CHAPTER XVII
PRIMATES
Order XIII. PRIMATES.
The highest of mammals, the Primates,[[389]] may be thus differentiated from other groups:—Completely hairy, generally arboreal mammals, with five digits on fore- and hind-limbs, provided with flat nails (except in the case of certain Lemurs and the Marmosets), the phalanges that bear these being flattened at the extremity and expanded rather than diminished in size. The fore-feet are grasping hands as a rule, and the hind-feet walking as well as (generally) grasping organs, and the mode of progression is plantigrade. The teats, except in Chiromys, are thoracic, and even axillary in position. The skull is characterised by the fact that the orbital and the temporal vacuities are, at least partly, separated by bone. The clavicles are always present. The carpus has separate lunar and scaphoid bones, and the centrale is often present. There is rarely an entepicondylar foramen in the humerus, except in some archaic Lemurs. The femur has no third trochanter. The stomach is usually simple, being sacculated only in Semnopithecinae. The caecum is always present, and often large.
This great group could be easily divided into two separate orders, the Apes and the Lemurs, if it were not for certain fossil types. As will be seen from the description of Nesopithecus and of Tarsius, the actual hard and fast lines between all Apes and all Lemurs are very few. On the other hand, it is a little difficult to draw a hard and fast line between the Primates as a whole—or at least between the Lemurine section—and the Creodonta, a
group to which so many others appear to converge. It is disputed, for example, whether the Chriacidae among extinct Lemurs are rightly placed, or whether they should be referred to the Creodonta. The number of primitive characters seen among the Primates, even in Man himself, is remarkable. Of these the more important are the five digits of both limbs and the plantigrade walk, the presence of clavicles and of a centrale, and the absence of a third trochanter. All these features distinguish the early Eutheria.
Sub-Order 1. LEMUROIDEA.[[390]]
The animals known as Lemurs, from their nocturnal and ghostly habits, are on a lower level of organisation than the other division of the Primates. Even the external form enables the members of the present sub-order to be readily distinguished from the higher Anthropoidea. The head is more like that of a Fox, with a sharp muzzle; it lacks the human expression of the face of even the lower among the Apes. The long tail is never prehensile, and there is never any trace of cheek pouches or of integumental callosities, which are frequently so characteristic of the Apes. The Lemurs agree with the remainder of the Primates in having pectoral mammae (sometimes abdominal ones are present in addition, and in Hapalemur—in the male at least—there is a mamma upon each shoulder), in having opposable thumbs and toes, and in the flattened digits. The tail varies from complete absence (in the Loris) to a great length and bushiness in the Aye-aye. The pectoral limbs are always shorter than the hind-limbs; the reverse is occasionally the case in the Anthropoidea. A curious contrast between the two divisions of the Primates concerns the digits of the hands and feet. In the Anthropoidea it is the hallux or pollex which is subject to great variation. In the Lemurs, on the contrary, the thumb and great toe are always well developed, but the second or the third digit constantly shows some abnormality; thus the singular elongation of the third digit of the hand in Chiromys and the absence of the index in the Potto.[[391]] In all Lemurs the
second toe is furnished with a sharp nail, unlike the flattened nails of the other fingers and toes, and in Tarsius the third also is thus provided. As to osteology, the shape of the head, already referred to, indicates some of the differences in the skull which mark off the Lemurs from the Anthropoidea. The brain case is small relatively to the face; the orbital and temporal fossae are in communication, though the frontal and jugal bones are united behind the orbit. The two halves of the lower jaw are not invariably ossified to form one piece, as is the case with most Apes. The lachrymal foramen lies upon the face in front of the orbit. The teeth are characteristic, not so much in their number (the dental formula is usually I 2, C 1, Pm 3, M 3 = 36) as in the disposition of the incisors. The incisors of the lower jaw and the canines project forwards in a way only found in a few American Monkeys; as in the Apes there are four incisors in each jaw, but, with the exception of the highly aberrant Chiromys, there is a space in the upper jaw between the incisors of the two sides. The canines of the lower jaw, moreover, are often incisiform. There is a well-developed sublingua beneath the tongue (see p. [61]). The stomach is perfectly simple; and the caecum, always present and varying in length, never has a vermiform appendix. The gall-bladder is always present. The brain differs from that of the Anthropoidea in that the cerebellum is, as in the lower Mammalia, exposed. The convolutions upon the cerebral hemispheres are not greatly developed, a circumstance, however, which (see p. [77]) may have more relation to the size of the animals than to their mental development. Though the brain in its general outlines is not like that of the other Primates, there are certain resemblances; the most striking of these is perhaps the presence, though in rather a rudimentary condition, of the "Simian fissure."