The Scientific Evidences of Organic Evolution.
by George John Romanes.
PREFACE.
Several months ago I published in the _Fortnightly Review_ a lecture, which I had previously delivered at the Philosophical Inst.i.tutions of Edinburgh and Birmingham, and which bore the above t.i.tle. The late Mr.
Darwin thought well of the epitome of his doctrine which the lecture presented, and urged me so strongly to republish it in a form which might admit of its being "spread broadcast over the land," that I promised him to do so. In fulfilment of this promise, therefore--which I now regard as more binding than ever--I reproduce the essay in the "Nature Series" with such additions and alterations as appear to me, on second thoughts, to be desirable. The only object of the essay is that which is expressed in the opening paragraph.
LONDON,
_June 1, 1882._
Since this little Essay was published, it has been suggested to me that, in its mode of presenting the arguments in favour of Evolution, there is a similarity to that which has been adopted by Mr. Herbert Spencer in the third part of his _Principles of Biology_. I should therefore like to state, that while such similarity is no doubt in part due to the similarity of subject-matter, I think, upon reading again, after an interval of ten years, his admirable presentation of the evidence it may also in part be due to unconscious memory. This applies particularly to the headings of the chapters, which I find to be almost identical with those previously used by Mr. Spencer.
G. J. R.
THE SCIENTIFIC EVIDENCES
OF
ORGANIC EVOLUTION.
Although it is generally recognised that the _Origin of Species_ has produced an effect both on the science and the philosophy of our age which is without a parallel in the history of thought, admirers of Mr.
Darwin"s genius are frequently surprised at the ignorance of his work which is displayed by many persons who can scarcely be said to belong to the uncultured cla.s.ses. The reason of this ignorance is no doubt partly due to the busy life which many of our bread-winners are constrained to live; but it is also, I think, partly due to mere indolence. There are thousands of educated persons who, on coming home from their daily work, prefer reading literature of a less scientific character than that which is supplied by Mr. Darwin"s works; and therefore it is that such persons feel these works to belong to a category of books which is to them a very large one--the books, namely, which never are, but always to be, read. Under these circ.u.mstances I have thought it desirable to supply a short digest of the _Origin of Species_, which any man, of however busy a life, or of however indolent a disposition, may find both time and energy to follow.
With the general aim of the present abstract being thus understood, I shall start at the beginning of my subject by very briefly describing the theory of natural selection. It is a matter of observable fact that all plants and animals are perpetually engaged in what Mr. Darwin calls a "struggle for existence." That is to say, in every generation of every species a great many more individuals are born than can possibly survive; so that there is in consequence a perpetual battle for life going on among all the const.i.tuent individuals of any given generation.
Now, in this struggle for existence, which individuals will be victorious and live? a.s.suredly those which are best fitted to live: the weakest and the least fitted to live will succ.u.mb and die, while the strongest and the best fitted to live will be triumphant and survive.
Now it is this "survival of the fittest" that Mr. Darwin calls "natural selection." Nature, so to speak, _selects_ the best individuals out of each generation to live. And not only so, but as these favoured individuals transmit their favourable qualities to their offspring, according to the fixed laws of heredity, it follows that the individuals composing each successive generation have a general tendency to be better suited to their surroundings than were their forefathers.
And this follows, not merely because in every generation it is only the flower of the race that is allowed to breed, but also because if in any generation some new and beneficial qualities happen to appear as slight variations from the ancestral type, these will be seized upon by natural selection and added, by transmission in subsequent generations, to the previously existing type. Thus the best idea of the whole process will be gained by comparing it with the closely a.n.a.logous process whereby gardeners and cattlebreeders create their wonderful productions; for just as these men, by always selecting their best individuals to breed from, slowly but continuously improve their stock, so Nature, by a similar process of selection, slowly but continuously makes the various species of plants and animals better and better suited to the external conditions of their life.
