Popular Science Monthly/Volume 21/May 1882/Monkeys
|←Professor Goldwin Smith as a Critic||Popular Science Monthly Volume 21 May 1882 (1882)
By Alfred Russel Wallace
|The Development of the Senses→|
IF the skeletons of an orang-outang and a chimpanzee be compared with that of a man, there will be found to be the most wonderful resemblance, together with a very marked diversity. Bone for bone, throughout the whole structure, will be found to agree in general form, position, and function, the only absolute differences being that the orang has nine wrist-bones, whereas man and the chimpanzee have but eight; and the chimpanzee has thirteen pairs of ribs, whereas the orang, like man, has but twelve. With these two exceptions, the differences are those of shape, proportion, and direction only, though the resulting differences in the external form and motions are very considerable. The greatest of these are, that the feet of the anthropoid or man-like apes, as well as those of all monkeys, are formed like hands, with large opposable thumbs fitted to grasp the branches of trees, but unsuitable for erect walking, while the hands have weak small thumbs but very long and powerful fingers, forming a hook rather than a hand, adapted for climbing up trees and suspending the whole weight from horizontal branches. The almost complete identity of the skeleton, however, and the close similarity of the muscles and of all the internal organs, have produced that striking and ludicrous resemblance to man which every one recognizes in these higher apes and, in a less degree, in the whole monkey tribe; the face and features, the motions, attitudes, and gestures being often a strange caricature of humanity. Let us, then, examine a little more closely in what the resemblance consists, and how far, and to what extent, these animals really differ from us.
Besides the face, which is often wonderfully human—although the absence of any protuberant nose gives it often a curiously infantile aspect—monkeys, and especially apes, resemble us most closely in the hand and arm. The hand has well-formed fingers with nails, and the skin of the palm is lined and furrowed like our own. The thumb is, however, smaller and weaker than ours, and is not so much used in taking hold of anything. The monkey's hand is, therefore, not so well adapted as that of man for a variety of purposes, and can not be applied with such precision in holding small objects, while it is unsuitable for performing delicate operations such as tying a knot or writing with a pen. A monkey does not take hold of a nut with its forefinger and thumb as we do, but grasps it between the fingers and the palm in a clumsy way, just as a baby does before it has acquired the proper use of its hand. Two groups of monkeys—one in Africa and one in South America—have no thumbs on their hands, and yet they do not seem to be in any respect inferior to other kinds which possess it. In most of the American monkeys the thumb bends in the same direction as the fingers, and in none is it so perfectly opposed to the fingers as our thumbs are; and all these circumstances show that the hand of the monkey is, both structurally and functionally, a very different, and very inferior organ to that of man, since it is not applied to similar purposes, nor is it capable of being so applied.
When we look at the feet of monkeys we find a still greater difference, for these have much larger and more opposable thumbs, and are, therefore, more like our hands; and this is the case with all monkeys, so that even those which have no thumbs on their hands, or have them small and weak and parallel to the fingers, have always large and well-formed thumbs on their feet. It was on account of this peculiarity that the great French naturalist Cuvier named the whole group of monkeys Quadrumana, or four-handed animals, because, besides the two hands on their fore-limbs, they have also two hands in place of feet on their hind-limbs. Modern naturalists have given up the use of this term, because they say that the hind extremities of all monkeys are really feet, only these feet are shaped like hands; but this is a point of anatomy, or rather of nomenclature, which we need not here discuss.
Let us, however, before going further, inquire into the purpose and use of this peculiarity, and we shall then see that it is simply an adaptation to the mode of life of the animals which possess it. Monkeys, as a rule, live in trees, and are especially abundant in the great tropical forests. They feed chiefly upon fruits, and occasionally eat insects and birds' eggs, as well as young birds, all of which they find in the trees; and, as they have no occasion to come down to the ground, they travel from tree to tree by jumping or swinging, and thus pass the greater part of their lives entirely among the leafy branches of lofty trees. For such a mode of existence, they require to be able to move with perfect ease upon large or small branches, and to climb up rapidly from one bough to another. As they use their hands for gathering fruit and catching insects or birds, they require some means of holding on with their feet, otherwise they would be liable to continual falls, and they are able to do this by means of their long finger-like toes and large opposable thumbs, which grasp a branch almost as securely as a bird grasps its perch. The true hands, on the contrary, are used chiefly to climb with, and to swing the whole weight of the body from one branch or one tree to another, and for this purpose the fingers are very long and strong, and in many species they are further strengthened by being partially joined together, as if the skin of our fingers grew together as far as the knuckles. This shows that the separate action of the fingers, which is so important to us, is little required by monkeys, whose hand is really an organ for climbing and seizing food, while their foot is required to support them firmly in any position on the branches of trees, and for this purpose it has become modified into a large and powerful grasping hand.
