Popular Science Monthly/Volume 10/December 1876/Professor Huxley's Lectures II
|PROFESSOR HUXLEY'S LECTURES.|
THE NEGATIVE AND FAVORABLE EVIDENCE.
IN my lecture on Monday night I pointed out that there are three hypotheses which may be entertained, and which have been entertained, respecting the past history of life upon the globe. According to the first of these hypotheses, life, such as we now know it, has existed from all eternity upon this earth. We tested that hypothesis by the circumstantial evidence, as I called it, which is furnished by the fossil remains contained in the earth's crust, and we found that it was obviously untenable. I then proceeded to consider the second hypothesis, which I termed the Miltonic hypothesis, not because it is of any particular consequence to me whether John Milton seriously entertained it or not, but because it is stated in a clear and unmistakable manner in-his great poem. I pointed out to you that the evidence at our command as completely and fully negatives that hypothesis as it did the preceding one. And I confess that I had too much respect for your intelligence to think it necessary to add that that negation was equally strong and equally valid whatever the source from which that hypothesis might be derived, or whatever the authority by which it might be supported.
I further stated that, according to the hypothesis of evolution, the existing state of things was the last term of a long series of antecedent states, which, when traced back, would be found to show no interruption and no breach of continuity. I propose in this and a following lecture to test this hypothesis rigorously by the evidence at command, and to inquire how far that evidence could be said to be indifferent to it, how far it could be said to be favorable to it, and, finally, how far it could be said to be demonstrative. From almost the origin of these discussions upon the existing condition—and the causes which have led to it—of the animal and vegetable worlds, an argument has been put forward as an objection to evolution, which we shall have to consider very seriously. I think that that argument was first clearly stated by Cuvier in his criticism of the doctrines propounded by his great contemporary, Lamarck. At that time the French expedition to Egypt had called the attention of learned men to the wonderful stores of antiquities in that country, and there had been brought back to France numerous mummified corpses of animals which the ancient Egyptians revered and preserved, the date of which, at a reasonable computation, cannot be placed at less than 3,000 or 4,000 years before the time at which they were thus brought to light. Cuvier endeavored to ascertain, by a very just and proper method, what foundation there was for the belief in a gradual and progressive change of animals, by comparing the skeletons and all accessible parts of these animals, such as crocodiles, birds, dogs, cats, and the like, with those which are now found living in Egypt, and he came to the conclusion—a conclusion which has been verified by all subsequent research—that no appreciable change had taken place in the animals which inhabited Egypt. And he drew thence the conclusion, and a hasty one, that this fact was altogether opposed to the doctrine of evolution. The progress of research since Cuvier's time has furnished far stronger arguments than those which he drew from the mummified bodies of Egyptian animals. A remarkable case is to be found in your own country in the neighborhood of the magnificent falls of Niagara. In the immediate vicinity of the whirlpool, and again upon Goat Island, in the superficial deposits which cover the surface of the soil of the rock in those regions, there are found remains of animals in perfect preservation—shells belonging to exactly the same forms as at present inhabit the still waters of Lake Erie. It is evident from the formation of the country that these animal remains were deposited in the beds in which they occur, at the time at which the lake extended over the region in which they are found. This involves the necessity that they lived and died before the falls had cut their way back through the gorge of Niagara; and, indeed, it is possible to determine that at that time the falls of Niagara must have been at least six miles farther down the river than they are at present. Many computations have been made of the rate at which Niagara is thus cutting its way back. Those computations have varied greatly, but I believe I am speaking within the bounds of prudence if I assume that the tails of Niagara have not retreated at a greater pace than about a foot a year. Six miles, speaking roughly, are 30,000 feet; 30,000 feet, at a foot a year, are 30,000 years; and we are fairly justified in concluding that no less a period than this has passed since these shell-fish, whose remains are left in the beds to which we have referred, were deposited. But there is even still stronger evidence of the long duration of certain types than this. As we work our way through the great series of the Tertiary formations, we find species of animals identical with those which live at the present day, diminishing in numbers, it is true, but still existing in a certain number in the oldest of the Tertiary rocks. And not only so, but when