Among these fossil anthropoids, as well as among the four living forms, we discover no evidence of direct relationship to man but very strong evidence of descent from the same ancestral stock. These proofs of common ancestry, which have already been observed in the existing races of man, become far more conspicuous in the ancient Palæolithic races; in fact, we cannot interpret the anatomy of the men of the Old Stone Age without a survey of the principal characters of the existing anthropoid apes, the gibbon, the orang, the chimpanzee, and the gorilla. [Illustration: FIG. 17. The orang has a high rounded skull and long face. Photograph from the New York Zoological Park.] The gibbon is the most primitive of living apes in its skull and dentition, but the most specialized in the length of its arms and its other extreme adaptations to arboreal life. As in the other anthropoids, the face is abbreviated, the narial region is narrow, _i. e._, catarrhine, and the brain-case is widened, but the top of the skull is smooth, and the forehead lacks the prominent ridges above the orbits; thus the profile of the skull of the gibbon (Fig. 16) is more human than that of the other anthropoid apes. When on the ground the gibbon walks erect and is thus afforded the free use of its arms and independent movements of its fingers. In the brain there is a striking development of the centres of sight, touch, and hearing. It is these characteristics of the modern gibbon which preserve with relatively slight changes the type of the original ancestor of man, as noted by Elliot Smith.(2) [Illustration: FIG. 18. The chimpanzee. This figure illustrates the walking powers of the chimpanzee, the great length of the arms, and the abbreviation of the legs. Photograph from the New York Zoological Park.] The limbs of the orang are less elongated and less extremely specialized for arboreal life than those of the gibbon but more so than those of the chimpanzee and the gorilla. The skull is rounded and of great vertical height, with broad, bony ridges above the orbits and a great median crest on top of the skull in old males. The lower jaw of the orang is stout and deep, and, although used as a fighting weapon, the canine tusks are much less prominent than in either the gibbon, chimpanzee, or gorilla. Of all anthropoids this jaw most nearly resembles that of the Piltdown man. [Illustration: FIG. 19. The Chimpanzee. This figure shows certain facial characteristics which are preserved in the Neanderthal race of men. Note also the shortening of the thumb and the enlargement of the big toe. Photograph from the New York Zoological Park.]