CHAPTER II. Ancestors Before the Apes

ANCESTORS BEFORE THE APES

THE BRAIN FROM FISH TO MAN

Practical Significance of Evolution

There are many who still harbour resentment against the ape, especially in explaining man’s origin. As a result, hostilities often flare up against evolution. It cannot be denied that the unattractive ape is at the root of these reactions. He is the bar sinister and the real stumbling block in the evolutionary theory. He is also, to many people at least, the entire gist of it. That we are descended from monkeys is rather generally accepted as the meaning of evolution. This view, at best, is a superficial explanation of what evolution really means. No scientist to-day believes that any one of the living apes is ancestral to man. These animals belong to families totally divergent from the human family. They have ascended well up into the trees. Here doubtless they will remain, quite as unconcerned in human origin as they are innocent of participation in it. Our interest in evolution should not centre upon the ape kind. The line of our ancestry reaches far back of them through millions of years. We were in the making long before there were any apes on earth. They, in their tree life, merely afforded the last finishing touches which shaped our course toward humanity. If we wish to acknowledge our hereditary indebtedness properly, we would be compelled to recognize in our family tree that highly important line of mammals which first introduced the custom of arboreal living. Back of them are still older lines which deserve equal ancestral credit. Here are found those animals without the existence of which we should never have arrived. Among these is the vast assortment of reptiles, together with mammal-like reptilians which appeared in the Age of Reptiles. All of these reptilian forms were in their turn indebted for existence to earlier amphibians and fish, their progenitors during the long Age of Fish. Thus the true line of evolutionary descent leads us from fish to man. Not until we appreciate the meaning of this long vertebrate lineage through all its various phases does the vital significance of evolution become clear. If we view it in this way it is possible to sense the irresistible force that has carried animal life onward and upward through the ages from the earliest times. This force may still carry us onward. In its broader applications such a viewpoint should make an urgent appeal for thoughtful consideration. It offers many suggestions concerning further advances and readjustments in human behaviour.

Evidence of Evolution in Our Bodies

The brain is one of the best witnesses testifying to this long evolutionary development of man. It contains convincing evidence of this process in three striking particulars. First, it gives numerous signs indicating its primitive origin from the lowest of the vertebrates, the fish. Second, it bears identifying marks of intimate association with animals of its own class, the mammals. Third, it has a large number of details in its special mechanisms possessed in common with all of the primate order, to which man belongs together with the lemurs, tarsiers, monkeys, and apes. This evidence is not circumstantial. It is direct and unimpeachable. It leaves no point in the line of man’s long descent to be decided by inference. It embodies factors which led, step by step, to the upbuilding of the human brain.

Other tissues and organs of the body tell the same story of slow, steady progress upward, from some low and simple phase of life, through many graded stages of improvement until the human form at length came in sight.

The blood has been an especially positive witness concerning this progressive development. Tests with many different kinds of animals show that the blood of man is much nearer to that of the great apes than to the lower Old World monkeys. The relation between the human blood and that of the New World monkeys is still more remote. In general, these blood tests are among the most convincing proofs of evolution.

The bony system of the body is another decisive witness. The skeleton of the fore and hind limbs sheds much light on the changing adjustments which have been made in the motor apparatus. The use of the limbs as fins, paddles, wings, hoofs, paws, claws, hands, or feet, indicates the broad family relations and kinship of various animals. The size and shape of the skull and the character of the teeth reveal the manner in which this evolutionary process has passed through its several stages. The muscular system, the system for eliminating waste products of the body, the heart and the lungs, all afford important evidence of vertebrate kinship and evolution. The increase in the complexity of the breathing apparatus, from the early gill stages of the fish to the lung of the mammal, through all its many intermediate phases, discloses with astonishing clearness the course of this progress.

The Embryo as a Witness

Testimony from another source also stands undisputed. This corroboration comes from the manner in which all vertebrates are conceived and formed. The witness in this case is the embryo, which in all animals begins in the same way. Embryonic existence starts from a single cell. It holds true to the earliest beginnings of animal life that first appeared in a single cell such as the amœba. In the higher animals this cell is called the ovum. From it, after fertilization, two cells are derived, then four, then eight, then sixteen, until it has an appearance closely resembling some of the colonized protozoan animals. Here again, even in man, is seen that decisive stage in which a critical cellular distinction is made between outside and inside cells. From this time specializing progress in the growing individual goes forward. Each new phase repeats in a general way a stage of development previously attained in the evolution of life. All embryos of vertebrate animals pass through such phases. The fish embryo carries the process up to the stage characterized by those improvements which developed during the Age of Fish. The amphibian embryo takes the process one step farther. It adds new features essential to living on land. Embryos of reptiles and of birds introduce the progressive advancements peculiar to their kinds. The mammal embryo takes the final step, prior to which it passes successively through the several phases of the lower grades of life. The human embryo follows the mammalian plan and puts the finishing touches of development upon what the mammal has gained from all the stages below it. Fish, amphibians, reptiles--all have their beginning in a single cell. Regardless of the differences in body form, in mode of life, and in behaviour, all are cast in a mold of development based on a common design. Thus, while the blood, the bony system, the muscles, the teeth, the eliminating system, the heart, and the lungs tell the story of progressive development, the embryo gives a summary of this process by disclosing the general plan which underlies the manner in which every backboned animal is formed.

