The leg became longer, and only the tip of the toes struck the ground.  The middle toe (digit number three), originally the longest of the five, steadily enlarged, while the remaining digits dwindled and disappeared.  The inner digit, corresponding to the great toe and thumb, was the first to go.  Next number five, the little finger, was also dropped.  By the end of the Eocene a three-toed genus of the horse family had appeared, as large as a sheep.  The hoof of digit number three now supported most of the weight, but the slender hoofs of digits two and four were still serviceable.  In the Miocene the stature of the ancestors of the horse increased to that of a pony.  The feet were still three-toed, but the side hoofs were now mere dewclaws and scarcely touched the ground.  The evolution of the family was completed in the Pliocene.

The middle toe was enlarged still more, the side toes were dropped, and the palm and foot bones which supported them were reduced to splints.

While these changes were in progress the radius and ulna of the fore limb became consolidated to a single bone; and in the hind limb the fibula dwindled to a splint, while the tibia was correspondingly enlarged.  The molars, also gradually lengthened, and became more and more complex on their grinding surface; the neck became longer; the brain steadily increased in size and its convolutions became more abundant.  The evolution of the horse has made for greater fleetness and intelligence.

The rhinoceros and tapir.  These animals, which are grouped with the horse among the Odd-toed (perissodactyl) mammals, are now verging toward extinction.  In the rhinoceros, evolution seems to have taken the opposite course from that of the horse.  As the animal increased in size it became more clumsy, its limbs became shorter and more massive, and, perhaps because of its great weight, the number of digits were not reduced below the number three.  Like other large herbivores, the rhinoceros, too slow to escape its enemies by flight, learned to withstand them.  It developed as its means of defense a nasal horn.

Peculiar offshoots of the line appeared at various times in the Tertiary.  A rhinoceros, semiaquatic in habits, with curved tusks, resembling in aspect the hippopotamus, lived along the water courses of the plains east of the Rockies, and its bones are now found by the thousands in the Miocene of Kansas.  Another developed along a line parallel to that of the horse, and herds of these light-limbed and swift-footed running rhinoceroses ranged the Great Plains from the Dakotas southward.

The tapirs are an ancient family which has changed but little since it separated from the other perissodactyl stocks in the early Tertiary.  At present, tapirs are found only in South America and southern Asia,—­a remarkable distribution which we could not explain were it not that the geological record shows that during Tertiary times tapirs ranged throughout the northern hemisphere, making their way to South America late in that period.  During the Pleistocene they became extinct over all the intervening lands between the widely separated regions where now they live.  The geographic distribution of animals, as well as their relationships and origins, can be understood only through a study of their geological history.