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. [Illustration: 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.