Handbook to the Primates/Introduction
ALLEN'S NATURALIST'S LIBRARY.
MAMMALS.
ORDER PRIMATES.
LEMURS, MONKEYS AND APES.
INTRODUCTION.
Of the varied forms of animal life that people the globe, those that possess a back-bone and two pairs of limbs (the Vertebrata) are considered the highest in the scale. Of the Vertebrata, those are held to be of superior organisation which possess warm red blood and suckle their young with milk from the breast (i.e., Mammalia). Our present volume deals with the highest and most specialised group of the Mammalia, and, therefore, of the whole Animal Kingdom.
Man, in respect of his mental endowments, stands alone and unapproachable among living creatures. Considered as to his "place in nature," however, he must be described as an erect-walking Mammal, possessing anterior extremities developed into hands of great perfection, for exclusive use as tactile and grasping organs, and posterior limbs, on which his body is perfectly balanced and entirely supported, exclusively devoted to locomotion, as well as highly specialised cerebral characters. These attributes in part constitute the standard by which we estimate superiority in animal structure, and fitness of adaptation.
Notwithstanding the numerous varieties and races of mankind distributed over every region of the globe, each exhibiting differences in habits, customs and superficial complexion, Man forms but one species, Homo sapiens, the sole representative of the unique genus of his family. Though the genus Homo is thus far apparently zoologically isolated, there is a remarkable group of animals, which we designate "Apes," and which, possessing many of the same structural characters more or less modified, stand apart from all the other Mammalia, and make a distinct approach to Man. Between Man, however, and the Apes, even the untrained eye at once perceives, amid obvious marks of inferiority, unmistakable resemblances, while anatomical investigations reveal that "the points in which Man differs from the Apes most nearly resembling him, are not of greater importance than those in which the Ape differs from other and universally acknowledged members of the group." (Flower and Lydekker.) The Apes, on the other hand, are so nearly related to the Monkeys, the Baboons and the Marmosets, by characters which insensibly merge into each other that they, along with Man, must logically be embraced in the same zoological division. The animals known to us as Lemurs, called by the Germans "Half-Apes" and by the French "False-Monkeys," are the nearest to the Apes and Man of all the remaining Mammals, though there are many points of divergence from the above-named groups. The Lemurs, in fact, exhibit considerable affinity to lower forms of Mammalia, especially to the Insectivora, but in internal structure and habit they approach the Anthropiform[1] group just referred to—in the flattened form of the digits, the opposable great toe, with its ankle-bone (the ento-cuneiform) rounded for its articulation, as in the higher Apes and Man.
The Lemurs have, by many distinguished naturalists, been relegated to a distinct Order quite separate from the latter; but by such pre-eminent authorities as Linnæus, Lesson, Huxley, Broca and Flower, they have been assigned a subordinate position within that great Order, on which has been conferred the rank of the Primates of the Animal Kingdom.
The Order Primates, therefore, comprises two very homogeneous sub-orders—(1) The Lemur-like animals (Lemuroidea) including the Aye-Aye, the Tarsier, and the True Lemurs; and (2) the Man-like animals (the Anthropoidea), which embrace the Marmosets, the Baboons, the great Apes, and Man.
In common with all other Mammals, the Primates are furnished with an epidermal covering, which, except in Man, consists of a woolly or hairy fur. They possess four limbs and a tail, which may be long, short, or concealed, and which is often used as a prehensile organ. The young are born in a condition of greater or less helplessness, with their eyes, as a rule, unopened, and the framework of their bodies incompletely ossified, and consequently requiring protective care and entire nourishment from the mother, for a considerable period. At maturity this skeleton consists of a skull, a breast- and a back-bone of many pieces, ribs, jointed limbs, and a pair of collar-bones. As a knowledge of many of these bones and some of the more prominent organs of the body are necessary for an accurate comprehension of the description and classification of the animals discussed in this volume, a few of the more important must be briefly referred to.