The brain contains a comprehensive record of this progress. There are reasons why this is the case. Brain influences pervade and dominate all other systems. This organ is the great transformer of energy, which so assembles other parts in operation that the body as a whole becomes a smoothly acting machine. It receives sensory impressions from its environment. It controls the reactions incited by these impressions. In this dual capacity the brain has been especially sensitive to those influences of change and adjustment, of action, reaction, and interaction that have affected animal life during its long existence. It has responded to these changes and has retained the impression of such responses. In many cases it has been structurally improved. Gradually it became capable of sensing the world more effectively. It acquired the capacity to react on a broader scale. Developing along certain progressive lines it has served to transform impressions received from the senses in such a way as to produce an increasingly more effective turnover of nervous energy. For this reason it is necessary for us to estimate the value of such senses as were utilized in this way. Without going too extensively into detail, it may be said that, with extremely few exceptions, vertebrate animals possess four chief varieties of sense. Each of these supplies the brain with stimuli necessary to its proper reaction.

Value of Our Senses

First, chemical sense, through special organs for smell and taste, conveys information concerning certain chemical conditions in the surroundings. The sense of smell derives its impressions from gaseous or volatile substances which, among other things, may create a pleasant or a disagreeable odour important in selecting food. The sense of taste gathers its information from substances in solution. It depends upon acid, sweet, bitter, salty, or other similar stimuli. The primitive headquarters for taste are in the hindbrain, while the endbrain serves in this capacity for the sense of smell.

Second, body sense furnishes information concerning what transpires within the body, as in the heart and lungs, in the stomach and intestines, and in other special organs. It also supplies equally important information concerning what contraction is occurring in the muscles, how the bones are being moved, what postures the different parts are assuming, and how the body as a whole is being balanced.

Third, contact sense makes known what is going on immediately outside the body. It depends upon many things which touch the body surfaces, such as the touch and pressure of a handclasp, the temperature of water upon the hand, the vibration of a heavy vehicle running over the ground. Body and contact senses had their original headquarters in the midbrain and interbrain.

Fourth, distance sense supplies information concerning objects in the world outside of the body more or less remote from it. The information which this sense brings is news from abroad. It is gathered by the sense of sight and the sense of hearing. Sight, in a way, is touch at a distance. When an animal sees its enemy a long way off it, so to speak, touches this enemy with its eyes and thus gives the brain the needed information while there is yet time for escape. Sight depends upon light waves, and hearing upon sound waves. By such means these two highly specialized agents of distance sense gather their information. The central offices of sight and hearing were at first situated in the midbrain.

All impressions obtained from these senses were and still are the raw materials utilized in the energy turnover produced by the brain.

Improvement was not always the result of the great struggle for adjustment. There were many ups and downs, many trials, many failures. Yet a certain insistent tendency toward progress was constantly in evidence. By means not entirely clear, this tendency ultimately succeeded in finding some way to become effective. It appears to have exerted its influence by selecting definite parts of the animal machinery for emphasis or repression.

Often some highly selective improvement was developed in the brain to meet special conditions. Such is the expansion in the bird’s brain by which the sense of sight is greatly amplified. This special increase makes it possible for the bird to see its prey from great distances in the air, as the hawk sees the fish in the water, or the vulture detects the presence of carrion by its keen eyesight. The sense of smell in birds is much less developed than vision.

In scenting animals, like the dog, the fox, and the cat, selective improvement has affected the sense of smell. In a few instances the addition of a relatively new sense was the means by which improvement manifested itself. Such an addition is seen in that transition when fish life first began to assume the characters of living upon land. At that juncture the sense of hearing was added in some amphibious animals belonging to the same class as the frog. These and other methods for getting a better supply of raw materials through the senses contributed to progressive development in the brain.

The Sense Combiner

Still more effective was the improvement which came as a new mechanism. It provided a special apparatus that may for convenience be called the “sense combiner.” The office of this mechanism was to assemble sense impressions in the brain, to make composite pictures of sight, hearing, taste, smell, and all other senses. This sense combiner served also as an effective depository for impressions already received. It held them in readiness for use as a background of experience that would be needed for new or subsequent situations. At a glance it is evident that the brain having the best sense combiner would outstrip all others in its efficiency and output. In the earliest vertebrates this new mechanism did not acquire a centralized headquarters. Its operations were controlled from several scattered stations in the brain. Obviously such division of responsibility could not be considered an efficient method of control. Centralization was needed, and certain stages in the development of the brain from fish to man illustrate how this improvement was gradually brought about.

