CHAPTER VI. Dawn of the Primate Brain

DAWN OF THE PRIMATE BRAIN

THE LOWEST OF THE MONKEY KIND

We are now approaching a critical period in the history of the brain. It is a period that contains many incidents of the utmost importance. Particularly noteworthy are the episodes which favoured the production of human characteristics in the animal kingdom. These characteristics showed many manlike tendencies that much later were to appear full fledged in the human race. They were from the first limited to a single, highly interesting order of mammals. And this seems especially strange because from the beginning of the Age of Mammals (65,000,000 years ago) a great variety of new animals came into existence. The fact that a single group out of all this vast number was picked out to develop human resemblances must hold the secret of some potent selective influence. Such an influence was definitely at work. Its operations were slow but steady. Little by little it changed and reshaped the structure of the body until at length there appeared a race of animals so human in their organization that they might well have been the forerunners of mankind.

It would be difficult to conceive the kind of modification in structure that could produce the form of man from a horse, from a whale, or even from a dog. But this difficulty becomes far less in the case of the animals usually referred to as the monkey kind. In many features of their structure these animals resemble men. Existing in a great variety of forms, they manifest numerous modifications in the different parts of their bodies and exhibit a wide range of behaviour in their habits. Only a superficial acquaintance with them is necessary to reveal their many progressive traits. But their progress, like all other progress, had its humble beginnings. At first the apes were very simple creatures. Their coming, however, marked the dawn of a new day in animal life. We shall be interested to follow the advances that occurred in their mental capacities as they slowly made their progressive strides forward. We shall be particularly struck by those changes which gradually led up to the development of a brain capable to control all of the complex activities of human behaviour.

Naturally we may expect to find a simple controlling organ in the lowest of the monkey kind. As we pass upward, however, into the higher families of the apes, we shall not only observe a pronounced increase in manlike tendencies but, as the great anthropoids at length become human in miniature and then almost human, we shall recognize in these animals a brain which very closely resembles that of man.

Class Distinctions in the Monkey World

In the ape world there are animals of high and low degree. Some are so humble that it is hard to decide whether they actually belong to the monkeys or not. With few exceptions they all prefer to live in the jungles and tropical forests. We could not fail to be impressed by the striking resemblance that many of them bear to man. Yet there are such marked differences among them that they cannot all be regarded as members of the same family. If we grouped them as we do human races, we might most advantageously assign them to certain large classes according to their nearness to man.

Monkeys of lowest degree include the lemurs, the tarsiers, and all of the New World monkeys.

The intermediate monkeys in the next higher grade are those which live in the Old World, with the exception of the three great manlike apes.

The higher anthropoids occupy the top rank and are the nearest to man both in their appearance and in their habits.

These three ranks in apedom did not appear at the same time. One rank, so to speak, successively developed from another. By a process of selection and adjustment the higher forms arose from the lower. The ranking great apes owe their superiority to many traits and characters which they inherited from more humble forerunners and which they improved by the process of progressive development. The lowest monkeys likewise had their day of upward progress, during which they emerged from some mammal still lower in the animal scale. These forerunners of the earliest primates, the lemurs and tarsiers, had in all probability been gradually specializing during the latter part of the Age of Reptiles. Their ancestors came from that stock of mammal-like reptiles which started from lowly beginnings and remained modestly in the background during the reptilian period.

The Lemurs

In the endeavour to get some conception of these distant predecessors of the monkeys and apes, it is believed that the tree shrews possess those simple characters necessary for the proper starting point. The shrew is an insect-eater and lives in the trees. It has many specializations in its legs, in its head, and in its trunk. These special adjustments might serve as the beginning of those important changes in the body which later distinguished the monkey kind. In the first place, the small size of the tree shrew was particularly favourable for this purpose. Then, in the second place, its habit of living in the trees foreshadowed advantages of great promise. Such an epoch-making adjustment made its appearance when paws were replaced by hands and when definite hand-like feet appeared. If an animal like the tree shrew were the forerunner of the monkeys, it is not difficult to appreciate how the lemurs arose from this stock. They and their kind may be looked upon as the first chapter in the history of the ape world and the ape brain. At present they live exclusively in Madagascar and its small adjacent islands. They are not known in any other part of the world, although fossils of them indicate that they were widespread throughout the globe in earlier times. The reasons for their present exclusiveness and their insular homes are not clear. Geologists claim that the parts in which they live originally had land connection with the continent thus permitting their wide dissemination. The later disappearance of this connection accounts for their present isolation.

