DARWIN, AND AFTER DARWIN
AN EXPOSITION OF THE DARWINIAN THEORY AND A DISCUSSION OF POST-DARWINIAN QUESTIONS
BY
GEORGE JOHN ROMANES, M.A., LL.D., F.R.S.
Honorary Fellow of Gonville and Caius College, Cambridge
I
THE DARWINIAN THEORY
FOURTH EDITION

Chicago
THE OPEN COURT PUBLISHING COMPANY.
1910

The Illustrations of this book (with the exception of the Frontispiece and the colored plate facing page 332) are copyrighted under the title “Darwinism Illustrated.”

The Open Court Publishing Co.

PRESS OF THE
BLAKELY-OSWALD PRINTING CO.
CHICAGO

Ch. Ch. Oxford:
March 15th 1892.
My dear Sir,
As we have now agreed that
the Open Court Publishing Company is to
undertake the American edition of my
work entitled “Darwin and after
Darwin,” I have much pleasure
in transferring to you the copyright
thereof, with all that this
includes.
Thanking you very much for
the kindness and liberality which
have marked your conduct of these
negotiations,
I remain,
Yours very faithfully,
George J. Romanes
To
Edward C. Hegeler Esq.
La Salle, Ill. U. S.

PREFACE

Several years ago Lord Rosebery founded, in the University of Edinburgh, a lectureship on “The Philosophy of Natural History,” and I was invited by the Senatus to deliver the lectures. This invitation I accepted, and subsequently constituted the material of my lectures the foundation of another course, which was given in the Royal Institution, under the title “Before and after Darwin.” Here the course extended over three years—namely from 1888 to 1890. The lectures for 1888 were devoted to the history of biology from the earliest recorded times till the publication of the “Origin of Species” in 1859; the lectures for 1889 dealt with the theory of organic evolution up to the date of Mr. Darwin’s death, in 1882; while those of the third year discussed the further developments of this theory from that date till the close of the course in 1890.

It is from these two courses—which resembled each other in comprising between thirty and forty lectures, but differed largely in other respects—that the present treatise has grown. Seeing, however, that it has Grown much beyond the bulk of the original lectures, I have thought it desirable to publish the whole in the form of three separate works. Of these the first—or that which deals with the purely historical side of biological science—may be allowed to stand over for an indefinite time. The second is the one which is now brought out and which, as its sub-title signifies, is devoted to the general theory of organic evolution as this was left by the stupendous labours of darwin. as soon as the translations shall have been completed, the third portion will follow (probably in the autumn season), under the sub-title, “post-darwinian questions.”

As the present volume is thus intended to be merely a systematic exposition of what may be termed the Darwinism of Darwin, and as on this account it is likely to prove of more service to general readers than to professed naturalists, I have been everywhere careful to avoid assuming even the most elementary knowledge of natural science on the part of those to whom the exposition is addressed. The case, however, will be different as regards the next volume, where I shall have to deal with the important questions touching Heredity, Utility, Isolation, &c., which have been raised since the death of Mr. Darwin, and which are now being debated with such salutary vehemence by the best naturalists of our time.

My obligations to the Senatus of the University of Edinburgh, and to the Board of Management of the Royal Institution, have already been virtually expressed; but I should like to take this opportunity of also expressing my obligations to the students who attended the lectures in the University of Edinburgh. For alike in respect of their large numbers, their keen intelligence, and their generous sympathy, the members of that voluntary class yielded a degree of stimulating encouragement, without which the labour of preparing the original lectures could not have been attended with the interest and the satisfaction that I found in it. My thanks are also due to Mr. R. E. Holding for the painstaking manner in which he has assisted me in executing most of the original drawings with which this volume is illustrated; and likewise to Messrs. Macmillan and Co. for kindly allowing me to reprint—without special acknowledgment in every case—certain passages from an essay which they published for me many years ago, under the title “Scientific Evidences of Organic Evolution.” Lastly, I must mention that I am indebted to the same firm for permission to reproduce an excellent portrait of Mr. Darwin, which constitutes the frontispiece.

G. J. R.

Christ Church, Oxford,
April 19th, 1892.

CONTENTS

LIST OF ILLUSTRATIONS

SECTION I
EVOLUTION

CHAPTER I.
Introductory.

Among the many and unprecedented changes that have been wrought by Mr. Darwin’s work on the Origin of Species, there is one which, although second in importance to no other, has not received the attention which it deserves. I allude to the profound modification which that work has produced on the ideas of naturalists with regard to method.

Having had occasion of late years somewhat closely to follow the history of biological science, I have everywhere observed that progress is not so much marked by the march of discovery per se, as by the altered views of method which the march has involved. If we except what Aristotle called “the first start” in himself, I think one may fairly say that from the rejuvenescence of biology in the sixteenth century to the stage of growth which it has now reached in the nineteenth, there is a direct proportion to be found between the value of work done and the degree in which the worker has thereby advanced the true conception of scientific working. Of course, up to a certain point, it is notorious that the revolt against the purely “subjective methods” in the sixteenth century revived the spirit of inductive research as this had been left by the Greeks; but even with regard to this revolt there are two things which I should like to observe.

In the first place, it seems to me, an altogether disproportionate value has been assigned to Bacon’s share in the movement. At most, I think, he deserves to be regarded but as a literary exponent of the Zeitgeist of his century. Himself a philosopher, as distinguished from a man of science, whatever influence his preaching may have had upon the general public, it seems little short of absurd to suppose that it could have produced any considerable effect upon men who were engaged in the practical work of research. And those who read the Novum Organon with a first-hand knowledge of what is required for such research can scarcely fail to agree with his great contemporary Harvey, that he wrote upon science like a Lord Chancellor.

The second thing I should like to observe is, that as the revolt against the purely subjective methods grew in extent and influence it passed to the opposite extreme, which eventually became only less deleterious to the interests of science than was the bondage of authority, and addiction to a priori methods, from which the revolt had set her free. For, without here waiting to trace the history of this matter in detail, I think it ought now to be manifest to everyone who studies it, that up to the commencement of the present century the progress of science in general, and of natural history in particular, was seriously retarded by what may be termed the Bugbear of Speculation. Fully awakened to the dangers of web-spinning from the ever-fertile resources of their own inner consciousness, naturalists became more and more abandoned to the idea that their science ought to consist in a mere observation of facts, or tabulation of phenomena, without attempt at theorizing upon their philosophical import. If the facts and phenomena presented any such import, that was an affair for men of letters to deal with; but, as men of science, it was their duty to avoid the seductive temptations of the world, the flesh, and the devil, in the form of speculation, deduction, and generalization.

I do not allege that this ideal of natural history was either absolute or universal; but there can be no question that it was both orthodox and general. Even Linnæus was express in his limitations of true scientific work in natural history to the collecting and arranging of species of plants and animals. In accordance with this view, the status of a botanist or a zoologist was estimated by the number of specific names, natural habitats, &c., which he could retain in his memory, rather than by any evidences which he might give of intellectual powers in the way of constructive thought. At the most these powers might legitimately exercise themselves only in the direction of taxonomic work; and if a Hales, a Haller, or a Hunter obtained any brilliant results in the way of observation and experiment, their merit was taken to consist in the discovery of facts per se: not in any endeavours they might make in the way of combining their facts under general principles. Even as late in the day as Cuvier this ideal was upheld as the strictly legitimate one for a naturalist to follow; and although Cuvier himself was far from being always loyal to it, he leaves no doubt regarding the estimate in which he held the still greater deviations of his colleagues, St. Hilaire and Lamarck.

Now, these traditional notions touching the severance between the facts of natural history and the philosophy of it, continued more or less to dominate the minds of naturalists until the publication of the Origin of Species, in 1859. Then it was that an epoch was marked in this respect, as in so many other respects where natural history is concerned. For, looking to the enormous results which followed from a deliberate disregard of such traditional canons by Darwin, it has long since become impossible for naturalists, even of the strictest sect, not to perceive that their previous bondage to the law of a mere ritual has been for ever superseded by what verily deserves to be regarded as a new dispensation. Yet it cannot be said, or even so much as suspected, that Darwin’s method in any way resembled that of pre-scientific days, the revolt against which led to the straight-laced—and for a long time most salutary—conceptions of method that we have just been noticing. Where, then, is the difference? To me it seems that the difference is as follows; and, if so, that not the least of our many obligations to Darwin as the great organizer of biological science arises from his having clearly displayed the true principle which ought to govern biological research.

To begin with, he nowhere loses sight of the primary distinction between fact and theory; so that, thus far, he loyally follows the spirit of revolt against subjective methods. But, while always holding this distinction clearly in view, his idea of the scientific use of facts is plainly that of furnishing legitimate material for the construction of theories. Natural history is not to him an affair of the herbarium or the cabinet. The collectors and the species-framers are, as it were, his diggers of clay and makers of bricks: even the skilled observers and the trained experimentalists are his mechanics. Valuable as the work of all these men is in itself, its principal value, as he has finally demonstrated, is that which it acquires in rendering possible the work of the architect. Therefore, although he has toiled in all the trades with his own hands, and in each has accomplished some of the best work that has ever been done, the great difference between him and most of his predecessors consists in this,—that while to them the discovery or accumulation of facts was an end, to him it is the means. In their eyes it was enough that the facts should be discovered and recorded. In his eyes the value of facts is due to their power of guiding the mind to a further discovery of principles. And the extraordinary success which attended his work in this respect of generalization immediately brought natural history into line with the other inductive sciences, behind which, in this most important of all respects, she has so seriously fallen. For it was the Origin of Species which first clearly revealed to naturalists as a class, that it was the duty of their science to take as its motto, what is really the motto of natural science in general,

Felix qui potuit rerum cognoscere causas.

Not facts, then, or phenomena, but causes or principles, are the ultimate objects of scientific quest. It remains to ask, How ought this quest to be prosecuted?

Well, in the second place, Darwin has shown that next only to the importance of clearly distinguishing between facts and theories on the one hand, and of clearly recognising the relation between them on the other, is the importance of not being scared by the Bugbear of Speculation. The spirit of speculation is the same as the spirit of science, namely, as we have just seen, a desire to know the causes of things. The hypotheses non fingo of Newton, if taken to mean what it is often understood as meaning, would express precisely the opposite spirit from that in which all scientific research must necessarily take its origin. For if it be causes or principles, as distinguished from facts or phenomena, that constitute the final aim of scientific research, obviously the advancement of such research can be attained only by the framing of hypotheses. And to frame hypotheses is to speculate.

Therefore, the difference between science and speculation is not a difference of spirit; nor, thus far, is it a difference of method. The only difference between them is in the subsequent process of verifying hypotheses. For while speculation, in its purest form, is satisfied to test her explanations only by the degree in which they accord with our subjective ideas of probability—or with the “Illative Sense” of Cardinal Newman,—science is not satisfied to rest in any explanation as final until it shall have been fully verified by an appeal to objective proof. This distinction is now so well and so generally appreciated that I need not dwell upon it. Nor need I wait to go into any details with regard to the so-called canons of verification. My only object is to make perfectly clear, first, that in order to have any question to put to the test of objective verification, science must already have so far employed the method of speculation as to have framed a question to be tested; and, secondly, that the point where science parts company with speculation is the point where this testing process begins.

Now, if these things are so, there can be no doubt that Darwin was following the truest method of inductive research in allowing any amount of latitude to his speculative thought in the direction of scientific theorizing. For it follows from the above distinctions that the danger of speculation does not reside in the width of its range, or even in the impetuosity of its vehemence. Indeed, the wider its reach, and the greater its energy, the better will it be for the interests of science. The only danger of speculation consists in its momentum being apt to carry away the mind from the more laborious work of adequate verification; and therefore a true scientific judgment consists in giving a free rein to speculation on the one hand, while holding ready the break of verification with the other. Now, it is just because Darwin did both these things with so admirable a judgment, that he gave the world of natural history so good a lesson as to the most effectual way of driving the chariot of science.

This lesson we have now all more or less learnt to profit by. Yet no other naturalist has proved himself so proficient in holding the balance true. For the most part, indeed, they have now all ceased to confound the process of speculation per se with the danger of inadequate verification; and therefore the old ideal of natural history as concerned merely with collecting species, classifying affinities, and, in general, tabulating Facts, has been well-nigh universally superseded. But this great gain has been attended by some measure of loss. For while not a few naturalists have since erred on the side of insufficiently distinguishing between fully verified principles of evolution and merely speculative deductions therefrom, a still larger number have formed for themselves a darwinian creed, and regard any further theorizing on the subject of evolution as ipso facto unorthodox.

Having occupied the best years of my life in closely studying the literature of Darwinism, I shall endeavour throughout the following pages to avoid both these extremes. No one in this generation is able to imitate Darwin, either as an observer or a generalizer. But this does not hinder that we should all so far endeavour to follow his method, as always to draw a clear distinction, not merely between observation and deduction, but also between degrees of verification. At all events, my own aim will everywhere be to avoid dogmatism on the one hand, and undue timidity as regards general reasoning on the other. For everything that is said justification will be given; and, as far as prolonged deliberation has enabled me to do so, the exact value of such justification will be rendered by a statement of at least the main grounds on which it rests. The somewhat extensive range of the present treatise, however, will not admit of my rendering more than a small percentage of the facts which in each case go to corroborate the conclusion. But although a great deal must thus be necessarily lost on the one side, I am disposed to think that more will be gained on the other, by presenting, in a terser form than would otherwise be possible, the whole theory of organic evolution as I believe that it will eventually stand. My endeavour, therefore, will be to exhibit the general structure of this theory in what I take to be its strictly logical form, rather than to encumber any of its parts by a lengthy citation of facts. Following this method, I shall in each case give only what I consider the main facts for and against the positions which have to be argued; and in most cases I shall arrange the facts in two divisions, namely, first those of largest generality, and next a few of the most special character that can be found.

As explained in the Preface, the present instalment of the treatise is concerned with the theory of evolution, from the appearance of the Origin of Species in 1859, to the death of its author in 1882; while the second part will be devoted to the sundry post-Darwinian questions which have arisen in the subsequent decade. To the possible criticism that a disproportionate amount of space will thus be allotted to a consideration of these post-Darwinian questions, I may furnish in advance the following reply.

In the first place, besides the works of Darwin himself, there are a number of others which have already and very admirably expounded the evidences, both of organic evolution as a fact, and of natural selection as a cause. Therefore, in the present treatise it seemed needless to go beyond the ground which was covered by my original lectures, namely, a condensed and connected, while at the same time a critical statement of the main evidences, and the main objections, which have thus far been published with reference to the distinctively Darwinian theory. Indeed while re-casting this portion of my lectures for the present publication, I have felt that criticism might be more justly urged from the side of impatience at a reiteration of facts and arguments already so well known. But while endeavouring, as much as possible, to avoid overlapping the previous expositions, I have not carried this attempt to the extent of damaging my own, by omitting any of the more important heads of evidence; and I have sought to invest the latter with some measure of novelty by making good what appears to me a deficiency which has hitherto obtained in the matter of pictorial illustration. In particular, there will be found a tolerably extensive series of woodcuts, serving to represent the more important products of artificial selection. These, like all the other original illustrations, have been drawn either direct from nature or from a comparative study of the best authorities. Nevertheless, I desire it to be understood that the first part of this treatise is intended to retain its original character, as a merely educational exposition of Darwinian teaching—an exposition, therefore, which, in its present form, may be regarded as a compendium, or hand-book, adapted to the requirements of a general reader, or biological student as distinguished from those of a professed naturalist.

The case, however, is different with the second instalment, which will be published at no very distant date. Here I have not followed with nearly so much closeness the material of my original lectures. On the contrary, I have had in view a special class of readers; and, although I have tried not altogether to sacrifice the more general class, I shall desire it to be understood that I am there appealing to naturalists who are specialists in Darwinism. One must say advisedly, naturalists who are specialists in Darwinism, because, while the literature of Darwinism has become a department of science in itself, there are nowadays many naturalists who, without having paid any close attention to the subject, deem themselves entitled to hold authoritative opinions with regard to it. These men may have done admirable work in other departments of natural history, and yet their opinions on such matters as we shall hereafter have to consider may be destitute of value. As there is no necessary relation between erudition in one department of science and soundness of judgment in another, the mere fact that a man is distinguished as a botanist or zoologist does not in itself qualify him as a critic where specially Darwinian questions are concerned. Thus it happens now, as it happened thirty years ago, that highly distinguished botanists and zoologists prove themselves incapable as judges of general reasoning. It was Darwin’s complaint that for many years nearly all his scientific critics either could not, or would not, understand what he had written—and this even as regarded the fundamental principles of his theory, which with the utmost clearness he had over and over again repeated. Now the only difference between such naturalists and their successors of the present day is, that the latter have grown up in a Darwinian environment, and so, as already remarked, have more or less thoughtlessly adopted some form of Darwinian creed. But this scientific creed is not a whit less dogmatic and intolerant than was the more theological one which it has supplanted; and while it usually incorporates the main elements of Darwin’s teaching, it still more usually comprises gross perversions of their consequences. All this I shall have occasion more fully to show in subsequent parts of the present work; and allusion is made to the matter here merely for the sake of observing that in future I shall not pay attention to unsupported expressions of opinion from any quarter: I shall consider only such as are accompanied with some statement of the grounds upon which the opinion is held. And, even as thus limited, I do not think it will be found that the following exposition devotes any disproportional amount of attention to the contemporary movements of Darwinian thought, seeing, as we shall see, how active scientific speculation has been in the field of Darwinism since the death of Mr. Darwin.

Leaving, then, these post-Darwinian questions to be dealt with subsequently, I shall now begin a systematic résumé of the evidences in favour of the Darwinian theory, as this was left to the world by Darwin himself.

There is a great distinction to be drawn between the fact of evolution and the manner of it, or between the evidence of evolution as having taken place somehow, and the evidence of the causes which have been concerned in the process. This most important distinction is frequently disregarded by popular writers on Darwinism; and, therefore, in order to mark it as strongly as possible, I will effect a complete separation between the evidence which we have of evolution as a fact, and the evidence which we have as to its method. In other words, not until I shall have fully considered the evidence of organic evolution as a process which somehow or another has taken place, will I proceed to consider how it has taken place, or the causes which Darwin and others have suggested as having probably been concerned in this process.

Confining, then, our attention in the first instance to a proof of evolution considered as a fact, without any reference at all to its method, let us begin by considering the antecedent standing of the matter.

First of all we must clearly recognise that there are only two hypotheses in the field whereby it is possible so much as to suggest an explanation of the origin of species. Either all the species of plants and animals must have been supernaturally created, or else they must have been naturally evolved. There is no third hypothesis possible; for no one can rationally suggest that species have been eternal.

Next, be it observed, that the theory of a continuous transmutation of species is not logically bound to furnish a full explanation of all the natural causes which it may suppose to have been at work. The radical distinction between the two theories consists in the one assuming an immediate action of some supernatural or inscrutable cause, while the other assumes the immediate action of natural—and therefore of possibly discoverable—causes. But in order to sustain this latter assumption, the theory of descent is under no logical necessity to furnish a full proof of all the natural causes which may have been concerned in working out the observed results. We do not know the natural causes of many diseases; but yet no one nowadays thinks of reverting to any hypothesis of a supernatural cause, in order to explain the occurrence of any disease the natural causation of which is obscure. The science of medicine being in so many cases able to explain the occurrence of disease by its hypothesis of natural causes, medical men now feel that they are entitled to assume, on the basis of a wide analogy, and therefore on the basis of a strong antecedent presumption, that all diseases are due to natural causes, whether or not in particular cases such causes happen to have been discovered. And from this position it follows that medical men are not logically bound to entertain any supernatural theory of an obscure disease, merely because as yet they have failed to find a natural theory. And so it is with biologists and their theory of descent. Even if it be fully proved to them that the causes which they have hitherto discovered, or suggested, are inadequate to account for all the facts of organic nature, this would in no wise logically compel them to vacate their theory of evolution, in favour of the theory of creation. All that it would so compel them to do would be to search with yet greater diligence for the natural causes still undiscovered, but in the existence of which they are, by their independent evidence in favour of the theory, bound to believe.

In short, the issue is not between the theory of a supernatural cause and the theory of any one particular natural cause, or set of causes—such as natural selection, use, disuse, and so forth. The issue thus far—or where only the fact of evolution is concerned—is between the theory of a supernatural cause as operating immediately in numberless acts of special creation, and the theory of natural causes as a whole, whether these happen, or do not happen, to have been hitherto discovered.

This much by way of preliminaries being understood, we have next to notice that whichever of the two rival theories we choose to entertain, we are not here concerned with any question touching the origin of life. We are concerned only with the origin of particular forms of life—that is to say, with the origin of species. The theory of descent starts from life as a datum already granted. How life itself came to be, the theory of descent, as such, is not concerned to show. Therefore, in the present discussion, I will take the existence of life as a fact which does not fall within the range of our present discussion. No doubt the question as to the origin of life is in itself a deeply interesting question, and although in the opinion of most biologists it is a question which we may well hope will some day fall within the range of science to answer, at present, it must be confessed, science is not in a position to furnish so much as any suggestion upon the subject; and therefore our wisdom as men of science is frankly to acknowledge that such is the case.

We are now in a position to observe that the theory of organic evolution is strongly recommended to our acceptance on merely antecedent grounds, by the fact that it is in full accordance with what is known as the principle of continuity. By the principle of continuity is meant the uniformity of nature, in virtue of which the many and varied processes going on in nature are due to the same kind of method, i. e. the method of natural causation. This conception of the uniformity of nature is one that has only been arrived at step by step through a long and arduous course of human experience in the explanation of natural phenomena. The explanations of such phenomena which are first given are always of the supernatural kind; it is not until investigation has revealed the natural causes which are concerned that the hypotheses of superstition give way to those of science. Thus it follows that the hypotheses of superstition which are the latest in yielding to the explanations of science, are those which refer to the more recondite cases of natural causation; for here it is that methodical investigation is longest in discovering the natural causes. Thus it is only by degrees that fetishism is superseded by what now appears a common-sense interpretation of physical phenomena; that exorcism gives place to medicine; alchemy to chemistry; astrology to astronomy; and so forth. Everywhere the miraculous is progressively banished from the field of explanation by the advance of scientific discovery; and the places where it is left longest in occupation are those where the natural causes are most intricate or obscure, and thus present the greatest difficulty to the advancing explanations of science. Now, in our own day there are but very few of these strongholds of the miraculous left. Nearly the whole field of explanation is occupied by naturalism, so that no one ever thinks of resorting to supernaturalism except in the comparatively few cases where science has not yet been able to explore the most obscure regions of causation. One of these cases is the origin of life; and, until quite recently, another of these cases was the origin of Species. But now that a very reasonable explanation of the origin of species has been offered by science, it is but in accordance with all previous historical analogies that many minds should prove themselves unable all at once to adjust themselves to the new ideas, and thus still linger about the more venerable ideas of supernaturalism. But we are now in possession of so many of these historical analogies, that all minds with any instincts of science in their composition have grown to distrust, on merely antecedent grounds, any explanation which embodies a miraculous element. Such minds have grown to regard all these explanations as mere expressions of our own ignorance of natural causation; or, in other words, they have come to regard it as an a priori truth that nature is everywhere uniform in respect of method or causation; that the reign of law universal; the principle of continuity ubiquitous.

Now, it must be obvious to any mind which has adopted this attitude of thought, that the scientific theory of natural descent is recommended by an overwhelming weight of antecedent presumption, as against the dogmatic theory of supernatural design.

To begin with, we must remember that the fact of evolution—or, which is the same thing, the fact of continuity in natural causation—has now been unquestionably proved in so many other and analogous departments of nature, that to suppose any interruption of this method as between species and species becomes, on grounds of such analogy alone, well-nigh incredible. For example, it is now a matter of demonstrated fact that throughout the range of inorganic nature the principles of evolution have obtained. It is no longer possible for any one to believe with our forefathers that the earth’s surface has always existed as it now exists. For the science of geology has proved to demonstration that seas and lands are perpetually undergoing gradual changes of relative positions—continents and oceans supplanting each other in the course of ages, mountain-chains being slowly uplifted, again as slowly denuded, and so forth. Moreover, and as a closer analogy, within the limits of animate nature we know it is the universal law that every individual life undergoes a process of gradual development; and that breeds, races, or strains, may be brought into existence by the intentional use of natural processes—the results bearing an unmistakeable resemblance to what we know as natural species. Again, even in the case of natural species themselves, there are two considerations which present enormous force from an antecedent point of view. The first is that organic forms are only then recognised as species when intermediate forms are absent. If the intermediate forms are actually living, or admit of being found in the fossil state, naturalists forthwith regard the whole series as varieties, and name all the members of it as belonging to the same species. Consequently it becomes obvious that naturalists, in their work of naming species, may only have been marking out the cases where intermediate or connecting forms have been lost to observation. For example, here we have a diagram representing a very unusually complete series of fossil shells, which within the last few years has been unearthed from the Tertiary lake basins of Slavonia. Before the series was completed, some six or eight of the then disconnected forms were described as distinct species; but as soon as the connecting forms were found—showing a progressive modification from the older to the newer beds,—the whole were included as varieties of one species.

Fig. 1.—Successive forms of Paludina, from the Tertiary deposits of Slavonia (after Neumayr).

Of course, other cases of the same kind might be adduced, and therefore, as just remarked, in their work of naming species naturalists may only have been marking out the cases where intermediate forms have been lost to observation. And this possibility becomes little less than a certainty when we note the next consideration which I have to adduce, namely, that in all their systematic divisions of plants and animals in groups higher than species—such as genera, families, orders, and the rest—naturalists have at all times recognised the fact that the one shades off into the other by such imperceptible gradations, that it is impossible to regard such divisions as other than conventional. It is important to remember that this fact was fully recognised before the days of Darwin. In those days the scientifically orthodox doctrine was, that although species were to be regarded as fixed units, bearing the stamp of a special creation, all the higher taxonomic divisions were to be considered as what may be termed the artificial creation of naturalists themselves. In other words, it was believed, and in many cases known, that if we could go far enough back in the history of the earth, we should everywhere find a tendency to mutual approximation between allied groups of species; so that, for instance, birds and reptiles would be found to be drawing nearer and nearer together, until eventually they would seem to become fused in a single type; that the existing distinctions between herbivorous and carnivorous mammals would be found to do likewise; and so on with all the larger group-distinctions, at any rate within the limits of the same sub-kingdoms. But although naturalists recognised this even in the pre-Darwinian days, they stoutly believed that a great exception was to be made in the case of species. These, the lowest or initial members of their taxonomic series, they supposed to be permanent—the miraculously created units of organic nature. Now, all that I have at present to remark is, that this pre-Darwinian exception which was made in favour of species to the otherwise recognised principle of gradual change, was an exception which can at no time have been recommended by any antecedent considerations. At all times it stood out of analogy with the principle of continuity; and, as we shall fully find in subsequent chapters, it is now directly contradicted by all the facts of biological science.

There remains one other fact of high generality to which prominent attention should be drawn from the present, or merely antecedent, point of view. On the theory of special creation no reason can be assigned why distinct specific types should present any correlation, either in time or in space, with their nearest allies; for there is evidently no conceivable reason why any given species, A, should have been specially created on the same area and at about the same time as its nearest representative, B,—still less, of course, that such should be a general rule throughout all the thousands and millions of species which have ever inhabited the earth. But, equally of course, on the theory of a natural evolution this is so necessary a consequence, that if no correlation of such a two-fold kind were observable, the theory would be negatived. Thus the question whether there be any indication of such a two-fold correlation may be regarded as a test-question as between the two theories; for although the vast majority of extinct species have been lost to science, there are a countless number of existing species which furnish ample material for answering the question. And the answer is so unequivocal that Mr. Wallace, who is one of our greatest authorities on geographical distribution, has laid it down as a general law, applicable to all the departments of organic nature, that, so far as observation can extend, “every species has come into existence coincident both in space and time with a pre-existing and closely allied species.” As it appears to me that the significance of these words cannot be increased by any comment upon them, I will here bring this introductory chapter to a close.

CHAPTER II.
Classification.

The first line of direct evidence in favour of organic evolution which I shall open is that which may be termed the argument from Classification.

It is a matter of observable fact that different forms of plants and animals present among themselves more or less pronounced resemblances. From the earliest times, therefore, it has been the aim of philosophical naturalists to classify plants and animals in accordance with these resemblances. Of course the earliest attempts at such classification were extremely crude. The oldest of these attempts with which we are acquainted—namely, that which is presented in the books of Genesis and Leviticus—arranges the whole vegetable kingdom in three simple divisions of Grass, Herbs, and Trees; while the animal kingdom is arranged with almost equal simplicity with reference, first to habitats in water, earth, or air, and next as to modes of progression. These, of course, were what may be termed common-sense classifications, having reference merely to external appearances and habits of life. But when Aristotle laboriously investigated the comparative anatomy of animals, he could not fail to perceive that their entire structures had to be taken into account in Order to classify them scientifically; and, also, that for this purpose the internal parts were of quite as much importance as the external. indeed, he perceived that they were of greatly more importance in this respect, inasmuch as they presented so many more points for comparison; and, in the result, he furnished an astonishingly comprehensive, as well as an astonishingly accurate classification of the larger groups of the animal kingdom. On the other hand, classification of the vegetable kingdom continued pretty much as it had been left by the book of genesis—all plants being divided into three groups, herbs, shrubs, and trees. Nor was this primitive state of matters improved upon till the sixteenth century, when gesner (1516-1565), and still more cæsalpino (1519-1603), laid the foundations of systematic botany.

But the more that naturalists prosecuted their studies on the anatomy of plants and animals, the more enormously complex did they find the problem of classification become. Therefore they began by forming what are called artificial systems, in contradistinction to natural systems. An artificial system of classification is a system based on the more or less arbitrary selection of some one part, or set of parts; while a natural classification is one that is based upon a complete knowledge of all the structures of all the organisms which are classified.

Thus, the object of classification has been that of arranging organisms in accordance with their natural affinities, by comparing organism with organism, for the purpose of ascertaining which of the constituent organs are of the most invariable occurrence, and therefore of the most typical signification. A porpoise, for instance, has a large number of teeth, and in this feature resembles most fish, while it differs from all mammals. But it also gives suck to its young. Now, looking to these two features alone, should we say that a porpoise ought to be classed as a fish or as a mammal? Assuredly as a mammal; because the number of teeth is a very variable feature both in fish and mammals, whereas the giving of suck is an invariable feature among mammals, and occurs nowhere else in the animal kingdom. This, of course, is chosen as a very simple illustration. Were all cases as obvious, there would be but little distinction between natural and artificial systems of classification. But it is because the lines of natural affinity are, as it were, so interwoven throughout the organic world, and because there is, in consequence, so much difficulty in following them, that artificial systems have to be made in the first instance as feelers towards eventual discovery of the natural system. In other words, while forming their artificial systems of classification, it has always been the aim of naturalists—whether consciously or unconsciously—to admit as the bases of their systems those characters which, in the then state of their knowledge, seemed most calculated to play an important part in the eventual construction of the natural system. If we were dealing with the history of classification, it would here be interesting to note how the course of it has been marked by gradual change in the principles which naturalists adopted as guides to the selection of characters on which to found their attempts at a natural classification. Some of these changes, indeed, I shall have to mention later on; but at present what has to be specially noted is, that through all these changes of theory or principle, and through all the ever-advancing construction of their taxonomic science, naturalists themselves were unable to give any intelligible reason for the faith that was in them—or the faith that over and above the artificial classifications which were made for the mere purpose of cataloguing the living library of organic nature, there was deeply hidden in nature itself a truly natural classification, for the eventual discovery of which artificial systems might prove to be of more or less assistance.

Linnæus, for example, expressly says—“You ask me for the characters of the natural orders; I confess that I cannot give them.” Yet he maintains that, although he cannot define the characters, he knows, by a sort of naturalist’s instinct, what in a general way will subsequently be found to be the organs of most importance in the eventual grouping of plants under a natural system. “I will not give my reasons for the distribution of the natural orders which I have published,” he said: “you, or some other person, after twenty or after fifty years, will discover them, and see that I was right.”

Thus we perceive that in forming their provisional or artificial classifications, naturalists have been guided by an instinctive belief in some general principle of natural affinity, the character of which they have not been able to define; and that the structures which they selected as the bases of their classifications when these were consciously artificial, were selected because it seemed that they were the structures most likely to prove of use in subsequent attempts at working out the natural system.

This general principle of natural affinity, of which all naturalists have seen more or less well-marked evidence in organic nature, and after which they have all been feeling, has sometimes been regarded as natural, but more often as supernatural. Those who regarded it as supernatural took it to consist in a divine ideal of creation according to types, so that the structural affinities of organisms were to them expressions of an archetypal plan, which might be revealed in its entirety when all organisms on the face of the earth should have been examined. Those, on the other hand, who regarded the general principle of affinity as depending on some natural causes, for the most part concluded that these must have been utilitarian causes; or, in other words, that the fundamental affinities of structure must have depended upon fundamental requirements of function. According to this view, the natural classification would eventually be found to stand upon a basis of physiology. Therefore all the systems of classification up to the earlier part of the present century went upon the apparent axiom, that characters which are of most importance to the organisms presenting them must be characters most indicative of natural affinities. But the truth of the matter was eventually found to be otherwise. For it was eventually found that there is absolutely no correlation between these two things; that, therefore, it is a mere chance whether or not organs which are of importance to organisms are likewise of importance as guides to classification; and, in point of fact, that the general tendency in this matter is towards an inverse instead of a direct proportion. More often than not, the greater the value of a structure for the purpose of indicating natural affinities, the less is its value to the creatures presenting it.

