CAMELIDIE, Foisil. The evolution of the camel (q.v.) through the Tertiary and Quaternary periods is nearly as completely known as that of the horse, and is hardly less instructive. The camels now inhabit central Asia and northern Africa, the llamas, South America. No fossil camels or llamas are found in these countries in deposits much older 'than the Quaternary. But in the Tertiary strata of North America have been found a series of animals which appear to be the direct an cestors of this family, and connect them with the primitive hoofed animals of the earliest Eocene. The earliest member of this series, ancestral probabbr to the camels among other ruminants, is Trig onolestes of the Lower Eocene, smaller than a cotton-tail rabbit, with the complete series of incisor, premolar and molar teeth, the molars of the primitive buno dont type (see BusonoNT) and probably five complete toes, the side toes very slender, and the metapodials all separate. In the Upper Eocene stage, Protylo pus, as large as a jack rabbit, the molars have become selenodont (q.v.), as in modern camels, but with shorter crowns, and the side toes in the hind leg are represented only by splints. In the Oligocene stage, (Poilmotheriuns), as large as a gazelle, the molars have longer crowns, the splints are re duced to small nodules of bone and the meta podials though still separate, are closely ap pressed. In the Miocene stage (Pro camelu.s, .etc.) the metapodials are sometimes separate, sometimes united; the incisors and premolars are generally reduced in size and the anterior upper incisors are often lost; and the form of the teeth and skull comes closer to the modern type. The Pliocene camels (Pliauchenia, etc.) are still closer to the modern type, all with united metapodials and reduced incisors and premolars, and at this epoch they spread to South America and the Old World, the gradual rising of the continents having made land con nections between them about this time. During
the Pleistocene epoch the camels all became extinct in their original home, although they still survive in the alien continents to which they had wandered.
The most remarkable peculiarity of the camels is the adaptation of the stomach, which enables the animal to go a long time without water (see Coast) ; palaeontology gives no direct evidence of the evolution of this charac ter. But the cushioned foot, equally an adapta tion to desert life, is not indicated (by the form iof the toe bones) in any ancestral' camel pre vious to the Miocene, from which time it be came gradually more marked. We may sup pose, therefore, that the earlier ancestors of the camel were antelope- or deer-like in their habitat, and were gradually adapted to desert life.
Besides the main line of descent there were, especially in the Miocene, side branches now extinct, one of which (Aleicanselus) was sin gularly giraffe-like in proportions, although not related to the giraffes, which were evolved in the Old World at the same epoch.
It is a general law in the evolution of any race of animals that at each succeeding stage in its development the progressive characters appear at an earlier period in the lifetime of the individual. The young individuals of one stage resemble the adults of the preceding stage, while the old individuals take on some of the characters of that next succeeding. This is well illustrated in the camels, especially of the Miocene epoch; in young individuals the meta podials are always separate, as they are in all adult camels of the Oligocene, and they are usually not completely consolidated until a com paratively advanced age. In modern camels and llamas they are consolidated before birth. The anterior incisors and premolars usually drop out in old individuals of Miocene camels; in the later stages they are minute stumps or scales which disappear early in life.