Classes, then, it will be admitted, have undoubtedly become what we now know them by a very gradual process following laws identical (as we have already seen, page 19) with the laws of evolution. Shall we try orders? Of the class Mammalia there are two well-recognized and widely-distinct _orders_, viz., the Carnivores and the Herbivores. We all know how widely diverse these are in form, in structure, in habits, and in food. Has it always been so? Have these been made so at once? By no means. They have gradually become so. The earliest mammals were neither the one nor the other distinctively. They were _omnivores_, completely intermediate in food, habits, form, and structure. From this common stock the two orders have gradually separated, the carnivores becoming more and more adapted to one mode of life and the herbivores to another, by a process following the laws of evolution, as already explained. Shall we try _families_ and _genera_? Marsh and Huxley have shown us how completely the horse family (_Equidæ_) and the horse-genus (_Equus_) illustrate the process of gradual becoming and the law of evolution. Under their guidance, we see that the earliest traceable ancestor of the horse family, before it was distinctively a horse family at all, had on the fore-foot five toes in the Lower Eocene, four toes in the Upper Eocene, and three toes in the Miocene; then we see the two side-toes shortening up more and more in the Pliocene and becoming rudimentary splints, leaving only one toe in the Quaternary and present epochs. Thus, the side-splints in the foot of the modern horse tell the story of its three-toed ancestry. Similar gradual changes are clearly traceable in size, shape, structure of limbs, of teeth, and of brain. In all respects the members of the horse family have become more and more horse-like in the course of time. This subject will be taken up and more fully illustrated, under the head of special evidences, in a subsequent chapter. We here touch it only sufficiently to illustrate this universal law of gradual becoming.