Enough has now been said to show three things. First, that long before the theory of descent was entertained by naturalists, naturalists perceived the fact of natural affinities, and did their best to construct a natural system of classification for the purpose of expressing such affinities. Second, that naturalists had a kind of instinctive belief in some one principle running through the whole organic world, which thus served to bind together organisms in groups subordinate to groups—that is, into species, genera, orders, families, classes, sub-kingdoms, and kingdoms. Third, that they were not able to give any very intelligible reason for this faith that was in them; sometimes supposing the principle in question to be that of a supernatural plan of organization, sometimes regarding it as dependent on conditions of physiology, and sometimes not attempting to account for it at all.

Of course it is obvious that the theory of descent furnishes the explanation which is required. For it is now evident to evolutionists, that although these older naturalists did not know what they were doing when they were tracing these lines of natural affinity, and thus helping to construct a natural classification—I say it is now evident to evolutionists that these naturalists were simply tracing the lines of genetic relationship. The great principle pervading organic nature, which was seen so mysteriously to bind the whole creation together as in a nexus of organic affinity, is now easily understood as nothing more or less than the principle of Heredity. Let us, therefore, look a little more closely at the character of this network, in order to see how far it lends itself to this new interpretation.

The first thing that we have to observe about the nexus is, that it is a nexus—not a single line, or even a series of parallel lines. In other words, some time before the theory of descent was seriously entertained, naturalists for the most part had fully recognised that it was impossible to arrange either plants or animals, with respect to their mutual affinities, in a ladder-like series (as was supposed to be the type of classification by the earlier systematists), or even in map-like groups (as was supposed to be the type by Linnæus). And similarly, also, with respect to grades of organization. In the case of the larger groups, indeed, it is usually possible to say that the members of this group as a whole are more highly organized than the members of that group as a whole; so that, for instance, we have no hesitation in regarding the Vertebrata as more highly organized than the Invertebrata, Birds than Reptiles, and so on. But when we proceed to smaller subdivisions, such as genera and species, it is usually impossible to say that the one type is more highly organized than another type. A horse, for instance, cannot be said to be more highly organized than a zebra or an ass; although the entire horse-genus is clearly a more highly organized type than any genus of animal which is not a mammal.

In view of these facts, therefore, the system of classification which was eventually arrived at before the days of Darwin, was the system which naturalists likened to a tree; and this is the system which all naturalists now agreed upon as the true one. According to this system, a short trunk may be taken to represent the lowest organisms which cannot properly be termed either plants or animals. This short trunk soon separates into two large trunks, one of which represents the vegetable and the other the animal kingdom. Each of these trunks then gives off large branches signifying classes, and these give off smaller, but more numerous branches, signifying families, which ramify again into orders, genera, and finally into the leaves, which may be taken to represent species. Now, in such a representative tree of life, the height of any branch from the ground may be taken to indicate the grade of organization which the leaves, or species, present; so that, if we picture to ourselves such a tree, we may understand that while there is a general advance of organization from below upwards, there are many deviations in this respect. Sometimes leaves growing on the same branch are growing at a different level—especially, of course, if the branch be a large one, corresponding to a class or sub-kingdom. And sometimes leaves growing on different branches are growing at the same level: that is to say, although they represent species belonging to widely divergent families, orders, or even classes, it cannot be said that the one species is more highly organized than the other.

Now, this tree-like arrangement of species in nature is an arrangement for which Darwin is not responsible. For, as we have seen, the detecting of it has been due to the progressive work of naturalists for centuries past; and even when it was detected, at about the commencement of the present century, naturalists were confessedly unable to explain the reason of it, or what was the underlying principle that they were engaged in tracing when they proceeded ever more and more accurately to define these ramifications of natural affinity. But now, as just remarked, we can clearly perceive that this underlying principle was none other than Heredity as expressed in family likeness,—likeness, therefore, growing progressively more unlike with remoteness of ancestral relationship. For thus only can we obtain any explanation of the sundry puzzles and apparent paradoxes, which a working out of their natural classifications revealed to botanists and zoologists during the first half of the present century. It will now be my endeavour to show how these puzzles and paradoxes are all explained by the theory that natural affinities are merely the expression of genetic affinities.

First of all, and from the most general point of view, it is obvious that the tree-like system of classification, which Darwin found already and empirically worked out by the labours of his predecessors, is as suggestive as anything could well be of the fact of genetic relationship. For this is the form that every tabulation of family pedigree must assume; and therefore the mere fact that a scientific tabulation of natural affinities was eventually found to take the form of a tree, is in itself highly suggestive of the inference that such a tabulation represents a family tree. If all species were separately created, there can be no assignable reason why the ideas of earlier naturalists touching the form which a natural classification would eventually assume should not have represented the truth—why, for example, it should not have assumed the form of a ladder (as was anticipated in the seventeenth century), or of a map (as was anticipated in the eighteenth), or, again, of a number of wholly unrelated lines, circles, &c. (as certain speculative writers of the present century have imagined). But, on the other hand, if all species were separately and independently created, it becomes virtually incredible that we should everywhere observe this progressive arborescence of characters common to larger groups into more and more numerous, and more and more delicate, ramifications of characters distinctive only of smaller and smaller groups. A man would be deemed insane if he were to attribute the origin of every branch and every twig of a real tree to a separate act of special creation; and although we have not been able to witness the growth of what we may term in a new sense the Tree of Life, the structural relations which are now apparent between its innumerable ramifications bear quite as strong a testimony to the fact of their having been due to an organic growth, as is the testimony furnished by the branches of an actual tree.

Or, to take another illustration. Classification of organic forms, as Darwin, Lyell, and Häckel have pointed out, strongly resembles the classification of languages. In the case of languages, as in the case of species, we have genetic affinities strongly marked; so that it is possible to some extent to construct a Language-tree, the branches of which shall indicate, in a diagrammatic form, the progressive divergence of a large group of languages from a common stock. For instance, Latin may be regarded as a fossil language, which has given rise to a group of living languages—Italian, Spanish, French, and, to a large extent, English. Now what would be thought of a philologist who should maintain that English, French, Spanish, and Italian were all specially created languages—or languages separately constructed by the Deity, and by as many separate acts of inspiration communicated to the nations which now speak them—and that their resemblance to the fossil form, Latin, must be attributed to special design? Yet the evidence of the natural transmutation of species is in one respect much stronger than that of the natural transmutation of languages—in respect, namely, of there being a vastly greater number of cases all bearing testimony to the fact of genetic relationship.

But, quitting now this most general point of view—or the suggestive fact that what we have before us is a tree—let us next approach this tree for the purpose of examining its structure more in detail. When we do this, the fact of next greatest generality which we find is as follows.

In cases where a very old form of life has continued to exist unmodified, so that by investigation of its anatomy we are brought back to a more primitive type of structure than that of the newer forms growing higher up upon the same branch, two things are observable. In the first place, the old form is less differentiated than the newer ones; and, in the next place, it is seen much more closely to resemble types of structure belonging to some of the other and larger branches of the tree. The organization of the older form is not only simpler; but it is, as naturalists say, more generalized. It comprises within itself characters belonging to its own branch, and also characters belonging to neighbouring branches, or to the trunk from which allied branches spring. Hence it becomes a general rule of classification, that it is by the lowest, or by the oldest, forms of any two natural groups that the affinities between the two groups admit of being best detected. And it is obvious that this is just what ought to be the case on the theory of descent with divergent modification; while, upon the alternative theory of special creation, no reason can be assigned why the lowest or the oldest types should thus combine the characters which afterwards become severally distinctive of higher or newer types.

Again, I have already alluded to the remarkable fact that there is no correlation between the value of structures to the organisms which present them, and their value to the naturalist for the purpose of tracing natural affinity; and I have remarked that up to the close of the last century it was regarded as an axiom of taxonomic science, that structures which are of most importance to the animals or plants possessing them must likewise prove of most importance in any natural system of classification. On this account, all attempts to discover the natural classification went upon the supposition that such a direct proportion must obtain—with the result that organs of most physiological importance were chosen as the bases of systematic work. And when, in the earlier part of the present century, De Candolle found that instead of a direct there was usually an inverse proportion between the functional and the taxonomic value of a structure, he was unable to suggest any reason for this apparently paradoxical fact. For, upon the theory of special creation, no reason can be assigned why organs of least importance to organisms should prove of most importance as marks of natural affinity. But on the theory of descent with progressive modification the apparent paradox is at once explained. For it is evident that organs of functional importance are, other things equal, the organs which are most likely to undergo different modifications in different lines of family descent, and therefore in time to have their genetic relationships in these different lines obscured. On the other hand, organs or structures which are of no functional importance are never called upon to change in response to any change of habit, or to any change in the conditions of life. They may, therefore, continue to be inherited through many different lines of family descent, and thus afford evidence of genetic relationship where such evidence fails to be given by any of the structures of vital importance, which in the course of many generations have been required to change in many ways according to the varied experiences of different branches of the same family. Here, then, we have an empirically discovered rule in the science of classification, the raison d’être of which we are at once able to appreciate upon the theory of evolution, whereas no possible explanation of why it should ever have become a rule could be furnished upon the theory of special creation.

Here, again, is another empirically determined rule. The larger the number, as distinguished from the importance, of structures which are found common to different groups, the greater becomes their value as guides to the determination of natural affinity. Or, as Darwin puts it, “the value of an aggregate of characters, even when none are important, alone explains the aphorism enunciated by Linnæus, namely, that the characters do not give the genus, but the genus gives the characters; for this seems founded on the appreciation of many trifling points of resemblance, too slight to be defined[1].”

Now it is evident, without comment, of how much value aggregates of characters ought to be in classification, if the ultimate meaning of classification be that of tracing lines of pedigree; whereas, if this ultimate meaning were that of tracing divine ideals manifested in special creation, we can see no reason why single characters are not such sure tokens of a natural arrangement as are aggregates of characters, even though the latter be in every other respect unimportant. For, on the special creation theory, we cannot explain why an assemblage, say of four or five trifling characters, should have been chosen to mark some unity of plan, rather than some one character of functional importance, which would have served at least equally well any such hypothetical purpose. On the other hand, as Darwin remarks, “we care not how trifling a character may be—let it be the mere inflection of the angle of the jaw, the manner in which an insect’s wing is folded, whether the skin be covered with hair or feathers—if it prevail throughout many and different species, especially those having very different habits of life, it assumes high value; for we can account for its presence in so many forms, with such different habits, only by inheritance from a common parent. We may err in this respect in regard to single points of structure, but when several characters, let them be ever so trifling, concur throughout a large group of beings having different habits, we may feel almost sure, on the theory of descent, that these characters have been inherited from a common ancestor; and we know that such aggregated characters have especial value in classification[2].”

It is true that even a single character, if found common to a large number of forms, while uniformly absent from others, is also regarded by naturalists as of importance for purposes of classification, although they recognise it as of a value subordinate to that of aggregates of characters. But this also is what we should expect on the theory of descent. If even any one structure be found to run through a number of animals presenting different habits of life, the readiest explanation of the fact is to be found in the theory of descent; but this does not hinder that if several such characters always occur together, the inference of genetic relationship is correspondingly confirmed. And the fact that before this inference was ever drawn, naturalists recognised the value of single characters in proportion to their constancy, and the yet higher value of aggregates of characters in proportion to their number—this fact shows that in their work of classification naturalists empirically observed the effects of a cause which we have now discovered, to wit, hereditary transmission of characters through ever-widening groups of changing species.

There is another argument which appears to tell strongly in favour of the theory of descent. We have just seen that non-adaptive structures, not being required to change in response to change of habits or conditions of life, are allowed to persist unchanged through many generations, and thus furnish exceptionally good guides in the science of classification—or, according to our theory, in the work of tracing lines of pedigree. But now, the converse of this statement holds equally true. For it often happens that adaptive structures are required to change in different lines of descent in analogous ways, in order to meet analogous needs; and, when such is the case, the structures concerned have to assume more or less close resemblances to one another, even though they have severally descended from quite different ancestors. The paddles of a whale, for instance, most strikingly resemble the fins of a fish as to their outward form and movements; yet, on the theory of descent, they must be held to have had a widely different parentage. Now, in all such cases where there is thus what is called an analogous (or adaptive) resemblance, as distinguished from what is called an homologous (or anatomical) resemblance—in all such cases it is observable that the similarities do not extend further into the structure of the parts than it is necessary that they should extend, in order that the structures should both perform the same functions. The whole anatomy of the paddles of a whale is quite unlike that of the fins of a fish—being, in fact, that of the fore-limb of a mammal. The change, therefore, which the fore-limb has here undergone to suit it to the aquatic habits of this mammal, is no greater than was required for that purpose: the change has not extended to any one feature of anatomical significance. This, of course, is what we should expect on the theory of descent with modification of ancestral characters; but on the theory of special creation it is not intelligible why there should always be so marked a distinction between resemblances as analogical or adaptive, and resemblances as homological or of meaning in reference to a natural classification. To take another and more detailed instance, the Tasmanian wolf is an animal separated from true wolves in a natural system of classification. Yet its jaws and teeth bear a strong general resemblance to those of all the dog tribe, although there are differences of anatomical detail. In particular, while the dogs all have on each side of the upper jaw four pre-molars and two molars, the Tasmanian wolf has three pre-molars and four molars. Now there is no reason, so far as their common function of dealing with flesh is concerned, why the teeth of the Tasmanian wolf should not have resembled homologically as well as analogically the teeth of a true wolf; and therefore we cannot assign any intelligible reason why, if all the species of the dog genus were separately created with one pattern of teeth, the unallied Tasmanian wolf should have been furnished with what is practically the same pattern from a functional point of view, while differing from a structural point of view. But, of course, on the theory of descent with modification, we can well understand why similarities of habit should have led to similarities of structural appearance of an adaptive kind in different lines of descent, without there being any trace of such real or anatomical similarities as could possibly point to genetic relationship.

Lastly, to adduce the only remaining argument from classification which I regard as of any considerable weight, naturalists have found it necessary, while constructing their natural classifications, to set great store on what Mr. Darwin calls “chains of affinities.” Thus, for instance, “nothing can be easier than to define a number of characters common to all birds; but with crustaceans any such definition has hitherto been found impossible. There are crustaceans at the opposite ends of the series, which have hardly a character in common; yet the species at both ends, from being plainly allied to others, and these to others, and so onwards, can be recognised as unequivocally belonging to this, and to no other class of the articulata[3].” Now it is evident that this progressive modification of specific types—where it cannot be said that the continuity of resemblance is anywhere broken, and yet terminates in modification so great that but for the connecting links no one could divine a natural relationship between the extreme members of the series,—it is evident that such chains of affinity speak most strongly in favour of a transmutation of the species concerned, while it is impossible to suggest any explanation of the fact in terms of the rival theory. For if all the links of such a chain were separately forged by as many acts of special creation, we can see no reason why B should resemble A, C resemble B, and so on, but with ever slight though accumulating differences, until there is no resemblance at all between A and Z.

I hope enough has now been said to show that all the general principles and particular facts appertaining to the natural classification of plants and animals, are precisely what they ought to be according to the theory of genetic descent; while no one of them is such as might be—and, indeed, used to be—expected upon the theory of special creation. Therefore, the only possible way in which all this uniform body of direct evidence can be met by a supporter of the latter theory, is by falling back upon the argument from ignorance. We do not know, it may be said, what hidden reasons there may have been for following all these general principles in the separate creation of specific types. Now, it is evident that this is a form of argument which admits of being brought against all the actual—and even all the possible—lines of evidence in favour of evolution. Therefore I deem it desirable thus early in our proceedings to place this argument from ignorance on its proper logical footing.

If there were any independent evidence in favour of special creation as a fact, then indeed the argument from ignorance might be fairly used against any sceptical cavils regarding the method. In this way, for example, Bishop Butler made a legitimate use of the argument from ignorance when he urged that it is no reasonable objection against a revelation, otherwise accredited, to show that it has been rendered in a form, or after a method, which we should not have antecedently expected. But he could not have legitimately employed this argument, except on the supposition that he had some independent evidence in favour of the revelation; for, in the absence of any such independent evidence, appeal to the argument from ignorance would have become a mere begging of the question, by simply assuming that a revelation had been made. And thus it is in the present case. A man, of course, may quite legitimately say, Assuming that the theory of special creation is true, it is not for us to anticipate the form or method of the process. But where the question is as to whether or not the theory is true, it becomes a mere begging of this question to take refuge in the argument from ignorance, or to represent in effect that there is no question to be discussed. And if, when the form or method is investigated, it be found everywhere charged with evidence in favour of the theory of descent, the case becomes the same as that of a supposed revelation, which has been discredited by finding that all available evidence points to a natural growth. In short, the argument from ignorance is in any case available only as a negative foil against destructive criticism: in no case has it any positive value, or value of a constructive kind. Therefore, if a theory on any subject is destitute of positive evidence, while some alternative theory is in possession of such evidence, the argument from ignorance can be of no logical use to the former, even though it maybe of such use to the latter. For it is only the possession of positive evidence which can furnish a logical justification of the argument from ignorance: in the absence of such evidence, even the negative value of the argument disappears, and it then implies nothing more than the gratuitous assumption of a theory.

I will now sum up the various considerations which have occupied us during the present chapter.