There is much in the appearance of the lemurs that distinguishes them from the monkeys and apes. Their most distinctive feature, the head, is much like that of a fox and is drawn out in a long pointed muzzle. Many characters appear in lemurs not seen in monkeys. They have no cheek pouches. Their tails, never prehensile, are usually furry. They develop no gluteal pads, which many apes possess in consequence of their squatting postures. It is in their hands that they resemble monkeys most strikingly. They have fingers and toes with finger nails and toe nails. The thumb and great toe are always well developed, but the second or third digit is often greatly modified. They also have mammary glands like the monkeys. In the female these glands assume certain definitely human characters. The lemur is a little smaller than the domestic cat. Its fur is thick and woolly. Its large and prominent eyes are more widely separated than in monkeys. The ears are long and have tufts of hair on their upper portions. The arms are not quite so long as the legs. The tail is long and often bushy. Fleshy pads appear on the palms of the hands and soles of the feet, as well as upon the palmar surface of the fingers. These enable the animal to grasp the branch of a tree with great tenacity.

Little is known of the lemur’s habits in the wild state. It is not strictly nocturnal, for some of these animals are known to seek their food during the day. Often they travel about in troupes consisting of many individuals. Most of them live in the forest. Their food consists of fruits, insects, birds’ eggs, and birds themselves, which latter they are most skillful in catching. During the heat of the day they sleep with the head beneath the arm and the tail curled about the neck. When walking they go upon the hands and feet, both when on the ground and in the trees. The tail is used in the manner of a balancing or steering organ. Sometimes they assume a semi-erect posture on the hind legs, or sit in a half-crouching position. Both hands and feet are employed primarily for climbing or running about on the ground.

The lemur has great ability in leaping from tree to tree. Its movements are so rapid that it can only with difficulty be followed by the eye. Hunters say that it is easier to kill a bird on the wing than a lemur when leaping. If pursued and shot at it has a habit of dropping suddenly from the topmost branches into the bushes, giving the hunter the impression that he has succeeded in killing the animal. This impression is soon dissipated upon seeing the lemur in another tree at a considerable distance from the spot where it fell. When wild the animals are said to subsist largely upon bananas. They also seem to be fond of the brains of birds. After fracturing the bird’s skull with their teeth, as they might puncture a nutshell, they suck the brain out of the brain case. The lemur, however, does not eat the rest of the bird. We may see from this description that this is an animal of great agility. Not only does it possess much speed in its locomotion, but it also manifests the utmost nicety in balancing and remarkable precision in all movements.

Tarsius

Another of these lowest monkeys is a strange little animal called tarsius, which has acquired a notable reputation. Several learned authorities have singled it out as the standard bearer of human origin from some lower mammal. The tarsius is about as large as a small squirrel. Its appearance is peculiar because of its closely set bulging eyes, its long tufted tail, its protruding ears, and the small circular pads on the end of each finger and toe. Tarsiers have two curious habits that attract attention at once. They can leap with astonishing swiftness from tree to tree, often in pursuit of insects, and when they turn their heads they seem at one instant to be looking forward and the next directly backward. Their eyes, though very large and prominent, do not seem to give them the best of eyesight. The animals can see well at night, but during the day they appear to be almost stupid because in the sunlight their vision is imperfect. Tarsius lives in the jungle, usually in the low countries of the Malay Islands. During the day it passes most of its time clinging in a vertical position to the trunks of the smaller trees and underbrush. The way in which it supports itself is interesting and peculiar. With its fingers and toes it firmly grasps its slender support, at the same time pressing inward with its long tail, which acts like a spring against the tree. If its tail is pulled away from this support the tarsier at once tends to slip backward. The tail, which has no grasping power, is used like a rudder for balancing and steering when the animal is in motion.

