Fig. 12.—Hypohippus, a forest-living horse, rear view, showing large lateral digits on the fore and hind feet, adapted to prevent the animal from sinking into the soft soil.

Fig. 13.—Neohipparion, a plains-living horse with very slender limbs and lateral digits small and well raised from the ground, adapted to a dry, hard soil.

Laws of Local Adaptive Radiation and Polyphyletic Evolution, illustrated by two Upper Miocene Horses of the Plains Region of North America. These horses are of the same geologic age (Upper Miocene) and were found in the same geographic region (South Dakota, U.S.A.). One is supposed to have lived in the forests along the stream borders, and the other in the open plains.

(Illustrations reproduced by permission of the American Museum of Natural History, New York.)

Fig. 14.—Restoration of Hypohippus. (From a drawing by Charles R. Knight, made under the direction of Professor Osborn.)

Fig. 15.—Restoration of Neohipparion. (From a drawing by Charles R. Knight, made under the direction of Professor Osborn.)