Following Professor Huxley's guidance, we trace the pedigree downwards, thus:
Firstly, there is the true horse. Next we have the American Pliocene form, Pliohippus. In the conformation of its limbs it presents some very slight deviations from the ordinary horse. Then comes Protohippus, which represents the European Hipparion, having one large digit and two small ones on each foot.... But it is more valuable than Hipparion, for certain peculiarities tend to show that the latter is rather a member of a collateral branch, than a form in the direct line of succession. Next, in the backward order in time, is the Miohippus, [Miocene], which corresponds pretty nearly[{243}] with the Anchitherium of Europe. It presents three complete toes—one large median and two smaller lateral ones; and there is a rudiment of that digit which answers to the little finger of the human hand. The European record stops here: in the American Tertiaries, the series of ancestral equine forms is continued into the Eocene. An older Miocene form, Mesohippus, has three toes in front, with a large splint-like rudiment representing the little finger, and three toes behind. The radius and ulna, tibia and fibula,[278] are distinct. Most important of all is the Orohippus, from the Eocene. Here we find four complete toes on the front limb, three toes on the hind-limb, a well developed ulna, a well developed fibula.
Here, when the lecture which we are considering was delivered, the series terminated:—and upon the facts as above given Professor Huxley thus commented:
Thus, it has become evident that, so far as our present knowledge extends, the history of the horse-type is exactly and precisely that which could have been predicted from a knowledge of the principles of Evolution. And the knowledge we now possess justifies us completely in the anticipation, that when the still lower Eocene deposits, and those which belong to the Cretaceous Epoch have yielded up their remains, we shall find, first, a form with four complete toes and a rudiment of the innermost or first digit in front, with probably a[{244}] rudiment of the fifth digit in the hind foot; while, in still older forms, the series of the digits will be more and more complete, until we come to the five-toed animals, in which, if the doctrine of Evolution is well founded, the whole series must have taken its origin.
Finally he was able to add in a note that since the delivery of the lecture, Professor Marsh had discovered a new genus of Equine Mammals, Eohippus, corresponding very nearly to his description of what might first be looked for. "This," adds Professor Huxley, "is what I mean by demonstrative evidence of Evolution.... In fact, the whole evidence is in favour of Evolution, and there is none against it."
That these facts are indeed most remarkable and deserving of all attention, cannot be questioned. But before we can agree that they are conclusive and demonstrative in Professor Huxley's sense a good many considerations require to be carefully weighed.
(i.) It is obvious, in the first place, that here as in all other instances which we have seen, the one thing is lacking which is really wanted in order to prove Evolution, namely evidence of one species gradually shading off into another. The creatures of which we have heard, are each isolated from the rest, and indeed very much isolated, for each belongs to a different genus,[279] which shows that the differences[{245}] between them are substantial. They are, in fact, farther apart from one another, than the zebra or the donkey from the horse, for both of these are classed in the genus equus,—or than the Bengal tiger is from the domestic pussy-cat, both belonging to the genus felis.
These various ungulate forms thus stand a long way from one another, and if they were once connected together by a bridge, or rather a causeway, we ought certainly to find some traces of it, and not always of those particular types which require to be united. If we suppose the very distinct species actually known to have been the piers of such a bridge, yet what has become of the arches? Till some vestiges of these be found, or, at least, some positive evidence that arches there actually were, can it be said that the story of the fossil equidae furnishes convincing testimony on behalf of the supposed evolution? Affinities these various forms undoubtedly exhibit: it has yet to be shown that affinities necessarily imply descent.
There is, however, something even more remarkable. We have seen that Professor Huxley prognosticated beforehand the discovery of Eohippus, and specified pretty nearly the features it would be found to present. In the same way, Professor Marsh[280] anticipates and describes a still more remote ancestral form, for which, though it has not yet been[{246}] found, he has provided an appellation, Hippops. But if either Professor really believes in Evolution, why does he take for granted that we shall chance upon one particular form, standing like a solitary outpost by itself, and not upon any other trace of the stream of life whereof it was but one transient phase? Such predictions may be evidence that the occurrence of these progressive forms is regulated by something analogous to Bode's Law of interplanetary distances, and that their discovery may be looked for at certain intervals. But the very fact that their actual position can be so accurately specified serves to show that it is very definitely fixed.
(ii.) Moreover, a very grave difficulty at once suggests itself, of which Professor Huxley makes no mention. The horse as we now have him, Equus caballus, is a native of the Old World, and has been introduced to America only since the time of Columbus. There had, it is true, been horses in America previously,—belonging to the genus Equus, perhaps even to the species caballus,—they had, however, been long extinct, and no memory of them remained. But, as will be noticed, the pedigree given by Professor Huxley consists almost entirely of American animals, to which category belong all whose names terminate in -hippus, and these cannot with any reason be assigned as progenitors to the European horse. As Sir J. W. Dawson observes:[281][{247}]