova acta, st. petersburg,' , p. ; , pp. , , ; , p. . [ ] 'de la fécondation,' , p. . [ ] 'amaryllidaceæ,' , p. . [ ] abstracted in 'gard. chronicle,' , p. . [ ] this was the opinion of the elder de candolle, as quoted in 'dic. class. d'hist. nat.,' tom. viii. p. . puvis, in his work, 'de la dégénération,' , p. , has discussed this same point. [ ] 'comptes rendus,' novembre , , p. . [ ] 'nova acta, st. petersburg,' , p. . [ ] 'bastarderzeugung,' s. , , . [ ] 'die bastardbefruchtung,' &c., , s. . [ ] 'bastarderzeugung,' s. , . [ ] 'die bastardbefruchtung,' s. . [ ] 'bastarderzeugung,' s. . [ ] 'dritte fortsetzung,' &c., , s. . [ ] 'die bastardbefruchtung,' &c., , s. ; _see_ also the rev. m. j. berkeley on the same subject, in 'journal of royal hort. soc.,' , p. . [ ] dr. p. lucas has given a history of opinion on this subject: 'héréd. nat.,' , tom. i. p. . [ ] 'hist. des anomalies,' tom. iii. p. . [ ] idem., tom. iii. pp. , . [ ] _see_ his interesting work, 'métamorphoses de l'homme,' &c., , p. . [ ] 'dritte fortsetzung,' &c., s. ; 'bastarderzeugung,' s. . [ ] 'gardener's chronicle,' , p. . [ ] mr. wildman, 'floricultural soc.,' feb. , , reported in 'gard. chron.,' , p. . [ ] mr. robson, in 'journal of horticulture,' feb. th, , p. . [ ] 'journal of horticulture,' , p. . [ ] ibid., , p. . [ ] 'gard. chron.,' , p. . [ ] ibid., , p. . [ ] 'journal of hort.,' , pp. , . [ ] 'des variétés,' &c., p. . [ ] engel, 'sur les prop. médicales des plantes,' , pp. , . on changes in the odours of plants, _see_ dalibert's experiments, quoted by beckman, 'inventions,' vol. ii. p. ; and nees, in ferussac, 'bull. des sc. nat.,' , tom. i. p. . with respect to the rhubarb, &c., _see_ also 'gardener's chronicle,' , p. ; , p. . [ ] hooker, 'flora indica,' p. . [ ] naudin, 'annales des sc. nat.,' th series, bot., tom. xi., , p. . 'gardener's chronicle,' , p. . [ ] moorcroft's 'travels,' &c., vol. ii. p. . [ ] 'gardener's chronicle,' , p. . [ ] royle, 'productive resources of india,' p. . [ ] 'personal narrative,' eng. translat., vol. v. p. . this statement has been confirmed by karsten ('beitrag zur kenntniss der rhynchoprion:' moscow, . s. ), and by others. [ ] 'organic chemistry,' eng. translat., st edit., p. . [ ] prichard, 'phys. hist. of mankind,' , vol. i. p. . [ ] darwin, 'journal of researches,' , p. . [ ] these statements on disease are taken from dr. boudin's 'géographie et de statistique médicales,' , tom. i. p. xliv. and lii.; tom. ii. p. . [ ] e. desor, quoted in the 'anthrop. rev.,' , p. . for much confirmatory evidence, _see_ quatrefages, 'unité de l'espèce humaine,' , p. . [ ] 'ceylon,' by sir j. e. tennent, vol. i., , p. . [ ] godron, 'de l'espèce,' tom. ii. p. . [ ] 'journal of horticultural soc.,' vol. vii., , p. . [ ] 'journal of hort. soc.,' vol. i. p. . [ ] _see_ lecoq on the villosity of plants, 'geograph. bot.,' tom. iii. pp. , ; gärtner, 'bastarderz.,' s. ; mr. musters, on the opuntia, in 'gard. chronicle,' , p. . [ ] 'pom. phys.,' p. . [ ] 'ampelographie,' , p. . [ ] gärtner, 'bastarderz.,' s. , has collected nearly all recorded facts. andrew knight (in 'transact. hort. soc.,' vol. ii. p. ) goes so far as to maintain that few varieties are absolutely permanent in character when propagated by buds or grafts. [ ] mr. blyth, in 'annals and mag. of nat. hist.,' vol. xx., , p. . [ ] 'natural history review,' , p. . [ ] 'journal of roy. geographical soc.,' vol. ix., , p. . [ ] 'travels in bokhara,' vol. iii. p. . [ ] _see_ also, on the influence of marshy pastures on the wool, godron, 'l'espèce,' tom. ii. p. . [ ] isidore geoffroy st. hilaire, 'hist. nat. gén.,' tom. iii. p. . [ ] azara has made some good remarks on this subject, 'quadrupèdes du paraguay,' tom. ii. p. . _see_ an account of a family of naked mice produced in england, 'proc. zoolog. soc.,' , p. . [ ] 'die fauna der pfahlbauten,' , s. . [ ] 'schweinschædel,' , s. . [ ] 'travels in siberia,' eng. translat., vol. i. p. . [ ] a. r. wallace, 'travels on the amazon and rio negro,' p. . [ ] 'naturgeschichte der stubenvögel,' , s. , . [ ] 'hist. nat. gén.,' tom. iii. p. . [ ] 'bull. de la soc. imp. d'acclimat.,' tom. viii. p. . [ ] _see_ an account of mr. gregson's experiments on the _abraxus grossulariata_, 'proc. entomolog. soc.,' jan. th, : these experiments have been confirmed by mr. greening, in 'proc. of the northern entomolog. soc.,' july th, . for the effects of food on caterpillars, see a curious account by m. michely, in 'bull. de la soc. imp. d'acclimat.,' tom. viii. p. . for analogous facts from dahlbom on hymenoptera, _see_ westwood's 'modern class. of insects,' vol. ii. p. . _see_ also dr. l. möller, 'die abhängigkeit der insecten,' , s. . [ ] 'the principles of biology,' vol. ii. . the present chapters were written before i had read mr. herbert spencer's work, so that i have not been able to make so much use of it as i should otherwise probably have done. [ ] 'proc. acad. nat. soc. of philadelphia,' jan. th, . [ ] _see_ mr. b. d. walsh's excellent papers in 'proc. entomolog. soc. philadelphia,' dec. , p. . with respect to the willow, _see_ idem, , p. . [ ] _see_ his admirable histoire des galles, in 'annal. des sc. nat. bot.,' rd series, tom. xix., , p. . [ ] kirby and spence's 'entomology,' , vol. i. p. ; lucaze-duthiers, idem, p. . [ ] 'proc. entomolog. soc. philadelphia,' , p. . [ ] mr. b. d. walsh, idem, p. ; and dec. , p. . [ ] mr. b. d. walsh, idem, , p. , , ; and dec. , p. . _see_ also lucaze-duthiers. [ ] lucaze-duthiers, idem, pp. , . [ ] 'linnæa,' vol. xvii., ; quoted by dr. m. t. masters, royal institution, march th, . [ ] hewett c. watson, 'cybele britannica,' vol. i., , p. . [ ] 'gardener's chronicle,' , p. . [ ] 'mémoire sur la production artificielle des monstrosités,' , pp. - ; 'recherches sur les conditions, &c., chez les monstres,' , p. . an abstract is given of geoffroy's experiments by his son, in his 'vie, travaux, &c.,' , p. . [ ] paget, 'lectures on surgical pathology,' , vol. i. p. . [ ] 'researches upon the venom of the rattle-snake,' jan. , by dr. mitchell, p. . [ ] mr. sedgwick, in 'british and foreign medico-chirurg. review,' july , p. . [ ] 'an essay on generation,' eng. translat., p. ; paget, 'lectures on surgical pathology,' , vol. i. p. . [ ] 'an essay on animal reproduction,' eng. translat., , p. . [ ] carpenter's 'principles of comp. physiology,' , p. . [ ] charlesworth's 'mag. of nat. hist.,' vol. i., , p. . [ ] paget, 'lectures on surgical pathology,' vol. i. p. . [ ] quoted by carpenter, 'comp. phys.,' p. . [ ] paget, 'lectures,' &c., p. . [ ] these cases are given by blumenbach in his 'essay on generation,' pp. , . [ ] 'cellular pathology,' trans. by dr. chance, , pp. , . [ ] paget, 'lectures on pathology,' vol. i., , p. . [ ] paget, idem, p. . [ ] 'the principles of biology,' vol. ii., , chap. - . [ ] 'lectures on pathology,' , vol. i. p. . [ ] 'comptes rendus,' sept. th, , p. . [ ] 'the principles of biology,' vol. ii. p. . [ ] idem, vol. ii. p. . [ ] idem, vol. ii. p. . [ ] paget, 'lectures on pathology,' vol. ii. p. . [ ] müller's 'phys.,' eng. translat., pp. , . prof. reed has given ('physiological and anat. researches,' p. ) a curious account of the atrophy of the limbs of rabbits after the destruction of the nerve. [ ] quoted by lecoq, in 'geograph. bot.,' tom. i., , p. . [ ] 'das abändern der vögel,' , s. . [ ] nathusius, 'die racen des schweines,' , s. , ; 'vorstudien ... schweineschædel,' , s. , , . [ ] 'journal of agriculture of highland soc.,' july, , p. . [ ] 'principles of biology,' vol. ii. p. . [ ] 'natural history review,' vol. iv., oct. , p. . [ ] 'lectures on surgical pathology,' , vol. i. p. . [ ] andersson, 'travels in south africa,' p. . for analogous cases in south america, _see_ aug. st. hilaire, 'voyage dans le province de goyaz,' tom. i. p. . [ ] brickell's 'nat. hist. of north carolina,' , p. . [ ] livingstone, quoted by youatt on sheep, p. . hodgson, in 'journal of asiatic soc. of bengal,' vol. xvi., , p. , &c. &c. [ ] 'naturalist library,' dogs, vol. ii. , p. . [ ] 'de l'espèce,' tom. i., , p. . [ ] 'ceylon,' by sir j. e. tennent, , vol. ii. p. . [ ] for the foregoing statements, _see_ hunter's 'essays and observations,' , vol. ii. p. ; dr. edmondston, as quoted in macgillivray's 'british birds,' vol. v. p. ; menetries, as quoted in bronn's 'geschichte der natur,' b. ii. s. . [ ] these statements on the intestines are taken from isidore geoffroy st. hilaire, 'hist. nat. gén.,' tom. iii. pp. , . [ ] gilbert white, 'nat. hist. selbourne,' , vol. ii. p. . [ ] burdach, 'traité de phys.,' tom. ii. p. , as quoted by dr. p. lucas, 'l'héréd. nat.,' tom. i. p. . [ ] this and several other cases are given by colin, 'physiologie comp. des animaux dom.,' , tom. i. p. . [ ] m. michely de cayenne, in 'bull. soc. d'acclimat.,' tom. viii., , p. . [ ] quatrefages, 'unité de l'espèce humaine,' , p. . [ ] 'flora,' , b. ii. p. . [ ] alph. de candolle, 'géograph. bot.,' tom. ii. p. . [ ] royle, 'illustrations of the botany of the himalaya,' p. . [ ] 'gardener's chronicle,' , pp. , . [ ] rev. r. everest, 'journal as. soc. of bengal,' vol. iii. p. . [ ] youatt on sheep, , p. . [ ] royle, 'prod. resources of india,' p. . [ ] tegetmeier, 'poultry book,' , p. . [ ] dr. r. paterson, in a paper communicated to bot. soc. of canada, quoted in the 'reader,' . nov. th. [ ] _see_ remarks by editor in 'gard. chronicle,' , p. . [ ] 'gard. chronicle,' , p. . remarks by editor and quotation from decaisne. [ ] j. de jonghe, of brussels, in 'gard. chronicle,' , p. . [ ] ch. martius, 'voyage bot. côtes sept. de la norvège,' p. . [ ] 'journal de l'acad. hort. de gand,' quoted in 'gard. chron.,' , p. . [ ] 'gard. chronicle,' , p. . [ ] idem., , p. . [ ] on the authority of labat, quoted in 'gard. chron.,' , p. . [ ] mm. edwards and colin, 'annal. des sc. nat.,' nd series, bot., tom. v. p. . [ ] 'géograph. bot.,' p. . [ ] 'swedish acts,' eng. translat., - , vol. i. kalm, in his 'travels,' vol. ii. p. , gives an analogous case with cotton-plants raised in new jersey from carolina seed. [ ] de candolle, 'géograph. bot.,' p. . [ ] 'gard. chronicle,' , p. . [ ] gallesio, 'teoria della riproduzione veg.,' , p. ; and 'traité du citrus,' , p. . [ ] 'essai sur l'hist. des orangers,' , p. , &c. [ ] alph. de candolle, 'géograph. bot.,' p. . [ ] 'ch. darwin's lehre von der entstehung,' &c., , s. . [ ] decaisne, quoted in 'gard. chronicle,' , p. . [ ] for the magnolia, _see_ loudon's 'gard. mag.,' vol. xiii., , p. . for camellias and roses, _see_ 'gard. chron.,' , p. . for the yew, 'journal of hort.,' march rd, , p. . for sweet potatoes, _see_ col. von siebold, in 'gard. chron.,' , p. . [ ] the editor, 'gard. chron.,' , p. . [ ] loudon's 'gard. mag.,' vol. xii., , p. . [ ] 'gardeners chron.,' , p. . [ ] 'arboretum et fruticetum,' vol. iii. p. . [ ] mr. robson, in 'journal of horticulture,' , p. . [ ] dr. bonavia, 'report of the agri.-hort. soc. of oudh,' . [ ] 'cottage gardener,' , april, th, p. . [ ] 'gardener's chronicle,' , p. . [ ] mr. beaton, in 'cottage gardener,' march th, , p. . queen mab will also stand stove heat, _see_ 'gard. chronicle,' , p. . [ ] 'gardener's chronicle,' , p. . [ ] quoted by asa gray, in 'am. journ. of sci.,' nd series, jan. , p. . [ ] for china, _see_ 'mémoire sur les chinois,' tom, xi., , p. . columella is quoted by carlier, in 'journal de physique,' tom. xxiv. . [ ] messrs. hardy and son, in 'gard. chronicle,' , p. . [ ] isid. geoffroy st. hilaire, 'hist. nat. des anomalies,' , tom. ii. pp. , , , ; 'philosoph. transact.,' , p. . [ ] pallas, quoted by youatt on sheep, p. . [ ] youatt on cattle, , p. . [ ] 'encyclop. méthod.,' , p. : _see_ p. , on the indian zebu casting its horns. similar cases in european cattle were given in the third chapter. [ ] pallas, 'travels,' eng. translat., vol. i. p. . [ ] mr. beaton, in 'journal of horticulture,' may , , p. . [ ] lecoq, 'de la fécondation,' , p. . [ ] 'annales du muséum,' tom. vi. p. . [ ] 'hist. des anomalies,' tom. iii. p. . prof. huxley applies the same principle in accounting for the remarkable, though normal, differences in the arrangement of the nervous system in the mollusca, in his great paper on the morphology of the cephalous mollusca, in 'phil. transact.,' , p. . [ ] 'eléments de tératologie veg.,' , p. . [ ] prof. j. b. simonds, on the age of the ox, sheep, &c., quoted in 'gard. chronicle,' , p. . [ ] 'hist. des anomalies,' tom. i. p. . [ ] quoted by isid. geoffroy, idem, tom. i. p. . [ ] 'the poultry book,' by w. b. tegetmeier, , p. . [ ] a. walker on intermarriage, , p. . [ ] 'the farrier and naturalist,' vol. i., , p. . [ ] godron, 'sur l'espèce,' tom. ii. p. . [ ] 'quadrupèdes du paraguay,' tom. ii. p. . [ ] on sheep, p. . [ ] 'ueber racen, kreuzungen, &c.,' , s. . [ ] quoted from conolly, in 'the indian field,' feb. , vol. ii. p. . [ ] 'domesticated animals of the british islands,' pp. , . [ ] 'proceedings zoolog. soc.,' , p. . [ ] sedgwick, 'brit. and foreign medico-chirurg. review,' april , p. . [ ] 'gard. chronicle,' , p. . [ ] 'embassy to the court of ava,' vol. i. p. . [ ] 'narrative of a mission to the court of ava in ,' p. . [ ] those statements are taken from mr. sedgwick, in the 'medico-chirurg. review,' july , p. ; april , pp. and . liebreich is quoted by professor devay, in his 'mariages consanguins,' , p. . [ ] loudon's 'mag. of nat. hist.,' vol. i., , pp. , . _see_ also dr. p. lucas, 'l'héréd. nat.,' tom. i. p. , on the inheritance of deafness in cats. [ ] 'annales des sc. nat.' zoolog., rd series, , tom. viii. p. . [ ] 'gardener's chron.,' , p. . [ ] verlot gives several other instances, 'des variétés,' , p. . [ ] 'arbres fruitiers,' , tom. ii. pp. , . [ ] 'annales du muséum,' tom. xx. p. . [ ] 'gardener's chron.,' , p. . [ ] ibid., , p. . [ ] 'hist. des anomalies,' tom. iii. p. . _see_ also m. camille dareste, 'recherches sur les conditions,' &c., , pp. , . [ ] rev. e. s. dixon, 'ornamental poultry,' , p. ; isidore geoffroy, 'hist. anomalies,' tom. i. p. . [ ] 'on the breeding of domestic animals,' , p. . [ ] youatt on cattle, , p. . [ ] mr. herbert spencer ('principles of biology,' , vol. i. pp. , ) takes a different view; and in one place remarks: "we have seen reason to think that, as fast as essential faculties multiply, and as fast as the number of organs that co-operate in any given function increases, indirect equilibration through natural selection becomes less and less capable of producing specific adaptations; and remains fully capable only of maintaining the general fitness of constitution to conditions." this view that natural selection can do little in modifying the higher animals surprises me, seeing that man's selection has undoubtedly effected much with our domesticated quadrupeds and birds. [ ] dr. prosper lucas apparently disbelieves in any such connexion, 'l'héréd. nat.,' tom. ii. pp. - . [ ] 'british medical journal,' , p. . [ ] boudin, 'geograph. médicale,' tom. i. p. . [ ] this fact and the following cases, when not stated to the contrary, are taken from a very curious paper by prof. heusinger, in 'wochenschrift für heilkunde,' may , s. . [ ] mr. mogford, in the 'veterinarian,' quoted in 'the field,' jan. , , p. . [ ] 'edinburgh veterinary journal,' oct. , p. . [ ] 'hist. des anomalies,' , tom. i. pp. , - ; tom. iii. p. . [ ] 'comptes rendus,' , pp. , . [ ] carpenter's 'comp. phys.,' , p. ; _see_ also camille dareste, 'comptes rendus,' march th, , p. . [ ] 'elements of physiology,' eng. translat, vol. i., , p. . with respect to vrolik, _see_ todd's 'cyclop. of anat. and phys.,' vol. iv., - , p. . [ ] 'tératologie vég.,' , livre iii. [ ] 'hist. des anomalies,' tom. iii. pp. , , . [ ] 'tératologie vég.,' p. . _see_ also my paper on climbing plants