CREATION BY EVOLUTION
America, however, they flourished, and they continued to multiply and to grow larger during the following epoch, the Oligocene.
The Oligocene horses were larger than their predecessors, and each of their feet bore three toes. This three-toed horse is called Mesohippus. It appears to have been confined to the American continent, a fact suggesting the temporary severance of land connection between North America and Asia. The later Oligocene horses (Miohippus) were larger and had longer teeth and smaller side toes than the earlier forms.
During the Miocene epoch, which followed the Oligocene, Western North America was inhabited by many kinds of horses, among them one that has been called the “forest horse.” The Miocene horses were also three toed and had low-crowned teeth. The forest horses spread from America to Asia and Europe, where their remains are found; but they appear to have died out in America in mid-Miocene time and later in Europe. The main line of the horses, however, continued to exist and underwent great changes, all originating in America. These changes appear to have been determined by environment. The teeth became longer and harder to adapt them better to grazing; the feet, which in the earlier horses were first five-toed, then four-toed, and then three-toed, advanced toward a single-toed form, the side toes becoming useless (Fig. 2). These changes indicate growing adaptation to life on grassy plains. The grass of these plains is harsher than that in or near forests, containing more silica, and horses that feed on it must have hard teeth. A hard, small hoof is also peculiar to plains horses, as well as long legs, for the horse must be able to escape from enemies, such as wolves and other carnivorous animals.
In their adaptation to life on plains in Miocene time the horses differentiated into three groups, the types of which
[ 228 ]