First of all we must take note that the classification of plants and animals in groups subordinate to groups is not merely arbitrary, or undertaken only for a matter of convenience and nomenclature—such, for instance, as the classification of stars in constellations. On the contrary, the classification of a naturalist differs from that of an astronomer, in that the objects which he has to classify present structural resemblances and structural differences in numberless degrees; and it is the object of his classification to present a tabular statement of these facts. Now, long before the theory of evolution was entertained, naturalists became fully aware that these facts of structural resemblances running through groups subordinate to groups were really facts of nature, and not merely poetic imaginations of the mind. No one could dissect a number of fishes without perceiving that they were all constructed on one anatomical pattern, which differed considerably from the equally uniform pattern on which all mammals were constructed, even although some mammals bore an extraordinary resemblance to fish in external form and habits of life. And similarly with all the smaller divisions of the animal and vegetable kingdoms. Everywhere investigation revealed the bonds of close structural resemblances between species of the same genus, resemblance less close between genera of the same family, resemblance still less close between families of the same order, resemblance yet more remote between orders of the same class, and resemblance only in fundamental features between classes of the same sub-kingdom, beyond which limit all anatomical resemblance was found to disappear—the different sub-kingdoms being formed on wholly different patterns. Furthermore, in tracing all these grades of structural relationship, naturalists were slowly led to recognise that the form which a natural classification must eventually assume would be that of a tree, wherein the constituent branches would display a progressive advance of organization from below upwards.

Now we have seen that although this tree-like arrangement of natural groups was as suggestive as anything could well be of all the forms o£ life being bound together by the ties of genetic relationship, such was not the inference which was drawn from it. Dominated by the theory of special creation, naturalists either regarded the resemblance of type subordinate to type as expressive of divine ideals manifested in such creation, or else contented themselves with investigating the facts without venturing to speculate upon their philosophical import. But even those naturalists who abstained from committing themselves to any theory of archetypal plans, did not doubt that facts so innumerable and so universal must have been due to some one co-ordinating principle—that, even though they were not able to suggest what it was, there must have been some hidden bond of connexion running through the whole of organic nature. Now, as we have seen, it is manifest to evolutionists that this hidden bond can be nothing else than heredity; and, therefore, that these earlier naturalists, although they did not know what they were doing, were really tracing the lines of genetic descent as revealed by degrees of structural resemblance,—that the arborescent grouping of organic forms which their labours led them to begin, and in large measure to execute, was in fact a family tree of life.

Here, then, is the substance of the argument from classification. The mere fact that all organic nature thus incontestably lends itself to a natural arrangement of group subordinate to group, when due regard is paid to degrees of anatomical resemblance—this mere fact of itself tells so weightily in favour of descent with progressive modification in different lines, that even if it stood alone it would be entitled to rank as one of our strongest pieces of evidence. But, as we have seen, it does not stand alone. When we look beyond this large and general fact of all the innumerable forms of life being thus united in a tree-like system by an unquestionable relationship of some kind, to those smaller details in the science of classification which have been found most useful as guides for this kind of research, then we find that all these details, or empirically discovered rules, are exactly what we should have expected them to be, supposing the real meaning of classification to have been that of tracing lines of pedigree.

In particular, we have seen that the most archaic types are both simpler in their organization and more generalized in their characters than are the more recent types—a fact of which no explanation can be given on the theory of special creation. But, upon the theory of natural evolution, we can without difficulty understand why the earlier forms should have been the simpler forms, and also why they should have been the most generalized. For it is out of the older forms that the newer must have grown; and, as they multiplied, they must have become more and more differentiated.

Again, we have seen that there is no correlation between the importance of any structure from a classificatory point of view, and the importance of that structure to the organism which presents it. On the contrary, it is a general rule that “the less any part of the organization is concerned with special habits, the more important it becomes for classification.” Now, from the point of view of special creation it is unintelligible why unity of ideal should be most manifested by least important structures, whereas from the point of view of evolution it is to be expected that these life-serving structures should have been most liable to divergent modification in divergent lines of descent, or in adaptation to different conditions of life, while the trivial or less important characters should have been allowed to remain unmodified. Thus we can now understand why all primitive classifications were wrong in principle when they went upon the assumption that divine ideals were best exhibited by resemblances between life-serving (and therefore adaptive) structures, with the result that whales were classed with fishes, birds with bats, and so on. Nevertheless, these primitive naturalists were quite logical; for, from the premises furnished by the theory of special creation, it is much more reasonable to expect that unity of ideal should be shown in plainly adaptive characters than in trivial and more or less hidden anatomical characters. Moreover, long after biological science had ceased consciously to follow any theological theory, the apparent axiom continued to be entertained, that structures of most importance to organisms must also be structures of most importance to systematists. And when at last, in the present century, this was found not to be the case, no reason could be suggested why it was not the case. But now we are able fully to explain this apparent anomaly.

Once more, we have seen that aggregates of characters presenting resemblances to one another have always been found to be of special importance as guides to classification. This, of course, is what we should have expected, if the real meaning of classification be that of tracing lines of pedigree; but on the theory of special creation no reason can be assigned why single characters are not such sure tokens of a natural arrangement as are aggregates of characters, however trivial the latter may be. For it is obvious that unity of ideal might have been even better displayed by everywhere maintaining the pattern of some one important structure, than by doing so in the case of several unimportant structures. Take an analogous instance from human contrivances. Unity of ideal in the case of gun-making would be shown by the same principles of mechanism running through all the different sizes and shapes of gun-locks, rather than by the ornamental patterns engraved upon the outside. Yet it must be supposed that in the mechanisms assumed to have been constructed by special creation, it was the trivial details rather than the fundamental principles of these mechanisms which were chosen by the Divinity to display his ideals.

And this leads us to the next consideration—namely, that when in two different lines of descent animals happen to adopt similar habits of life, the modifications which they undergo in order to fit them for these habits often induces striking resemblances of structure between the two animals, as in the case of whales and fish. But in all such instances it is invariably found that the resemblance is only superficial and apparent: not anatomical or real. In other words, the resemblance does not extend further than it is necessary that it should, if both sets of organs are to be adapted to perform the same functions. Now this, again, is just what one would expect to find as the universal rule on the theory of descent, with modification of ancestral characters. But, on the opposite theory of special creation, I know not how it is to be explained that among so many instances of close superficial resemblance between creatures belonging to different branches of the tree of life, there are no instances of any real or anatomical resemblance. So far as their structures are adapted to perform a common function, there is in all such cases what may be termed a deceptive appearance of some unity of ideal; but, when carefully examined, it is always found that two apparently identical structures occurring on different branches of the classificatory tree are in fact fundamentally different in respect of their structural plan.

Lastly, we have seen that one of the guiding principles of classification has been empirically found to consist in setting a high value on “chains of affinities.” That is to say, naturalists not unfrequently meet with a long series of progressive modifications of type, which, although it cannot be said that the continuity is anywhere broken, at last leads to so much divergence of character that, but for the intermediate links, the members at each end of the chain could not be suspected of being in any way related. Well, such cases of chains of affinity obviously tell most strongly in favour of descent with continuous modification; while it is impossible to suggest why, if all the links were separately forged by as many acts of special creation, there should have been this gradual transmutation of characters carried to the point where the original creative ideal has been so completely transformed that, but for the accident of the chain being still complete, no one of nature’s interpreters could possibly have discovered the connexion. For, as we have seen, this is not a case in which any appeal can be logically made to the argument from ignorance of divine method, unless some independent evidence could be adduced in favour of special creation. And that no such independent evidence exists, it will be the object of future chapters to show.

CHAPTER III.
Morphology.

The theory of evolution supposes that hereditary characters admit of being slowly modified wherever their modification will render an organism better suited to a change in its conditions of life. Let us, then, observe the evidence which we have of such adaptive modifications of structure, in cases where the need of such modification is apparent. We may begin by again taking the case of the whales and porpoises. The theory of evolution infers, from the whole structure of these animals, that their progenitors must have been terrestrial quadrupeds of some kind, which gradually became more and more aquatic in their habits. Now the change in the conditions of their life thus brought about would have rendered desirable great modifications of structure. These changes would have begun by affecting the least typical—that is, the least strongly inherited—structures, such as the skin, claws, and teeth. But, as time went on, the adaptation would have extended to more typical structures, until the shape of the body would have become affected by the bones and muscles required for terrestrial locomotion becoming better adapted for aquatic locomotion, and the whole outline of the animal more fish-like in shape. This is the stage which we actually observe in the seals, where the hind legs, although retaining all their typical bones, have become shortened up almost to rudiments, and directed backwards, so as to be of no use for walking, while serving to complete the fish-like taper of the body. (Fig. 2.) But in the whales the modification has gone further than this so that the hind legs have ceased to be apparent externally, and are only represented internally—and even this only in some species—by remnants so rudimentary that it is difficult to make out with certainty the homologies of the bones; moreover, the head and the whole body have become completely fish-like in shape. (Fig. 3.) But profound as are these alterations, they affect only those parts of the organism which it was for the benefit of the organism to have altered, so that it might be adapted to an aquatic mode of existence. Thus the arm, which is used as a fin, still retains the bones of the shoulder, fore-arm, wrist, and fingers, although they are all enclosed in a fin-shaped sack, so as to render them useless for any purpose other than swimming (Fig. 4.) Similarly, the head, although it so closely resembles the head of a fish in shape, still retains the bones of the mammalian skull in their proper anatomical relations to one another; but modified in form so as to offer the least possible resistance to the water. In short, it may be said that all the modifications have been effected with the least possible divergence from the typical mammalian type, which is compatible with securing so perfect an adaptation to a purely aquatic mode of life.

Fig. 2.—Skeleton of Seal, 1/8 nat. size. Drawn from nature (R. Coll. Surg. Mus.).

Fig. 3.—Skeleton of Greenland Whale, 1/100 nat. size. The rudimentary bones of the pelvis are shown on a larger scale in the upper drawing. (From Prof. Flower.)

Now I have chosen the case of the whale and porpoise group, because they offer so extreme an example of profound modification of structure in adaptation to changed conditions of life. But the same thing may be seen in hundreds and hundreds of other cases. For instance, to confine our attention to the arm, not only is the limb modified in the whale for swimming, but in another mammal—the bat—it is modified for flying, by having the fingers enormously elongated and overspread with a membranous web.

Fig. 4.—Paddle of Whale compared with Hand of Man. Drawn from nature (R. Coll. Surg. Mus.).

In birds, again, the arm is modified for flight in a wholly different way—the fingers here being very Short and all run together, while the chief expanse of the wing is composed of the shoulder and fore-arm. In frogs and lizards, again, we find hands more like our own; but in an extinct species of flying reptile the modification was extreme, the wing having been formed by a prodigious elongation of the fifth finger, and a membrane spread over it and the rest of the hand. (Fig. 5.) lastly, in serpents the hand and arm have disappeared altogether.

Fig. 5.—Wing of Reptile, Mammal, and Bird. Drawn from nature (Brit. Mus.).

Thus, even if we confine our attention to a single organ, how wonderful are the modifications which it is seen to undergo, although never losing its typical character. Everywhere we find the distinction between homology and analogy which was explained in the last chapter—the distinction, that is, between correspondence of structure and correspondence of function. On the one hand, we meet with structures which are perfectly homologous and yet in no way analogous: the structural elements remain, but are profoundly modified so as to perform wholly different functions. On the other hand, we meet with structures which are perfectly analogous, and yet in no way homologous: totally different structures are modified to perform the same functions. How, then, are we to explain these things? By design manifested in special creation, or by descent with adaptive modification? If it is said by design manifested in special creation, we must suppose that the Deity formed an archetypal plan of certain structures, and that he determined to adhere to this plan through all the modifications which those structures exhibit. But, if so, why is it that some structures are selected as typical and not others? Why should the vertebral skeleton, for instance, be tortured into every conceivable variety of modification in order to subserve as great a variety of functions; while another structure, such as the eye, is made in different sub-kingdoms on fundamentally different plans, notwithstanding that it has throughout to perform the same function? Will any one have the hardihood to assert that in the case of the skeleton the Deity has endeavoured to show his ingenuity, by the manifold functions to which he has made the same structure subservient; while in the case of the eye he has endeavoured to show his resources, by the manifold structures which he has adapted to serve the same function? If so, it becomes a most unfortunate circumstance that, throughout both the vegetable and animal kingdoms, all cases which can be pointed to as showing ingenious adaptation of the same typical structure to the performance of widely different functions—or cases of homology without analogy,—are cases which come within the limits of the same natural group of plants and animals, and therefore admit of being equally well explained by descent from a common ancestry; while all cases of widely different structures performing the same function—or cases of analogy without homology,—are to be found in different groups of plants or animals, and are therefore suggestive of independent variations arising in the different lines of hereditary descent.

To take a specific illustration. The octopus, or devil-fish, belongs to a widely different class of animals from a true fish; and yet its eye, in general appearance, looks wonderfully like the eye of a true fish. Now, Mr. Mivart pointed to this fact as a great difficulty in the way of the theory of evolution by natural selection, because it must clearly be a most improbable thing that so complicated a structure as the eye of a fish should happen to be arrived at through each of two totally different lines of descent. And this difficulty would, indeed, be a formidable one to the theory of evolution, if the similarity were not only analogical but homological. Unfortunately for the objection, however, Darwin clearly showed in his reply that in no one anatomical or homologous feature do the two structures resemble one another; so that, in point of fact, the two organs do not resemble one another in any particular further than it is necessary that they should, if both are to be analogous, or to serve the same function as organs of sight. But now, suppose that this had not been the case, and that the two structures, besides presenting the necessary superficial or analogical resemblance, had also presented an anatomical or homologous resemblance, with what force might it have then been urged,—Your hypothesis of hereditary descent with progressive modification being here excluded by the fact that the animals compared belong to two widely different branches of the tree of life, how are we to explain the identity of type manifested by these two complicated organs of vision? The only hypothesis open to us is intelligent adherence to an ideal plan or mechanism. But as this cannot now be urged in any comparable case throughout the whole organic world, we may on the other hand present it as a most significant fact, that while within the limits of the same large branch of the tree of life we constantly find the same typical structures modified so as to perform very different functions, we never find any of these particular types of structure in other large branches of the tree. That is to say, we never find typical structures appearing except in cases where their presence may be explained by the hypothesis of hereditary descent; while in thousands of such cases we find these structures undergoing every conceivable variety of adaptive modification.

Consequently, special creationists must fall back upon another position and say,—Well, but it may have pleased the Deity to form a certain number of ideal types, and never to have allowed the structures occurring in one type to appear in any of the others. We answer,—Undoubtedly such may have been the case; but, if so, it is a most unfortunate thing for your theory, because the fact implies that the Deity has planned his types in such a way as to suggest the counter-theory of descent. For instance, it would seem most capricious on the part of the Deity to have made the eyes of an innumerable number of fish on exactly the same ideal type, and then to have made the eye of the octopus so exactly like these other eyes in superficial appearance as to deceive so accomplished a naturalist as Mr. Mivart, and yet to have taken scrupulous care that in no one ideal particular should the one type resemble the other. However, adopting for the sake of argument this great assumption, let us suppose that God did lay down these arbitrary rules for his own guidance in creation, and then let us see to what the assumption leads. If the Deity formed a certain number of ideal types, and determined that on no account should he allow any part of one type to appear in any part of another, surely we should expect that within the limits of the same type the same typical structures should always be present. Thus, remember what efforts, so to speak, have been made to maintain the uniformity of type in the case of the fore-limb as previously explained, and should we not expect that in other and similar cases a similar method should have been followed? Yet we repeatedly find that this is not the case. Even in the whale, as we have seen, the hind-limbs are either altogether absent or dwindled almost to nothing; and it is impossible to see in what respect the hind-limbs are of any less ideal value than the fore-limbs—which are carefully preserved in all vertebrated animals except the snakes, and the extinct Dinornis, where again we meet in this particular with a sudden and sublime indifference to the maintenance of a typical structure. (Fig. 6.)[4] Now I say that if the theory of ideal types is true, we have in these facts evidence of a most unreasonable inconsistency. But the theory of descent with continued adaptive modification fully explains all the known cases; for in every case the degree of divergence from the typical structure which an organism presents corresponds, in a general way, with the length of time during which the divergence has been going on. Thus we scarcely ever meet with any great departure from the typical form with respect to one of the organs, without some of the other organs being so far modified as of themselves to indicate, on the supposition of descent with modification, that the animal or plant must have been subject to the modifying influences for an enormously long series of generations. And this combined testimony of a number of organs in the same organism is what the theory of descent would lead us to expect, while the rival theory of design can offer no explanation of the fact, that when one organ shows a conspicuous departure from the supposed ideal type, some of the other organs in the same organism should tend to keep it company by doing likewise.

Fig. 6.—Skeleton of Dinornis gravis, 1/16 nat. size. Drawn from nature (Brit. Mus.). As separate cuts on a larger scale are shown, 1st, the sternum, as this appears in mounted skeletons, and, 2nd, the same in profile, with its (hypothetical) scapulo-coracoid attached.