we examine the rocks of the Cretaceous epoch itself, we find the remains of some animals which the closest scrutiny cannot show to be in any respect different from those which live at the present time. That is the case with one of the lamp-shells, a Terebratula which is found in the chalk, and which has continued as it was found, or with insignificant variation, through to the present day. Such is the case with the Globigerinæ, the skeletons of which, aggregated together, form the great mass of our chalk in England. That Globigerina can be traced down to the Globigerinæ which live at the surface of our great oceans, and the remains of which, falling to the bottom of the sea, give rise to a chalky material. So that it must be admitted that certain species of creatures living at the present day show no sign of modification or transformation in the course of a lapse of time as great as that which carries us back to the period of chalk. There are groups of species so closely allied together that it needs the eye of a naturalist to distinguish them one from another. If we pay attention to these, we find that a vastly greater period must be allotted, in some cases, to these persistent forms. In the chalk itself, for example, there is the fish belonging to the highest and the most differentiated of osseous fishes, which go by the name of Beryx. That fish is one of the most beautiful of fossils found in our English chalk. It can be studied anatomically, so far as the hard parts are concerned, almost as well as if it were a recent fish. We find that that fish is represented at the present day by very closely-allied species which are living in the Pacific and Atlantic Oceans. But we may go still farther back, and we find, as I mentioned to you yesterday, that the Carboniferous formations in Europe and in America contain the remains of scorpions in an admirable state of preservation, and those scorpions are hardly distinguishable from such as now live. I do not mean to say that they are not distinguishable, but they require close scrutiny to distinguish them from the scorpions which exist at the present day.
More than that. At the very bottom of the Silurian series, in what is by some authorities termed the Cambrian formation, where all signs of life appear to be dying out—even there, among the few and scanty animal remains which exist, we find species of molluscous animals which are so closely allied to existing forms that at one time they were grouped under the same generic name. I refer to the well known Lingula of the Lingula flags, lately, in consequence of some slight differences, placed in the new genus Lingulella. Practically it belongs to the same great generic group as the Lingula, which you will find at the present day upon the shores of Australia. And the same thing is exemplified if we turn to certain great periods of the earth's history—as, for example, throughout the whole of the Mesozoic period. There are groups of reptiles which begin shortly after the commencement of this period, as the Ichthyosauria and the Plesiosauria, and they abound in vast numbers. They disappear with the chalk, and throughout the whole of that great series of rocks they present no important modifications. Facts of this kind are undoubtedly fatal to any form of the doctrine of evolution, which necessitates the supposition that there is an intrinsic necessity on the part of animal forms which once come into existence to undergo modification; and they are still more distinctly opposed to any view which should lead to the belief that the modification in different types of animal or vegetable life goes on equally and evenly. The facts, as I have placed them before you, would obviously contradict directly any such form of the hypothesis of evolution as laid down in these two postulates.
Now, the service that has been rendered by Mr. Darwin in the doctrine of evolution in general is this: that he has shown that there are two great factors in the process of evolution: one of them is the tendency to vary, the existence of which may be proved by observation in all living forms; the other is the influence of surrounding conditions upon what I may call the parent form and the variations which are thus evolved from it. The cause of the production of variations is a matter not at all properly understood at present. Whether it depends upon some intricate machinery—if I may use the phrase—of the animal form itself, or whether it arises through the influence of conditions upon that form, is not certain, and the question may for the present be left open. But the important point is the tendency to the production of variations; then, whether the variations which are produced shall survive and supplant the parent, or whether the parent form shall survive and supplant the variations, is a matter which depends entirely on surrounding conditions. If the surrounding conditions are such that the parent form is more competent to deal with them and flourish in them than the derived forms, then, in the struggle for existence, the parent form will maintain itself and the derived forms will be exterminated. But if, on the contrary, the conditions are such as to be better for the derived than for the parent form, the parent form will be extirpated and the derived form will take its place.