in 'journal of linn. soc. bot.,' vol. ix., , p. . [ ] 'mémoires du muséum,' &c., tom. viii. p. . [ ] loudon's 'encyclop. of gardening,' p. . [ ] prichard, 'phys. hist. of mankind,' , vol. i. p. . [ ] 'annales des sc. nat.,' st series, tom. xix. p. . [ ] 'comptes rendus,' dec. , p. . [ ] ueber fötale rachites, 'würzburger medicin. zeitschrift,' , b. i. s. . [ ] 'tératologie vég.,' p. . dr. m. masters informs me that he doubts the truth of this conclusion; but the facts to be given seem to be sufficient to establish it. [ ] 'journal of horticulture,' july nd, , p. . [ ] it would be worth trial to fertilise with the same pollen the central and lateral flowers of the pelargonium, and of some other highly cultivated plants, protecting them of course from insects: then to sow the seed separately, and observe whether the one or the other lot of seedlings varied the most. [ ] quoted in 'journal of horticulture,' feb. , , p. . [ ] 'gardener's chronicle,' , p. . for the phalænopsis, _see_ idem, , p. . [ ] mémoires ... des végétaux,' , tom. ii. p. . [ ] 'journal of horticulture,' july , , p. . [ ] 'nouvelles archives du muséum,' tom. i. p. . [ ] hugo von mohl, 'the vegetable cell,' eng. tr., , p. . [ ] the rev. h. h. dombrain, in 'journal of horticulture,' , june th, p. ; and june th, p. ; , april th, p. . [ ] 'transact. linn. soc.,' vol. xxiii., , p. . [ ] 'die getreidearten,' , s. , . [ ] 'gardener's chronicle,' , p. . [ ] quoted in 'gardener's chron.,' , p. . [ ] 'ueber den begriff der pflanzenart,' , s. . [ ] 'domesticated animals,' , p. . [ ] bechstein, 'naturgeschichte deutschlands,' band iv., , s. . [ ] 'proc. entomolog. soc. of philadelphia,' oct. , p. . [ ] quoted by paget, 'lectures on pathology,' , p. . [ ] dr. lachmann, also, observes ('annals and mag. of nat. history,' nd series, vol. xix., , p. ) with respect to infusoria, that "fissation and gemmation pass into each other almost imperceptibly." again, mr. w. c. minor ('annals and mag. of nat. hist.,' rd series, vol. xi. p. ) shows that with annelids the distinction that has been made between fission and budding is not a fundamental one. _see_ bonnet, 'oeuvres d'hist. nat.,' tom. v., , p. , for remarks on the budding-out of the amputated limbs of salamanders. _see_, also, professor clark's work 'mind in nature,' new york, , pp. , . [ ] paget, 'lectures on pathology,' , p. . [ ] idem, pp. , . [ ] on the asexual reproduction of cecydomyide larvæ, translated in 'annals and mag. of nat. hist.,' march , pp. , . [ ] _see_ some excellent remarks on this head by quatrefages, in 'annales des sc. nat.,' zoolog., rd series, , p. . [ ] 'annals and mag. of nat. hist.,' nd series, vol. xx., , pp. - . [ ] 'annales des sc. nat.,' rd series, , tom. xiii. [ ] 'transact. phil. soc.,' , pp. , , ; , p. , . [ ] 'beitrage zur kenntniss,' &c., , s. . [ ] 'nouvelles archives du muséum,' tom. i. p. . [ ] as quoted by sir j. lubbock in 'nat. hist. review,' , p. . [ ] 'transact. linn. soc.,' vol. xxiv., , p. . [ ] 'parthenogenesis,' , pp. - . prof. huxley has some excellent remarks ('medical times,' , p. ) on this subject, in reference to the development of star-fishes, and shows how curiously metamorphosis graduates into gemmation or zoid-formation, which is in fact the same as metagenesis. [ ] prof. j. reay greene, in günther's 'record of zoolog. lit.,' , p. . [ ] fritz müller's 'für darwin,' , s. , . the highest authority on crustaceans, prof. milne edwards, insists ('annal. des sci. nat.,' nd series, zoolog., tom. iii. p. ) on their metamorphoses differing even in closely allied genera. [ ] prof. allman, in 'annals and mag. of nat. hist.,' rd series, vol. xiii., , p. ; dr. s. wright, idem, vol. viii., , p. . _see_ also p. for analogous statements by sars. [ ] 'tissus vivants,' , p. . [ ] 'cellular pathology,' translat. by dr. chance, , pp. , , , . [ ] paget, 'surgical pathology,' vol. i., , pp. - . [ ] idem, p. . [ ] mantegazza, quoted