Another striking difference between monkeys and men is, that the former never walk with ease in an erect posture, but always use their arms in climbing or in walking on all-fours like most quadrupeds. The monkeys that we see in the streets, dressed up and walking erect, only do so after much drilling and teaching, just as dogs may be taught to walk in the same way; and the posture is almost as unnatural to the one animal as it is to the other. The largest and most man-like of the apes—the gorilla, chimpanzee, and orang-outang—also walk usually on all-fours; but in these the arms are so long and the legs so short that the body appears half erect when walking; and they have the habit of resting on the knuckles of the hands, not on the palms like the smaller monkeys, whose arms and legs are more nearly of an equal length, which tends still further to give them a semi-erect position. Still, they are never known to walk of their own accord on their hind-legs only, though they can do so for short distances, and the story of their using a stick and walking erect by its help in the wild state is not true. Monkeys, then, are both four-handed and four-footed beasts; they possess four hands formed very much like our hands, and capable of picking up or holding any small object in the same manner; but they are also four-footed, because they use all four limbs for the purpose of walking, running, or climbing; and, being adapted to this double purpose, the hands want the delicacy of touch and the freedom as well as the precision of movement which ours possess. Man alone is so constructed that he walks erect with perfect ease, and has his hands free for any use to which he wishes to apply them; and this is the great and essential bodily distinction between monkeys and men.
We will now give some account of the different kinds of monkeys and the countries they inhabit.
The Different Kinds of Monkeys and the Countries they inhabit.—Monkeys are usually divided into three kinds—apes, monkeys, and baboons; but these do not include the American monkeys, which are really more different from all those of the Old World than any of the latter are from each other. Naturalists, therefore, divide the whole monkey-tribe into two great families, inhabiting the Old and the New Worlds respectively; and, if we learn to remember the kind of differences by which these several groups are distinguished, we shall be able to understand something of the classification of animals, and the difference between important and unimportant characters.
Taking first the Old World groups, they may be thus defined: apes have no tails; monkeys have tails, which are usually long; while baboons have short tails, and their faces, instead of being round and with a man-like expression as in apes and monkeys, are long and more dog-like. These differences are, however, by no means constant, and it is often difficult to tell whether an animal should be classed as an ape, a monkey, or a baboon. The Gibraltar ape, for example, though it has no tail, is really a monkey, because it has callosities, or hard pads of bare skin on which it sits, and cheek-pouches in which it can stow away food; the latter character being always absent in the true apes, while both. are present in most monkeys and baboons. All these animals, however, from the largest ape to the smallest monkey, have the same number of teeth as we have, and they are arranged in a similar manner, although the tusks, or canine teeth, of the males are often large, like those of a dog.
The American monkeys, on the other hand, with the exception of the marmosets, have four additional grinding-teeth (one in each jaw on either side), and none of them have callosities or cheek-pouches. They never have prominent snouts like the baboons; their nostrils are placed wide apart and open sideways on the face; the tail, though sometimes short, is never quite absent; and the thumb bends the same way as the fingers, is generally very short and weak, and is often quite wanting. We thus see that these American monkeys differ in a great number of characters from those of the Eastern hemisphere; and they have this further peculiarity, that many of them have prehensile or grasping tails, which are never found in the monkeys of any other country. This curious organ serves the purpose of a fifth hand. It has so much muscular power that the animal can hang by it easily with the tip curled round a branch, while it can also be used to pick up small objects with almost as much ease and exactness as an elephant's trunk. In those species which have it most perfectly formed it is very long and powerful, and the end has the underside covered with bare skin, exactly resembling that of the finger or palm of the hand, and apparently equally sensitive. One of the common kinds of monkeys that accompany street organ-players has a prehensile tail, but not of the most perfect kind; since in this species the tail is entirely clad with hair to the tip, and seems to be used chiefly to steady the animal when sitting on a branch by being twisted round another branch near it. The statement is often erroneously made that all American monkeys have prehensile tails; but the fact is that rather less than half the known kinds have them so, the remainder having this organ either short and bushy or long and slender, but entirely without any power of grasping. All prehensile-tailed monkeys are American, but all American monkeys are not prehensile-tailed.
