With regard to the brain, the anthropoid cerebrum, or fore-brain, is greatly convoluted, and differs from that of the Lemurs by its proportionately larger size, the cerebellum, or hind-brain, being as a rule entirely covered by it.
The uterus and structures for the nutrition of the young prior to birth differ greatly in this Sub-order from the conditions existing in the Lemuroidea. The uterus is a simple and not a two-horned sac, and its inner layer, in which the fœtal and maternal structures intermingle during the growth of the embryo, is shed after the birth of the young, which is not the case in the Lemurs.
"The resemblance of Monkeys to Man," says Mr. Darwin, "is greatly caused by the relative position of the features of the face. The eyes are arched over; they are separated by a long nose, the end of which in some is very human. The mouth is not carried back, but occupies the same general position as in Man, and the forehead, so often wrinkled, is usually prominent and like that of a child. The likeness is increased by the fact that anger, sorrow, pleasure, and satisfaction, are displayed by the Monkey by nearly similar movements of the muscles and skin, chiefly above the eyebrows and round the mouth. Some few expressions are indeed almost the same, as in the weeping of certain kinds of Monkeys, and in the laughing noise made by others, during which the corners of the mouth are drawn backward and the eyelids wrinkled. In Man the nose is much more prominent than in most Monkeys; but we may trace the commencement of an aquiline curvature in the nose of the Hoolock Gibbon, and this in the Great-nosed Monkey (Nasalis larvatus) is carried to a ridiculous extreme."
In regard to the distribution of the Anthropoidea, excluding Man (Hominidæ), two families (the Hapalidæ and Cebidæ) are known only from the New World; and two others (the Cercopithecidæ and Simiidæ) are exclusively confined to the Old World. No fossil remains of Eastern Hemisphere forms have as yet been found in the Western, or vice versâ, a fact which indicates, doubtless, a separation of great antiquity between the two groups. The various species of these families are to be found chiefly in the warmer regions on both sides of the equator. In the New World some species range as far north as to 20° N. lat. in Mexico; and South, to 30° below the equator. In the Eastern Hemisphere, the Old World species predominate in the tropical and sub-tropical regions; but certain forms have spread as far north as Thibet and Japan, and others have made the high altitudes of the Himalaya Mountains their home; while to the southward they extend in Africa nearly to the Cape of Good Hope. No indigenous species have ever been found in New Guinea, Australia, New Zealand, or in the Pacific, or West Indian Islands.
The Apes of the Old World differ in many important characters from those of the New. Among the former, as already mentioned, the openings of the nostrils are directed downwards, as in Man; the nose is narrow, and the nostrils themselves are set close together, being separated from each other by a thin septum, or partition, of cartilage. On this account, they have received the name of Catarrhine Monkeys (Catarrhini).[[7]] The New World Monkeys, on the other hand, have the nose flat and the opening of their nostrils directed outwards, and the one nostril widely separated from the other by a broad cartilaginous septum, and they are therefore designated Platyrrhine Monkeys (Platyrrhini).[[8]]
The dental formula of the Old World forms is I22, C11, P22, M33, making a total of thirty-two teeth in all; but those of the Western Hemisphere differ in having invariably three pre-molars, and sometimes two molars, instead of three, so that they possess either thirty-two or thirty-six teeth altogether. There is always a gap, or diastema, in the series of the teeth in front of the upper and behind the lower canines; the latter teeth being taller than the rest. Many of the Catarrhine Apes have large cheek-pouches as well as bare patches, or callosities, often brightly coloured, on the part they apply to the ground when sitting. None of the Platyrrhine group have cheek-pouches or callosities, but in many of them the tail is marvellously prehensile, which is not the case in any of the Old World species. Again, in the Apes of the Eastern Hemisphere, the ear-capsules of the skull have an external bony channel (or meatus) for conveying the sound vibrations into the ear, which is absent in the American species.
As a rule the Platyrrhine Monkeys have the fore-limbs shorter than the hind-, and are more quadrupedal than those of the Old World. Their thumb is also more like a finger than the same digit in their Eastern brethren.
Of the New World Monkeys, the Hapalidæ, or Marmosets, have thirty-two teeth, and the Cebidæ, with several sub-families, have thirty-six teeth. The former include the Marmosets (Hapale) and the Tamarins (Midas). The latter comprise the Capuchins (Cebus), which may be taken as the representative genus of American Monkeys, the Woolly Monkeys (Lagothrix), the Spider-Monkeys (Ateles and the allied Eriodes), the Howlers (Mycetes), the Sakis (Pithecia and Brachyurus), the Night-Monkeys or Douroucolis (Nyctipithecus), and the Squirrel Monkeys or Saimiris (Chrysothrix), with the allied Callithrix.
"The extensive equatorial forests of the Amazon and Orinoco, and their tributaries, constitute par excellence the home of the American Monkeys, but the majority of the genera have a very extended range, appearing in one or more species throughout the greater portion of the tract covered by the entire family. This is more particularly the case with the Sapajous (Cebus), Spider-Monkeys, Howlers, and the species of Callithrix. The range of the species, on the other hand, is not unfrequently very sharply defined, as, for example, when a natural barrier, offering insurmountable obstacles to further migration, suddenly interposes itself. Examples of such limitation, as brought about by the dominant water-courses of the equatorial forests," are numerous. Mr. Wallace cites the case of certain species of Saki Monkey (Pithecia), found on either side of the Amazon river, whose range, either southward or northward, appears to be limited by that river. "The number of species of these American Apes found in, and north of, the Isthmus of Panama is ten, of which only one (Ateles vellerosus) extends into Mexico; Mycetes villosus, the Guatemalan Howler, or 'Mono,' has thus far been found only in Guatemala and Honduras. It is a little surprising that the range of only two of the species—the Black-faced Spider-Monkey (Ateles ater) and one of the Night-Apes (Nyctipithecus vociferans)—extends beyond Colombia, in South America."
"None of the South American Monkeys appear to pass west of the Andean chain of mountains south of Ecuador, and even north of the Peruvian boundary the number of such transgressional forms is very limited. Indeed, even among the wooded slopes, a habitation along the basal line of the mountain axis seems to be much preferred. The greatest altitude at which Monkeys were observed by Tschudi in Peru was 3,000 feet (Lagothrix humboldti); Ateles ater and Cebus robustus were found at 2,500 feet. On the other hand, Salvin and Godman state that in the district of Vera Paz, in Guatemala, the 'Mono' or Howler is most abundant at an elevation of 6,000 feet; and on the Volcano of Atitlan, in the same country, Mr. Salvin found troops of the Mexican Spider-Monkey (Ateles vellerosus) in the forest region of 7,000 feet elevation.