in 'popular science review,' july , p. . [ ] 'de la production artificielle des os,' p. . [ ] isidore geoffroy st. hilaire, 'hist. des anomalies,' tom. ii. pp. , , ; virchow, idem, p. . [ ] for the most recent classification of cells, _see_ ernst häckel's 'generelle morpholog.,' band ii., , s. . [ ] 'the structure and growth of tissues,' , p. , &c. [ ] dr. w. turner, 'the present aspect of cellular pathology,' 'edinburgh medical journal,' april, . [ ] this term is used by dr. e. montgomery ('on the formation of so-called cells in animal bodies,' , p. ), who denies that cells are derived from other cells by a process of growth, but believes that they originate through certain chemical changes. [ ] prof. huxley has called my attention to the views of buffon and bonnet. the former ('hist. nat. gén.,' edit. of , tom. ii. pp. , , , , , ) supposes that organic molecules exist in the food consumed by every living creature; and that these molecules are analogous in nature with the various organs by which they are absorbed. when the organs thus become fully developed, the molecules being no longer required collect and form buds or the sexual elements. if buffon had assumed that his organic molecules had been formed by each separate unit throughout the body, his view and mine would have been closely similar. bonnet ('oeuvres d'hist. nat.,' tom. v., part i., , to edit., p. ) speaks of the limbs having germs adapted for the reparation of all possible losses; but whether these germs are supposed to be the same with those within the buds and sexual organs is not clear. his famous but now exploded theory of _emboîtement_ implies that perfect germs are included within germs in endless succession, pre-formed and ready for all succeeding generations. according to my view, the germs or gemmules of each separate part were not originally pre-formed, but are continually produced at all ages during each generation, with some handed down from preceding generations. prof. owen remarks ('parthenogenesis,' , pp. - ), "not all the progeny of the primary impregnated germ-cell are required for the formation of the body in all animals: certain of the derivative germ-cells may remain unchanged and become included in that body which has been composed of their metamorphosed and diversely combined or confluent brethren: so included, any derivative germ-cell, or the nucleus of such, may commence and repeat the same processes of growth by imbibition, and of propagation by spontaneous fission, as those to which itself owed its origin;" &c. by the agency of these germ-cells prof. owen accounts for parthenogenesis, for propagation by self-division during successive generations, and for the repairs of injuries. his view agrees with mine in the assumed transmission and multiplication of his germ-cells, but differs fundamentally from mine in the belief that the primary germ-cell was formed within the ovarium of the female and was fertilised by the male. my gemmules are supposed to be formed, quite independently of sexual concourse, by each separate cell or unit throughout the body, and to be merely aggregated within the reproductive organs. lastly, mr. herbert spencer ('principles of biology,' vol. i., - , chaps. iv. and viii.) has discussed at considerable length what he designates as physiological units. these agree with my gemmules in being supposed to multiply and to be transmitted from parent to child; the sexual elements are supposed to serve merely as their vehicles; they are the efficient agents in all the forms of reproduction and in the repairs of injuries; they account for inheritance, but they are not brought to bear on reversion or atavism, and this is unintelligible to me; they are supposed to possess polarity, or, as i call it, affinity; and apparently they are believed to be derived from each separate part of the whole body. but gemmules differ from mr. spencer's physiological units, inasmuch as a certain number, or mass of them, are, as we shall see, requisite for the development of each cell or part. nevertheless i should have concluded that mr. spencer's views were fundamentally the same with mine, had it not been for several passages which, as far as i understand them, indicate something quite different. i will quote some of these passages from pp. - . "in the fertilised germ we have two groups of physiological units, slightly different in their structures."