Fig. 158.—Right manus and pes of †Eohippus.
This is the merest outline sketch of a most wonderful series of gradual and progressive modifications, a sketch that might readily be expanded into a volume, were all the details filled in. While each set of organs, teeth, skull, neck, body, limbs and feet, might appear to advance independently of the others, in reality there was no such independence, for at every stage of the progression all the parts must have been so coördinated into a harmonious whole, that the animal could thrive and hold its own in the stress of competition. Could we but discover all the facts of environment, on the one hand, and organization, on the other, we should doubtless learn that the little †Eohippus was as exquisitely fitted to its place in the Wasatch world, as are the horses, asses and zebras of the present day to theirs. It was the response to changing needs, whether of food, climate, disease or competition, that was the main factor of development.
Fig. 159.—Skeleton of a Pampean horse (†Hippidion neogæum). National Museum, Buenos Aires. For restoration, see [Fig. 119, p. 214]. Note the splint-like nasal bones attached only at the hinder end.
2. †Titanotheriidæ. †Titanotheres
This family, all of whose members vanished from the earth ages ago, was a comparatively short-lived group and nearly the whole of its recorded history was enacted in North America; only a few belated stragglers reached the eastern hemisphere, though the family may, nevertheless, have originated there.
Fig. 160.—White River †titanothere (†Titanotherium robustum) males fighting. Restored from a skeleton in the American Museum of Natural History.