By remembering these characters it is easy, with a little observation, to tell whether any strange monkey comes from America or from the Old World. If it has bare seat-pads, or if when eating it fills its mouth till its cheeks swell out like little bags, we may be sure it comes from some part of Africa or Asia; while, if it can curl up the end of its tail so as to take hold of anything, it is certainly American. As all the tailed monkeys of the Old World have seat-pads (or ischial callosities as they are called in scientific language), and as all the American monkeys have tails, but no seat-pads, this is the most constant external character by which to distinguish them; and, having done so, we can look for the other peculiarities of the American monkeys, especially the distance apart of the nostrils and their lateral position.
The whole monkey-tribe is especially tropical, only a few kinds being found in the warmer parts of the temperate zone. One inhabits the Rock of Gibraltar, and there is one very like it in Japan, and these are the two monkeys which live farthest from the equator. In the tropics they become very abundant, and increase in numbers and variety as we approach the equator, where the climate is hot, moist, and equable, and where flowers, fruits, and insects are to be found throughout the year. Africa has about fifty-five different kinds, Asia and its islands about sixty, while America has one hundred and fourteen, or almost exactly the same as Asia and Africa together. Australia and its islands have no monkeys, nor has the great and luxuriant Island of New Guinea, whose magnificent forests seem so well adapted for them. We will now give a short account of the different kinds of monkeys inhabiting each of the tropical continents.
Africa possesses two of the great man-like apes—the gorilla and the chimpanzee, the former being the largest ape known, and the one which, on the whole, perhaps most resembles man, though its countenance is less human than that of the chimpanzee. Both are found in West Africa, near the equator, but they also inhabit the interior wherever there are great forests; and Dr. Schweinfurth states that the chimpanzee inhabits the country about the sources of the Shari River, in 28º east longitude and 4º north latitude.
The long-tailed monkeys of Africa are very numerous and varied. One group has no cheek-pouches and no thumb on the hand, and many of these have long, soft fur of varied colors. The most numerous group are the guenons, rather small, long-tailed monkeys, very active and lively, and often having their faces curiously marked with white or black, or ornamented with whiskers or other tufts of hair; and they all have large cheek-pouches and good-sized thumbs. Many of them are called green monkeys, from the greenish-yellow tint of their fur, and most of them are well-formed, pleasing animals. They are found only in tropical Africa.
The baboons are larger, but less numerous. They resemble dogs in the general form and the length of the face or snout, but they have hands with well-developed thumbs on both the fore and hind limbs; and this, with something in the expression of the face, and their habit of sitting up and using their hands in a very human fashion, at once shows that they belong to the monkey-tribe. Many of them are very ugly, and in their wild state they are the fiercest and most dangerous of monkeys. Some have the tail very long, others of medium length, while it is sometimes reduced to a mere stump, and all have large cheek-pouches and bare seat-pads. They are found all over Africa, from Egypt to the Cape of Good Hope; while one species, called the hamadryas, extends from Abyssinia across the Red Sea into Arabia, and is the only baboon found out of Africa. This species was known to the ancients, and it is often represented in Egyptian sculptures, while mummies of it have been found in the catacombs. The largest and most remarkable of all the baboons is the mandrill of West Africa, whose swollen and hog-like face is ornamented with stripes of vivid blue and scarlet. This animal has a tail scarcely two inches long, while in size and strength it is not much inferior to the gorilla. These large baboons go in bands, and are said to be a match for any-other animals in the African forests, and even to attack and drive away the elephants from the districts they inhabit.
