Darwin, and After Darwin Volume I Part 2

Again, we have seen that there is no correlation between the importance of any structure from a cla.s.sificatory 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 cla.s.sification." 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 cla.s.sifications were wrong in principle when they went upon the a.s.sumption that divine ideals were best exhibited by resemblances between life-serving (and therefore adaptive) structures, with the result that whales were cla.s.sed 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 cla.s.sification. This, of course, is what we should have expected, if the real meaning of cla.s.sification be that of tracing lines of pedigree; but on the theory of special creation no reason can be a.s.signed 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 a.n.a.logous 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 a.s.sumed 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 cla.s.sificatory tree are in fact fundamentally different in respect of their structural plan.

Lastly, we have seen that one of the guiding principles of cla.s.sification 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 trans.m.u.tation 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 h.o.m.ologies 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.

[Ill.u.s.tration: FIG. 2.--Skeleton of Seal, 1/8 nat. size. Drawn from nature (_R. Coll. Surg. Mus._).]

[Ill.u.s.tration: 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.)]

[Ill.u.s.tration: FIG. 4.--Paddle of Whale compared with Hand of Man.

Drawn from nature (_R. Coll. Surg. Mus._).]

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, 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.

[Ill.u.s.tration: 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 h.o.m.ology and a.n.a.logy 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 h.o.m.ologous and yet in no way a.n.a.logous: 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 a.n.a.logous, and yet in no way h.o.m.ologous: 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 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 adapted to serve the same function? If so, it becomes 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--or cases of h.o.m.ology without a.n.a.logy,--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 a.n.a.logy without h.o.m.ology,--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 a formidable one to the theory of evolution, if the similarity were not only a.n.a.logical but h.o.m.ological. Unfortunately for the objection, however, Darwin clearly showed in his reply that in no one anatomical or h.o.m.ologous 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 a.n.a.logous, 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 a.n.a.logical resemblance, had also presented an anatomical or h.o.m.ologous 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 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 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 a.s.sumption, let us suppose that G.o.d did lay down these arbitrary rules for his own guidance in creation, and then let us see to what the a.s.sumption 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.

[4] It is, however, probable that all species of the genus retained a tiny rudiment of wings in greatly dwindled scapulo-coracoid bones.

And Mr. H. O. Forbes has detected, in a recently exhumed specimen of the latter, an indication of the glenoid cavity, for the articulation of an extremely aborted humerus. (See _Nature_, Jan.

14th, 1892.)

[Ill.u.s.tration: 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 ill.u.s.tration 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 sh.e.l.ls of mollusks. In a.s.sociation 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-sh.e.l.l, 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-sh.e.l.l; 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-sh.e.l.l. 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 sh.e.l.l 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-sh.e.l.ls; 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-sh.e.l.l. 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-sh.e.l.l 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 sh.e.l.l 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.

[Ill.u.s.tration: FIG. 7.--Hermit-crabs compared with the cocoa-nut crab. On the left of the ill.u.s.tration one hermit-crab is represented as occupying a mollusk-sh.e.l.l, and another (larger specimen) as it appears when withdrawn from such a sh.e.l.l. On the right of the ill.u.s.tration 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 pa.s.sage 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."

[Ill.u.s.tration: 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, 1/4 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 a.s.signed, 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 const.i.tutes 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 antic.i.p.ated 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.

[Ill.u.s.tration: 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 a.n.a.logous to this reduction of wings in birds which have no further use for them, are to be met with also in insects under similar circ.u.mstances. 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 a.s.sume 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 cla.s.s of facts.

It is now many years ago since the editors of _Silliman's Journal_ requested the late Professor Aga.s.siz 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. Aga.s.siz 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. Aga.s.siz, who was a believer in special creation, allowed that this ought to const.i.tute a crucial test as between the two theories of special design and hereditary descent. "If physical circ.u.mstances," 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 circ.u.mstances 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 cla.s.ses, 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 l.u.s.trous 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. Aga.s.siz in regarding these cave animals as furnis.h.i.+ng 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 relations.h.i.+ps, 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 a.n.a.logy 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 att.i.tude 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 nict.i.tating 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 cla.s.ses 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.)

[Ill.u.s.tration: FIG. 10.--Ill.u.s.trations of the nict.i.tating 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.

(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.