Whence came the mammals? That, again, is one of the questions which time alone can completely answer. But the opinion of most geologists is that they arose and developed in Asia first of all, and then spread to other continents. The rise of the mammals, which, unlike the reptiles, bring forth their young relatively mature and nourish and protect them, was contributing to the downfall of the reptiles, though it cannot be considered an actual cause. The mammals' young had a better chance of living and surviving than had the eggs of reptiles. Moreover, the mammals began with superior agility and higher brain-power. It is not surprising, therefore, that the invasion of the mammals resulted in the clumsy, affectionless, small-brained reptiles being driven either into extinction, or into the sedges and rushes, the swamps and lagoons, the coverts of the jungles, the crevices of the rocks, and the various by-ways which the mammals cared least to frequent, and that they have been kept there to this day. At first the mammals were not very different in habit or type from one another. Small animals, which, like the shrews, moles, and hedgehogs lived on insects were among the earliest. There were others whose toes were turning to hoofs in order to fit them for fleetness; and there were some curious creatures called _Coryphodons_, which were like the modern tapir, though they were tusked like boars. The _Coryphodon_ was a slow beast, with toes like those of an elephant, though it was much smaller. In America appeared a small animal not much bigger than a fox-terrier, which was the ancestor of the horse, and of which we shall have more to say. The birds increased, and forms like those of the heron mingled in the swamps with other goose-like birds that kept in their serrated bills some traces of the teeth of their early ancestors. Others, like kingfishers, flitted over the streams; and the emu, ostrich, and moa, as well as the albatross, find their earliest representatives in the Eocene times.