Turning now to Asia, we have first one of the best known of the large man-like apes—the orang-outang, found only in the two large islands, Borneo and Sumatra. The name is Malay, signifying "man of the woods," and it should be pronounced órang-óotang, the accent being on the first syllable of both words. It is a very curious circumstance that, whereas the gorilla and chimpanzee are both black, like the negroes of the same country, the orang-outang is red or reddish-brown, closely resembling the color of the Malays and Dyaks who live in the Bornean forests. Though very large and powerful, it is a harmless creature, feeding on fruit, and never attacking any other animal except in self-defense. A full-grown male orang-outang is rather more than four feet high, but with a body as large as that of a stout man, and with enormously long and powerful arms.
Another group of true apes inhabit Asia and the larger Asiatic islands, and are in some respects the most remarkable of the whole family. These are the gibbons, or long-armed apes, which are generally of small size and of a gentle disposition, but possessing the most wonderful agility. In these creatures the arms are as long as the body and legs together, and are so powerful that a gibbon will hang for hours suspended from a branch, or swing to-and-fro, and then throw itself a great distance through the air. The arms, in fact, completely take the place of the legs for traveling. Instead of jumping from bough to bough, and running on the branches, like other apes and monkeys, the gibbons move along while hanging suspended in the air, stretching their arms from bough to bough, and thus going hand over hand as a very active sailor will climb along a rope. The strength of their arms is, however, so prodigious, and their hold so sure, that they often loose one hand before they have caught a bough with the other, thus seeming almost to fly through the air by a series of swinging leaps; and they travel among the network of interlacing boughs a hundred feet above the earth with as much ease and certainty as we walk or run upon level ground, and with even greater speed. These little animals scarcely ever come down to the ground of their own accord; but, when obliged to do so, they run along almost erect, with their long arms swinging round and round, as if trying to find some tree or other object to climb upon. They are the only apes who naturally walk without using their hands as well as their feet; but this does not make them more like men, for it is evident that the attitude is not an easy one, and is only adopted because the arms are habitually used to swing by, and are therefore naturally held upward instead of downward, as they must be when walking on them.
The tailed monkeys of Asia consist of two groups, the first of which have no cheek-pouches, but always have very long tails. They are true forest monkeys, very active, and of a shy disposition. The most remarkable of these is the long-nosed monkey of Borneo, which is very large, of a pale-brown color, and distinguished by possessing a long, pointed, fleshy nose, totally unlike that of all other monkeys. Another interesting species is the black and white entellus monkey of India, called "Hanuman" by the Hindoos, and considered sacred by them. These animals are petted and fed, and at some of the temples numbers of them come every day for the food which the priests, as well as the people, provide for them.
The next group of Eastern monkeys are the Macaques, which are more like baboons, and often run upon the ground. They are more bold and vicious than the others. All have cheek-pouches, and though some have long tails, in others the tail is short, or reduced to a mere stump. In some few this stump is so very short that there appears to be no tail, as in the magot of North Africa and Gibraltar, and in an allied species that inhabits Japan.
American Monkeys.—The monkeys which inhabit America form three very distinct groups: 1. The Sapajous, which have prehensile or grasping tails; 2. The Sagouins, which have ordinary tails, either long or short; and, 3. The Marmosets, very small creatures, with sharp claws, long tails, which are not prehensile, and a smaller number of teeth than all other American monkeys. Each of these three groups contains several sub-groups, or genera, which often differ remarkably from each other, and from all the monkeys of the Old World.
We will begin with the howling monkeys, which are the largest found in America, and are celebrated for the loud voice of the males. Often in the great forests of the Amazon, or Orinoco, a tremendous noise is heard in the night or early morning, as if a great assemblage of wild beasts were all roaring and screaming together. The noise may be heard for miles, and it is louder and more piercing than that of any other animals, yet it is all produced by a single male howler sitting on the branches of some lofty tree. They are enabled to make this extraordinary noise by means of an organ that is possessed by no other animal. The. lower jaw is unusually deep, and this makes room for a hollow bony vessel about the size of a large walnut, situated under the root of the tongue, and having an opening into the windpipe by which the animal can force air into it. This increases the power of its voice, acting something like the hollow case of a violin, and producing those marvelous rolling and reverberating sounds which caused the celebrated traveler Waterton to declare that they were such as might have had their origin in the infernal regions. The howlers are large and stout-bodied monkeys with bearded faces, and very strong and powerfully grasping tails. They inhabit the wildest forests; they are very shy, and are seldom taken captive, though they are less active than many other American monkeys.