The cranium, formed of many bones firmly united together, consists of a cerebral region, or box, containing and guarding the brain, and a facial region, in which are situated, besides the mouth, the organs of sight and smell. The bones connected with the mouth are the two maxillæ, along the margins of which are placed the grinding- or cheek-teeth; the two pre-maxillæ, in which are set the cutting- and the eye-teeth; and lastly, the palatine bones which form the roof of the mouth. Hinged on to the sides of the cranium is the toothed mandible, or lower jaw, composed of two halves, which may be solidly or loosely joined together in the mid-line, or symphysis. Along the under surface of the skull, there are, besides the great (often posterior) orifice for the entrance of the spinal cord, numerous foramina, or openings, for the passage of blood-vessels for the nourishment of the brain, and of nerves which bring all parts of the body into relation with the supreme directing centre. Conspicuous near its posterior part, on each side, is an ivory-like capsule, the periotic bone, containing the essential organ of hearing. Lying beneath the lower jaw is the hyoid arch, a slender framework of bones, supporting the tongue and the upper end of the windpipe with the organ of voice. In a few of the Monkeys and Apes certain of the bones of this arch are much enlarged and hollowed for increasing the volume of sound emitted by them. On either side of the great opening which is so conspicuous at the hinder part of the skull, for the reception of the spinal cord, is a smooth kidney-shaped surface, called a "condyle." These two condyles serve for the articulation of the first segment of the back-bone to the cranium, and by the possession of this pair of condyles the Mammalian skull can always be distinguished from that of Birds and Reptiles. The pieces of which the back-bone are composed are named the vertebræ. Those of the neck, the "cervical" vertebræ, are recognised by having no true ribs attached to them, and are, in all Primates, seven in number. Those of the back, or "dorsal" vertebræ, may be distinguished by having articulated to them, on each side, a movable rib, the other end of which is attached to the breast-bone; they follow next to the cervical vertebræ, while to them succeed the "lumbar" vertebræ which carry no complete ribs. The dorsal and lumbar segments vary in number, but together they rarely exceed seventeen. Behind these extend the "sacral" vertebræ—completely ossified together, and lastly, the bones of the tail or "caudal" vertebræ, which may be many or few, according to the length of that appendage.
The fore-limb is composed of three segments, the arm, fore-arm, and hand, together with a block by which it is attached to the side of the body. To this block—the blade-bone or scapula—is articulated the arm-bone, or humerus, which at its elbow-joint hinges with the two bones, the ulna and the radius, of the fore-arm, on which in turn the hand is rotated. The hand is made up of three parts, the wrist-bones, or carpus, closely united together in two transverse rows with a central bone intervening between them; next the elongated bones of the palm of the hand, or metacarpus, one to each finger, and lastly the phalanges, or finger-bones, three to each digit, except in the thumb, where there are but two. The hind-limb is formed on exactly the same plan. It has a connecting block—the pelvis; giving suspension to the thigh, with its single bone, the femur, to which articulates the leg, with two bones (tibia and fibula), and the tripartite foot, composed of tarsus, metatarsus, and phalanges.
Of the digestive organs of the Primates the teeth present very important characters, from the point of view of the classification of the Order. They differ in form and number, and have distinct functions to perform. The teeth situated in front are the incisors and canines, sharp and pointed, for seizing, cutting, and holding the food. Behind them come the pre-molars, and still further back the molars, both with broad crowns of complicated tubercles and ridges for milling the hard portions contained in the food. Animals provided—as all the Primates are—with these different sorts of teeth, are said to be Heterodont,[2] in contradistinction to forms like the Dolphins and Whales, which are termed Homodont,[3] because the whole of these teeth are of the same pattern. The Primates are Diphyodont[4] as well, because many of their permanent teeth are preceded by another set, commonly known as the milk-teeth. In order to present to the eye at a glance the number of each sort that any species possesses, a dental formula has been adopted by naturalists. Such a formula as I 22, C 11, P 33, M 33 = 36, indicates that in one half of the mouth, above and below, there are 2 incisors, 1 canine, 3 pre-molars, and 3 molars = 18; and therefore in the two halves of the mouth together there are 36 teeth in all.
The masticated food, partially digested by the saliva of the mouth, descends the gullet by the muscular contractions of its walls to the simple, sac-like, stomach, and thence to the intestines. These latter consist of two portions, one smaller and narrower, nearer to the stomach, and a second portion further down, larger and wider; the junction of the two portions being marked by a process of varying length, the cæcum. The stomach and intestines, with other important structures, such as the liver, kidneys and generative organs, are contained in a lower cavity, separated by a muscular midriff, the diaphragm, from the upper part or thorax, containing the blood-purifying and pumping organs, the lungs and the heart.
The upper part of the windpipe is, in all Primates, modified to form the larynx, or organ of voice, constituted by fibrous strings stretched across its orifice, where they may be set in vibration by the air, in its passage to and from the lungs.
The brain is relatively large in proportion to the body, and attains in the higher of the two sub-orders its most perfect development. The main brain (or cerebral hemispheres), when viewed from above, in size preponderates over, and conceals (except in the Lemurs) all the other parts of that organ. The surface of its lateral halves, which are connected by transverse bands so as to insure harmony of action between them, is marked by fissures and foldings, or convolutions, which vary in number and complexity, evidently in relation to the intelligence of the animal. The brain within the skull gives origin to the nerves for the chief organs of sense; while from its posterior part it is continued along the back—within a canal formed by the neural arches of the vertebræ—as the spinal column, from which arise the rest of the nerves for the body.
The young of all the Primates are nourished in the mother's womb by the passage of material from the blood-vessels of the parent through an organ known as the placenta. They are all born in a helpless condition, and remain unable to look after themselves for a considerable period, during which they are dependent on the milk secreted on the ventral surface of the mother by two or four glands, the teats or mammæ—those characteristic organs from which the "Mammalia" have derived their name. These glands are present in both sexes, but are functional only in the female.
We shall now proceed to describe more minutely the first of the two sub-orders of the Primates—the Lemur-like animals.