In the first case, there will be no progression, no advance of type, through any imaginable series of ages; in the second place, there will be modification and change of form. Thus the existence of these persistent types of life is no obstacle in the way of the theory of evolution at all. Take the case of the scorpions to which I have just referred. No doubt, since the Carboniferous epoch conditions have existed such as existed when the scorpions of that epoch flourished, in which they find themselves better off, more competent to deal with the difficulties in their way than any kind of variation from the scorpion type; and for that reason the scorpion has persisted, and has not been supplanted by any other form. And there is no reason in the nature of things why, as long as this world exists, if there be conditions more favorable to scorpions than any variation which may arise from them, these forms of life should not persist.
Therefore, this objection is no objection at all. The facts of this character—and they are numerous—belong to that class of evidence which I have called indifferent. That is to say, they may afford no direct support to the doctrine of evolution, but they are perfectly capable of being interpreted in consistency with it. There is another order of facts of the same kind, and susceptible of the same interpretation. The great group of Lizards, which abound so much at the present day, extends through the whole series of formations as far back as what is called the Permian epoch, which is represented by the strata lying just above the coal. These Permian lizards differ astonishingly little from the lizards which exist at the present day. Comparing the amount of difference between these Permian lizards and the lizards of the present day with the prodigious lapse of time between the Permian epoch and the present age, it may be said that there has been no appreciable change.
But when you carry your researches farther back in time you find no trace whatever of lizards nor of any true reptile whatever in the whole mass of formations beneath the Permian. Now, it is perfectly clear that if our existing paleontological collections, our existing species of stratified rock, exhaust the whole series of events which have ever taken place upon the surface of the globe, such a fact as this directly contravenes the whole theory of evolution, because this theory postulates that the existence of every form must have been preceded by that of some form comparatively little different from it. Here, however, we have to take into consideration that important fact so well insisted upon by Lyell and Darwin—the imperfection of the geological record. It can be demonstrated as a matter of fact that the geological record must be incomplete, that it can only preserve remains found in certain favorable localities and under particular conditions; that it must be destroyed by processes of denudation, and obliterated by processes of metamorphosis—by which I mean that beds of rock of any thickness crammed full of organic remains may yet, either by the percolation of water through them or the influence of subterranean heat (if they descend far enough toward the centre of the earth), lose all trace of these remains, and present the appearance of beds of rock formed under conditions in which there was no trace of living forms. Such metamorphic rocks occur in formations of all ages, and we know with perfect certainty when they do appear that they have contained organic remains, and that those remains have been absolutely obliterated.
I insist upon the defects of the geological record the more because those who have not attended to these matters are apt to say to us, "It is all very well, but, when you get into difficulty with your theory of evolution, you appeal to the incompleteness and the imperfection of the geological record;" and I want to make it perfectly clear to you that that imperfection is a vast fact which must be taken into account in all our speculations, or we shall constantly be going wrong.
You will all see that singular series of tracks which is copied of its natural size in the large diagram hanging up here, which I owe to the kindness of my friend Prof. Marsh, with whom I had the opportunity recently of visiting the precise locality in Massachusetts in which these tracks occur. I am, therefore, able to give you my own testimony, if needed, that they accurately represent the state of
Fig. 1.—Tracks of Brontozoum.
things which we saw. The valley of the Connecticut is classical ground for the geologist. It contains great beds of sandstone, covering many square miles, and which present this peculiarity, that they have evidently formed a part of an ancient sea-shore, or, it may be, lake-shore, and that they have been sufficiently soft for a certain period of time to receive the impressions of whatever animals walked over them, and to preserve them afterward in exactly the same way, as such impressions are at this very moment preserved on the shores of the bay of Fundy and elsewhere. We have there the tracks of some gigantic animal (pointing to the diagram), which walked on its hind-legs. You see the series of marks made alternately by the right foot and by the left foot; so that from one impression to the other of the three-toed foot on the same side is one stride, and that stride, as we measured it, is six feet nine inches. I leave you, therefore, to form an impression of the magnitude of the creature which must have walked along the ancient shore, and which made these impressions.
Now, of such impressions there are untold thousands upon these shores. Fifty or sixty different kinds have been discovered, and they cover vast areas. But up to this present time not a bone, not a fragment, of any one of the great creatures which certainly made these impressions has been found; and the only skeleton which has been met with in all these deposits to the present day—though they have been carefully hunted over—is one fragmentary skeleton of one of the smaller forms. What has become of all these bones? You see we are not dealing with little creatures, but animals that make a step of six feet nine inches; and their remains must have been left somewhere. The probability is, that they have been dissolved away, and absolutely lost.
