"To anyone who is acquainted with the morphology of vertebrated animals, these characteristic structures of the horse show that it deviates widely from the general structure of mammals; and that the horse type is, in many respects, an extreme modification of the general mammalian plan. The least modified mammals, in fact, have the radius and ulna, the tibia and fibula, distinct and separate. They have five distinct and complete digits on each foot, and no one of these digits is very much larger than the rest. Moreover, in the least modified mammals, the total number of the teeth is very generally forty-four, while in the horse the usual number is forty, and, in the absence of the canines, it may be reduced to thirty-six; the incisor teeth are devoid of the fold seen in those of the horse; the grinders regularly diminish in size from the middle of the series to its front end; while their crowns are short, early attain their full length, and exhibit simple ridges or tubercles, in place of the complex foldings of the horse's grinders.
"Hence the general principles of the hypothesis of evolution lead to the conclusion that the horse must have been derived from some quadruped which possessed five complete digits on each foot; which had the bones of the forearm and of the leg complete and separate; and which possessed forty-four teeth, among which the crown of the incisors and grinders had a simple structure; while the latter gradually increased in size from before backwards, at any rate in the anterior part of the series, and had short crowns.
"And if the horse had been thus evolved, and the remains of the different stages of its evolution have been preserved, they ought to present us with a series of forms in which the number of the digits becomes reduced; the bones of the forearm and leg gradually take on the equine condition; and the form and arrangement of the teeth successively approximate to those which obtain in existing horses.
"Let us turn to the facts and see how far they fulfill these requirements of the doctrine of evolution.
"In Europe abundant remains of horses are found in the Quaternary and later Tertiary strata as far as the Pliocene formation. But these horses, which are so common in the cave-deposits and in the gravel of Europe, are in all essential respects like existing horses, and that is true of all the horses of the later part of the Pliocene epoch. But, in the deposits which belong to the earlier Pliocene, and later Miocene epochs, and which occur in Britain, in France, in Germany, in Greece, in India, we find animals which are extremely like horses—which in fact are so similar to horses, that you may follow descriptions given in works upon the anatomy of the horse, upon the skeletons of these animals—but which differ in some important particulars. For example, the structure of their fore and hind limbs is somewhat different. The bones, which, in the horse are represented by two long splints, imperfect below, are as long as the middle metacarpal and metatarsal bones; and, attached to the extremity of each, is a digit with three joints of the same general character as those of the middle digit, only very much smaller. These small digits are so disposed that they could have had but very little functional importance, and they must have been rather of the nature of the dew-claws, such as are to be found in many ruminant animals. The Hipparion, as the extinct European three-toed horse is called, in fact presents a foot similar to that of the American Protohippus except that in Hipparion the smaller digits are situated further back, and are of smaller proportional size than in the Protohippus.
"The ulna is slightly more distinct than in the horse; and the whole length of it, as a very slender shaft, intimately united with the radius, is completely traceable. The fibula appears to be in the same condition as in the horse. The teeth of the Hipparion are essentially similar to those of the horse, but the pattern of the grinders is in some respects a little more complex, and there is a depression on the face of the skull in front of the orbit, which is not seen in existing horses.
"In the earlier Miocene and perhaps in the Eocene deposits of some parts of Europe, another distinct animal has been discovered, which Cuvier, who first described some fragments of it, considered to be a Palæotherium, but as further discoveries threw new light on its structure, it was recognised as a distinct genus, under the name of Anchitherium.
"In its general characters the skeleton of Anchitherium is very similar to that of the horse, in fact Lartet and De Blainville called it Palæotherium equinum or Hippoides; and De Cristol, in 1847, said that it differed from Hipparion in little more than the characters of the teeth, and gave it the name of Hipparitherium. Each foot possesses three complete toes: while the lateral toes are much larger in proportion to the middle toe than in Hipparion, and doubtless rested on the ground in ordinary locomotion. The ulna is complete and quite distinct from the radius, although firmly united with the latter. The fibula seems also to have been complete; its lower end, though intimately united with that of the tibia, is clearly united with that of the latter bone. There are forty-four teeth; the incisors have no strong pit. The canines seem to have been well developed in both sexes. The first of the seven grinders, which, as I have said, is frequently absent, and, when it does exist, is small in the horse, is a good-sized and permanent tooth, while the grinder which follows it is but little larger than the hinder ones. The crowns of the grinders are short, and, although the fundamental pattern of the horse-tooth is discernible, the front and back ridges are less curved, the accessory pillars are wanting, and the valleys, much shallower, are not filled up with cement."
Then, after describing his early efforts to trace the descent of the horse from European fossils, Huxley goes on to relate the new light thrown on the matter from the American discoveries of Professor Marsh:
"You are all aware that, when your country was first discovered by Europeans, there were no traces of the existence of the horse in any part of the American continent. The accounts of the conquest of Mexico dwell on the astonishment of the natives of that country when they first became acquainted with that astounding phenomenon, a man seated upon a horse. Nevertheless, the investigations of American geologists have proved that the remains of horses occur in the most superficial deposits of both North and South America, just as they do in Europe. Therefore, for some reason or other,—no feasible suggestion on that subject, so far as I know, has been made,—the horse must have died out on this continent at some period preceding the discovery of America. Of late years there has been discovered in your Western territories that marvellous accumulation of deposits, admirably adapted for the preservation of organic remains, to which I referred the other evening, and which furnishes us with a consecutive series of records of the fauna of the older half of the Tertiary epoch, for which we have no parallel in Europe. The researches of Leidy and others have shewn that forms allied to the Hipparion and the Anchitherium are to be found among these remains. Rut it is only recently that the admirably conceived and most thoroughly and patiently worked-out investigations of Professor Marsh have given us a just idea of the vast fossil wealth and of the scientific importance of these deposits. I have had the advantage of glancing over the collections in Yale Museum; and I can truly say that, so far as my knowledge extends, there is no collection from any one region and series of strata comparable, for extent, or for care with which the remains have been got together, or for their scientific importance, to the series of fossils which he has deposited there. This vast collection has yielded evidence bearing on the question of the pedigree of the horse of the most striking character. It tends to show that we must look to America rather than to Europe for the original seat of the equine series; and that the archaic forms and successive modifications of the horse's ancestry are far better preserved here than in Europe.