New World Monkeys

A group of closely related monkeys comprises the woolly monkeys Lagothrix, the spider-monkeys Ateleus (or Ateles) and, intermediate between these, the woolly spider-monkey Brachy teles. These are fruit-eating monkeys, all considerably larger than Cebus. Lagothrix has particularly soft, woolly fur, greyish or brownish in colour, and a naked black peculiarly human face. The spider-monkeys have long hair and remarkably slender elongate limbs. But while woolly monkeys have the thumb well developed, it is entirely absent externally in the spider-monkeys, though bony and muscular vestiges are present under the skin. The other fingers are elongated and form a highly serviceable hand for rapid progression in the treetops. Spider-monkeys in captivity frequently walk erect on their hind-legs for a few feet. Brachyteles forms a link between these two genera in cer tain respects. Its fur is woolly like that of Lagothrix; the thumb is intermediate, greatly reduced but usually still visible externally. In its attenuated limbs it approximates Ateleus. These three genera all have long and powerful prehensile tails, of which the extremity is naked on the under side and provided with friction-ridges like a finger. This organ is used not merely in climbing but also for picking up objects. These monkeys are easily tamed, affectionate and intelligent. The brain, especially in Ateleus, is exceptionally large and complex.

The last Cebid genus to be mentioned is Alouatta (Mycetes), the howler monkey, with several species. The group is notorious for its extraordinary vocal powers, their roars being audible for several miles. The howlers are gregarious, living on leaves and fruit and are the largest and heaviest American monkeys, though the limbs are shorter than those of the spider-monkey. The powerful prehensile tail is very similar to that of the monkeys last described but this may be an example of parallelism in view of numerous differences in other respects. The most noteworthy anatomical feature is the vocal apparatus. The hyoid bone is expanded into a great bony cup, the thyroid cartilage greatly dilated and the angle of the jaw enlarged to protect these struc tures. The intelligence of the howlers is low and the brain ex tremely small and poorly developed. Their relation to other genera is unsettled.

Callitrichidae.

The second family of the American monkeys is that of the marmosets, Callitrichidae or Hapalidae, including the diminutive, squirrel-like marmosets proper, Callitrichus or Hapale, and the closely related tamarins, Midas. Some zoologists have considered these the most primitive American monkeys, but they probably represent dwarfed descendants of primitive Cebi dae. In addition to small size, they are characterized by reduction of the jaws with a correlated loss of the third molar, leaving but 32 teeth. The digits, except the hallux, bear sharp curved claws in place of nails, forming, in such small creatures, highly efficient organs for arboreal locomotion. The hands are used for grasp ing food, but lack the varied functions of these members in larger monkeys, a fact which may be correlated with the inferior intelli gence of marmosets. In the hind foot the great toe is relatively smaller and less opposable than in the Cebidae. The cerebrum, though very large, is almost devoid of convolutions. Marmosets are usually found in small groups and subsist on insects and fruit. They have occasionally bred in captivity, producing up to three young at a birth, instead of one as in monkeys in general. Callimico, a peculiar type previously assigned to the genus Midas, has most of the external features, including claws, like the Calli trichidae, but the skull, though essentially that of a marmoset, has certain likenesses to the Cebidae, the most important being the retention of the third molars. 0. Thomas proposes a new sub

family, Callimiconinae, which he is inclined to place in the Cebidae, but its affinities seem to be with the marmosets.

Origin of the New World Monkeys.

The monkeys of South and Central America form an entirely different series from the monkeys of the Old World and in spite of their general similarity to the latter, they probably have been derived from a different ancestral stock.

In the first place the face in New World monkeys differs widely from that in Old World monkeys, especially in the fact that in the New World or platyrrhine series the nostrils are usually widely separated at the base and are directed laterally. Other external differences from the Old World monkeys have been noted above.

The New World monkeys may also be at once distinguished by the fact that there are three bicuspid or premolar teeth on each side, both in the upper and in the lower jaws, whereas in Old World monkeys there are two. In the region of the middle ear, on the lower side of the skull, New World monkeys have a large ring-like tympanic bone, whereas in Old World monkeys the same element forms a bony gutter, completely covering the drum membrane on the lower side of the skull. Moreover, the cheek bone of New World monkeys has a broad contact with the parietal bone, which is never the case in Old World monkeys. The placenta in New World monkeys is disc-like, without the second ary placenta seen in Old World monkeys.

All known New World monkeys both living and fossil stand on a rather high plane of evolution and there are no "living fossils" to connect the group definitely with any older fossil family. It is also difficult to be sure which is the most primitive living genus, but after repeated analyses of the characters of the skull and teeth it seems probable that the most primitive are certain small mon keys of the family Cebidae, especially the owl monkey (Aotus) and Callicebus. As noted above, the olfactory region of Aotus is less reduced than in most Cebidae and marmosets. These little monkeys have large orbits, short deep lower jaws, short muzzles and unreduced molar teeth. They agree with certain North Ameri can Eocene tarsioids in their dental formula (If x 2 = 36 and in the general form of the skull, but they differ from all known tarsioids in that the tarsal bones are not elongated. On the other hand, it is not impossible that the skeleton of the South American monkeys may have been derived from the type illus trated in the North American Eocene Notharctus, but in the present state of knowledge this inference is unsafe in view of the prevailing resemblances of the dentition and skull of Aotus to the tarsioids rather than to the notharctids. Finally there is the possibility that the New World monkeys may be derived from early extinct lemuroids resembling Archaeolemur.

Typical platyrrhine monkeys (Homunculus) in the Lower Miocene of Patagonia have been found in association with a peculiar mammalian fauna which for millions of years had its headquarters in South America; but some of the ancestral stocks of this fauna have been found in far earlier (Eocene) deposits of North America. In view of other available evidence, an ulti mate North American origin for the New World primates seems quite possible. The monkeys now inhabiting Central America are closely related to those of Guiana and Brazil.

The New World monkeys as a whole exhibit a profound adap tation to arboreal life and probably originated in some heavily forested region such as they now occupy.