Next come the spider-monkeys, so called from their slender bodies and enormously long limbs and tail. In these monkeys the tail is so long, strong, and perfect, that it completely takes the place of a fifth hand. By twisting the end of it round a branch the animal can swing freely in the air with complete safety; and this gives them a wonderful power of climbing and passing from tree to tree, because the distance they can stretch is that of the tail, body, and arm added together, and these are all unusually long. They can also swing themselves through the air for great distances, and are thus able to pass rapidly from tree to tree without ever descending to the ground, just like the gibbons in the Malayan forests. Although capable of feats of wonderful agility, the spider-monkeys are usually slow and deliberate in their motions, and have a timid, melancholy expression, very different from that of most monkeys. Their hands are very long, but have only four fingers,, being adapted for hanging on to branches rather than for getting hold of small objects. It is said that, when they have to cross a river the trees on the opposite banks of which do not approach near enough for a leap, several of them form a chain, one hanging by its tail from a lofty overhanging branch and seizing hold of the tail of the one below it, then gradually swinging themselves backward and forward till the lower one is able to seize hold of a branch on the opposite side. He then climbs up the tree, and, when sufficiently high, the first one lets go, and the swing either carries him across to a bough on the opposite side or he climbs up over his companions.
Closely allied to the last are the woolly monkeys, which have an equally well-developed prehensile tail, but better proportioned limbs, and a thick, wholly fur of a uniform gray or brownish color. They have well-formed fingers and thumbs, both on the hands and feet, and are rather deliberate in their motions, and exceedingly tame and affectionate in captivity. They are great eaters, and are usually very fat. They are found only in the far interior of the Amazon Valley, and, having a delicate constitution, seldom live long in Europe. These monkeys are not so fond of swinging themselves about by their tails as are the spider-monkeys, and offer more opportunities of observing how completely this organ takes the place of a fifth hand. When walking about a house, or on the deck of a ship, the partially curled tail is carried in an horizontal position on the ground, and the moment it touches anything it twists round it and brings it forward, when, if eatable, it is at once appropriated; and when fastened up the animal will obtain any food that may be out of reach of its hands with the greatest facility, picking up small bits of biscuit, nuts, etc., much as an elephant does with the tip of his trunk.
We now come to a group of monkeys whose prehensile tail is of a less perfect character, since it is covered with hair to the tip, and is of no use to pick up objects. It can, however, curl round a branch, and serves to steady the animal while sitting or feeding, but is never used to hang and swing by, in the manner so common with the spider-monkeys and their allies. These are rather small-sized animals, with round heads and with moderately long tails. They are very active and intelligent, their limbs are not so long as in the preceding group, and, though they have five fingers on each hand and foot, the hands have weak and hardly opposable thumbs. Some species of these monkeys are often carried about by itinerant organ-men, and are taught to walk erect and perform many amusing tricks. They form the genus Cebus of naturalists.
The remainder of the American monkeys have nonprehensile tails, like those of the monkeys of the Eastern hemisphere; but they consist of several distinct groups, and differ very much in appearance and habits. First we have the Sakis, which have a bushy tail and usually very long and thick hair, something like that of a bear. Sometimes the tail is very short, appearing like a rounded tuft of hair; many of the species have fine bushy whiskers, which meet under the chin, and appear as if they had been dressed and trimmed by a barber, and the head is often covered with thick, curly hair, looking like a wig. Others, again, have the face quite red, and one has the head nearly bald—a most remarkable peculiarity among monkeys. This latter species was met with by Mr. Bates on the upper Amazon, and he describes the face as being of a vivid scarlet, the body clothed from neck to tail with very long, straight, and shining white hair, while the head was nearly bald, owing to the very short crop of thin, gray hairs. As a finish to their striking physiognomy, these monkeys have bushy whiskers, of a sandy color, meeting under the chin, and yellowish-gray eyes. The color of the face is so vivid that it looks as if covered with a thick coat of bright scarlet paint. These creatures are very delicate, and have never reached Europe alive, though several of the allied forms have lived some time in our Zoölogical Gardens.