As an illustration both of this and of other points which have been mentioned, I may draw attention to what seems to me a particularly suggestive case. So-called soldier-or hermit-crabs, are crabs which have adopted the habit of appropriating the empty shells of mollusks. In association with this peculiar habit, the structure of these animals differs very greatly from that of all other crabs. In particular, the hinder part of the body, which occupies the mollusk-shell, and which therefore has ceased to require any hard covering of its own, has been suffered to lose its calcareous integument, and presents a soft fleshy character, quite unlike that of the more exposed parts of the animal. Moreover, this soft fleshy part of the creature is specially adapted to the particular requirements of the creature by having its lateral appendages—i. e. appendages which in other crustacea perform the function of legs—modified so as to act as claspers to the inside of the mollusk-shell; while the tail-end of the part in question is twisted into the form of a spiral, which fits into the spiral of the mollusk-shell. Now, in Keeling Island there is a large kind of crab called Birgus latro, which lives upon land and there feeds upon cocoa-nuts. The whole structure of this crab, it seems to me, unmistakeably resembles the structure of a hermit-crab (see drawings on the next page, Fig. 7). Yet this crab neither lives in the shell of a mollusk, nor is the hinder part of its body in the soft and fleshy condition just described: on the contrary, it is covered with a hard integument like all the other parts of the animal. Consequently, I think we may infer that the ancestors of Birgus were hermit-crabs living in mollusk-shells; but that their descendants gradually relinquished this habit as they gradually became more and more terrestrial, while, concurrently with these changes in habit, the originally soft posterior parts acquired a hard protective covering to take the place of that which was formerly supplied by the mollusk-shell. So that, if so, we now have, within the limits of a single organism, evidence of a whole series of morphological changes in the past history of its species. First, there must have been the great change from an ordinary crab to a hermit-crab in all the respects previously pointed out. Next, there must have been the change back again from a hermit-crab to an ordinary crab, so far as living without the necessity of a mollusk-shell is concerned. From an evolutionary point of view, therefore, we appear to have in the existing structure of Birgus a morphological record of all these changes, and one which gives us a reasonable explanation of why the animal presents the extraordinary appearance which it does. But, on the theory of special creation, it is inexplicable why this land-crab should have been formed on the pattern of a hermit-crab, when it never has need to enter the shell of a mollusk. In other words, its peculiar structure is not specially in keeping with its present habits, although so curiously allied to the similar structure of certain other crabs of totally different habits, in relation to which the peculiarities are of plain and obvious significance.

Fig. 7.—Hermit-crabs compared with the cocoa-nut crab. On the left of the illustration one hermit-crab is represented as occupying a mollusk-shell, and another (larger specimen) as it appears when withdrawn from such a shell. On the right of the illustration the cocoa-nut crab is represented in its natural habitat on land. When full-grown, however, it is much larger than our hermit-crabs. The latter are drawn from life, natural size, the former from a specimen in the British Museum, 1/6 natural size.

I will devote the remainder of this chapter to considering another branch of the argument from morphology, to which the case of Birgus serves as a suitable introduction: I mean the argument from rudimentary structures.

Throughout both the animal and vegetable kingdoms we constantly meet with dwarfed and useless representatives of organs, which in other and allied kinds of animals and plants are of large size and functional utility. Thus, for instance, the unborn whale has rudimentary teeth, which are never destined to cut the gums; and throughout its life this animal retains, in a similarly rudimentary condition, a number of organs which never could have been of use to any kind of creature save a terrestrial quadruped. The whole anatomy of its internal ear, for example, has reference to hearing in air—or, as Hunter long ago remarked, “is constructed upon the same principle as in the quadruped"; yet, as Owen says, “the outer opening and passage leading therefrom to the tympanum can rarely be affected by sonorous vibrations of the atmosphere, and indeed they are reduced, or have degenerated, to a degree which makes it difficult to conceive how such vibrations can be propagated to the ear-drum during the brief moments in which the opening may be raised above the water.”

Fig. 8.—Rudimentary or vestigial hind-limbs of Python, as exhibited in the skeleton and on the external surface of the animal. Drawn from nature, ¼ nat. size (Zoological Gardens).

Now, rudimentary organs of this kind are of such frequent occurrence, that almost every species presents one or more of them—usually, indeed, a considerable number. How, then, are they to be accounted for? Of course the theory of descent with adaptive modification has a simple answer to supply—namely, that when, from changed conditions of life, an organ which was previously useful becomes useless, it will be suffered to dwindle away in successive generations, under the influence of certain natural causes which we shall have to consider in future chapters. On the other hand, the theory of special creation can only maintain that these rudiments are formed for the sake of adhering to an ideal type. Now, here again the former theory appears to be triumphant over the latter; for, without waiting to dispute the wisdom of making dwarfed and useless structures merely for the whimsical motive assigned, surely if such a method were adopted in so many cases, we should expect that in consistency it would be adopted in all cases. This reasonable expectation, however, is far from being realized. We have already seen that in numberless cases, such as that of the fore-limbs of serpents, no vestige of a rudiment is present. But the vacillating policy in the matter of rudiments does not end here; for it is shown in a still more aggravated form where within the limits of the same natural group of organisms a rudiment is sometimes present and sometimes absent. For instance, although in nearly all the numerous species of snakes there are no vestiges of limbs, in the Python we find very tiny rudiments of the hind-limbs. (Fig. 8.) Now, is it a worthy conception of Deity that, while neglecting to maintain his unity of ideal in the case of nearly all the numerous species of snakes, he should have added a tiny rudiment in the case of the Python—and even in that case should have maintained his ideal very inefficiently, inasmuch as only two limbs, instead of four, are represented? How much more reasonable is the naturalistic interpretation; for here the very irregularity of their appearance in different species, which constitutes rudimentary structures one of the crowning difficulties to the theory of special design, furnishes the best possible evidence in favour of hereditary descent; seeing that this irregularity then becomes what may be termed the anticipated expression of progressive dwindling due to inutility. Thus, for example, to return to the case of wings, we have already seen that in an extinct genus of bird, Dinornis, these organs were reduced to such an extent as to leave it still doubtful whether so much as the tiny rudiment hypothetically supplied to Fig. 6 (p. [61]) was present in all the species. And here is another well-known case of another genus of still existing bird, which, as was the case with Dinornis, occurs only in New Zealand. (Fig. 9.) Upon this island there are no four-footed enemies—either existing or extinct—to escape from which the wings of birds would be of any service. Consequently we can understand why on this island we should meet with such a remarkable dwindling away of wings.

Fig. 9.—Apteryx Australis. Drawn from life in the Zoological Gardens, 1/8 nat. size. The external wing is drawn to a scale in the upper part of the cut. The surroundings are supplied from the most recent descriptions.

Similarly, the logger-headed duck of South America can only flap along the surface of the water, having its wings considerably reduced though less so than the Apteryx of New Zealand. But here the interesting fact is that the young birds are able to fly perfectly well. Now, in accordance with a general law to be considered in a future chapter, the life-history of an individual organism is a kind of condensed recapitulation of the life-history of its species. Consequently, we can understand why the little chickens of the logger-headed duck are able to fly like all other ducks, while their parents are only able to flap along the surface of the water.

Facts analogous to this reduction of wings in birds which have no further use for them, are to be met with also in insects under similar circumstances. Thus, there are on the island of Madeira somewhere between 500 and 600 species of beetles, which are in large part peculiar to that island, though related to other—and therefore presumably parent—species on the neighbouring continent. Now, no less than 200 species—or nearly half the whole number—are so far deficient in wings that they cannot fly. And, if we disregard the species which are not peculiar to the island—that is to say, all the species which likewise occur on the neighbouring continent, and therefore, as evolutionists conclude, have but recently migrated to the island,—we find this very remarkable proportion. There are altogether 29 peculiar genera, and out of these no less than 23 have all their species in this condition.

Similar facts have been recently observed by the Rev. A. E. Eaton with respect to insects inhabiting Kerguelen Island. All the species which he found on the island—viz. a moth, several flies, and numerous beetles—he found to be incapable of flight; and therefore, as Wallace observes, “as these insects could hardly have reached the islands in a wingless state, even if there were any other known land inhabited by them, which there is not, we must assume that, like the Madeiran insects, they were originally winged, and lost their power of flight because its possession was injurious to them"—Kerguelen Island being “one of the stormiest places on the globe,” and therefore a place where insects could rarely afford to fly without incurring the danger of being blown out to sea.

Here is another and perhaps an even more suggestive class of facts.

It is now many years ago since the editors of Silliman’s Journal requested the late Professor Agassiz to give them his opinion on the following question. In a certain dark subterranean cave, called the Mammoth cave, there are found some peculiar species of blind fishes. Now the editors of Silliman’s Journal wished to know whether Prof. Agassiz would hold that these fish had been specially created in these caves, and purposely devoided of eyes which could never be of any use to them; or whether he would allow that these fish had probably descended from other species, but, having got into the dark cave, gradually lost their eyes through disuse. Prof. Agassiz, who was a believer in special creation, allowed that this ought to constitute a crucial test as between the two theories of special design and hereditary descent. “If physical circumstances,” he said, “ever modified organized beings, it should be easily ascertained here.” And eventually he gave it as his opinion, that these fish “were created under the circumstances in which they now live, within the limits over which they now range, and with the structural peculiarities which now characterise them.”

Since then a great deal of attention has been paid to the fauna of this Mammoth cave, and also to the faunas of other dark caverns, not only in the New, but also in the Old World. In the result, the following general facts have been fully established.

(1) Not only fish, but many representatives of other classes, have been found in dark caves.

(2) Wherever the caves are totally dark, all the animals are blind.

(3) If the animals live near enough to the entrance to receive some degree of light, they may have large and lustrous eyes.

(4) In all cases the species of blind animals are closely allied to species inhabiting the district where the caves occur; so that the blind species inhabiting American caves are closely allied to American species, while those inhabiting European caves are closely allied to European species.

(5) In nearly all cases structural remnants of eyes admit of being detected, in various degrees of obsolescence. In the case of some of the crustaceans of the Mammoth cave the foot-stalks of the eyes are present, although the eyes themselves are entirely absent.

Now, it is evident that all these general facts are in full agreement with the theory of evolution, while they offer serious difficulties to the theory of special creation. As Darwin remarks, it is hard to imagine conditions of life more similar than those furnished by deep limestone caverns under nearly the same climate in the two continents of America and Europe; so that, in accordance with the theory of special creation, very close similarity in the organizations of the two sets of faunas might have been expected. But, instead of this, the affinities of these two sets of faunas are with those of their respective continents—as of course they ought to be on the theory of evolution. Again, what would have been the sense of creating useless foot-stalks for the imaginary support of absent eyes, not to mention all the other various grades of degeneration in other cases? So that, upon the whole, if we agree with the late Prof. Agassiz in regarding these cave animals as furnishing a crucial test between the rival theories of creation and evolution, we must further conclude that the whole body of evidence which they now furnish is weighing on the side of evolution.

So much, then, for a few special instances of what Darwin called rudimentary structures, but what may be more descriptively designated—in accordance with the theory of descent—obsolescent or vestigial structures. It is, however, of great importance to add that these structures are of such general occurrence throughout both the vegetable and animal kingdoms, that, as Darwin has observed, it is almost impossible to point to a single species which does not present one or more of them. In other words, it is almost impossible to find a single species which does not in this way bear some record of its own descent from other species; and the more closely the structure of any species is examined anatomically, the more numerous are such records found to be. Thus, for example, of all organisms that of man has been most minutely investigated by anatomists; and therefore I think it will be instructive to conclude this chapter by giving a list of the more noteworthy vestigial structures which are known to occur in the human body. I will take only those which are found in adult man, reserving for the next chapter those which occur in a transitory manner during earlier periods of his life. But, even as thus restricted, the number of obsolescent structures which we all present in our own persons is so remarkable, that their combined testimony to our descent from a quadrumanous ancestry appears to me in itself conclusive. I mean, that even if these structures stood alone, or apart from any more general evidences of our family relationships, they would be sufficient to prove our parentage. Nevertheless, it is desirable to remark that of course these special evidences which I am about to detail do not stand alone. Not only is there the general analogy furnished by the general proof of evolution elsewhere, but there is likewise the more special correspondence between the whole of our anatomy and that of our nearest zoological allies. Now the force of this latter consideration is so enormous, that no one who has not studied human anatomy can be in a position to appreciate it. For without special study it is impossible to form any adequate idea of the intricacy of structure which is presented by the human form. Yet it is found that this enormously intricate organization is repeated in all its details in the bodies of the higher apes. There is no bone, muscle, nerve, or vessel of any importance in the one which is not answered to by the other. Hence there are hundreds of thousands of instances of the most detailed correspondence, without there being any instances to the contrary, if we pay due regard to vestigial characters. The entire corporeal structure of man is an exact anatomical copy of that which we find in the ape.

My object, then, here is to limit attention to those features of our corporeal structure which, having become useless on account of our change in attitude and habits, are in process of becoming obsolete, and therefore occur as mere vestigial records of a former state of things. For example, throughout the vertebrated series, from fish to mammals, there occurs in the inner corner of the eye a semi-transparent eye-lid, which is called the nictitating membrane. The object of this structure is to sweep rapidly, every now and then, over the external surface of the eye, apparently in order to keep the surface clean. But although the membrane occurs in all classes of the sub-kingdom, it is more prevalent in some than in others—e.g. in birds than in mammals. Even, however, where it does not occur of a size and mobility to be of any use, it is usually represented, in animals above fishes, by a functionless rudiment, as here depicted in the case of man. (Fig. 10.)

Fig. 10.—Illustrations of the nictitating membrane in the various animals named drawn from nature. The letter N indicates the membrane in each case. In man it is called the plica semilunaris, and is represented in the two lower drawings under this name. In the case of the shark (Galeus) the muscular mechanism is shown as dissected.

Now the organization of man presents so many vestigial structures thus referring to various stages of his long ancestral history, that it would be tedious so much as to enumerate them. Therefore I will yet further limit the list of vestigial structures to be given as examples, by not only restricting these to cases which occur in our own organization; but of them I shall mention only such as refer us to the very last stage of our ancestral history—viz. structures which have become obsolescent since the time when our distinctively human branch of the family tree diverged from that of our immediate forefathers, the Quadrumana.

Fig. 11.—Rudimentary, or vestigial and useless, muscles of the human ear. (From Gray’s Anatomy.)

(1) Muscles of the external ear.—These, which are of large size and functional use in quadrupeds, we Retain in a dwindled and useless condition (Fig. 11). this is likewise the case in anthropoid apes; but in not a few other quadrumana (e.g. baboons, macacus, magots, &c.) degeneration has not proceeded so far, and the ears are voluntarily moveable.

(2) Panniculus carnosis.—A large number of the mammalia are able to move their skin by means of sub-cutaneous muscle—as we see, for instance, in a horse, when thus protecting himself against the sucking of flies. We, in common with the Quadrumana, possess an active remnant of such a muscle in the skin of the forehead, whereby we draw up the eyebrows; but we are no longer able to use other considerable remnants of it, in the scalp and elsewhere,—or, more correctly, it is rarely that we meet with persons who can. But most of the Quadrumana (including the anthropoids) are still able to do so. There are also many other vestigial muscles, which occur only in a small percentage of human beings, but which, when they do occur, present unmistakeable homologies with normal muscles in some of the Quadrumana and still lower animals[5].

(3) Feet.—It is observable that in the infant the feet have a strong deflection inwards, so that the soles in considerable measure face one another. This peculiarity, which is even more marked in the embryo than in the infant (see p. [153]), and which becomes gradually less and less conspicuous even before the child begins to walk, appears to me a highly suggestive peculiarity. For it plainly refers to the condition of things in the Quadrumana, seeing that in all these animals the feet are similarly curved inwards, to facilitate the grasping of branches. And even when walking on the ground apes and monkeys employ to a great extent the outside edges of their feet, as does also a child when learning to walk. The feet of a young child are also extraordinarily mobile in all directions, as are those of apes. In order to show these points, I here introduce comparative drawings of a young ape and the portrait of a young male child. These drawings, moreover, serve at the same time to illustrate two other vestigial characters, which have often been previously noticed with regard to the infant’s foot. I allude to the incurved form of the legs, and the lateral extension of the great toe, whereby it approaches the thumb-like character of this organ in the Quadrumana. As in the case of the incurved position of the legs and feet, so in this case of the lateral extensibility of the great toe, the peculiarity is even more marked in embryonic than in infant life. For, as Prof. Wyman has remarked with regard to the fœtus when about an inch in length, “The great toe is shorter than the others; and, instead of being parallel to them, is projected at an angle from the side of the foot, thus corresponding with the permanent condition of this part in the Quadrumana[6].” So that this organ, which, according to Owen, “is perhaps the most characteristic peculiarity in the human structure,” when traced back to the early stages of its development, is found to present a notably less degree of peculiarity.

Fig. 12.—Portrait of a young male gorilla (after Hartmann).

Fig. 13.—Portrait of a young male child. Photographed from life, when the mobile feet were for a short time at rest in a position of extreme inflection.