Now, if this process of continuously adapting organisms to their environment takes place in nature at all, there is no reason why we should set any limits on the extent to which it is able to go up to the point at which a complete and perfect adaptation is achieved. Therefore we might suppose that all species would attain to this condition of perfect adjustment to their environment, and there remain fixed. And so undoubtedly they would, if the environment were itself unchanging. But forasmuch as the environment--or the sum total of the external conditions of life--of almost every organic type alters more or less from century to century (whether from astronomical, geological, and geographical changes, or from the immigrations and emigrations of other species living on contiguous geographical areas), it follows that the process of natural selection need never reach a terminal phase. And forasmuch as natural selection may thus continue, _ad infinitum_, slowly to alter a specific type in adaptation to a gradually changing environment, if in any case the alteration thus effected is sufficient in amount to lead naturalists to denote the specific type by some different name, it follows that natural selection has trans.m.u.ted one specific type into another. And so the process is supposed to go on over all the countless species of plants and animals simultaneously--the world of organic types being thus regarded as in a state of perpetual, though gradual, flux.
Such, then, is the theory of natural selection, or survival of the fittest; and the first thing we have to notice with regard to it is, that it offers to our acceptance a scientific explanation of the numberless cases of apparent design which we everywhere meet with in organic nature. For all such cases of apparent design consist only in the _adaptation_ which is shown by organisms to their environment, and it is obvious that the facts are covered by the theory of natural selection no less completely than they are covered by the theory of intelligent design. Perhaps it may be answered,--"The fact that these innumerable cases of adaptation may be accounted for by natural selection is no proof that they are not really due to intelligent design." And, in truth, this is an objection which is often urged by minds--even highly cultured minds--which have not been accustomed to scientific modes of thought. I have heard an eminent professor tell his cla.s.s that the many instances of adaptation which Mr. Darwin discovered and described as occurring in orchids, seemed to him to tell more in favour of contrivance than in favour of natural causes; and another eminent professor once wrote to me that although he had read the _Origin of Species_ with care, he could see in it no evidence of natural selection which might not equally well be adduced in favour of intelligent design. But here we meet with a radical misconception of the whole logical att.i.tude of science. For, be it observed, the exception _in limine_ to the evidence which we are about to consider, does not question that natural selection _may_ not be able to do all that Mr.
Darwin ascribes to it: it merely objects to his interpretation of the facts, because it maintains that these facts might _equally well_ be ascribed to intelligent design. And so undoubtedly they might, if we were all childish enough to rush into a supernatural explanation whenever a natural explanation is found sufficient to account for the facts. Once admit the glaringly illogical principle that we may a.s.sume the operation of higher causes where the operation of lower ones is sufficient to explain the observed phenomena, and all our science and all our philosophy are scattered to the winds. For the law of logic which Sir William Hamilton called the law of parsimony--or the law which forbids us to a.s.sume the operation of higher causes when lower ones are found sufficient to explain the observed effects--this law const.i.tutes the only logical barrier between science and superst.i.tion. For it is manifest that it is always possible to give a hypothetical explanation of any phenomenon whatever, by referring it immediately to the intelligence of some supernatural agent; so that the only difference between the logic of science and the logic of superst.i.tion consists in science recognising a validity in the law of parsimony which superst.i.tion disregards. Therefore I have no hesitation in saying that this way of looking at the evidence in favour of natural selection is not a scientific or a reasonable way of looking at it, but a purely superst.i.tious way. Let us take, for instance, as an ill.u.s.tration, a perfectly parallel case. When Kepler was unable to explain by any known causes the paths described by the planets, he resorted to a supernatural explanation, and supposed that every planet was guided in its movements by some presiding angel. But when Newton supplied a beautifully simple physical explanation, all persons with a scientific habit of mind at once abandoned the metaphysical explanation. Now, to be consistent, the above-mentioned professors, and all who think with them, ought still to adhere to Kepler"s hypothesis in preference to Newton"s explanation; for, excepting the law of parsimony, there is certainly no other logical objection to the statement that the movements of the planets afford as good evidence of the influence of guiding angels as they do of the influence of gravitation.
So much, then, for the absurdly illogical position that, granting the evidence in favour of natural selection and supernatural design to be equal and parallel, we should hesitate for one moment in our choice.