In some respects tarsiers are quite human. They go about in pairs and are not gregarious like most of the monkeys. Furthermore, they give birth to but a single offspring at a time. After the breeding season the female and her young find a home by themselves. There are no indications that these animals build nests or even live in holes of trees. The tarsier often falls asleep in its characteristic clinging position, and the head then sinks downward much as that of an old man asleep in his chair. Often the young tarsius will perch upon the mother’s head while she is asleep, and in this position fall asleep itself. The general behaviour of the animal is extremely stereotyped and limited. It learns but little under training. In captivity it is able to make but few new adjustments. During the day its enormous bulging eyes give it an almost ridiculous appearance as it gropes awkwardly for food. This no doubt is due to the fact that its eyes are constructed for hunting at night and do not contain the specialization essential to the sharpest kind of vision. On the ground tarsius leaps like a frog but is very awkward. In the trees, however, it is extremely agile, and is probably the quickest jumper of all mammals. While grasping a small branch it can turn its head so as to look directly backward and jump more quickly than the human eye can follow. It seems to be looking in one direction and jumping in another. This is due to the great rapidity with which it turns its head. In captivity it is pugnacious and cannot be tamed. It performs its toilet much as a cat does and thus keeps itself scrupulously clean. It is not known to make vocal sounds indicating fear or anger. On rare occasions, and particularly when young, it has been heard to squeak. The infant tarsius clings to the hair of the mother’s chest like other young monkeys. The eyes are open at birth, and many reactions appear at once that are long delayed in such animals as the rat, cat, dog, and higher apes.

The Marmosets

Another lowly monkey is the marmoset. It has less renown than tarsius but is nevertheless an interesting animal. It is often carried around in the pocket of its owner and fits conveniently inside of the old-fashioned fur muff. The marmosets belong to the group of the New World monkeys. They inhabit South America and Central America. Their chief interest arises from the fact that they represent one of those moments of faltering experienced by the monkeys in their upward strivings. These little animals have an almost pathetic expression and features that are in many ways quite human. Yet in spite of this human-like appearance they indicate an actual backsliding in the attempts at progress. This backsliding is most apparent in their fingers and toes. In fact, the entire hand and foot have lost most of their human resemblance. The finger nails are now replaced by sharp, talon-like claws, and the toes are equipped in the same way. The marmosets, both because of their diminutive size and the imperfections in their hands and feet, are now looked upon as monkeys that show signs of retrogression.

The marmoset is as large as a small squirrel and covered with a thick, silky fur. It is naturally very timid but soon becomes friendly to those with whom it is familiar in captivity. The female produces two or three young at a birth and in this respect is unlike most of the monkeys. The marmoset’s facial appearance and shape of head are certainly more ape-like than the lemur’s. The eyes are set much closer together, and are separated by a flat, narrow nose suggesting that vision now depends on the simultaneous operation of both eyes. The animal has a long, bushy tail. It lives in the tree tops or small underbrush and climbs the trees in a manner similar to the squirrel. Although it has a cat-like agility, it does not make the long and daring leaps characteristic both of lemur and tarsius. It often loses its grip on the branches and falls from a considerable height to the ground. In captivity it shows little inclination to develop new actions. It is not easily trained, and to teach it to do tricks of any kind is most difficult. It lives upon worms, insects, and fruit. It is known also to invade birds’ nests and suck the eggs. Very rarely does it prey upon bird life and then only when it is able to overpower one of the smaller birds or unprotected young.

South American Howling Monkeys

In this group of lower monkeys we encounter one with a highly interesting personality, known as the “red howling monkey of South America.” He is a real monkey, noisy and disagreeable, often attaining the size of a fox-terrier. He always seems to be in an unpleasant mood, showing his teeth and howling on the slightest provocation. In spite of all this ill temper, he belongs to the progressive party of the monkeys. There is not the slightest doubt that he has made definite advances along the lines of progress. If we should question this progress we would soon have our doubts set at rest when we saw the astonishing manner in which he uses his tail like a fifth hand. Even more convincing in this respect is the almost human appearance of his hands. Not long ago a young woman visiting the ape house in the zoölogical gardens was struck by these human similarities. She was still more impressed when a large howling monkey thrust his long tail through the bars and deftly tossed her hat into the air.

The howling monkeys enjoy this gift of a capable, grasping tail in common with most of their fellows who live in South America. The prehensile tail is especially well developed in the spider monkeys and in the woolly monkeys. At its end this tail looks like a long, tapering finger. It is a highly developed sense organ and gives the monkey a new instrument for locomotion and for exploring. These monkeys are able to swing themselves from the branches by their tails and thus leave the hands and feet free for other purposes.

