THE EVOLUTION OF THE HORSE
are known as Pliohippus, Protohippus, and Hipparion, each probably representing adaptation to life on a certain type of plain—the grassy plain, the brushy plain, and the desert plain.
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Fig. 2.—Principal stages in the evolution of the teeth and the fore foot of the horse. Showing the increase in the complexity of the grinding teeth and the gradual loss of toes on the front foot.
- 1. Four-toed horse (Eohippus). Eocene epoch.
- 2. Early three-toed horse (Mesohippus). Oligocene epoch.
- 3. Later three-toed horse (Merychippus). Miocene epoch.
- 4. One-toed horse (Equus) . Pleistocene and Recent epochs.
At the beginning of the next geological epoch, the Pliocene, the three types just named were living in America. During this epoch America was again united to Asia by a stretch of land that extended across Bering Sea. Over this land horses migrated from America to Asia and from Asia to Europe, where they became abundant and were differentiated into several species. Horses also found their way from North America to South America across the Isthmus of Panama, then recently emerged.
The Pliocene horses were all plains horses of the three
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