(4) Hands.—Dr. Louis Robinson has recently observed that the grasping power of the whole human hand is so surprisingly great at birth, and during the first few weeks of infancy, as to be far in excess of present requirements on the part of a young child. Hence he concludes that it refers us to our quadrumanous ancestry—the young of anthropoid apes being endowed with similar powers of grasping, in order to hold on to the hair of the mother when she is using her arms for the purposes of locomotion. This inference appears to me justifiable, inasmuch as no other explanation can be given of the comparatively inordinate muscular force of an infant’s grip. For experiments showed that very young babies are able to support their own weight, by holding on to a horizontal bar, for a period varying from one half to more than two minutes[7]. With his kind permission I here reproduce one of Dr. Robinson’s instantaneous, and hitherto unpublished, photographs of a very young infant. This photograph was taken after the above paragraph (3) was written, and I introduce it here because it serves to show incidentally—and perhaps even better than the preceding figure—the points there mentioned with regard to the feet and great toes. Again, as Dr. Robinson observes, the attitude, and the disproportionately large development of the arms as compared with the legs, give all the photographs a striking resemblance to a picture of the chimpanzee “Sally” at the Zoological Gardens. For “invariably the thighs are bent nearly at right angles to the body, and in no case did the lower limbs hang down and take the attitude of the erect position.” He adds, “In many cases no sign of distress is evinced, and no cry uttered, until the grasp begins to give way.”

Fig. 14.—An infant, three weeks old, supporting its own weight for over two minutes. The attitude of the lower limbs, feet, and toes, is strikingly simian. Reproduced from an instantaneous photograph, kindly given for the purpose by Dr. L. Robinson.

Fig. 15.—Sacrum of Gorilla compared with that of Man, showing the rudimentary tail-bones of each. Drawn from nature (R. Coll. Surg. Mus.).

(5) Tail.—The absence of a tail in man is popularly supposed to constitute a difficulty against the doctrine of his quadrumanous descent. As a matter of fact, however, the absence of an external tail in man is precisely what this doctrine would expect, seeing that the nearest allies of man in the quadrumanous series are likewise destitute of an external tail. Far, then, from this deficiency in man constituting any difficulty to be accounted for, if the case were not so—i. e. if man did possess an external tail,—the difficulty would be to understand how he had managed to retain an organ which had been renounced by his most recent ancestors. Nevertheless, as the anthropoid apes continue to present the rudimentary vestiges of a tail in a few caudal vertebræ below the integuments, we might well expect to find a similar state of matters in the case of man. And this is just what we do find, as a glance at these two comparative illustrations will show. (Fig. 15.) Moreover, during embryonic life, both of the anthropoid apes and of man, the tail much more closely resembles that of the lower kinds of quadrumanous animals from which these higher representatives of the group have descended. For at a certain stage of embryonic life the tail, both of apes and of human beings, is actually longer than the legs (see Fig. 16). And at this stage of development, also, the tail admits of being moved by muscles which later on dwindle away. Occasionally, however, these muscles persist, and are then described by anatomists as abnormalities. The following illustrations serve to show the muscles in question, when thus found in adult man.

Fig. 16.—Diagrammatic outline of the human embryo when about seven weeks old, showing the relations of the limbs and tail to the trunk (after Allen Thomson), r, the radial, and u, the ulnar, border of the hand and fore-arm; t, the tibial, and f, the fibular, border of the foot and lower leg; au, ear; s, spinal cord; v, umbilical cord; b, branchial gill-slits; c, tail.

Fig. 17.—Front and back view of adult human sacrum, showing abnormal persistence of vestigial tail-muscles. (The first drawing is copied from Prof. Watson’s paper in Journl. Anat. and Physiol., vol. 79: the second is compiled from different specimens.)

(6) Vermiform Appendix of the Cæcum.—This is of large size and functional use in the process of digestion among many herbivorous animals; while in man it is not only too small to serve any such purpose, but is even a source of danger to life—many persons dying every year from inflammation set up by the lodgement in this blind tube of fruit-stones, &c.

In the orang it is longer than in man (Fig. 18), as it is also in the human fœtus proportionally compared with the adult. (Fig. 19.) In some of the lower herbivorous animals it is longer than the entire body.

Fig. 18.—Appendix vermiformis in Orang and in Man. Drawn from dried inflated specimens in the Cambridge Museum by Mr. J. J. Lister. Il, ilium; Co, colon; C, cæcum; W, a window cut in the wall of the cæcum; X X X, the appendix.

Fig. 19.—The same, showing variation in the Orang. Drawn from a specimen in the Museum of the Royal College of Surgeons.

Like vestigial structures in general, however, this one is highly variable. Thus the above cut (Fig. 19) serves to show that it may sometimes be almost as short in the orang as it normally is in man—both the human subjects of this illustration having been normal.

(7) Ear.—Mr. Darwin writes:—

The celebrated sculptor, Mr. Woolner, informs me of one little peculiarity in the external ear, which he has often observed both

Fig. 20.—Human ear, modelled and drawn by Mr. Woolner. a, the projecting point. in men and women.... The peculiarity consists in a little blunt point, projecting from the inwardly folded margin, or helix. When present, it is developed at birth, and, according to Prof. Ludwig Meyer, more frequently in man than in woman. Mr. Woolner made an exact model of one such case, and sent me the accompanying drawing.... The helix obviously consists of the extreme margin of the ear folded inwards; and the folding appears to be in some manner connected with the whole external ear being permanently pressed backwards. In many monkeys, which do not stand high in the order, as baboons and some species of macacus, the upper portion of the ear is slightly pointed, and the margin is not at all folded inwards; but if the margin were to be thus folded, a slight point would necessarily project towards the centre.... The following wood-cut is an accurate copy of a photograph of the fœtus of an orang (kindly sent me by Dr. Nitsche), in which it may be seen how different the pointed outline of the ear is at this period from its adult condition, when it bears a close general resemblance to that of man [including even the occasional appearance of the projecting point shown in the preceding woodcut]. It is evident that the folding over of the tip of such an ear, unless it changed greatly during its further development, would give rise to a point projecting inwards[8].

Fig. 21.—Fœtus of an Orang. Exact copy of a photograph, showing the form of the ear at this early stage.

The following woodcut serves still further to show vestigial resemblances between the human ear and that of apes. The last two figures illustrate the general resemblance between the normal ear of fœtal man and the ear of an adult orang-outang. The other two figures on the lower line are intended to exhibit occasional modifications of the adult human ear, which approximate simian characters somewhat more closely than does the normal type. It will be observed that in their comparatively small lobes these ears resemble those of all the apes; and that while the outer margin of one is not unlike that of the Barbary ape, the outer margin of the other follows those of the chimpanzee and orang. Of course it would be easy to select individual human ears which present either of these characters in a more pronounced degree; but these ears have been chosen as models because they present both characters in conjunction. The upper row of figures likewise shows the close similarity of hair-tracts, and the direction of growth on the part of the hair itself, in cases where the human ear happens to be of an abnormally hirsute character. But this particular instance (which I do not think has been previously noticed) introduces us to the subject of hair, and hair-growth, in general.

Fig. 22.—Vestigial characters of human ears. Drawn from nature.

(8) Hair.—Adult man presents rudimentary hair over most parts of the body. Wallace has sought to draw a refined distinction between this vestigial coating and the useful coating of quadrumanous animals, in the absence of the former from the human back. But even this refined distinction does not hold. On the one hand, the comparatively hairless chimpanzee which died last year in the Zoological Gardens (T. calvus) was remarkably denuded over the back; and, on the other hand, men who present a considerable development of hair over the rest of their bodies present it also on their backs and shoulders. Again, in all men the rudimentary hair on the upper and lower arm is directed towards the elbow—a peculiarity which occurs nowhere else in the animal kingdom, with the exception of the anthropoid apes and a few American monkeys, where it presumably has to do with arboreal habits. For, when sitting in trees, the orang, as observed by Mr. Wallace, places its hands above its head with its elbows pointing downwards: the disposition of hair on the arms and fore-arms then has the effect of thatch in turning the rain. Again, I find that in all species of apes, monkeys, and baboons which I have examined (and they have been numerous), the hair on the backs of the hands and feet is continued as far as the first row of phalanges; but becomes scanty, or disappears altogether, on the second row; while it is invariably absent on the terminal row. I also find that the same peculiarity occurs in man. We all have rudimentary hair on the first row of phalanges, both of hands and feet: when present at all, it is more scanty on the second row; and in no case have I been able to find any on the terminal row. In all cases these peculiarities are congenital, and the total absence or partial presence of hair on the second phalanges is constant in different species of Quadrumana. For instance, it is entirely absent in all the chimpanzees, which I have examined, while scantily present in all the orangs. As in man, it occurs in a patch midway between the joints.

Fig. 23.—Hair-tracts on the arms and hands of Man, as compared with those on the arms and hands of Chimpanzee. Drawn from life.

Besides showing these two features with regard to the disposition of hair on the human arm and hand, the above woodcut illustrates a third. By looking closely at the arm of the very hairy man from whom the drawing was taken, it could be seen that there was a strong tendency towards a whorled arrangement of the hairs on the backs of the wrists. This is likewise, as a general rule, a marked feature in the arrangement of hair on the same places in the gorilla, orang, and chimpanzee. In the specimen of the latter, however, from which the drawing was taken, this characteristic was not well marked. The downward direction of the hair on the backs of the hands is exactly the same in man as it is in all the anthropoid apes. Again, with regard to hair, Darwin notices that occasionally there appears in man a few hairs in the eyebrows much longer than the others; and that they seem to be representative of similarly long and scattered hairs which occur in the chimpanzee, macacus, and baboons.

Lastly, it may be here more conveniently observed than in the next chapter on Embryology, that at about the sixth month the human fœtus is often thickly coated with somewhat long dark hair over the entire body, except the soles of the feet and palms of the hands, which are likewise bare in all quadrumanous animals. This covering, which is called the lanugo, and sometimes extends even to the whole forehead, ears, and face, is shed before birth. So that it appears to be useless for any purpose other than that of emphatically declaring man a child of the monkey.

(9) Teeth.—Darwin writes:—

It appears as if the posterior molar or wisdom-teeth were tending to become rudimentary in the more civilized races of man. These teeth are rather smaller than the other molars, as is likewise the case with the corresponding teeth in the chimpanzee and orang; and they have only two separate fangs.... They are also much more liable to vary, both in structure and in the period of their development, than the other teeth. In the Melanian races, on the other hand, the wisdom-teeth are usually furnished with three separate fangs, and are usually sound [i. e. not specially liable to decay]; they also differ from the other molars in size, less than in the Caucasian races.

Now, in addition to these there are other respects in which the dwindling condition of wisdom-teeth is manifested—particularly with regard to the pattern of their crowns. Indeed, in this respect it would seem that even in the anthropoid apes there is the beginning of a tendency to degeneration of the molar teeth from behind forwards. For if we compare the three molars in the lower jaw of the gorilla, orang, and chimpanzee, we find that the gorilla has five well-marked cusps on all three of them; but that in the orang the cusps are not so pronounced, while in the chimpanzee there are only four of them on the third molar. Now in man it is only the first of these three teeth which normally presents five cusps, both the others presenting only four. So that, comparing all these genera together, it appears that the number of cusps is being reduced from behind forwards; the chimpanzee having lost one of them from the third molar, while man has not only lost this, but also one from the second molar,—and, it may be added, likewise partially (or even totally) from the first molar, as a frequent variation among civilized races. But, on the other hand, variations are often met with in the opposite direction, where the second or the third molar of man presents five cusps—in the one case following the chimpanzee, in the other the gorilla. These latter variations, therefore, may fairly be regarded as reversionary. For these facts I am indebted to the kindness of Mr. C. S. Tomes.

Fig. 24.—Molar teeth of lower jaw in Gorilla, Orang, and Man. Drawn from nature, nat. size (R. Mus. Coll. Surg.).

(10) Perforations of the humerus.—The peculiarities which we have to notice under this heading are two in number. First, the supra condyloid foramen is a normal feature in some of the lower Quadrumana (Fig. 25), where it gives passage to the great nerve of the fore-arm, and often also to the great artery. In man, however, it is not a normal feature. Yet it occurs in a small percentage of cases—viz., according to Sir W. Turner, in about one per cent., and therefore is regarded by Darwin as a vestigial character. Secondly, there is inter-condyloid foramen, which is also situated near the lower end of the humerus, but more in the middle of the bone. This occurs, but not constantly, in apes, and also in the human species. From the fact that it does so much more frequently in the bones of ancient—and also of some savage—races of mankind (viz. in 20 to 30 per cent. of cases), Darwin is disposed to regard it also as a vestigial feature. On the other hand, Prof. Flower tells me that in his opinion it is but an expression of impoverished nutrition during the growth of the bone.

Fig. 25.—Perforation of the humerus (supra-condyloid foramen) in three species of Quadrumana where it normally occurs, and in Man, where it does not normally occur. Drawn from nature (R. Coll. Surg. Mus.).

(11) Flattening of tibia.—In some very ancient human skeletons, there has also been found a lateral flattening of the tibia, which rarely occurs in any existing human beings, but which appears to have been usual among the earliest races of mankind hitherto discovered. According to Broca, the measurements of these fossil human tibiæ resemble those of apes. Moreover, the bone is bent and strongly convex forwards, while its angles are so rounded as to present the nearly oval section seen in apes. It is in association with these ape-like human tibiæ that perforated humeri of man are found in greatest abundance.

On the other hand, however, there is reason to doubt whether this form of tibia in man is really a survival from his quadrumanous ancestry. For, as Boyd-Dawkins and Hartmann have pointed out, the degree of flattening presented by some of these ancient human bones is greater than that which occurs in any existing species of anthropoid ape. Of course the possibility remains that the unknown species of ape from which man descended may have had its tibia more flattened than is now observable in any of the existing species. Nevertheless, as some doubt attaches to this particular case, I do not press it—and, indeed, only mention it at all in order that the doubt may be expressed.

Similarly, I will conclude by remarking that several other instances of the survival of vestigial structures in man have been alleged, which are of a still more doubtful character. Of such, for example, are the supposed absence of the genial tubercle in the case of a very ancient jaw-bone of man, and the disposition of valves in human veins. From the former it was argued that the possessor of this very ancient jaw-bone was probably speechless, inasmuch as the tubercle in existing man gives attachment to muscles of the tongue. From the latter it has been argued that all the valves in the veins of the human body have reference, in their disposition, to the incidence of blood-pressure when the attitude of the body is horizontal, or quadrupedal. Now, the former case has already broken down, and I find that the latter does not hold. But we can well afford to lose such doubtful and spurious cases, in view of all the foregoing unquestionable and genuine cases of vestigial structures which are to be met with even within the limits of our own organization—and even when these limits are still further limited by selecting only those instances which refer to the very latest chapter of our long ancestral history.

CHAPTER IV.
Embryology.

We will next consider what of late years has become the most important of the lines of evidence, not only in favour of the general fact of evolution, but also of its history: I mean the evidence which has been yielded by the newest of the sciences, the science of Embryology. But here, as in the analogous case of adult morphology, in order to do justice to the mass of evidence which has now been accumulated, a whole volume would be necessary. As in that previous case, therefore, I must restrict myself to giving an outline sketch of the main facts.

First I will display what in the language of Paley we may call “the state of the argument.”

It is an observable fact that there is often a close correspondence between developmental changes as revealed by any chronological series of fossils which may happen to have been preserved, and developmental changes which may be observed during the life-history of now existing individuals belonging to the same group of animals. For instance, the successive development of prongs in the horns of deer-like animals, which is so clearly shown in the geological history of this tribe, is closely reproduced in the life-history of existing deer. Or, in other words, the antlers of an existing deer furnish in their development a kind of résumé, or recapitulation, of the successive phases whereby the primitive horn was gradually superseded by horns presenting a greater and greater number of prongs in successive species of extinct deer (Fig. 26). Now it must be obvious that such a recapitulation in the life-history of an existing animal of developmental changes successively distinctive of sundry allied, though now extinct species, speaks strongly in favour of evolution. For as it is of the essence of this theory that new forms arise from older forms by way of hereditary descent, we should antecedently expect, if the theory is true, that the phases of development presented by the individual organism would follow, in their main outlines, those phases of development through which their long line of ancestors had passed. The only alternative view is that as species of deer, for instance, were separately created, additional prongs were successively added to their antlers; and yet that, in order to be so added to successive species every individual deer belonging to later species was required to repeat in his own lifetime the process of successive additions which had previously taken place in a remote series of extinct species. Now I do not deny that this view is a possible view; but I do deny that it is a probable one. According to the evolutionary interpretation of such facts, we can see a very good reason why the life-history of the individual is thus a condensed résumé of the life-history of its ancestral species. But according to the opposite view no reason can be assigned why such should be the case. In a previous chapter—the chapter on Classification—we have seen that if each species were created separately, no reason can be assigned why they should all have been turned out upon structural patterns so strongly suggestive of hereditary descent with gradual modifications, or slow divergence—the result being group subordinated to group, with the most generalized (or least developed) forms at the bottom, and the highest products of organization at the top. And now we see—or shall immediately see—that this consideration admits of being greatly fortified by a study of the developmental history of every individual organism. If it would be an unaccountable fact that every separately created species should have been created with close structural resemblances to a certain limited number of other species, less close resemblances to certain further species, and so backwards; assuredly it would be a still more unaccountable fact that every individual of every species should exhibit in its own person a history of developmental change, every term of which corresponds with the structural peculiarities of its now extinct predecessors—and this in the exact historical order of their succession in geological time. The more that we think about this antithesis between the naturalistic and the non-naturalistic interpretations, the greater must we feel the contrast in respect of rationality to become; and, therefore, I need not spend time by saying anything further upon the antecedent standing of the two theories in this respect. The evidence, then, which I am about to adduce from the study of development in the life-histories of individual organisms, will be regarded by me as so much unquestionable evidence in favour of similar processes of development in the life-histories of their respective species—in so far, I mean, as the two sets of changes admit of being proved parallel.

Fig. 26.—Antlers of Stag, showing successive addition of branches in successive years. Drawn from nature (Brit. Mus.).

In the only illustration hitherto adduced—viz. that of deers’ horns—the series of changes from a one-pronged horn to a fully developed arborescent antler, is a series which takes place during the adult life of the animal; for it is only when the breeding age has been attained that horns are required to appear. But seeing that every animal passes through most of the phases of its development, not only before the breeding age has been attained, but even before the time of its own birth, clearly the largest field for the study of individual development is furnished by embryology. For instance, there is a salamander which differs from most other salamanders in being exclusively terrestrial in its habits. Now, the young of this salamander before their birth are found to be furnished with gills, which, however, they are never destined to use. Yet these gills are so perfectly formed, that if the young salamanders be removed from the body of their mother shortly before birth, and be then immediately placed in water, the little animals show themselves quite capable of aquatic respiration, and will merrily swim about in a medium which would quickly drown their own parent. Here, then, we have both morphological and physiological evidence pointing to the possession of gills by the ancestors of the land salamander.

It would be easy to devote the whole of the present chapter to an enumeration of special instances of the kinds thus chosen for purposes of illustration; but as it is desirable to take a deeper, and therefore a more general view of the whole subject, I will begin at the foundation, and gradually work up from the earliest stages of development to the latest. Before starting, however, I ask the reader to bear in mind one consideration, which must reasonably prevent our anticipating that in every case the life-history of an individual organism should present a full recapitulation of the life-history of its ancestral line of species. Supposing the theory of evolution to be true, it must follow that in many cases it would have been more or less disadvantageous to a developing type that it should have been obliged to reproduce in its individual representatives all the phases of development previously undergone by its ancestry—even within the limits of the same family. We can easily understand, for example, that the waste of material required for building up the useless gills of the embryonic salamanders is a waste which, sooner or later, is likely to be done away with; so that the fact of its occurring at all is in itself enough to show that the change from aquatic to terrestrial habits on the part of this species must have been one of comparatively recent occurrence. Now, in as far as it is detrimental to a developing type that it should pass through any particular ancestral phases of development, we may be sure that natural selection—or whatever other adjustive causes we may suppose to have been at work in the adaptation of organisms to their surroundings—will constantly seek to get rid of this necessity, with the result, when successful, of dropping out the detrimental phases. Thus the foreshortening of developmental history which takes place in the individual lifetime may be expected often to take place, not only in the way of condensation, but also in the way of excision. Many pages of ancestral history may be recapitulated in the paragraphs of embryonic development, while others may not be so much as mentioned. And that this is the true explanation of what embryologists term “direct” development—or of a more or less sudden leap from one phase to another, without any appearance of intermediate phases—is proved by the fact that in some cases both direct and indirect development occur within the same group of organisms, some genera or families having dropped out the intermediate phases which other genera or families retain.

The argument from embryology must be taken to begin with the first beginning of individual life in the ovum. And, in order to understand the bearings of the argument in this its first stage, we must consider the phenomena of reproduction in the simplest form which these phenomena are known to present.

The whole of the animal kingdom is divided into two great groups, which are called the Protozoa and the Metazoa. Similarly, the whole of the vegetable kingdom is divided into the Protophyta and the Metaphyta. The characteristic feature of all the Protozoa and Protophyta is that the organism consists of a single physiological cell, while the characteristic of all the Metazoa and Metaphyta is that the organism consists of a plurality of physiological cells, variously modified to subserve different functions in the economy of the animal or plant, as the case may be. For the sake of brevity, I shall hereafter deal only with the case of animals (Protozoa and Metazoa); but it may throughout be understood that everything which is said applies also to the case of plants (Protophyta and Metaphyta).

A Protozoön (like a Protophyton) is a solitary cell, or a “unicellular organism,” while a Metazoön (like a Metaphyton) is a society of cells, or a “multicellular organism.” Now, it is only in the multicellular organisms that there is any observable distinction of sex. In all the unicellular organisms the phenomena of reproduction appear to be more or less identical with those of growth. Nevertheless, as these phenomena are here in some cases suggestively peculiar, I will consider them more in detail.

A Protozoön is a single corpuscle of protoplasm which in different species of Protozoa varies in size from more than one inch to less than 1/1000 of an inch in diameter. In some species there is an enveloping cortical substance; in other species no such substance can be detected. Again, in most species there is a nucleus, while in other species no such differentiation of structure has hitherto been observed. Nevertheless, from the fact that the nucleus occurs in the majority of Protozoa, coupled with the fact that the demonstration of this body is often a matter of extreme difficulty, not only in some of the Protozoa where it has been but recently detected, but also in the case of certain physiological cells elsewhere,—from these facts it is not unreasonable to suppose that all the Protozoa possess a nucleus, whether or not it admits of being rendered visible by histological methods thus far at our disposal. If this is the case, we should be justified in saying, as I have said, that a Protozoön is an isolated physiological cell, and, like cells in general, multiplies by means of what Spencer and Häckel have aptly called a process of discontinuous growth. That is to say, when a cell reaches maturity, further growth takes place in the direction of a severance of its substance—the separated portion thus starting anew as a distinct physiological unit. But, notwithstanding the complex changes which have been more recently observed to take place in the nucleus of some Protozoa prior to their division, the process of multiplication by division may still be regarded as a process of growth, which differs from the previous growth of the individual cell in being attended by a severance of continuity. If we take a suspended drop of gum, and gradually add to its size by allowing more and more gum to flow into it, a point will eventually be reached at which the force of gravity will overcome that of cohesion, and a portion of the drop will fall away from the remainder. Here we have a rough physical simile, although of course no true analogy. In virtue of a continuous assimilation of nutriment, the protoplasm of a cell increases in mass, until it reaches the size at which the forces of disruption overcome those of cohesion—or, in other words, the point at which increase of size is no longer compatible with continuity of substance. Nevertheless, it must not be supposed that the process is thus merely a physical one. The phenomena which occur even in the simplest—or so-called “direct"—cell-division, are of themselves enough to prove that the process is vital, or physiological; and this in a high degree of specialization. But so, likewise, are all processes of growth in organic structures; and therefore the simile of the drop of gum is not to be regarded as a true analogy: it serves only to indicate the fact that when cell-growth proceeds beyond a certain point cell-division ensues. The size to which cells may grow before they thus divide is very variable in different kinds of cells; for while some may normally attain a length of ten or twelve inches, others divide before they measure 1/1000 of an inch. This, however, is a matter of detail, and does not affect the general physiological principles on which we are at present engaged.

Now, as we have seen, a Protozoön is a single cell; for even although in some of the higher forms of protozoal life a colony of cells may be bound together in organic connexion, each of these cells is in itself an “individual,” capable of self-nourishment, reproduction, and, generally, of independent existence. Consequently, when the growth of a Protozoön ends in a division of its substance, the two parts wander away from each other as separate organisms. (Fig. 27.)

Fig. 27.—Fission of a Protozoön. In the left-hand drawing the process is represented as having advanced sufficiently far to have caused a division and segregation both of the nucleus and the vesicle. In the right-hand drawing the process is represented as complete. n, N, severed nucleus; vc, severed vesicle; ps, pseudopodia; f, ingested food.

The next point we have to observe is, that in all cases where a cell or a Protozoön multiplies by way of fissiparous division, the process begins in the nucleus. If the nucleus divides into two parts, the whole cell will eventually divide into two parts, each of which retains a portion of the original nucleus, as represented in the above figure. If the nucleus divides into three, four, or even, as happens in the development of some embryonic tissues, into as many as six parts, the cell will subdivide into a corresponding number, each retaining a portion of the nucleus. Therefore, in all cases of fissiparous division, the seat or origin of the process is the nucleus.

Thus far, then, the phenomena of multiplication are identical in all the lowest or unicellular organisms, and in the constituent cells of all the higher or multicellular. And this is the first point which I desire to make apparent. For where the object is to prove a continuity between the phenomena of growth and reproduction, it is of primary importance to show—1st, that there is such a continuity in the case of all the unicellular organisms, and, 2nd, that there are all the above points of resemblance between the multiplication of cells in the unicellular and in the multicellular organisms.

It remains to consider the points of difference, and, if possible, to show that these do not go to disprove the doctrine of continuity which the points of resemblance so forcibly indicate.

The first point of difference obviously is, that in the case of all the multicellular organisms the two or more “daughter-cells,” which are produced by division of the “mother-cell,” do not wander away from one another; but, as a rule, they continue to be held in more or less close apposition by means of other cells and binding membranes,—with the result of giving rise to those various “tissues,” which in turn go to constitute the material of “organs.” I cannot suppose, however, that any advocate of discontinuity will care to take his stand at this point. But, if any one were so foolish as to do so, it would be easy to dislodge him by describing the state of matters in some of the Protozoa where a number of unicellular “individuals” are organically united so as to form a “colony.” These cases serve to bridge this distinction between Protozoa and Metazoa, of which therefore we may now take leave.

In the second place, there is the no less obvious distinction that the result of cell-division in the Metazoa is not merely to multiply cells all of the same kind: on the contrary, the process here gives rise to as many different kinds of cells as there are different kinds of tissue composing the adult organism. But no one, I should think, is likely to oppose the doctrine of continuity on the ground of this distinction. For the distinction is clearly one which must necessarily arise, if the doctrine of continuity between unicellular and multicellular organisms be true. In other words, it is a distinction which the theory of evolution itself must necessarily pre-suppose, and therefore it is no objection to the theory that its pre-supposition is realized. Moreover, as we shall see better presently, there is no difficulty in understanding why this distinction should have arisen, so soon as it became necessary (or desirable) that individual cells, when composing a “colony,” should conform to the economic principle of the division of labour—a principle, indeed, which is already foreshadowed in the constituent parts of a single cell, since the nucleus has one set of functions and its surrounding protoplasm another.

But now, in the third place, we arrive at a more important distinction, and one which lies at the root of the others still remaining to be considered. I refer to sexual propagation. For it is a peculiarity of the multicellular organisms that, although many of them may likewise propagate themselves by other means (Fig. 28), they all propagate themselves by means of sexual congress. Now, in its essence, sexual congress consists in the fusion of two specialized cells (or, as now seems almost certain, of the nuclei thereof), so that it is out of such a combination that the new individual arises by means of successive cell-divisions, which, beginning in the fertilized ovum, eventually build up all the tissues and organs of the body.

Fig. 28.—Hydra viridis, partly in section. M, mouth; O, ovary, or bud containing female reproductive cells; T, testis, or bud containing male reproductive cells. In addition to these buds containing germinal elements alone, there is another which illustrates the process of “gemmation"—i. e. the direct out-growth of a fully formed offspring.

This process clearly indicates very high specialization on the part of germ-cells. For we see by it that although these cells when young resemble all other cells in being capable of self-multiplication by binary division (thus reproducing cells exactly like themselves), when older they lose this power; but, at the same time, they acquire an entirely new and very remarkable power of giving rise to a vast succession of many different kinds of cells, all of which are mutually correlated as to their several functions, so as to constitute a hierarchy of cells—or, to speak literally, a multicellular co-organization. Here it is that we touch the really important distinction between the Protozoa and the Metazoa; for although I have said that some of the higher Protozoa foreshadow this state of matters in forming cell-colonies, it must now be noted that the cells composing such colonies are all of the same kind; and, therefore, that the principle of producing different kinds of cells which, by mutual co-adaptation of functions, shall be capable of constructing a multicellular Metazoön,—this great principle of co-organization is but dimly nascent in the cell-colonies of Protozoa. And its marvellous development in the Metazoa appears ultimately to depend upon the highly specialized character of germ-cells. Even in cases where multicellular organisms are capable of reproducing their kind without the need of any preceding process of fertilization (parthenogenesis), and even in the still more numerous cases where complete organisms are budded forth from any part of their parent organism (gemmation, Fig. 28), there is now very good reason to conclude that these powers of a-sexual reproduction on the part of multicellular organisms are all ultimately due to the specialized character of their germ-cells. For in all these cases the tissues of the parent, from which the budding takes place, were ultimately derived from germ-cells—no matter how many generations of budded organisms may have intervened. And that propagation by budding, &c., in multicellular organisms is thus ultimately due to their propagation by sexual methods, seems to be further shown by certain facts which will have to be discussed at some length in my next volume. Here, therefore, I will mention only one of them—and this because it furnishes what appears to be another important distinction between the Protozoa and the Metazoa.

In nearly all cases where a Protozoön multiplies itself by fission, the process begins by a simple division of the nucleus. But when a Metazoön is developed from a germ-cell, although the process likewise begins by a division of the nucleus, this division is not a simple or direct one; on the contrary, it is inaugurated by a series of processes going on within the nucleus, which are so enormously complex, and withal so beautifully ordered, that to my mind they constitute the most wonderful—if not also the most suggestive—which have ever been revealed by microscopical research. It is needless to say that I refer to the phenomena of karyokinesis. A few pages further on they will be described more fully. For our present purposes it is sufficient to give merely a pictorial illustration of their successive phases; for a glance at such a representation serves to reveal the only point to which attention has now to be drawn—namely, the immense complexity of the processes in question, and therefore the contrast which they furnish to the simple (or “direct") division of the nucleus preparatory to cell-division in the unicellular organisms. Here, then (Fig. 29), we see the complex processes of karyokinesis in the first two stages of egg-cell division. But similar processes continue to repeat themselves in subsequent stages; and this, there is now good reason to believe, throughout all the stages of cell-division, whereby the original egg-cell eventually constructs an entire organism. In other words, all the cells composing all the tissues of a multicellular organism, at all stages of its development, are probably originated by these complex processes, which differ so much from the simple process of direct division in the unicellular organisms[9]. In this important respect, therefore, it does at first sight appear that we have a distinction between the Protozoa and the Metazoa of so pronounced a character, as fairly to raise the question whether cell-division is fundamentally identical in unicellular and in multicellular organisms.

Fig. 29.—Successive stages in the division of the ovum, or egg-cell, of a worm. (After Strasburger.) a to d show the changes taking place in the nucleus and surrounding cell-contents, which result in the first segmentation of the ovum at e; f and g show a repetition of these changes in each of the two resulting cells, leading to the second segmentation stage at h.

Lastly, the only other distinction of a physiologically significant kind between a single cell when it occurs as a Protozoön and when it does so as the unfertilized ovum of a Metazoön is, that in the latter case the nucleus discharges from its own substance two minute protoplasmic masses ("polar bodies"), which are then eliminated from the cell altogether. This process, which will be more fully described later on, appears to be of invariable occurrence in the case of all egg-cells, while nothing resembling it has ever been observed in any of the Protozoa.

We must now consider these several points of difference seriatim.

First, with regard to sexual propagation, we have already seen that this is by no means the only method of propagation among the multicellular organisms; and it now remains to add that, on the other hand, there is, to say the least, a suggestive foreshadowing of sexual propagation among the unicellular organisms. For although simple binary fission is here the more usual mode of multiplication, very frequently two (rarely three or more) Protozoa of the same species come together, fuse into a single mass, and thus become very literally “one flesh.” This process of “conjugation” is usually (though by no means invariably) followed by a period of quiescent “encystation"; after which the contents of the cyst escape in the form of a number of minute particles, or “spores,” and these severally develope into the parent type. Obviously this process of conjugation, when it is thus a preliminary to multiplication, appears to be in its essence the same as fertilization. And if it be objected that encystation and spore-formation in the Protozoa are not always preceded by conjugation, the answer would be that neither is oviparous propagation in the Metazoa invariably preceded by fertilization.

Nevertheless, that there are great distinctions between true sexual propagation and this foreshadowing of it in conjugation I do not deny. The question, however, is whether they be so great as to justify any argument against an historical continuity between them. What, then, are these remaining distinctions? Briefly, as we have seen, they are the extrusion from egg-cells of polar bodies, and the occurrence, both in egg-cells and their products (tissue-cells), of the process of karyokinesis. But, as regards the polar bodies, it is surely not difficult to suppose that, whatever their significance may be, it is probably in some way or another connected with the high specialization of the functions which an egg-cell has to discharge. Nor is there any difficulty in further supposing that, whatever purpose is served by getting rid of polar bodies, the process whereby they are got rid of was originally one of utilitarian development—i. e. a process which at its commencement did not betoken any difference of kind, or breach of continuity, between egg-cells and cells of simpler constitution.