But, of course, if the evidence is supposed _not_ to be equal and parallel--_i.e._, if it is supposed that the theory of natural relation is not so competent a theory to explain the facts of adaptation as is that of intelligent design--then the objection is no longer the one that we are considering. It is quite another objection, and one which is not _prima facie_ absurd; it requires to be met by examining how far the theory of natural selection _is_ able to explain the facts. Let us state the problem clearly.
Innumerable cases of adaptation of organisms to their environment are the observed facts for which an explanation is required. To supply this explanation two, and only two, hypotheses are in the field. Of these two hypotheses one is, intelligent design manifested in creation; and the other is, natural selection manifested during the countless ages of the past. Now it would be proof positive of intelligent design if it could be shown that all species of plants and animals were _created_--that is _suddenly_ introduced into the complex conditions of their life; for it is quite inconceivable that any cause other than intelligence could be competent to adapt an organism to its environment _suddenly_. On the other hand, it would be proof presumptive of natural selection if it could be shown that one species becomes slowly trans.m.u.ted into another--_i.e._, that one set of adaptations may be gradually transformed into another set of adaptations according as changing circ.u.mstances require. This would be proof presumptive of natural selection, because it would then become amply probable that natural selection might have brought about many, or most, of the cases of adaptations which we see; and if so, the law of parsimony excludes the rival hypothesis of intelligent design. Thus the whole question as between natural selection and supernatural design resolves itself into this--Were all the species of plants and animals separately created, or were they slowly evolved? For if they were specially created, the evidence of supernatural design remains unrefuted and irrefutable; whereas if they were slowly evolved, that evidence has been utterly and for ever destroyed. The doctrine of natural selection therefore depends for its validity on the doctrine of organic evolution; for if once the fact of organic evolution were established, no one would dispute that much of the adaptation was probably effected by natural selection. _How_ much we cannot say--probably never shall be able to say; for even Mr.
Darwin himself does not doubt that other causes besides that of natural selection have a.s.sisted in the modifying of specific types. For the sake of simplicity, however, I shall not go into this subject; but shall always speak of natural selection as the only cause of organic evolution. Let us, then, weigh the evidence in favour of organic evolution. If we find it wanting, we need have no complaints to make of natural theologians of to-day; but if we find it to be full measure, shaken together and running over, we ought to maintain that natural theologians can no longer adhere to the arguments of such writers as Paley, Bell, and Chalmers, without deliberately violating the only logical principle which separates science from fetishism.
To avoid misapprehension, however, I may here add that while Mr.
Darwin"s theory is thus in plain and direct contradiction to the theory of design, or system of teleology, as presented by the school of writers which I have named, I hold that Mr. Darwin"s theory has no point of logical contact with the theory of design in the larger sense, that behind all secondary causes of a physical kind, there is a primary cause of a mental kind. Therefore throughout this essay I refer to design in the sense understood by the narrower forms of teleology, or as an _immediate_ cause of the observed phenomena. Whether or not there is an _ultimate_ cause of a psychical kind pervading all nature, a _causa causarum_ which is the final _raison d"etre_ of the cosmos, this is another question which, as I have said, I take to present no point of logical contact with Mr. Darwin"s theory, or, I may add, with any of the methods and results of natural science. The only position, therefore, which I here desire to render plain is that, if the doctrine of evolution is seen to be established by sufficient evidence, and therefore the causes which it sets forth are recognised as adequate to furnish a scientific explanation of the results observed, then the facts of organic nature necessarily fall into the same logical category, with reference to any question of design, as that of all or any other series of facts in the physical universe.
This being understood, I shall now proceed to render an epitome of the evidence in favour of organic evolution, and I shall do so by cla.s.sifying the arguments in a way tending to show their distinct or independent character, and therefore calculated to display the additional force which they acquire from their c.u.mulative nature.
I.
THE ARGUMENT FROM CLa.s.sIFICATION.