An allied group consists of the elegant squirrel-monkeys, with long, straight, hairy tails, and often adorned with prettily variegated colors. They are usually small animals; some have the face marked with black and white, others have curious whiskers, and their nails are rather sharp and claw-like. They have large, round heads, and their fur is more glossy and smooth than in most other American monkeys, so that they more resemble some of the smaller monkeys of Africa. These little creatures are very active, running about the trees like squirrels, and feeding largely on insects as well as on fruit.
Closely allied to these are the small group of night-monkeys, which have large eyes, and a round face surrounded by a kind of ruff of whitish fur, so as to give it an owl like appearance, whence they are sometimes called owl-faced monkeys. They are covered with soft, gray fur, like that of a rabbit, and sleep all day long, concealed in hollow trees. The face is also marked with white patches and stripes, giving it a rather carnivorous or cat-like aspect, which, perhaps, serves as a protection, by causing the defenseless creature to be taken for an arboreal tiger-cat, or some such beast of prey.
This finishes the series of such of the American monkeys as have a larger number of teeth than those of the Old World. But there is another group, the Marmosets, which have the same number of teeth as Eastern monkeys, but differently distributed in the jaws, a premolar being substituted for a molar tooth. In other particulars they resemble the rest of the American monkeys. These are very small and delicate creatures, some having the body only seven inches long. The thumb of the hands is not opposable, and instead of nails they have sharp, compressed claws. These diminutive monkeys have long, non-prehensile tails, and they have a silky fur, often of varied and beautiful colors. Some are striped with gray and white, or are of rich brown or golden-brown tints, varied by having the head or shoulders white or black, while in many there are crests, frills, manes, or long ear-tufts, adding greatly to their variety and beauty. These little animals are timid and restless; their motions are more like those of a squirrel than a monkey. Their sharp claws enable them to run quickly along the branches, but they seldom leap from bough to bough, like the larger monkeys. They live on fruits and insects, but are much afraid of wasps, which they are said to recognize even in a picture. This completes our sketch of the American monkeys, and we see that, although they possess no such remarkable forms as the gorilla or the baboons, yet they exhibit a wonderful diversity of external characters, considering that all seem equally adapted to a purely arboreal life. In the howlers we have a specially developed voice-organ, which is altogether peculiar; in the spider-monkeys we find the adaptation to active motion among the topmost branches of the forest-trees carried to an extreme point of development; while the singular nocturnal monkeys, the active squirrel-monkeys, and the exquisite little marmosets, show how distinct are the forms under which the same general type may be exhibited, and in how many varied ways existence may be sustained under almost identical conditions.
Lemurs.—In the general term, monkeys, considered as equivalent to the order Primates, or the Quadrumana of naturalists, we have to include another sub-type, that of the lemurs. These animals are of a lower grade than the true monkeys, from which they differ in so many points of structure that they are considered to form a distinct sub-order, or, by some naturalists, even a separate order. They have usually a much larger head and more pointed muzzle than monkeys; they vary considerably in the number, form, and arrangement of the teeth; their thumbs are always well developed, but their fingers vary much in size and length; their tails are usually long, but several species have no tail whatever, and they are clothed with a more or less woolly fur, often prettily variegated with white and black. They inhabit the deep forests of Africa, Madagascar, and Southern Asia, and are more sluggish in their movements than true monkeys, most of them being of nocturnal or crepuscular habits. They feed largely on insects, eating also fruits and the eggs or young of birds.
