Such declarations clearly challenge consideration,[{240}] especially when it is remembered how strict were the views which Professor Huxley professed as to the necessity of proofs for our beliefs,—"that it is wrong for a man to say he is certain of the objective truth of any proposition unless he can produce evidence which logically justifies that certainty."[276]
We therefore turn naturally to his lectures on Evolution, wherein he treats the palæontological argument ex professo, and we find that his verdict is based upon a few selected instances, such as that of the reptilian birds already mentioned, which he considers favourable to Evolution, and one which he terms demonstrative,—namely that of the Horse. This he treats in some detail; in regard of it he delivers the positive judgment which we have just heard, and it therefore in a special manner demands our attention.
As furnishing evidence for the history of the horse, two features are of special importance, his limbs, and his teeth. Of these we may confine our attention to the former, as being, at once, sufficient for our purpose, and within the scope of ordinary observation.
The horse family, or Equidae, belong to the tribe of Ungulates, or hoofed animals, some points of whose anatomy require to be considered in relation to our own.
Taking first the fore-limbs. What we call the "knee" of a horse is in reality the wrist,—the true[{241}] knee, or rather elbow, being what we call the "shoulder." Below the knee comes the "cannon bone," corresponding to the middle bone of the hand, and below it the "pastern," "coronary," and "coffin" bones, representing the joints of the solitary middle-finger, while the hoof is its greatly enlarged and thickened nail. Similarly, in the hind-limbs; the "hock" is veritably the ankle, and again the lateral digits are suppressed, the middle toe alone remaining.
It thus appears that an Ungulate such as the horse, is an extreme modification of the general Mammalian plan, his members being highly specialized for a certain kind of work. His leg and hoof, as the theory of genetic Evolution declares, have been gradually fashioned to their present shape from an original limb in the common Mammalian ancestor, which by other modifications has equally produced the totally different members possessed by other mammals.
That the horse is descended from a race bearing more than one digit on each extremity, seems to be indicated by the splint-bones which are found on the cannon-bone of both fore and hind legs, and which represent the second and fourth finger and toe, and also by recorded occurrences of polydactyle horses, one of which has a distinguished place in history as Julius Cæsar's charger.[277][{242}]
That the animal as we now know him is the lineal descendant of various other ungulates, in whom the digits were gradually reduced from the normal number of five, to their present solitary representative, Professor Huxley and other Evolutionists hold to be demonstrated by the discovery in due succession of various equine specimens, in which this diminution is gradually exhibited.
The remains of these animals are all found in Tertiary strata, of which, it will be remembered, there are three great divisions, the Eocene, Miocene, and Pliocene, the first named being the most ancient, and the last the most recent.
The genus Equus, or at least our modern horse, Equus caballus, can be traced no further back than the Post-tertiary period. The succession of forms leading up thither commences at the bottom of the Eocene, and extends to the upper Pliocene.