I shall first take the argument from cla.s.sification. Naturalists find that all species of plants and animals present among themselves structural affinities. According as these structural affinities are more or less p.r.o.nounced, the various species are cla.s.sified under genera, orders, families, cla.s.ses, sub-kingdoms, and kingdoms. Now in such a cla.s.sification it is found impossible to place all the species in a linear series, according to the grade of their organization. For instance, we cannot say that a wolf is more highly organized than a fox or a jackal; we can only say that the specific points wherein it differs from these animals are without significance as proving the one type to be more highly organized than the others. But of course in many cases, and especially in the cases of the larger divisions, it is often possible to say--The members in this division are more highly organized than are the members in that division. Our system of cla.s.sification therefore may be likened to a tree, in which 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 cla.s.ses, 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 will understand that while there is a general advance of organization from below upwards, there are numberless slight variations in this respect between leaves growing even on the same branch; but in a still greater number of cases, leaves growing on the same branch are growing on the same level--that is, although they represent different species, it cannot be said that one is more highly organized than the other. Now, this tree-like arrangement of specific organisms in nature is an arrangement for which Mr. Darwin is not responsible. I mean that the framing of this natural cla.s.sification has been the work of naturalists for centuries past; and although they did not know what they were doing, it is now evident to evolutionists that they were tracing the lines of genetic relationship. For, be it observed, a scientific or natural cla.s.sification differs very much from a popular or hap-hazard cla.s.sification, and the difference consists in this, that while a popular cla.s.sification is framed with exclusive reference to the external appearance of organisms, a scientific cla.s.sification is made with reference to the whole structure. A whale, for instance, is often thought to be a fish, because it resembles a fish in form and habits; whereas dissection shows that it is beyond all comparison more unlike a fish than it is like a horse or a man. This is, of course, an extreme case; but it was cases such as this that first led naturalists to see that there are resemblances between organisms much more deep and important than appear upon the surface; and consequently, that if a natural cla.s.sification was possible at all, it must be made with reference to these deeper resemblances. Of course, it took time to perceive this distinction between fundamental and superficial resemblances. I remember once reading a very comical disquisition in one of Buffon"s works on the question as to whether or not a crocodile was to be cla.s.sified as an insect; and the instructive feature in the disquisition was this, that although a crocodile differs from an insect as regards every conceivable particular of its internal anatomy, no allusion at all is made to this fact, while the whole discussion is made to turn on the hardness of the external casing of a crocodile resembling the hardness of the external casing of a beetle; and when at last Buffon decides that, on the whole, a crocodile had better not be cla.s.sified as an insect, the only reason given is, that as a crocodile is so very large an animal, it would make "altogether too terrible an insect."
But now, when at last it came to be recognised that internal anatomy rather than external appearance was to be taken as a guide to cla.s.sification, the question was, What features in the internal anatomy are to take precedence over the other features? And this question it was not hard to answer. 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, and in this feature it differs from all fish, while it resembles all mammals. Now, looking at those two features alone, should we say that a porpoise ought to be cla.s.sed as a fish or as a mammal? a.s.suredly as a mammal, and for this reason: The number of teeth is a very variable feature both in fish and in mammals, whereas the giving of suck is an invariable feature among mammals, and occurs nowhere else in the animal kingdom. This, of course, is purposely chosen as a very simple ill.u.s.tration; but it exemplifies the general fact that the guiding principle of scientific cla.s.sification is the comparing of organism with organism, with the view of seeing which of the const.i.tuent organs are of the most invariable occurrence, and therefore of the most typical signification.
Now, since the days of Linnaeus this principle has been carefully followed, and it is by its aid that the tree-like system of cla.s.sification has been established. No one, even long before Darwin"s days, ever dreamed of doubting that this system is in reality, what it always has been in name, a _natural_ system. What, then, is the inference we are to draw from it? An evolutionist answers, that it is just such a system as his theory of descent would lead him to expect as a natural system. For this tree-like system is as clear an expression as anything could be of the fact that all species are bound together by the ties of genetic relationship. If all species were separately created, it is almost incredible that we should everywhere observe this progressive shading off of characters common to larger groups, into more and more specialized characters distinctive only of smaller and smaller groups.
At any rate, to say the least, the law of parsimony forbids us to ascribe such effects to a supernatural cause, acting in so whimsical a manner, when the effects are precisely what we should expect to follow from the action of a highly probable natural cause. The cla.s.sification of animal forms, indeed, as Darwin, Lyell, and Haeckel have pointed out, strongly resembles the cla.s.sification 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, by way of genetic descent, to a group of living languages--Italian, Spanish, French, and, to a large extent, English.
