Geology . er THE EOCENE PERIOD. 235 used for grasping or digging, as in the case of the carnivores, they gradu-ally dwindled away. On the whole, the two-toed system seems tohave proved the best; at least the artiodactyls are now much the morenumerous. The evolution of the perissodactyls did not pass beyond the three-toed form during the Eocene period. The three present types, thetapir, the horse, and the rhinoceros, were, however, distinctly fore-shadowed. The most undifferentiated of the early perissodactyls werethe lophiodonts, which seem to have graded almost insensibly into theancestral ta
Geology . er THE EOCENE PERIOD. 235 used for grasping or digging, as in the case of the carnivores, they gradu-ally dwindled away. On the whole, the two-toed system seems tohave proved the best; at least the artiodactyls are now much the morenumerous. The evolution of the perissodactyls did not pass beyond the three-toed form during the Eocene period. The three present types, thetapir, the horse, and the rhinoceros, were, however, distinctly fore-shadowed. The most undifferentiated of the early perissodactyls werethe lophiodonts, which seem to have graded almost insensibly into theancestral tapirs (Systemodon), horses (Hyracotherium), and rhinoceroses(Hyrochinus). The first definite steps in the development of the horse,which has become a classic example of evolution, appeared in the secondstage of the earlier Eocene (Wasatch), no traces having yet been foundof the equine line in the Puerco. The earliest recognized form was theHyracotherium (Fig. 431), whose equine characters are quite Fig. 431. — An early ancestor of the horse family, Hyracotherium (Protorohippus)venticolum, from the Lower Eocene (Wind River formation) of Wyoming, \ natu-ral size. (Skeleton restored by Cope.) Pachynolophus represented a slight step in advance, and the Orohippus(Epihippus) a more decided step. The latter was four-toed in front(three functional) and three-toed behind, and the limbs and teeth wereslightly modified in the direction of the horse. These forms were aboutthe size of a small dog, and as nearly canine as equine in evolution continued through the remaining periods of the Tertiary,the true horse only appearing in the Pliocene. The primitive Eocene 236 GEOLOGY. forms lived both in Europe and America, and the evolution seems tohave gone forward along much the same linos in both countries; huthow far this implies free Lntermigration and how far parallel evolutionis a mooted point. The rhinoceros family appears in the record a little later than the tapir
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