I have had occasion to work at series of fossil remains of which there was nothing whatever except the casts of the bones, the solid material of the bone having been dissolved out by percolating water. It was a chance in this case that the sandstone happened to be of such a constitution as to set, and to allow the bones to be afterward dissolved out, leaving cavities of the exact shape of the bones. Had that constitution been other than what it was, the bones would have been dissolved, the beds of sandstone would have fallen together, become one mass, and not the slightest indication that the animal had existed would have been discovered.
I know of no more striking evidence than this fact affords, of the caution which should be used in drawing the conclusion, from the absence of organic remains in a deposit, that animals did not exist at the time it was formed. I believe that, having the right understanding of the doctrine of evolution on the one hand, and having a just estimation of the importance of the imperfection of the geological record on the other, all difficulty from the kind of evidence to which I have adverted is removed; and we are justified in believing that all such cases are examples of what I have designated negative or indifferent evidence—that is to say, they in no way directly advance the theory of evolution, but they are no obstacle in the way of our belief in the doctrine.
I now pass on to the consideration of those cases which are not—for reasons which I will point out to you by-and-by—demonstrative of the truth of evolution, but which are such as must exist if evolution be true, and which therefore are, upon the whole, strongly in favor of the doctrine. If the doctrine of evolution be true, it follows that, however diverse the different groups of animals and of plants may be, they must have all, at one time or other, been connected by gradational forms; so that, from the highest animals, whatever they may be, down to the lowest speck of gelatinous matter in which life can be manifested, there must be a sure and progressive body of evidence—a series of gradations by which you could pass from one end of the series to the other. Undoubtedly that is a necessary postulate of the doctrine of evolution. But, when we look upon animated Nature as it at present exists, we find something totally different from this. We find that animals and plants fall into groups, the different members of which are pretty closely allied together, but which are separated by great breaks or intervals from other groups. And we cannot at present find any intermediate forms which bridge over these gaps or intervals. To illustrate what I mean: Let me call your attention to those vertebrate animals which are most familiar to you, such as mammals, and birds, and reptiles. At the present day these groups of animals are perfectly well defined from one another. We know of no animal now living which in any sense is intermediate between the mammal and the bird, or between the bird and reptile; but, on the contrary, there are actually some very distinct and anatomical peculiarities, well-defined marks, by which the mammal is separated from the bird, and the bird from the reptile. The distinctions are apparent and striking if you compare the definitions of these great groups as they now exist. At the present day there are numerous forms of what we may call broadly the pig tribe, and many varieties of ruminants. These latter have their definite characteristics, and the former have their distinguishing peculiarities. But there is nothing that fills up the gap between the ruminants and the pig tribe. The two are distinct. So also is this the case between the groups of another class—reptiles. We have crocodiles, lizards, snakes, and tortoises, and yet there is nothing—no connecting link—between the crocodile and lizard, or between the lizard and snake, or between the snake and the crocodile, or between any two of these groups. They are separated by absolute breaks. If, then, it could be shown that this state of things was from the beginning—had always existed—it would be fatal to the doctrine of evolution. If the intermediate gradations which the doctrine of evolution postulates must have existed between these groups—if they are not to be found anywhere in the records of the past history of the globe—all that is so far a strong and weighty argument against evolution; while, on the other hand, if such intermediate forms are to be found, that is so much to the good of evolution, although, for the reason which I will put before you by-and-by, we must be cautious in assuming such facts as proofs of the theory.
It is a very remarkable fact that, from the commencement of the serious study of paleontology, from the time in fact when Cuvier made his brilliant researches upon the fossil remains of animals found in the quarries of Montmartre, Paleontology has shown what she was going to do in this matter, and what kind of evidence it lay in her power to produce.
