THE EVOLUTION OF THE HORSE
many breeds), the wild horses of Mongolia, the half asses (the kiang and the onager), the asses, and the zebras. The genus includes eight to twenty species of living animals, the number discriminated depending upon the judgment or the fancy of the naturalist making the classification. Most of the later fossil horses are so nearly like the living horses that they have all been placed in the genus Equus. Any one who sees restorations of these animals at once calls them horses, zebras, asses, though some are much smaller than the living animals of these kinds.
North America was an early home of the horse, whose remains have been found in deposits in Wyoming that were laid down in Eocene time. (See the geological table.) At that time the climate of North America was warmer than it is now and Alaska was linked to Asia by land over which horses migrated. The Eocene lignite beds and gypsum deposits of France contain abundant bones of horses, and bones are found also in England, which was then connected with the Continent.
The earliest horses whose remains are found in America are the Eocene forms known as Eohippus (the “dawn horse”), some of which stood only about a foot high at the shoulder. The fore foot had four toes, the hind foot three toes, but each showed a vestige of an additional toe. The teeth were simple and short. Three kinds of Eocene horses have been distinguished, called Eohippus, Orohippus, and Epihippus. These horsed appear to have lived in Western North America and in England at nearly the same time. Some of them appear to have inhabited either park-like openings in forests or the forests themselves.
By the end of Eocene time or a little later all the European horses seem to have disappeared, for the deposits laid down in Europe about that time contain no bones of horses. In
[ 227 ]