Now what should we think 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 these several nations--and that their resemblance to the fossil form, Latin, is to be attributed to special design? Yet the evidence of the natural trans.m.u.tation of species, is, in one respect, much stronger than that of the natural trans.m.u.tation of languages--in respect, namely, of there being a vastly greater number of cases all bearing testimony to the fact of genetic relationship.
II.
THE ARGUMENT FROM MORPHOLOGY OR STRUCTURE.
I now pa.s.s to another line of argument. The theory of evolution by natural selection 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 we have of such adaptive modifications of structure, in cases where the need of such modification is apparent. For the sake of clearness, I shall 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 became aquatic in their habits. Now the change in the conditions of their life thus brought about would render desirable great modifications of structure. These changes would, in the first instance, begin to affect the least typical--that is, the least strongly inherited structures--such as the skin, claws, and teeth, &c. But as time went on, the adaptation would begin to extend to the more typical structures, until the shape of the body began to be 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, but serving to complete the fish-like taper of the body. But in the whales the modification has gone even further than this, so that the hind legs have ceased to be apparent externally, and are only represented internally by remnants so rudimentary that it is impossible to make out with certainty the h.o.m.ologies of the bones; moreover, the head and the whole body have become completely fish-like in shape. But profound as these changes are, they only affect 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 inclosed in a fin-shaped sack, so as to render them quite useless for any other purpose than swimming. 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 relation to one another, but modified in form so as to offer the least possible amount of 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.
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. In birds, again, the arm is modified for flight in a wholly different way--the fingers here being very short and all run together, and the chief expanse of the wing being 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.
Lastly, in serpents the hand and arm have disappeared altogether.
Thus, even if we confine our attention to a single structure, how wonderful are the modifications which it is seen to undergo, although never losing its typical character! How are we to explain this? 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. Now the difficulties in the way of this supposition are prodigious, if not quite insurmountable. In the first place, 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 make it serviceable for 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 a.s.sert 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 to subserve the same function? If so, it appears to me a most unfortunate circ.u.mstance, 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, 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 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 ill.u.s.tration. The octopus or devil-fish belongs to a widely different cla.s.s 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 almost fatal to the theory of evolution by natural selection, if the apparent similarity were a real one. Unfortunately for the objection, however, Mr. Darwin clearly showed, in his reply, that in no one anatomical feature of typical importance 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 serve as organs of sight. But now, suppose that this had not been the case, and that the two structures, besides presenting the necessary superficial resemblance, had also presented an anatomical resemblance; with what tremendous 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 ident.i.ty of type manifested by these two complicated organs of vision? The only hypothesis open to us is intelligent adherence to an ideal type." But as this cannot now be urged in any one 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 vestige of these particular types of structure in other large divisions of that tree. In other words, 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 allow the structures occurring in the one type to appear in any of the others." I answer, undoubtedly it may have done so; but if it did, it is a most unfortunate thing for your theory; for 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 to me a most capricious thing in the Deity to make the eyes of an innumerable number of fish on exactly the same ideal type, and then to make 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 take scrupulous care that in no one ideal particular should this solitary eye resemble all the host of other eyes. However, adopting for the sake of argument this gigantic a.s.sumption, let us suppose that G.o.d laid down these arbitrary rules for His own guidance in creation, and let us see to what it leads. If, as is a.s.sumed, 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 desperate efforts, so to speak, there have been made to maintain the uniformity of type in the case of the arm, and should we not expect that in other and similar cases similar efforts should be made? Yet we repeatedly find that this is not the case. Even in the whale, as we have seen, the hind-limbs are not apparent; and it is impossible to see in what respect the hind-limbs are of any less ideal value than the fore-limbs, which, as we have also seen, are so carefully preserved in nearly all vertebrated animals except the snakes, where again we meet in this particular with a sudden and sublime indifference to the maintenance of a typical structure. Now I say that if the theory of ideal types is true, we have in these facts evidence of the most unreasonable inconsistency; for no explanation can be a.s.signed why so much care should have been taken to maintain the type in some cases, while such reckless indifference should have been displayed towards maintaining it in others. 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 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--such as the absence of limbs--without some of the other organs in the body 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 a 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.[1]
[1] This consideration is, I believe, original. Several exceptions to its validity might be adduced, but as a general principle it certainly holds good.