The most curious species are—the slow lemurs of South India, small tailless nocturnal animals, somewhat resembling sloths in appearance and almost as deliberate in their movements, except when in the act of seizing their insect prey; the tarsier, or specter-lemur, of the Malay Islands, a small long-tailed nocturnal lemur, remarkable for the curious development of the hind-feet, which have two of the toes very short and with sharp claws, while the others have nails, the third toe being exceedingly long and slender, though the thumb is very large, giving the feet a very irregular and outré appearance; and, lastly, the aye-aye of Madagascar, the most remarkable of all. This animal has very large ears and a squirrel-like tail, with long, spreading hair. It has large curved incisor teeth, which add to its squirrel-like appearance and caused the early naturalists to class it among the rodents. But its most remarkable character is found in its fore-feet or hands, the fingers of which are all very long and armed with sharp, curved claws, but one of them, the second, is wonderfully slender, being not half the thickness of the others. This curious combination of characters shows that the aye-aye is a very specialized form—that is, one whose organization has been slowly modified to fit it for a peculiar mode of life. From information received from its native country, and from a profound study of its organization, Professor Owen believes that it is adapted for the one purpose of feeding on small, wood-boring insects. Its large feet and sharp claws enable it to cling firmly to the branches of trees in almost any position; by means of its large, delicate ears it listens for the sound of the insect gnawing within the branch, and is thus able to fix its exact position; with its powerful curved gnawing teeth it rapidly cuts away the bark and wood till it. exposes the burrow of the insect, most probably the soft larva of some beetle, and then comes into play the extraordinary long wire-like finger, which enters the small cylindrical burrow, and with the sharp bent claw hooks out the grub. Here we have a most complex adaptation of different parts and organs all converging to one special end, that end being the same as is reached by a group of birds, the wood-peckers, in a different way; and it is a most interesting fact that, although woodpeckers abound in all the great continents, and are especially common in the tropical forests of Asia, Africa, and America, they are quite absent from Madagascar. We may therefore consider that the aye-aye really occupies the same place in nature in the forests of this tropical island as do the woodpeckers in other parts of the world.
Distribution, Affinities, and Zoölogical Rank of Monkeys.—Having thus sketched an outline of the monkey-tribe as regards their more prominent external characters and habits, we must say a few words on their general relations as a distinct order of mammalia. No other group, so extensive and so varied as this, is so exclusively tropical in its distribution, a circumstance no doubt due to the fact that monkeys depend so largely on fruit and insects for their subsistence. A very few species extend into the warmer parts of the temperate zones, their extreme limits in the northern hemisphere being Gibraltar, the western Himalayas at eleven thousand feet elevation, East Thibet, and Japan. In America they are found in Mexico, but do not appear to pass beyond the tropic. In the southern hemisphere they are limited by the extent of the forests in South Brazil, which reach about 30º south latitude. In the East, owing to their entire absence from Australia, they do not reach the tropic; but in Africa some baboons range to the southern extremity of the continent.
But this extreme restriction of the order to almost tropical lands is only recent. Directly we go back to the Pliocene period of geology, we find the remains of monkeys in France, and even in England. In the earlier Miocene several kinds, some of large size, lived in France, Germany, and Greece, all more or less closely allied to living forms of Asia and Africa. About the same period monkeys of the South American type inhabited the United States. In the remote Eocene period the same temperate lands were inhabited by lemurs in the East, and by curious animals believed to be intermediate between lemurs and marmosets in the West. We know from a variety of other evidence that throughout these vast periods a mild and almost sub-tropical climate extended over all Central Europe and parts of North America, while one of a temperate character prevailed as far north as the Arctic Circle. The monkey tribe then enjoyed a far greater range over the earth, and perhaps filled a more important place in Nature than it does now. Its restriction to the comparatively narrow limits of the tropics is no doubt mainly due to the great alteration of climate which occurred at the close of the Tertiary period, but it may have been aided by the continuous development of varied forms of mammalian life better fitted for the contrasted seasons and deciduous vegetation of the north temperate regions. The more extensive area formerly inhabited by the monkey-tribe would have favored their development into a number of divergent forms, in distant regions and adapted to distinct modes of life. As these retreated southward and became concentrated in a more limited area, such as were able to maintain themselves became mingled together as we now find them, the ancient and lowly marmosets and lemurs subsisting side by side with the more recent and more highly developed howlers and anthropoid apes.