I said just now that at the present day the group of pig-like animals and the group of ruminants are entirely distinct; but one of the first of Cuvier's discoveries was an animal which he called the Anoplotherium, and which he showed to be, in a great many important respects, intermediate in its character between the pigs on the one hand and the ruminants on the other; that, in fact, research into the history of the past did so far—and to the extent which Cuvier indicated—tend to fill up the breach between the group of ruminants and the group of pigs. All subsequent research has also tended in this direction; and at the present day the investigations of such men as Rütimeyer and Gaudry have tended to fill up and connect, more and more, the gaps in our existing series of mammals. But I think it may have an especial interest if—instead of dealing with these cases, which would require a great deal of tedious osteological detail—I take the case of birds and reptiles—which groups, at the present day, are so clearly distinguished from one another that there are perhaps no classes of animals which in popular apprehension are more completely separated. Birds, as you are aware, are covered with feathers; they are provided with wings; they are specially and peculiarly modified as to their anterior extremities; and they walk perpendicularly upon two legs; and those limbs, when they are considered anatomically, present a great number of exceedingly remarkable peculiarities, to which I may have occasion to advert incidentally as I go on, but which are not met with even approximately in any existing forms of reptiles. On the other hand, reptiles, if they have a covering at all, have a covering of scales or bony plates. They possess no wings; they are not volatile, and they have no such modification of the limbs as we find in birds. It is impossible to imagine any two groups apparently more definitely and distinctly separated. As we trace the history of birds back in time, we find their remains abundant in the tertiary rocks throughout their whole extent, but, so far as our present knowledge goes, the birds of the tertiary rocks retain the same essential character as the birds of the present day—that is to say, the tertiary bird comes within the definition of our existing birds, and are as much separated from reptiles as our existing birds are. A few years ago no remains of birds had been found below the tertiary rocks, and I am not sure but that some persons were prepared to
Fig. 2.—Hesperornis Regalis. (Marsh.)
demonstrate that they could not have existed at an earlier period. But in the last few years such remains have been discovered in England, though, unfortunately, in a very imperfect condition. In your country the development of cretaceous rocks is enormous; the conditions under which the later cretaceous strata have been deposited are highly favorable to the preservation of organic remains, and the researches full of labor and toil, which have been carried on by Prof. Marsh in these Western cretaceous rocks, have rewarded him with the discovery of forms of birds of which we had hitherto no conception. By his kindness, I am enabled to place before you a restoration of one of these extraordinary birds, every part of which can be thoroughly
Fig. 3.—Hesperornis Regalis. (Marsh.)
justified. The remains exist in the greatest beauty in his collection. This Hesperornis stood about six feet high, and in a great many respects is astonishingly like an existing diver or grebe, so like it indeed that, had this skeleton been found in a museum, I suppose—if the head had not been known—it would have been placed in the same general group as the divers and grebes of the present day. But this bird differs from all existing birds, and so far resembles reptiles in one important particular that it is provided with teeth. These
Fig. 4.—Ichthyornis Dispar. (Marsh.)
long jaws are beset by teeth, as in this diagram, in which one of the teeth is represented separately. In possessing true teeth, the Hesperornis differs entirely from any existing bird, and in view of the characteristics of this bird we are obliged to modify the definition of the class of birds and reptiles. Before the discovery of a creature such as this, it might have been said that birds were characterized by the absence of teeth; but the discovery of a bird that had teeth shows at once that there were ancient birds that, in that particular respect, approached reptiles more nearly than any existing bird does.