In addition to this highly efficient tail, the howlers have developed a larynx and vocal cords with which they produce awe-inspiring sounds. Their mournful howlings are often audible for miles around, and it is supposed that they employ their cries as a means of defense to intimidate their enemies. The howling monkeys possess a slightly opposable thumb and well-developed fingers. While they are described as being the most ferocious of the South American monkeys, they are also credited with a low degree of intelligence. The face of this monkey is naked with the exception of a heavy beard that hangs beneath the chin. In captivity they are practically untamable and soon die. Their fur is usually black, but in some cases is brown or reddish brown. They live largely upon fruit, although like other South American monkeys they feed upon caterpillars and insects.

Measuring the Mentality of New World Monkeys

Professor Thorndike, of Columbia University, has made careful studies concerning the behaviour of several South American monkeys. He was chiefly interested in the manner in which monkeys differ from other animals in the mental capacities and methods of learning. In making his tests he devised certain experiments which utilized boxes with pegs, bolts, bars, and hooks. The object of these tests was to find out how the animal learned to release itself from confinement, or gain access to a goal containing food. Professor Thorndike concluded that these monkeys did not learn by reasoning. They do, however, form more and a greater variety of associations than other mammals. Their combinations of this kind are remarkably slow and ineffectual in providing any new behavioural accomplishment. Concerning the general mental development of the South American monkeys, Dr. Thorndike believes that they represent a certain advance from the generalized type of mammals toward man. This is particularly true of their sense equipment and their localized vision. All of this, he believes, is in reality an advance due to the brain acting with increased delicacy and bringing into line those activities which distinguish human mental faculty from that of all other animals. Here, at length, among the lower monkeys is well-attested proof of some progress toward the development of human capacity.

Monkey Behaviour

The way in which these lower members of the monkey kind behave deserves particular attention. It gives us the opportunity to observe certain striking resemblances to our own human behaviour. This question is one of primary importance. It acquires especial interest as we compare the brains of the monkeys and apes one with another. As the brain continues to improve from one stage to the next, we should be on the lookout for new developments in behaviour. It might perhaps be impossible to appreciate all of these minute changes among the monkey kind. It is even somewhat questionable whether such an exact comparison at the present time is necessary or possible. Yet there are certain outstanding traits of conduct that may be easily traced from stage to stage. One of the most important of these traits depends upon the development of the tail from the time when it first acted as a rudder-like organ for steering and balancing the animal until it acquired all of its great facilities as a fifth hand. After this it began to recede in importance and finally disappeared. The tail thus created a special cycle of behaviour which had important bearing upon the final outcome of man’s adjustment.

Another group of reactions centre upon the manner in which the hand made its appearance, including the progressive changes in behaviour when the monkeys first became four-handed. All of these changes were dependent upon living in the trees and gradually found their culmination in an animal that could stand upon two feet and use its hands. Such usage as this foretold the beginning of human skill, of human right-handedness, and of human speech.

Very important were the changes in behaviour that made their appearance as the eyes worked more in harmony with each other. They produced a kind of vision better able to guide the movements of the hand and give more complete information concerning distance, direction, and perspective. But far exceeding all other changes for getting a better control over the surroundings were those progressive advances introduced for making the fullest combinations of sense impressions. These advances favoured the development of better powers for learning and for profiting from experience. Progress in all of these particulars concerning the behaviour of the monkeys may be clearly traced in corresponding expansions in their brains.

Brains of the Lower Monkeys

In the brains of these four very simple members of the monkey kind we may readily see the expansions that promoted development in the governing organ. It will be apparent at a glance that progress followed no direct or easy path. It met many rebuffs and obstacles. Often it faltered and even stumbled. But struggling on it finally reached solid ground and then went forward to real advances.

Placing the brains of the lemur, tarsius, marmoset, and howling monkey side by side we may see how this progress began. To guide our way in following this advance, certain signposts and milestones will prove serviceable. Three of these landmarks are deep grooves or clefts. They appear in the superbrain and indicate the places in which progress has been particularly active. Around these grooves the outer covering of the superbrain has been folded to make room for more brain cells. This folding produces convolutions with the result that the more convoluted a brain is, the more cells it has for the development of brain power. Each of these three grooves has its own special meaning as a landmark:

1. The “Sylvian groove” is a fissure that runs between the department for the sense of hearing, called the “temporal lobe,” and the department for body and contact sense, called the “parietal lobe.”