... "it is not obvious that change in the form of the part, caused by changed action, involves such change in the physiological units throughout the organism, that these, when groups of them are thrown off in the shape of reproductive centres, will unfold into organisms that have this part similarly changed in form. indeed, when treating of adaptation, we saw that an organ modified by increase or decrease of function can but slowly so react on the system at large as to bring about those correlative changes required to produce a new equilibrium; and yet only when such new equilibrium has been established, can we expect it to be _fully_ expressed in the modified physiological units of which the organism is built--only then can we count on a complete transfer of the modification to descendants."... "that the change in the offspring must, other things equal, be in the same direction as the change in the parent, we may dimly see is implied by the fact, that the change propagated throughout the parental system is a change towards a new state of equilibrium--a change tending to bring the actions of all organs, reproductive included, into harmony with these new actions." [ ] m. philipeaux ('comptes rendus,' oct. , , p. , and june, ) has lately shown that when the entire fore-limb, including the scapula, is extirpated, the power of regrowth is lost. from this he concludes that it is necessary for regrowth that a small portion of the limb should be left. but as in the lower animals the whole body may be bisected and both halves be reproduced, this belief does not seem probable. may not the early closing of a deep wound, as in the case of the extirpation of the scapula, prevent the formation or protrusion of the nascent limb? [ ] 'annal. des sc. nat.,' rd series, bot., tom. xiv., , p. . [ ] _see_ some very interesting papers on this subject by prof. lionel beale, in 'medical times and gazette,' sept. th, , pp. , . [ ] third report of the r. comm. on the cattle plague, as quoted in 'gard. chronicle,' , p. . [ ] in a cod-fish, weighing lb., mr. f. buckland ('land and water,' , p. ) calculated the above number of eggs. in another instance, harmer ('phil. transact.,' , p. ) found , , eggs. for the ascaris, _see_ carpenter's 'comp. phys.,' , p. . mr. j. scott, of the royal botanic garden of edinburgh, calculated, in the same manner as i have done for some british orchids ('fertilisation of orchids,' p. ), the number of seeds in a capsule of an acropera, and found the number to be , . now this plant produces several flowers on a raceme and many racemes during a season. in an allied genus, gongora, mr. scott has seen twenty capsules produced on a single raceme: ten such racemes on the acropera would yield above seventy-four millions of seed. i may add that fritz müller informs me that he found in a capsule of a maxillaria, in south brazil, that the seed weighed ½ grains: he then arranged half a grain of seed in a narrow line, and by counting a measured length found the number in the half-grain to be , , so that in the capsule there must have been , , seeds! the same plant sometimes produces half-a-dozen capsules. [ ] 'annals and mag. of nat. hist.,' rd series, vol. viii., , p. . [ ] paget, 'lectures on pathology,' p. ; virchow, 'cellular pathology,' translat. by dr. chance, pp. , , ; claude bernard, 'des tissus vivants,' pp. , , ; müller's 'physiology,' eng. translat., p. . [ ] virchow, 'cellular pathology,' trans. by