Lastly, with respect to karyokinesis, although it is true that the microscope has in comparatively recent years displayed this apparently important distinction between unicellular and multicellular organisms, two considerations have here to be supplied. The first is, that in some of the Protozoa processes very much resembling those of karyokinesis have already been observed taking place in the nucleus preparatory to its division. And although such processes do not present quite the same appearances as are to be met with in egg-cells, neither do the karyokinetic processes in tissue-cells, which in their sundry kinds exhibit great variations in this respect. Moreover, even if such were not the case, the bare fact that nuclear division is not invariably of the simple or direct character in the case of all Protozoa, is sufficient to show that the distinction now before us—like the one last dealt with—is by no means absolute. As in the case of sexual propagation, so in that of karyokinesis, processes which are common to all the Metazoa are not wholly without their foreshadowings in the Protozoa. And seeing how greatly exalted is the office of egg-cells—and even of tissue-cells—as compared with that of their supposed ancestry in protozoal cells, it seems to me scarcely to be wondered at if their specializations of function should be associated with corresponding peculiarities of structure—a general fact which would in no way militate against the doctrine of evolution. Could we know the whole truth, we should probably find that in order to endow the most primitive of egg-cells with its powers of marshalling its products into a living army of cell-battalions, such an egg-cell must have been passed through a course of developmental specialization of so elaborate a kind, that even the complex processes of karyokinesis are but a very inadequate expression thereof.

Probably I have now said enough to show that, remarkable and altogether exceptional as the properties of germ-cells of the multicellular organisms unquestionably show themselves to be, yet when these properties are traced back to their simplest beginnings in the unicellular organisms, they may fairly be regarded as fundamentally identical with the properties of living cells in general. Thus viewed, no line of real demarcation can be drawn between growth and reproduction, even of the sexual kind. The one process is, so to speak, physiologically continuous with the other; and hence, so far as the pre-embryonic stage of life-history is concerned, the facts cannot fairly be regarded as out of keeping with the theory of evolution.

I will now pass on to consider the embryogeny of the Metazoa, beginning at its earliest stage in the fertilization of the ovum. And here it is that the constructive argument in favour of evolution which is derived from embryology may be said properly to commence. For it is surely in itself a most suggestive fact that all the Metazoa begin their life in the same way, or under the same form and conditions. Omne vivum ex ovo. This is a formula which has now been found to apply throughout the whole range of the multicellular organisms. And seeing, as we have just seen, that the ovum is everywhere a single cell, the formula amounts to saying that, physiologically speaking, every Metazoön begins its life as a Protozoön, and every Metaphyton as a Protophyton[10].

Now, if the theory of evolution is true, what should we expect to happen when these germ-cells are fertilized, and so enter upon their severally distinct processes of development? Assuredly we should expect to find that the higher organisms pass through the same phases of development as the lower organisms, up to the time when their higher characters begin to become apparent. If in the life-history of species these higher characters were gained by gradual improvement upon lower characters, and if the development of the higher individual is now a general recapitulation of that of its ancestral species, in studying this recapitulation we should expect to find the higher organism successively unfolding its higher characters from the lower ones through which its ancestral species had previously passed. And this is just what we do find. Take, for example, the case of the highest organism, Man. Like that of all other organisms, unicellular or multicellular, his development starts from the nucleus of a single cell. Again, like that of all the Metazoa and Metaphyta, his development starts from the specially elaborated nucleus of an egg-cell, or a nucleus which has been formed by the fusion of a male with a female element[11]. When his animality becomes established, he exhibits the fundamental anatomical qualities which characterize such lowly animals as polyps and jelly-fish. And even when he is marked off as a Vertebrate, it cannot be said whether he is to be a fish, a reptile, a bird, or a beast. Later on it becomes evident that he is to be a Mammal; but not till later still can it be said to which order of mammals he belongs.

Here, however, we must guard against an error which is frequently met with in popular expositions of this subject. It is not true that the embryonic phases in the development of a higher form always resemble so many adult stages of lower forms. This may or may not be the case; but what always is the case is, that the embryonic phases of the higher form resemble the corresponding phases of the lower forms. Thus, for example, it would be wrong to suppose that at any stage of his development a man resembles a jelly-fish. What he does resemble at an early stage of his development is the essential or groundplan of the jelly-fish, which that animal presents in its embryonic condition, or before it begins to assume its more specialized characters fitting it for its own particular sphere of life. The similarities, therefore, which it is the function of comparative embryology to reveal are the similarities of type or morphological plan: not similarities of specific detail. Specific details may have been added to this, that, and the other species for their own special requirements, after they had severally branched off from the common ancestral stem; and so could not be expected to recur in the life-history of an independent specific branch. The comparison therefore must be a comparison of embryo with embryo; not of embryos with adult forms.

In order to give a general idea of the results thus far yielded by a study of comparative embryology in the present connexion, I will devote the rest of this chapter to giving an outline sketch of the most important and best established of these results.

Histologically the ovum, or egg-cell, is nearly identical in all animals, whether vertebrate or invertebrate. Considered as a cell it is of large size, but actually it is not more than 1/100, and may be less than 1/200 of an inch in diameter. In man, as in most mammals, it is about 1/120. It is a more or less spherical body, presenting a thin transparent envelope, called the zona pellucida, which contains—first, the protoplasmic cell-substance or “yolk,” within which lies, second, the nucleus or germinal vesicle, within which again lies, third, the nucleolus or germinal spot. This description is true of the egg-cells of all animals, if we add that in the case of the lowest animals—such as sponges, &c.—there is no enveloping membrane: the egg-cell is here a naked cell, and its constituent protoplasm, being thus unconfined, is free to perform protoplasmic movements, which it does after the manner, and with all the activity, of an amœba. But even with respect to this matter of an enveloping membrane, there is no essential difference between an ovum of the lowest and an ovum of the highest animals. For in their early stages of development within the ovary the ova of the highest animals are likewise in the condition of naked cells, exhibiting amœbiform movements; the enveloping membrane of an ovum being the product of a later development. Moreover this membrane, when present, is usually provided with one or more minute apertures, through which the spermatozoön passes when fertilizing the ovum. It is remarkable that the spermatozoa know, so to speak, of the existence of these gate-ways,—their snake-like movements being directed towards them, presumably by a stimulus due to some emanation therefrom[12]. In the mammalian ovum, however, these apertures are exceedingly minute, and distributed all round the circumference of the pellucid envelope, as represented in this illustration (Fig. 32).

Fig. 30.—Ovarian ovum of a Mammal, (a) magnified and viewed under pressure, (b) burst by increased pressure, with yolk and nucleus escaping: (c) the nucleus more freed from yolk-substance. (From Quain’s Anatomy, after Allen Thomson.)

Fig. 31.—Amœboid movements of young egg-cells, a, Amœboid ovum of Hydra (from Balfour, after Kleitnenberg); b, early ovum of Toxopneustes variegatus, with pseudopodia-like processes (from Balfour, after Selenka); c, ovum of Toxopneustes lividus, more nearly ripe (from Balfour, Hertwig). A1 to A4, the primitive egg-cell of a Chalk-Sponge (Leuculmis echinus), in four successive conditions of motion. B1 to B8, ditto of a Hermit-Crab (Chondracanthus cornutus), in eight successive stages (after E. von Beneden). C1 to C5, ditto of a Cat, in five successive stages (after Pflüger). D, ditto of Trout; E, of a Hen; F, of Man. The first series is taken from the Encycl. Brit.; the second from Häckel’s Evolution of Man.

Fig. 32.—Human ovum, mature and greatly magnified. (After Häckel.)

In thus saying that the ova of all animals are, so far as microscopes can reveal, substantially similar, I am of course speaking of the egg-cell proper, and not of what is popularly known as the egg. The egg of a bird, for example, is the egg-cell, plus an enormous aggregation of nutritive material, an egg-shell, and sundry other structures suited to the subsequent development of the egg-cell when separated from the parent’s body. But all these accessories are, from our present point of view, accidental or adventitious. What we have now to understand by the ovum, the egg, or the egg-cell, is the microscopical germ which I have just described. So far then as this germ is concerned, we find that all multicellular organisms begin their existence in the same kind of structure, and that this structure is anatomically indistinguishable from that of the permanent form presented by the lowest, or unicellular organisms. But although anatomically indistinguishable, physiologically they present the sundry peculiarities already mentioned.

Now I have endeavoured to show that none of these peculiarities are such as to exclude—or even so much as to invalidate—the supposition of developmental continuity between the lowest egg-cells and the highest protozoal cells. It remains to show in this place, and on the other hand, that there is no breach of continuity between the lowest and the highest egg-cells; but, on the contrary, that the remarkable uniformity of the complex processes whereby their peculiar characters are exhibited to the histologist, is such as of itself to sustain the doctrine of continuity in a singularly forcible manner. On this account, therefore, and also because the facts will again have to be considered in another connexion when we come to deal with Weismann’s theory of heredity, I will here briefly describe the processes in question.

Fig. 33.—Stages in the formation of the polar bodies in the ovum of a star-fish. (After Hertwig.) g.v., germinal vesicle transformed into a spindle-shaped system of fibres; p.′, the first polar body becoming extruded; p., p., both polar bodies fully extruded; f. pn., female pronucleus, or residue of the germinal vesicle.

We have already seen that the young egg-cell multiplies itself by simple binary division, after the manner of unicellular organisms in general—thereby indicating, as also by its amœbiform movements, its fundamental identity with such organisms in kind. But, as we have likewise seen, when the ovum ceases to resemble these organisms, by taking on its higher degree of functional capacity, it is no longer able to multiply itself in this manner. On the contrary, its cell-divisions are now of an endogenous character, and result in the formation of many different kinds of cells, in the order required for constructing the multicellular organism to which the whole series of processes eventually give rise. We have now to consider these processes seriatim.

Fig. 34.—Fertilization of the ovum of an echinoderm. (From Quain’s Anatomy, after Selenka.) S, spermatozoön; m. pr., male pronucleus; f. pr., female pronucleus. 1 to 4 correspond to D to G in the next figure.

First of all the nucleus discharges its polar bodies, as previously mentioned, and in the manner here depicted on the previous page. (Fig. 33.) It will be observed that the nucleus of the ovum, or the germinal vesicle as it is called, gets rid first of one and afterwards of the other polar body by an “indirect,” or karyokinetic, process of division. (Fig. 33.) Extrusion of these bodies from the ovum (or it may be only from the nucleus) having been accomplished, what remains of the nucleus retires from the circumference of the ovum, and is called the female pronucleus. (Fig. 33. f. pn.) The ovum is now ready for fertilization. A similar emission of nuclear substance is said by some good observers to take place also from the male germ-cell, or spermatozoön, at or about the close of its development. The theories to which these facts have given rise will be considered in future chapters on Heredity.

Turning now to the mechanism of fertilization, the diagrams (Figs. 34, 35) represent what happens in the case of star-fish.

Fig. 35.—Fertilization of the ovum of a star-fish. (From the Encycl. Brit. after Fol.) A, spermatozoa in the mucilaginous coat of the ovum; a prominence is rising from the surface of the ovum towards a spermatozoön; B, they have almost met; C, they have met; D, the spermatozoön enters the ovum through a distinct opening; H, the entire ovum, showing extruded polar bodies on its upper surface, and the moving together of the male and female pronuclei; E, F, G, meeting and coalescence of the pronuclei.

The sperm-cell, or spermatozoön, is seen in the act of penetrating the ovum. In the first figure it has already pierced the mucilaginous coat of the ovum, the limit of which is represented by a line through which the tail of the spermatozoön is passing: the head of the spermatozoön is just entering the ovum proper. It may be noted that, in the case of many animals, the general protoplasm of the ovum becomes aware, so to speak, of the approach of a spermatozoön, and sends up a process to meet it. (Fig. 35, A, B, C.) Several—or even many—spermatozoa may thus enter the coat of the ovum; but normally only one proceeds further, or right into the substance of the ovum, for the purpose of effecting fertilization. This spermatozoön, as soon as it enters the periphery of the yolk, or cell-substance proper, sets up a series of remarkable phenomena. First, its own head rapidly increases in size, and takes on the appearance of a cell-nucleus: this is called the male pronucleus. At the same time its tail begins to disappear, and the enlarged head proceeds to make its way directly towards the nucleus of the ovum which, as before stated, is now called the female pronucleus. The latter in its turn moves towards the former, and when the two meet they fuse into one mass, forming a new nucleus. Before the two actually meet, the spermatozoön has lost its tail altogether; and it is noteworthy that during its passage through the protoplasmic cell-contents of the ovum, it appears to exercise upon this protoplasm an attractive influence; for the granules of the latter in its vicinity dispose themselves around it in radiating lines. All these various phenomena are depicted in the above wood-cuts. (Figs. 34, 35.)

Fertilization having been thus effected by fusion of the male and female pronuclei into a single (or new) nucleus, this latter body proceeds to exhibit complicated processes of karyokinesis, which, as before shown, are preliminary to nuclear division in the case of egg-cells. Indeed the karyokinetic process may begin in both the pronuclei before their junction is effected; and, even when their junction is effected, it does not appear that complete fusion of the so-called chromatin elements of the two pronuclei takes place. For the purpose of explaining what this means, and still more for the purpose of giving a general idea of the karyokinetic processes as a whole, I will quote the following description of them, because, for terseness combined with lucidity, it is unsurpassable.

Fig. 36.—Karyokinesis of a typical tissue-cell (epithelium of Salamander). (After Flemming and Klein.) The series from A to I represents the successive stages in the movement of the chromatin fibres during division, excepting G, which represents the “nucleus-spindle” of an egg-cell. A, resting nucleus; D, wreath-form; E, single star, the loops of the wreath being broken; F, separation of the star into two groups of U-shaped fibres; H, diaster or double star; I, completion of the cell-division and formation of two resting nuclei. In G the chromatin fibres are marked a, and correspond to the “equatorial plate"; b, achromatin fibres forming the nucleus-spindle; c, granules of the cell-protoplasm forming a “polar star.” Such a polar star is seen at each end of the nucleus-spindle, and is not to be confused with the diaster H, the two ends of which are composed of chromatin.

Researches, chiefly due to Flemming, have shown that the nucleus in very many tissues of higher plants and animals consists of a capsule containing a plasma of “achromatin,” not deeply stained by re-agents, ramifying in which is a reticulum of “chromatin” consisting of fibres which readily take a deep stain. (Fig. 36, A). Further it is demonstrated that, when the cell is about to divide into two, definite and very remarkable movements take place in the nucleus, resulting in the disappearance of the capsule and in the arrangement of its fibres first in the form of a wreath (D), and subsequently (by the breaking of the loops formed by the fibres) in the form of a star (E). A further movement within the nucleus leads to an arrangement of the broken loops in two groups (F), the position of the open ends of the broken loops being reversed as compared with what previously obtained. Now the two groups diverge, and in many cases a striated appearance of the achromatin substance between the two groups of chromatin loops is observable (H). In some cases (especially egg-cells) this striated arrangement of the achromatin is then termed a “nucleus-spindle,” and the group of chromatin loops (G, a) is known as “the equatorial plate.” At each end of the nucleus-spindle in these cases there is often seen a star consisting of granules belonging to the general protoplasm of the cell (G, c). These are known as “polar stars.” After the separation of the two sets of loops (H) the protoplasm of the general substance of the cell becomes constricted, and division occurs, so as to include a group of chromatin loops in each of the two fission products. Each of these then rearranges itself together with the associated chromatin into a nucleus such as was present in the mother cell to commence with (I)[13].

Since the above was published, however, further progress has been made. In particular it has been found that the chromatin fibres pass from phase D to phase F by a process of longitudinal splitting (Fig. 37 g, h; Fig. 38, VI, VII)—which is a point of great importance for Weismann’s theory of heredity,—and that the protoplasm outside the nucleus seems to take as important a part in the karyokinetic process as does the nuclear substance. For the so-called “attraction-spheres” (Fig. 38 II a, III, III a, VIII to XII), which were at first supposed to be of subordinate importance in the process as a whole, are now known to take an exceedingly active part in it (see especially IX to XI). Lastly, it may be added that there is a growing consensus of authoritative opinion, that the chromatin fibres are the seats of the material of heredity, or, in other words, that they contain those essential elements of the cell which endow the daughter-cells with their distinctive characters. Therefore, where the parent-cell is an ovum, it follows from this view that all hereditary qualities of the future organism are potentially present in the ultra-microscopical structure of the chromatin fibres.

Fig. 37.—Study of successive changes taking place in the nucleus of an epithelium cell, preparatory to division of the cell. (From Quain’s Anatomy, after Flemming.) a, resting cell, showing the nuclear network; b, first stage of division, the chromatoplasm transformed into a skein of closely contorted filaments; c to f, further stages in the growth and looping arrangement of the filaments; g, stellate phase, or aster; h, completion of the splitting of the filaments, already begun in f and g; i, j, k, successive stages in separation of the filaments into two groups; l, the final result of this (diaster); m to q, stages in the division of the whole cell into two, showing increasing contortion of the filaments, until they reach the resting stage at q.