2. The “central groove” is a fissure between the department for body and contact senses and the department of supreme brain activity, called the “frontal lobe.” This lobe of the brain is situated immediately above the eyes and behind the bone of the forehead (frontal bone). A small frontal lobe means a low brow with a correspondingly inferior mentality. As this lobe of the brain increases from ape to man, the forehead gradually becomes higher and more prominent.

3. The “ape groove” separates the occipital lobe in the back of the head from the parietal lobe. In the occipital lobe is situated the department for sight.

The three grooves form the boundary lines between the four chief departments of the superbrain, each of which is known as a lobe; namely, (1) the parietal lobe, department of body and contact senses; (2) the temporal lobe, department of hearing; (3) the occipital lobe, department of sight; and (4) the frontal lobe, department of the high mental faculties like judgment and reason.

Further advances from this point will occupy our attention in tracing the brain of the monkey kind upward. Two other landmarks in the brain have special value. One of them is the bridge (pons) which connects the larger brain (cerebral hemispheres) with the lesser brain (cerebellum). This lesser brain acts as the chief muscle timer and adjuster. It balances one muscle’s action to that of another and adjusts the force of such action. All of our most exact movements, whether in walking or writing or speaking, depend upon the little brain. If it is injured or destroyed the movements of our hands and feet, head and trunk, become shaky, unsteady, and very irregular. For an animal to become highly skillful requires high development in the little brain. The animal having the highest intelligence also possesses the greatest capacity for skill in its actions. The size of the bridge reflecting the degree of this skill is a good index of the intelligence possessed by the animal.

The pyramid is another important indicator of progress. Like the bridge, it is found on the base of the brain. It is called pyramid because of its somewhat pyramidal shape. It acts as the main trunk line for getting the orders of the superbrain out to the muscles. It transmits, so to speak, the highest commands of the brain in controlling the motor machinery. By means of it we act according to the dictates of our wills. If both of these great pyramidal trunk lines are interrupted, we become completely paralyzed. The pyramids conduct the highest output of the brain’s activity and increase in direct proportion as the animal’s behaviour becomes more and more complex.

The brains of low monkeys are of small size: lemur, 18 grams; tarsius, 6 grams; marmoset, 6.2 grams; and howling monkey, 24.5 grams.

Size and weight of brain, we must bear in mind, vary to a considerable degree with the size of the body, so that certain other signs of expansion in the brain are more impressive. These signs clearly indicate that progress is under way as follows: First, the large superbrain begins to cover over the lesser brain. In lemur this extension backward has only just begun. It is only slightly more marked than in many of the lower animals, like the cat and the dog. In tarsius the large brain has extended backward over the lesser brain to a considerably greater degree. This is an important change because the tarsier has transferred much of its business of sight to a new department in the occipital lobe of the superbrain. The marmoset shows this transfer carried a little farther, for the large brain now overhangs the lesser brain. The great advance shown in the howling monkey reveals the way in which the superbrain has taken complete control of the situation. It now covers over the lesser brain entirely. All of this change in the superbrain has been mainly in the interest of making a better department for sight, but the departments for the sense of hearing and for body and contact senses have not been behindhand in expanding in these lower monkeys.

Another pronounced sign of progress is the gradual change in the position of the groove of Sylvius. In lemur it is almost vertical, as in the cat, in the dog, and other lower mammals. The arrangement of other smaller grooves around it is also similar to that in lower animals. In the tarsius this groove is equally primitive. It is beginning to tip backward a little in marmoset. Finally, in the howling monkey this groove has become quite oblique, as it is in most of the apes and man.

All of this change has occurred as a direct result of perfecting the organization in the department of hearing. The more tilted the Sylvian groove becomes, the better developed is the temporal lobe which carries on the business of hearing. The tilting backward of this groove also results from an increase in that part of the superbrain which lies immediately about the groove. This is the parietal lobe, the department of contact and body sense. It is in this department that the especially important information concerning the movements in the hands and feet is registered. Thus the tilting backward of the Sylvian groove plainly tells the story of improvements in the departments of hearing and of body and contact sense.