dr. chance, , pp. , , , , . [ ] idem, pp. - . [ ] _see_ rev. j. m. berkeley, in 'gard. chron.,' april th, , on a bud developed on the petal of the clarkia. _see_ also h. schacht, 'lehrbuch der anat.,' &c., , theile ii. s. , on adventitious buds. [ ] mr. herbert spencer ('principles of biology,' vol. ii. p. ) has fully discussed the antagonism between growth and reproduction. [ ] the male salmon is known to breed at a very early age. the triton and siredon, whilst retaining their larval branchiæ, according to filippi and duméril ('annals and mag. of nat. hist.,' rd series, , p. ), are capable of reproduction. ernst häckel has recently ('monatsbericht akad. wiss. berlin,' feb. nd, ) observed the surprising case of a medusa, with its reproductive organs active, which produces by budding a widely different form of medusa; and this latter also has the power of sexual reproduction. krohn has shown ('annals and mag. of nat. hist.,' rd series, vol. xix., , p. ) that certain other medusæ, whilst sexually mature, propagate by gemmæ. [ ] _see_ his excellent discussion on this subject in 'nouvelles archives du muséum,' tom. i. p. . [ ] various physiologists have insisted on this distinction between growth and development. prof. marshall ('phil. transact.,' , p. ) gives a good instance in microcephalous idiots, in which the brain continues to grow after having been arrested in its development. [ ] 'compte rendu,' nov. , , p. . [ ] as previously remarked by quatrefages, in his 'metamorphoses de l'homme,' &c., , p. . [ ] günther's 'zoological record,' , p. . [ ] sedgwick, in 'medico-chirurg. review,' april , p. . [ ] isid. geoffroy st. hilaire, 'hist. des anomalies,' tom. i., , pp. , ; and tom. ii. p. . [ ] virchow, 'cellular pathology,' , p. . [ ] moquin-tandon, 'tératologie veg.,' , pp. , , . for the case of the pea, _see_ 'gardener's chron.,' , p. . [ ] müller's 'physiology,' eng. translat., vol. i. p. . [ ] _see_ some remarks to this effect by sir h. holland in his 'medical notes,' , p. . [ ] this is the view taken by prof. häckel, in his 'generelle morphologie' (b. ii. s. ), who says: "lediglich die partielle identität der specifischconstituirten materie im elterlichen und im kindlichen organismus, die theilung dieser materie bei der fortpflanzung, ist die ursache der erblichkeit." [ ] in these remarks i, in fact, follow naudin, who speaks of the elements or essences of the two species which are crossed. see his excellent memoir in the 'nouvelles archives du muséum,' tom. i. p. . [ ] godron, 'de l'espèce,' , tom. ii. p. , &c. [ ] journal proc. linn. soc., , vol. iii. p. . [ ] 'the quarterly journal of science,' oct. , p. . [ ] m. rufz de lavison, in 'bull. soc. imp. d'acclimat.,' dec. , p. . [ ] 'races of man,' , p. . [ ] 'travels in peru,' eng. translat., p. . [ ] youatt on cattle, , p : on pigs; _see_ 'gard. chronicle,' , p. . [ ] 'die pflanzen der pfahlbauten,' . [ ] morlot, 'soc. vaud. des scien. nat,' mars , p. . [ ] rütimeyer, 'die fauna der pfahlbauten,' , s. . [ ] godron, 'de l'espèce,' tom. i., , p. . [ ] 'géographie botan.,' , p. . [ ] pickering, 'races of man,' , p. . [ ] 'journal of a horticultural tour,' by a deputation of the caledonian hist. soc., , p. . * * * * * corrections made to printed original. p. iii. "appearance with advancing age": 'arpearance' in original. p. vi. "slight changes sufficient": 'sufficent' in original. p. . "bearing in mind what has been said": 'bearnig' in original. p. . "not attached to any particular period": 'particuliar' in original. p. . "it permits innumerable individuals to be born": 'permitts' in original. p. . "liable to complete absorption": 'absortion' in original. p. . "found that when the animal was compelled ...": 'found than ...' in original. p. . "branches in a rudimentary condition": 'rudimentry' in original. p. . "force themselves into a minute orifice": 'into' was printed on next line in original, after 'must'.