Camels and llamas.  The line of camels and llamas developed in North America, where the successive changes from an early Eocene ancestor, no larger than a rabbit, are traced step by step to the present forms, as clearly as is the evolution of the horse.  In the late Miocene some of the ancestral forms migrated to the Old World by way of a land connection where Bering Strait now is, and there gave rise to the camels and dromedaries.  Others migrated into South America, which had now been connected with our own continent, and these developed into the llamas and guanacos, while those of the race which remained in North America became extinct during the Pleistocene.

Some peculiar branches of the camel stem appeared in North America.  In the Pliocene arose a llama with the long neck and limbs of a giraffe, whose food was cropped from the leaves and branches of trees.  Far more generalized in structure was the Oreodon, an animal related to the camels, but with distinct affinities also with other lines, such as those of the hog and deer.  These curious creatures were much like the peccary in appearance, except for their long tails.  In the middle Eocene they roamed in vast herds from Oregon to Kansas and Nebraska.

The ruminants.  This division of the artiodactyls includes antelopes, deer, oxen, bison, sheep, and goats,—­all of which belong to a common stock which took its rise in Europe in the upper Eocene from ancestral forms akin to those of the camels.  In the Miocene the evolution of the two-toed artiodactyl foot was well-nigh completed.  Bonelike growths appeared on the head, and the two groups of the ruminants became specialized,—­the deer with bony antlers, shed and renewed each year, and the ruminants with hollow horns, whose two bony knobs upon the skull are covered with permanent, pointed, horny sheaths.

The ruminants evolved in the Old World, and it was not until the later Miocene that the ancestors of the antelope and of some deer found their way to North America.  Mountain sheep and goats, the bison and most of the deer, did not arrive until after the close of the Tertiary, and sheep and oxen were introduced by man.

The hoofed mammals of the Tertiary included many offshoots from the main lines which we have traced.  Among them were a number of genera of clumsy, ponderous brutes, some almost elephantine in their bulk.

The carnivores.  The ancestral lines of the families of the flesh eaters—­such as the cats (lions, tigers, etc.), the bears, the hyenas, and the dogs (including wolves and foxes)—­converge in the creodonts of the early Eocene,—­an order so generalized that it had affinities not only with the carnivores but also with the insect eaters, the marsupials, and the hoofed mammals as well.  From these primitive flesh eaters, with small and simple brains, numerous small teeth, and plantigrade tread, the different families of the carnivores of the present have slowly evolved.