A special class has to be made for the four genera of Old-World apes illustrated in Figs. 132 to 136. These are the Gorilla and Chimpanzee, which are African in distribution, and the Gibbon and Orang, which are Asiatic. They are termed the Anthropoid or man-like apes, and there can be no question that they are the nearest living relatives of the human species. This is seen in their general build, which is man-like in a high degree; by many similarities, even in the minutest details, in their skeleton and muscular system; and by the fact that their brain, while still greatly inferior to that of man, is by as much superior to that of any other animal. The voice is frequently well modulated, and the expression of the emotions, whether by it or by the countenance, is very man-like. The position of the group was summed up by Huxley, after a most thorough investigation of their anatomy, in these words: "Thus, whatever system of organs be studied, the comparison of their modification in the ape series leads to one and the same result—that the structural differences which separate man from the Gorilla and Chimpanzee are not so great as those which separate the Gorilla from the lower apes." An interesting physiological proof of the close relationship between the anthropoids and man has more lately been discovered. It is found that the blood serum of any animal destroys the blood corpuscles of any other when these are mixed with it, except those of closely related species. Now the human blood serum is destructive of the corpuscles of all the lower animals, so far as is known, except those of the anthropoids.
Fig. 136.—Skeletons (left to right) of Gibbon, Orang-Utan, Chimpanzee, Gorilla, and Man.
From Huxley's Man's Place in Nature.
As regards the inter-relationships of the four species, it is certain that the Gibbon is the lowest, and the nearest to the common ancestor of the other three and of man. It has indeed the man-like characteristic of walking in the erect position; but it has thirteen or fourteen pairs of ribs, as against the normal twelve in man; it has long arms, like the lower Old-World apes, and large canine teeth. It is further the smallest of the group, and in habits the most like the lower monkeys.
The Gorilla and Chimpanzee are closely related. The Gorilla is not only the largest ape of the four, but in shape and build the most man-like, which is accounted for by the fact that it is less strictly arboreal than the others, and confines itself largely to the ground. Its skull is superficially much less human in appearance than that of the Chimpanzee, due to the strongly developed crests, which serve as attachments for the powerful muscles of the lower jaw. The teeth of the Chimpanzee are more uniform in size, and the skull smoother. There is a marked difference in temperament between the two species, the Gorilla being fierce and gloomy and quite untamable, while the Chimpanzee is of a pleasant and lively disposition, and can, as is well known, be trained to wear clothes, eat with a fork and knife, etc.
The Orang-Utan is found inhabiting forest ground in Borneo and Sumatra, and living largely in trees, in which it builds nests as temporary sleeping-places. It is a clumsy-looking animal, supporting itself on the knuckles of its hands when travelling along the ground, and moving but slowly. It has twelve pairs of ribs, the same number as in man, and one fewer than in the Gorilla and Chimpanzee.
Practically all the important anatomical distinctions between man and the anthropoids are reducible to two causes: the change from the arboreal existence to a life on the ground and in the open country, and the great development of the intelligence. When the change in the mode of life occurred, it is obvious that an erect carriage would possess an advantage over the stooping gait of the ape, in which it is neither definitely a biped nor a quadruped. The erect position necessitated a better-developed heel, stronger calf and hip muscles, and a more parallel position and stronger development of the great toe. It also brought about a shortening of the fore-arms and a widening of the pelvis. Much has been made of the difference in the foot, between the condition of a grasping organ, with an opposable great toe, and that seen in man. But the change is just what we should devise in order to make the foot stronger for propelling the body on the ground. Moreover, the difference is more obvious than fundamental, and it is well known that in armless persons, who develop the possibilities of their feet, the latter members can still be turned into wonderfully efficient 'hands.'
Fig. 137.—Brains of (F) Cercopithecus (Eastern Monkey); (H) Australian Bushman; (L) Chimpanzee; (M) European.