Throughout the long ages of the Tertiary period monkeys must have been very abundant and very varied, yet it is but rarely that their fossil remains are found. This, however, is not difficult to explain. The deposits in which mammalian remains most abound are those formed in lakes or in caverns. In the former the bodies of large numbers of terrestrial animals were annually deposited, owing to their having been caught by floods in the tributary streams, swallowed up in marginal bogs or quicksands, or drowned by the giving way of ice. Caverns were the haunts of hyenas, tigers, bears, and other beasts of prey, which dragged into them the bodies of their victims, and left many of their bones to become imbedded in stalagmite or in the muddy deposit left by floods, while herbivorous animals were often carried into them by these floods, or by falling down the swallow-holes which often open into caverns from above. But, owing to their arboreal habits, monkeys were to a great extent freed from all these dangers. Whether devoured by beasts or birds of prey, or dying a natural death, their bones would usually be left on dry land, where they would slowly decay under atmospheric influences. Only under very exceptional circumstances would they become imbedded in aqueous deposits; and, instead of being surprised at their rarity, we should rather wonder that so many have been discovered in a fossil state.
Monkeys, as a whole, form a very isolated group, having no near relations to any other mammalia. This is undoubtedly an indication of great antiquity. The peculiar type which has since reached so high a development must have branched off the great mammalian stock at a very remote epoch, certainly far back in the Secondary period, since in the Eocene we find lemurs and lemurine monkeys already specialized. At this remoter period they were probably not separable from the insectivora, or (perhaps) from the ancestral marsupials. Even now we have one living form, the curious Galeopithecus, or flying lemur, which has only recently been separated from the lemurs, with which it was formerly united, to be classed as one of the insectivora; and it is only among the opossums and some other marsupials that we again find hand-like feet with opposable thumbs, which are such a curious and constant feature of the monkey-tribe.
This relationship to the lowest of the mammalian tribes seems inconsistent with the place usually accorded to these animals at the head of the entire mammalian series, and opens up the question whether this is a real superiority or whether it depends merely on the obvious relationship to ourselves. If we could suppose a being gifted with high intelligence, but with a form totally unlike that of man, to have visited the earth before man existed in order to study the various forms of animal life that were found there, we can hardly think he would have placed the monkey-tribe so high as we do. He would ob-serve that their whole organization was specially adapted to an arboreal life, and this specialization would be rather against their claiming the first rank among terrestrial creatures. Neither in size, nor strength, nor beauty, would they compare with many other forms, while in intelligence they would not surpass, even if they equaled, the horse or the beaver. The carnivora, as a whole, would certainly be held to surpass them in the exquisite perfection of their physical structure, while the flexible trunk of the elephant, combined with his vast strength and admirable sagacity, would probably gain for him the first rank in the animal creation.
But if this would have been a true estimate, the mere fact that the ape is our nearest relation does not necessarily oblige us to come to any other conclusion. Man is undoubtedly the most perfect of all animals, but he is so solely in respect of characters in which he differs from all the monkey-tribe—the easily erect posture, the perfect freedom of the hands from all part in locomotion, the large size and complete opposability of the thumb, and the well-developed brain, which enables him fully to utilize these combined physical advantages. The monkeys have none of these, and without them the amount of resemblance they have to us is no advantage, and confers no rank. We are biased by the too exclusive consideration of the man-like apes. If these did not exist, the remaining monkeys could not be thereby deteriorated as to their organization or lowered in their zoölogical position; but it is doubtful if we should then class them so high as we now do. We might then dwell more on their resemblances to lower types—to rodents, to insectivora, and to marsupials, and should hardly rank the hideous baboon above the graceful leopard or stately stag. The true conclusion appears to be, that the combination of external characters and internal structure which exists in the monkeys is that which, when greatly improved, refined, and beautified, was best calculated to become the perfect instrument of the human intellect, and to aid in the development of man's higher nature; while, on the other hand, in the rude, inharmonious, and undeveloped state which it has reached in the quadrumana, it is by no means worthy of the highest place, or can be held to exhibit the most perfect development of existing animal life.—Contemporary Review.