One fine day a revolution was effected. Just as an adaptation to arboreal life was produced at the expense of anterior species, an adaptation to terrestrial life was made, with a bipedal attitude favorable to a more extended vision, a diminution of the olfactory sense and of the facial prominence, a more perfect touch, and intelligence. Henceforth, all the living forces of adaptation tended toward the same end; the hind-thumb ceased to be opposable, the other toes diminished in length; what the feet lost the hands gained, and man was created, exclusively bimanous in front, exclusively bipedal behind, and all the accessory parts in the segments of the limbs confirming themselves in the types, less accented till now, which they had presented since the marsupials.
The peculiarity set forth by Cuvier of the opposable thumb perfectly characterizes what there is common and special among all the apes, the faculty of clinging to trees with the four extremities. It is true that this expresses only one of the details of that whole, perfect in man, which has given birth to the words hand and foot, but it is the essential one. It can not, however, be denied that the second characteristic necessary to the function of prehension—great mobility in every direction of the segments of the limb—is not very greatly developed in the hind-limbs of monkeys. Cuvier had, then, a perfect right to call all the monkeys quadrumana, although they were at the same time quadrupedal, and to oppose them to man. I, then, put the anthropoids and ordinary monkeys together under the name of monkeys, and will not recoil from the synonym of quadrumana if the term monkey does not suffice me.
The monkeys are divided into two groups, those of the old continent, also called catarrhinians, because their nostril-partitions are narrow and their nostrils are open below the nose (from κατα, low, and ῤιν, nose); and those of the new continent, also called platyrrhinians, because their nostril-partitions are broad, and their nostrils open on the side (from πλατνς, flat). The monkeys of the new continent are predominantly tree-dwellers, and are divided into two families—the monkeys proper of this continent, and the arctopitheci. The former are in turn divided into the diurnal—the howlers, the ateli, the sajous, etc.; and the nocturnal, including the sagoins, sakis, nyctipitheci, and the saimiris.
The arctopitheci or hapales are a group apart among monkeys, including the interesting wistit and the tamarin. They are tree-dwellers like the former group, and nocturnal like the latter. They afford an example of the imperfection of our modes of classification. They are monkeys, American monkeys, in many of their relations; but they lack the single characteristic that distinguishes all the monkeys, including the lemurs, and have the dentition neither of the American monkeys nor of the monkeys of
