The next division, called the Cercopithecidae, includes the baboons of the Old World.  These animals also run upon all fours, and their feet are handlike as before, but the tail is much reduced.  The general appearance of the head is doglike, and the brain-case arches little more than it does in the monkeys, but the face projects forward as a long muzzle, with terminal nostrils close together.  In some respects the baboons stand somewhat away from the line leading from the lower to higher anthropoids; in other characters they approach the latter, for in the teeth especially they are identical with the apes and with the human species.

The Simiidae, or true apes, possess an overwhelming importance, far beyond that of the baboons and monkeys.  There are only four principal kinds now existing, namely, the gibbon, orang-outang, chimpanzee, and the gorilla, of which the first is much less familiar than the others.  The known species of gibbons occur in Indo-China and the Malay Peninsula.  The typical animal stands about three feet high; its overarching braincase, enlarged in conformity with the much greater brain development, has pushed the eyes and face still further around underneath, so that if the animal walks upon all fours the eyes look almost straight into the ground.  Therefore it must bend back its head at an extremely uncomfortable angle if it is to remain upon all four feet, but it prefers to raise itself up into the human sitting posture, or, when it walks, it stands erect upon its hind limbs.  Hence we who are accustomed to think of ourselves as the only erect animals must revise our opinion, for we find in the gibbon an organism that is nearly, if not quite, as advanced in this respect as we are.  One peculiar difference may be pointed out,—­the walking gibbon stretches out its great long arms to the sides in order to preserve its balance.  The animal seems awkward to us, perhaps, but it is possible that the human method of balancing the body by vigorously swinging the arms might seem quite as awkward to a gibbon as its grotesque posture does to us.

The orang-outang comes next in this series.  It inhabits the islands of Borneo and Sumatra, where we find two distinct species.  It is a reddish colored animal standing about four feet four inches high, with rather long hair.  It is bulky, slow and deliberate in action, and when it walks in a semi-erect position it rests its knuckles upon the ground, swinging its long arms as crutch-like supports.  Like the gibbon, it does not walk upon all four feet in the way that the monkeys and baboons do, and we find in the still further development of the brain and the higher arch of the cranium the reasons for its semi-erectness.  It cannot remain with its hands and feet upon the ground and bend back its head so as to direct its vision forward.