Still another sign of progress appears in the central groove, which has an equally interesting history. In the lemur this groove is just discernible as a faint dent. In position it resembles a corresponding groove in animals like the cat and dog. Lemur in this respect suggests that in its striving to part company with the lower animals, to break away from ancient contacts, and to get on an independent new line of its own, it has not been entirely successful. This central groove shows where the chief department of the superbrain begins, that is, the frontal lobe. In the lemur this department is poorly developed. In tarsius it is impossible to find anything that looks like a central groove. This animal’s brain is an example of some of that hesitation which was encountered in the path of progress. The same faltering is also seen in the brain of marmoset, which has no central groove whatsoever. These little South American animals, it must be remembered, are thought to be backsliders, and this particular defect in their brain strongly supports that conclusion.

In the brain of the howling monkey we find the central groove now well developed. The superbrain shows that it is at length pursuing some definite policy of expansion in its most responsible department. Emphasis in growth is now obviously given to the frontal lobe for advancing the capacity to transact all higher mental faculties. In the howling monkey this department may not have attained any high degree of development, but its presence is undoubted, and from this relatively simple beginning it is only a matter of further expansion to bring into existence the most productive mechanism of the brain. The howling monkey shows its superiority over all lower monkeys in another respect. It has developed the ape groove, and by it the boundary between the department of sight and the department for body and contact sense is fully established.

Viewed as a whole, the brains of these four lower monkeys show distinct progress in the interests of developing a more efficient superbrain. Each of the sense departments has gradually become better defined in its boundaries, and doubtless correspondingly better organized for the administration of its duties. Rising supreme above them all there finally appeared the controlling department of the chief executive in the frontal lobe. We see this in its earliest stage in lemur. It assumes still more importance in the howling monkey. The departments of sight (occipital lobe), of hearing (temporal lobe), of body and contact sense (parietal lobe), show the effects of steady improvement from lemur up to the howling monkey. If there have been some hesitations, even some slipping back in the organization of efficiency, it is because some of these animals were rather uncertain disciples of progress. They may have been, as is probably true of tarsius, too close to the starting point where the real advances of the monkey kind began; or perhaps, like the marmosets, they ran into early difficulties along the upward climb. It seems probable that they were not able to extricate themselves with credit from these hazards or to overcome the obstacles that confronted them. For this reason their brain shows some actual backwardness. With these exceptions, however, the evidence of progress is undisputed. It seems sufficient to convince the most sceptical. The purpose of the progress is also sufficiently plain. It clearly appears to be that effort toward promoting organization in the superbrain so that the offices of the supreme executive might be established in the permanent quarters of the frontal lobe.

Measurable Improvements

Any doubts due to lack of measurable proofs may be easily overcome by several comparative measurements of the bridge and the pyramid. The size of these structures, both of which reveal the behavioural capacities of animals, has been carefully estimated. Accordingly the bridge has been assigned the following values: lemur, .055; tarsius, .057; marmoset, .095; howling monkey, .103. Thus the bridge, called by some authorities an index of intelligence, shows distinctly the advances made among these simple monkeys.

Quite as striking are the figures for the pyramid, which indicate the degree of voluntary control that the superbrain has over all actions: lemur, .110; tarsius, .032; marmoset, .064; howling monkey, .137.

From these figures the howling monkey stands in advance of his monkey associates in the index of his voluntary control. Doubtless much of this advantage is due to the high degree of hand-like specialization in this animal’s hands and feet. But the grasping tail of the howling monkey should not be overlooked. If tarsius and the marmoset appear to stand lower than the lemur, it is because one of them is a primitive type of animal with a much restricted repertoire of reactions, and the other, the marmoset, is a backslider less richly endowed in the more effective motor capacities.

All of these features in the brain seem to coincide with progress in the behaviour of the lower monkeys. They show the path which progressive advance has pursued. In the beginning, emerging from those strivings of lower mammals and with much of the mammalian heritage handed down by them, the lemurs took the first step of the monkey kind toward a new type of brain. There was prophecy in these early attempts made by the lemur. In some degree at least they foretold what this new kind of brain was to be. Obviously they had as their distant mark the ultimate upbuilding of the superbrain until an adequate department for the supreme executive of life was produced. If tarsius hesitated in reaching out toward this objective, it was none the less travelling in the right direction. The destination of this course was clearly visible in the brain of the howling monkey and other similar monkeys of the New World. In this manner the first primate steps toward a more highly efficient type of brain were taken. The conditions of tree life both incited and successfully urged them onward.