I will now briefly touch on another branch of the argument from morphology--the argument, namely, from rudimentary structures.
Throughout the animal and vegetable kingdoms we constantly meet with organs which are the 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 we all know that our own rudimentary tail is of no practical service. Now, rudimentary organs of this kind are of such common occurrence, that almost every species presents one or more of them. The question, therefore, is--How are they to be accounted for? Of course the theory of descent with adaptive modification has a delightfully simple answer to supply, viz., that when, from changed conditions of life, an organ which was previously useful becomes useless, natural selection, combined with disuse and so-called economy of growth, will cause it to dwindle till it becomes a rudiment. 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 is triumphant over the latter; for, without waiting to dispute the wisdom of making dwarfed and useless structures merely for the whimsical motive a.s.signed, surely if so extraordinary a method is 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 realised. 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, if possible, in a more aggravated form where, within the limits of the same natural group of organisms, a rudiment is sometimes present and sometimes absent. For instance, to take again the case of limbs, in nearly all the numerous species of snakes there are no vestiges of limbs at all; but in the python we find beneath the skin very tiny rudiments of the hind limbs. 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 type very inefficiently, inasmuch as only two limbs instead of four are represented? Or, again, take the case of the limb in other animals. Five toes seem to const.i.tute the ideal type, notwithstanding that in numberless cases this ideal fails in its structural expression.
Now, in the case of the horse, one toe appears to have become developed at the expense of the others; for the so-called knee of the horse is really the wrist or ankle, and the so-called shank the middle toe or finger very much enlarged. But on each side of this enlarged toe there are, beneath the skin, rudimentary bones of two other toes--the so-called splint-bones. So far good, but three toes are not five; so special creationists must suppose that while in this case the Deity has, so to speak, struggled to maintain the uniformity of His ideal, His efforts have nevertheless conspicuously failed. How much less strained is the scientific interpretation; for I may mention that in this particular case, besides the general inference that rudiments point us to a remote ancestry, we have direct palaeontological evidence that there have been a whole series of extinct horse-like animals, that began low down in the geological strata with five toes (on the fore-feet, one being rudimentary), which afterwards became reduced to four and then to three; after which the two lateral toes began to become rudimentary, as we now see them in oxen, and later on still more so. Lastly, as we come nearer to recent times, we find fossils of the existing horse, with the lateral toes shortened up to the condition of splint-bones. Thus we have some half-dozen different genera of horse, all standing in a linear series in time as in structure, between the earliest representative with the typical number of five toes, and the existing very aberrant form with only one toe.
It is sometimes said that a striking corroboration of a scientific theory is furnished when it enables us correctly to _predict_ discoveries. Such a corroboration is afforded in this instance; for Professor Huxley, speaking in 1870, said, "If the expectation raised by the splints of the horses that, in some ancestor of the horses, these splints would be found to be complete digits, has been verified, we are furnished with very strong reasons for looking for a no less complete verification that the three-toed _plagiolophus_-like "avus" of the horse must have had a five-toed "atavus" at some earlier period. No such five-toed "atavus," however, has yet made its appearance." But since then the "atavus" has made its appearance, if not with five complete toes, at least with four complete and one rudimentary; and any day we may hear that Professor Marsh has found in still earlier strata a more primitive form with all five toes complete.
I have no s.p.a.ce to go into the evidence of similar "missing links" which have been recently supplied by palaeontological researches in the case of several other groups of animals; but their consideration seems to me quite to justify a more recent utterance of Professor Huxley, who, in 1878, wrote in the _Encyclopaedia Britannica:_ "On the evidence of palaeontology, the evolution of many existing forms of animal life from their predecessors is no longer an hypothesis, but an historical fact; it is only the nature of the physiological factors to which that evolution is due which is still open to discussion."