The first or fish stage, as might be expected, expresses the beginning of this process of improvement in simplest terms. There are many who do not credit the fish with such a thing as a brain. These animals, however, are equipped with an effective organ of this kind. Its efficiency is not high according to human standards, yet, as we shall presently see, it has many characteristics of the human organ and reacts to similar stimuli.

In the fish brain there are nearly all of the working departments found in man. Much variation exists even among fish. Some of them have very simple brains. This is true of the earliest forms, but the more advanced types acquired brains thoroughly efficient for the special complexities of existence in which they had to live. The several departments in these brains are adjusted to their requirements. The sense of smell in the fish is particularly well developed. It has certain limitations, however, due to the fact that it must depend upon substances borne by the water. The department of this sense, nevertheless, occupies the major portion of what in these descriptions will be called the endbrain. The sense of taste is also well organized in fish. In certain of them, like the catfish, it has received special emphasis, because in addition to taste organs in the mouth there are organs of this kind scattered over the entire body from head to tail. The primitive central office of the sense of taste in fish is located in the hindbrain. Body sense is highly developed because most of the fishes are able to control their muscles and joints in an amazing way as they dart about in the water. Balancing of the body in swimming is another important problem in the locomotion of the fish. It is solved by means of certain highly specialized water levels (semicircular canals). The body sense department occupies the interbrain. The sense of sight in most fish is fairly well advanced, although it has distinct limitations. Being placed on the side of the head, each eye acts more or less independently of the other, and the fish, so to speak, gets a two-eyed picture of its surroundings. It will subsequently become clear that one of the most important events in the progress of the brain has been the development of that kind of vision in which both eyes receive the impression of an object at the same time. Then again, the medium in which the fish lives is in many respects less favourable for the passage of light rays than the air. The retina of the fish’s eye which first receives the light rays also indicates a relative simplicity in the organization of vision. For these and other reasons the fish’s sense of sight cannot be as effective as in the higher forms of life. This sense department is located in the midbrain.

Starting with the Fish

The fish stage in the development of the brain shows a striking deficiency in its lack of provision for a sense of hearing. Strictly speaking, fish have no ears. It is believed that the ability to hear which the human being possesses is denied to them. In still another respect, however, a more obvious deficiency makes itself apparent. The brain is poorly equipped in mechanisms that could specifically be called sense combiners. Some slight degree of combination between the senses does take place, but this at best is meagre and simple. Consequently the brain’s output, that is to say, its productive turnover, is limited. It confines itself to those reaction patterns with which we are familiar in the habits and behaviour of fish. The limitations by which these patterns are restricted are evident in the fact that the animal’s entire life programme is carried on largely under water. If an attempt were made to estimate the capabilities of the fish as a machine compared with other animals, it would almost certainly receive a low rating. The justification of this low estimate is obvious. The reasons for it are twofold: first, the relatively low degree of development in each of the sense departments including the lack in one department (sense of hearing); second, the poorly developed sense combiner.

Professor Gregory has devoted much time in the American Museum of Natural History to the study of the progressive stages from fish to man, and especially to those changes which appear in the head. He has shown that in this fish stage the animal at first had no lower jaw and no teeth. Its mouth served as a sucking organ, which thus obtained food in the form of minute organisms and small particles of organic matter. Certain new patterns were introduced with the appearance of primitive sharks. These animals had a lower jaw impregnated with lime salts, thus made effective for supporting many successive rows of formidable teeth. Such sharks also had well-developed gills. Certain lobe-finned fishes of a somewhat later period (Crossopterygian) began to live in streams and swamps. By means of their peculiar fins they were able to crawl over the surface of the land, and thus they were the forerunners of the next more completely air-breathing stage determined by the appearance of the amphibians.