The same rocks have yielded another bird (Ichthiyornis), which also has teeth in its jaws, the teeth in this case being situated in distinct sockets, while those of Hesperornis were not so lodged. The latter also had very small wings, while Ichthyornis has strong wings. Ichthyornis also differed in the fact that the joints of its backbone—its vertebræ had not the peculiar character that the vertebræ of existing birds have, but were concave at each end. This discovery leads us to make another modification in the definition of the group of birds, and to part with another of the characters by which they are distinguished from reptiles. We know nothing whatever of birds older than these until we come down to the Jurassic period, and from rocks of that age we have a single bird which was first made known by the finding of a fossil feather. It was thought wonderful that such a perishable thing as a feather should be discovered and nothing more, and so it was; and for a long time nothing was known of this bird except its feather. But, by-and-by one solitary specimen was discovered, which is now in the British Museum. That solitary specimen is unfortunately devoid of its head; but there is this wonderful peculiarity about the creature that, so far as its feet are known, it has all the characters of a bird, all those peculiarities by which a bird is distinguished from a reptile. Nevertheless, in other respects, it is unlike a bird and like a reptile. There is a long series of caudal vertebræ. The wing differs in some very remarkable respects from the structure it presents in a true bird. In a true bird the wing answers to these three fingers—the thumb and two fingers of my hand—the metacarpal bones are fused together into one mass—and the whole apparatus except the thumb is bound up in a sheath of integument, and the edge of the hand carries the principal quill-feathers. It is in that way that the bird's wing becomes an instrument of flight. In the Archæopteryx, the upper-arm bone is like that of a bird; these two forearm bones are more or less like those of a bird, but the fingers are not bound together—they are free, and they are all terminated by strong claws, not like such as are sometimes found in birds, but by such as reptiles possess, so that in the Archæopteryx you have an animal which, to a certain extent, occupies a midway place between a bird and a reptile. It is a bird so far as its foot and sundry other parts of its skeleton are concerned; it is essentially and thoroughly a bird in the fact that it possesses feathers, but it is much more properly a reptile in the fact that what represents the hand has separate bones resembling those which terminate the fore-limb of a reptile. Moreover, it had a long tail with a fringe of feathers on each side. All these cases, so far as they go, you will observe are in favor of evolution to this extent, that they show that in former periods of the world's history creatures existed which overstepped the bounds of all existing classes and groups, and tended to fill up the intervals which at present exist between them. But we can go further than this. It is possible to fill up the interval between birds and reptiles in a much more striking manner. I do not think that this is to be done by looking upon what are called the Pterodactyls as the intermediate form between
Fig. 5.—Pterodactylus Spectabilis. (Von Meyer.)
birds and reptiles. Throughout the whole series of the mesozoic rocks we meet with some exceedingly remarkable flying creatures, some of which attain a great size, their wings having a span of eighteen or twenty feet or more, and these are known as Pterosauria, or Pterodactyls. We find these with a bird-like head and neck, with a vertebral column sometimes terminated by a short and sometimes by a long tail, and in which the bones of the skeleton present one of the peculiarities which are often considered to be most characteristic of birds—that of having pneumatic cavities, which make the creature specifically light in its flight. Like a bird, this creature has a largish breastbone, but from that point onward, so far as I can see, special, particular resemblances end, and a careful examination of the fore-limbs shows you that they are not birds' wings; they are something totally different from a bird's wings. And then, again (pointing to a chart), those are not a bird's posterior extremities, but are rather a reptilian's hind-limbs. The vertebræ present nothing that I need dwell upon, but the bones of the hand are very wonderful.
There are four fingers represented. These four fingers are large, and three of them—these, which answer to these three in my hands—are terminated by claws, while the fourth is enormously prolonged into a great jointed style. You see at once from what I have stated about a bird's wing that there could be nothing more unlike a bird's wing than this is. It was concluded by general reasoning that this finger was made to support a great web like a bat's wing. Specimens now exist showing that this was really the case, that this creature was devoid of feathers, but the fingers supported a vast web like a bat's wing, and there can be no doubt that this ancient reptile flew after the fashion of a bat. Thus, though the pterodactyl is a flying reptile, although it presents some points of similarity to birds, yet is it so different from them that I do not think that we have any right to regard it as one of the forms intermediate between the reptile and the bird. Such intermediate forms are to be found, however, by looking in a different direction. Through the whole series of mesozoic rocks there occur reptiles, some of which are of gigantic dimensions; in fact, they are reckoned among the largest of terrestrial animals. Some of them are forty and fifty, possibly more, feet long. Such are the Iguanodon, the