"As to the convolutions, the brains of the Apes exhibit every stage of progress, from the almost smooth brain of the Marmoset, to the Orang and the Chimpanzee, which fall but little below Man. And it is most remarkable that as soon as all the principal sulci [or grooves] appear, the pattern according to which they are arranged is identical with that of the corresponding sulci of Man. The surface of the brain of a Monkey exhibits a sort of skeleton map of Man's, and in the Man-like Apes the details become more and more filled in, until it is only in minor characters, such as the greater excavation of the anterior lobes, the constant presence of fissures usually absent in Man, and the different disposition and proportions of some convolutions, that the Chimpanzee's or the Orang's brain can be structurally distinguished from Man's.... And the difference between the brains of the Chimpanzee and of Man is almost insignificant when compared with that between the Chimpanzee's brain and that of a Lemur." (Huxley.)
The Anthropoid Apes have no cheek-pouches. The larynx has large dilatations of the shallow depressions—called ventricles—of the mucous membrane on each side of its inner surface—which may extend down as far as the arm-pits, and be connected with powerful voice possessed in most of the species. The stomach is simple, like that of Man, and not sacculated, as in the last family (the Cercopithecidæ).
The uterus and other structures connected with the reproductive system resemble those in the human subject. The length of gestation varies probably in the different genera, and is unknown in many of the species. The period for which the young are suckled by the mother lasts about six months. "The proportions of the limbs to one another and to the body do not sensibly change after birth; but the body, limbs, and jaws enlarge to a much greater extent than the brain-case." (Huxley.) Observations are still required, in regard to most of the species, as to the age at which they arrive at maturity, and are able to reproduce.
The Simiidæ—the most intelligent of the animal kingdom—are all diurnal animals, and essentially arboreal. Many of the members of the family have, when walking, a tendency to tread on the outer edge of the foot, turning, therefore, the toe inward on account of the free motion which is possible between the various bones of its ankle, whereas, in the human foot, these bones are more solidly bound together. When climbing, the power of turning in the sole is, as is evident, of the greatest advantage to the Ape. Their food is chiefly vegetable; a few species exhibit slight carnivorous tendencies.
"Of the various genera of the Simiidæ, the Gibbons are most remote from Man. The Orangs come nearest in the number of the ribs, the form of the cerebral hemispheres, and certain other characters of the brain and skull; but they differ from him much more widely in other characters, especially in the limbs, than the Gorilla and the Chimpanzee do. Of the Chimpanzees the Gorilla is more Man-like in the proportions of the leg to the body, and of the foot to the hand; and likewise in the size of the heel, the curvature of the spine, and the absolute capacity of the cranium. The true Chimpanzees approach Man most closely in the skull, dentition, and proportionate length of the arms." (Huxley.)
The Simiidæ are confined to the Ethiopian and Indian Regions. The Gorillas and Chimpanzees live exclusively in the Tropical Regions of Western and Central Africa; the Gibbons range into all the four provinces of the Indian Region; while the Orangs are confined to two islands of the Indo-Malayan Sub-region.
THE GIBBONS. GENUS HYLOBATES.
Hylobates, Illiger, Prodr. Syst. Mamm., p. 67 (1811).
The group of Tree-walkers, as the term Hylobates signifies, embraces the smallest-sized, the slenderest-bodied, the longest-limbed, and the most perfectly arboreal of all the Man-like Apes. All are covered with thick woolly hair, which, on the arms and fore-arms, converges (except in H. agilis) towards the elbow.
Their head is small and round, and the face compressed. Except the Orangs, the Gibbons have the longest arms of all the Apes, so long that when they stand erect the points of their fingers can touch the ground. Compared with the spinal column, their arms are as 19 to 11, while the legs are one-third longer than it. The fore-arm is much longer than the arm itself; the hand is longer than the foot, and the thumb is very long in proportion to the hand. The knee is free from the side of the body, and the great-toe is well developed and nearly one-half the length of the foot. The nails of both the thumb and the great-toe are flat. Callosities, which are wanting in all the other genera, are present in Hylobates, but are very small.