From the point of view of the student of domesticated animals the races of men may well be divided into those which have and those which have not the use of the horse. Although there are half a score of other animals which have done much for man, which have indeed stamped themselves upon his history, no other creature has been so inseparably associated with the great triumphs of our kind, whether won on the battle-field or in the arts of peace. So far as material comfort, or even wealth, is concerned, we of the northern realms and present age could, perhaps, better spare the horse from our present life than either sheep or horned cattle; but without this creature it is certain that our civilization would never have developed in anything like its present form. Lacking the help which the horse gives, it is almost certain that, even now, it could not be maintained. We know the ancient natural history of the horse more completely than that of any other of our domesticated animals. We can trace the steps by which its singularly strong limbs and feet, on which rests its value to man, were formed in the great laboratory of geologic time. The story is so closely related to the interests of man that it will be well briefly to set it before the reader. In the first stages of the Tertiary period, in the age when we begin to trace the evolution of the suck-giving animals above the lowly grade in which the kangaroos and opossums belong, we find the ancestors of our mammalian series all characterized by rather weakly organized limbs fitted, as were those of their remoter kindred the marsupials, for tree climbing rather than for moving over the surface of the ground. The fact is, that all the creatures of this great clan acquired their properties of body in arboreal life, and with such relatively small and light bodies as were fitted for tree climbing. For this use the feet need to be loose-jointed, and so the system of five toes, each terminating in a sharp and strong nail or claw, became fixed in the inheritances. When, gaining strength and coming to possess a more important place in the world, these ancient tree-dwellers were able to occupy the ground which of old had been possessed by the great reptiles, the limbs that had served well for an arboreal life had to undergo many changes in order to fit them for progression in the new realm.