In America a series of horse-like animals has been selected, beginning with the Eohippus of the Eocene—an animal the size of a fox, and with four toes in front and three behind—and these have been marshalled as the ancestors of the fossil horses of America.... Yet all this is purely arbitrary, and dependent merely on a succession of genera more and more closely resembling the modern horse being procurable from successive Tertiary deposits often widely separated in time and place. In Europe, on the other hand, the ancestry of the horse has been traced back to Palæotherium—an entirely different form—by just as likely indications, the truth being that as the group to which the horse belongs culminated in the early Tertiary times, the animal has too many imaginary ancestors. Both genealogies can scarcely be true, and there is no actual proof of either. The existing American horses, which are of European origin, are, according to the theory, descendants of Palæotherium, not of Eohippus; but if we had not known this on historical evidence, there would have been nothing to prevent us from tracing them to the latter animal. This simple consideration alone is sufficient to show that such genealogies are not of the nature of scientific evidence.
(iii.) Even apart from this fundamental difficulty, there is much diversity as to the precise genealogy. We may compare together the lines of ancestry favoured—(1) by Professor Huxley, (2) In a case exhibited in our Museum of Natural History to illustrate the subject, (3) By Mr. Mivart,[282] (4) By[{248}] Mr. Lydekker,[283] (5) In The Evolution of the Horse, a pamphlet issued, January, 1903, by the American Museum. This last gives the very latest version of the pedigree, but, naturally, of the American Horse alone.
| Huxley. | British Museum Case. | Mivart. | Lydekker. | American Museum. | |
|---|---|---|---|---|---|
| Equus | Equus | Equus | Equus | Equus | |
| Pliohippus | |||||
| Protohippus | Hipparion | Hipparion Protohippus | Hipparion Protohippus | Hipparion Hypohippus | |
| { | Miohippus | Anchitherium | Anchitherium | Merychippus | |
| Anchitherium | Anchitherium | Pachynolophus | Anchilophus (form allied to) | Mesohippus (2 species) | |
| Mesohippus | Protohippus | Epihippus | |||
| Orohippus | {Mesohippus (2 species) | Protorohippus | |||
| Eohippus | Hyracotherium | Phenacodus | Hyracotherium Systemodon | Eohippus An undiscovered ancestor (Hippops) |
It will be observed, that whereas Hipparion is disallowed by Professor Huxley as not being in the direct line of descent, in all the other genealogies he appears as the immediate ancestor of Equus. Also that in all these tables, Old World and New World forms are used indifferently to supply progenitors for the same successor. Also that there is no agreement at all as to the earlier ancestry. It would likewise appear that even the existence of Eohippus himself is not beyond question, for in our Museum galleries and guide-book his name always has a note of interrogation appended. The American authorities give an anticipatory sketch of the limbs of the ancestor which still remains to be discovered.
There is something even more remarkable.[{249}]
Huxley's lecture exhibiting the pedigree we have been considering was delivered in 1876. We have already seen that six years earlier he had declared himself satisfied, after much search, that though other genealogies might be doubtful, we had in the case of the Horse something really satisfactory. But the pedigree of 1870—which he thus indicated as scientifically established—was totally different from that of 1876, and was acknowledged as erroneous by the very acceptance of the latter. In 1870 the ancestry presented for Equus consisted of Hipparion, Anchitherium, and Plagiolophus. Of these, Hipparion was in 1876 specifically disallowed as a direct ancestor: Anchitherium was displaced by Miohippus, and although we are told that these creatures "correspond pretty nearly," the Horse cannot be descended from both, especially as they dwelt in different hemispheres. Finally Plagiolophus disappears from the amended pedigree altogether. Nothing could more vividly illustrate the danger of such speculations than that an authority so clear-headed and conscientious as Professor Huxley should thus proclaim his acceptance of a genealogy which he had on after information to renounce. Nor to him alone have such misadventures happened. Mr. Darwin too thought the claim of Hipparion to ancestral equine rank to be beyond dispute. "No one will deny," he wrote,[284] "that the Hipparion is intermediate between the[{251}] existing horse and certain older ungulate forms." Yet, as we see, this has been denied by his champion Huxley himself.
(iv.) The materials available for the reconstruction of these various equine forms, are far less satisfactory than might easily be supposed. As a rule, each is known to us only by small fragments of its skeleton, so that we can have no assurance as to what the whole animal was really like, or even that all parts assigned to one creature really belonged to him. We can accordingly feel no certainty that if we could see any of these as a whole we should find it possible to suppose that the horse descended from it. Thus in Hippidium, an American genus closely allied to Equus, it is at least doubtful whether the digits did not terminate in claws.[285] One species of Hippidium is known only by a solitary tooth. Of Hyracotherium only the skull has been found: of Orohippus only parts of jaws and teeth and a forefoot: of Epihippus, "only incomplete specimens."[286] Accordingly, Professor Williamson, speaking of the discoveries of Professor Marsh and others, thus expresses himself:[287]
Beyond all question, some of the gaps that have hitherto separated the three animals [Anchitherium, Hipparion, and Equus] are filled up by these discoveries; but I want yet more evidence before I can arrive at the[{252}] conclusion that the doctrine of Evolution is proved by these facts beyond the possibility of question. It appears to me that before I can unhesitatingly give to the testimony of these fossil horses the full value I am asked to do, I must know more about them than is at present possible. It will not be enough that the limbs and teeth of these creatures indicate transmutation, but such transmutation must be evidenced by every part of the animal. This demand is especially applicable to the stages which intervene between the Hipparion and the horse.... Myriads of individuals must have existed to effect the gradual shading of the one into the other in every part of its body.