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] 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_. Following Professor Huxley's guidance, we trace the pedigree downwards, thus: