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. [Illustration: Fig. 346. A Tertiary Mastodon] [Illustration: Fig. 347. Head of Dinothere] =The proboscidians.= This unique order of hoofed mammals, of which the elephant is the sole survivor, has been traced back to the close of the Eocene. In the middle and later Tertiary it was represented by huge creatures so nearly akin to the mastodons of the Pleistocene that they are often included in that genus. The Tertiary _Mastodon_ was furnished with a long, flexible proboscis, and armed with two pairs of long, straight ivory tusks, the pair of the lower jaw being smaller. The _Dinothere_ was a curious offshoot of the line, which developed in the Miocene in Europe. In this immense proboscidian, whose skull was three feet long, the upper pair of tusks had disappeared, and those of the lower jaw were bent down with a backward curve in walrus fashion. [Illustration: Fig. 348. Crown of Mastodon Tooth] In the true _elephants_, which do not appear until near the close of the Tertiary, the lower jaw loses its tusks and the grinding teeth become exceedingly complex in structure. The grinding teeth of the mastodon had long roots and low crowns crossed by four or five peaked enameled ridges. In the teeth of the true elephants the crown has become deep, and the ridges of enamel have changed to numerous upright, platelike folds, their interspaces filled with cement. The two genera--Mastodon and Elephant--are connected by species whose teeth are intermediate in pattern. The proboscidians culminated in the Pliocene, when some of the giant elephants reached a height of fourteen feet. [Illustration: Fig. 349. Tooth of an Extinct Elephant, the Mammoth] [Illustration: Fig. 350. Evolution of the Artiodactyl Foot, Illustrated by Existing Families