Megalosaurus, and a number of others, with the names of which I will not trouble you. There are great diversities of structure among these great reptiles. Some of them resemble lizards in the proportions of their limbs, and have evidently walked on all-fours, in that respect resembling the existing crocodile; but in others you can trace a series of modifications in virtue of which the hind-limbs at length completely assumed the character of a bird's hind-limbs. I here indicate (pointing to a diagram) the hind limb of a crocodile, showing the bones of the hind-limbs and of the pelvis. These are the haunch-bones; these are the two leg-bones. Then comes the division of the foot which we call the tarsus, in which the component bones are separate and distinct from one another, from the bones of the leg and from those of the metatarsus. Then come the four toes, which alone exist in the hind-feet of the crocodile, and which are separate and distinct. The foot is flat on the ground, so that the legs spread out and the weight of the body hangs clumsily between them. Contrast this with what we find in the bird—the haunch-bone here is immensely elongated, and the joints of the back-bone, between the two haunch-bones, are united so as to form a solid support upon which the weight of the body rests. Then the thigh-bone becomes very short, and has a back ridge upon its outer articular surface. At the lower end the ridge tits in between the upper extremity of the small bone of the leg, near to the great bone,
and makes a kind of spring-joint. The small bone of the leg is quite large above, and becomes rudimentary below. It runs out into a style, instead of being long and large, as it is in the case of the crocodile. Then, when you come to the bones of the foot, you find there are no separate bones such as you have here, but the end of the tibia, or large bone of the leg, appears to end in a kind of pulley, a single bone follows the tibia and to the trifurcated extremity of this bone. Upon the extremity of that bone are attached three toes. It is obvious that the contrast between the crocodile's leg on the one hand, and the bird's leg on the other, is very striking. But this interval is completely filled up when you study the character of the hinder extremities in those ancient reptiles which are called the Dinosauria. In some of these the bones of the pelvis, and those of the hind-limb, become extraordinarily similar to those of birds, especially to those of young or foetal birds. Furthermore, in some of these reptiles, the fore-limbs become smaller and smaller, and thus the suspicion naturally arises that they may have assumed the erect position. That view was entertained by Mantel, and was also demonstrated to be probable by your own distinguished anatomist, Leidy, but the discoveries of late years show that in some of these forms the fact was actually so; that reptiles once existed which walked upon their hind-legs as birds now do. The diagram is a faithful and accurate representation of an existing fossil except for this, that, whereas in the existing fossil the bones are twisted about and out of place, I
Fig. 7.—Compsognathus Longipes. (Wagner.)
have put them here in the position that they must have had in nature. You see a creature with a long neck and bird-like head, with very small anterior extremities, and that compsognathus creature must assuredly have walked about upon its hind-legs, bird-fashion. Acid to this feathers, and the transition would be complete. Now to define it: The possession of teeth would, as we see, not separate this animal from the class of birds we have. We have had to stretch the class of birds so as to include birds possessed of teeth, and, so far as the character of the skeleton goes, we may fairly say that there needs here little more than the addition of feathers—and whether this creature had them or not we don't know—to convert it into a bird.
I have said that there can be no question, from their anatomical structure, that these animals walked upon their hind-legs, and, in fact, there are to be found in the Wealden strata of England gigantic foot-steps arranged in order like these of the Brontozoum, and which there can be no reasonable doubt were made by Dinosauria, the remains of which were found in the same rock. And, knowing that reptiles that walked upon their legs and shared many of the anatomical characters of birds did once exist, it becomes a very important question whether those tracks in Massachusetts—to which I referred just now, and which formerly used to be unhesitatingly ascribed to birds—may not all have been made either by reptiles similar to the Dinosauria, or whether, if we could get hold of the skeleton which made these tracks, some of which are marvelously like bird’s tracks, we should not come upon exactly that series of transitions by which in former days the reptile was connected with the bird.
I don’t think, ladies and gentlemen, that I need insist upon the value of evidence of this kind. You will observe that, although it does not prove that birds have originated from reptiles by the gradual modification of the ordinary reptile into a dinosaurian form, and so into a bird, yet it does show that such a process may possibly have taken place, and it does show that, in former times, there existed creatures which filled up one of the largest gaps in existing animate nature, and that was exactly the kind of evidence which I stated to you in starting we are bound to meet with in rocks if the hypothesis of evolution be correct.
In my third and last lecture I will take up what I venture to call the demonstrative evidence of evolution.
- The second of three lectures on "The Direct Evidence of Evolution," delivered at Chickering Hall, New York, September 20th. From the report of the New York Tribune, carefully revised by Prof. Huxley.