The Beginning of Life on Land

The second or amphibian stage came after those steps had been taken which led certain modified forms of fish life to attempt a partial adjustment to living on land and to breathing air. True amphibians then made their appearance. Animals called tetrapods, or four-footed creatures, were the result of this change. They were the forerunners of all higher animals. By the slow conversion of their fins and paddles into legs they acquired a new kind of transportation machinery. With the aid of these four legs the animal could now hop about on land and also swim in the water much as do the frogs. Such a transformation had a profound effect upon the entire body, which became greatly shortened and in many instances no longer possessed a tail (except in the polliwog stage). The head also changed. New devices were necessary for the purposes of air-breathing, which replaced the old method of getting oxygen out of the water. One of the most important changes, however, was the addition of the new sense of hearing. The amphibians, living partly on land, were now able to receive useful information by means of air waves. The advent of this new sense was destined to have momentous effects upon the further development of the brain. Each of the several sense departments is well represented in the frog. The sense of smell is highly organized. It contains some improvements over the fish for the reason that the animal is now able to scent odours borne by the air. The sense of taste shows little if any improvement. Compared with many of the fish it has actually receded. Body sense is well provided for and shows certain refinements due to the fact that it has taken on the new responsibility of sensing four legs. It also has the duty of supervising what is going on in the muscular machine when the animal performs its new kind of motion, hopping about over the ground, leaping into the water, or using the new frog-method of swimming. The department of the sense of sight shows some improvements when contrasted with that of the fish. The frog is able to adjust its vision both to air and water. While on land it is able to see many things that never come into the range of the fish’s field of vision. Some of the frogs even go so far as to have what is called a third eye in the middle of the forehead. This organ, however, is but poorly developed and serves more for light perception than for actual seeing. The introduction of the sense of hearing, by establishing certain innovations in the frog brain, provides an advantage over the fish. It is, however, in furthering the development of the sense combiner that the frog’s brain shows its most distinctive advance. The two great hemispheres are now clearly outlined. The endbrain, in consequence of land-living and air-breathing, has taken an important step forward. In all further advances this part will bear the chief burdens of progress and improvement.

The frog and his kind represent a machine that in many respects is not much better organized than the fish. But amphibians did serve to introduce advantages that were utilized in new adjustments to life; such, for example, as living on land, breathing air, getting about on four legs, and being able to hear. Besides this, the way was now opened for a better type of sense combiner. There was promise, if not actual profit, in these new amphibian endowments. Professor Gregory has shown that among the most important changes in the amphibian head were those which ultimately led to the formation of the ear. The skin in this region was already beginning to act as a tympanic member or eardrum.

Epoch of Giant Reptiles

The third or reptile stage witnessed that critical advance that came with the fully established habit of living on land. The amphibians, both those which retained and those which lost the tail, took the first somewhat hesitating steps in this direction. They were, however, essential predecessors to the next higher order, the reptiles, which upon their arrival stepped out boldly. During the remarkable Mesozoic period these reptiles covered the earth with their dominating and often hideous presence. No period compares with this one for the awe-inspiring inhabitants that peopled the world. It was then that the gigantic dinosaurs were the overlords of creation. Some of these monstrous creatures were composed of many tons of flesh and bone. They became the most terrific fighting machines ever produced by nature. Even the tail, which had disappeared in many of the amphibians, became prominent as part of the offensive equipment in these reptile monsters. Gigantic size was an outstanding structural feature. But these huge dimensions carried their own penalties. They were extremely hazardous and destined to bring catastrophe. Even if some of the great reptiles might have been thoroughly efficient fighting machines, they lacked the essential advantages of progressive brains and brain power. In this respect they had improved but little. That tremendous monster Tyrannosaurus rex, the most destructive engine ever created, had a body weighing many tons, with a brain of less than a pound.

The prolific Mesozoic reptiles inhabited the land and infested the waters of the earth, its oceans and inland seas, its lakes and rivers. They also for the first time attempted to realize the advantages of another mode of life. Having adjusted their weird bodies to the water and to the land, they next took to the air. Late in the Permian or Triassic times (150,000,000 years ago) some lizard-like reptiles, partially biped in habit and distantly related to the great two-legged dinosaurs, assumed habits of life adapted in part to the trees. Specialization of their fore limbs led to wing-like structures for purposes of volplaning to the ground. Such modified fore limbs eventually acquired the character of wings, and thus, according to some authorities, the most ancient of known birds had their origin in the Age of Reptiles. Many students of this subject believe that bird life may have begun at an even earlier period.

More conservative and also far less conspicuous was another tendency which developed in this reptilian age. For a long time it remained most unpretentious. The spectacular development of huge animals for land and sea held the centre of the stage. Mere size, however, is not always sufficient for success and progress. In any event, a certain number of relatively small reptiles began to show changes along entirely different lines. At first it was difficult to discern the signs of progress in them. Slowly, however, significant modifications came about in two important details: First, in the readjustment of the fore and hind legs, so that acting together they began to lift the body of the animal clear off the ground. The second great change was an alteration in the teeth, which were gradually specialized until they assumed the characters recognized in those later animals known as mammals. These two new traits, developed by relatively inconspicuous reptiles, led in time to animals that became the actual forerunners of the mammals. They are known as the pro-mammalian reptiles (Cynodont, Theriodont).

Reptile Forerunners of the Mammals

It is probable that while these momentous changes were in process an equally important modification had begun. This change affected the blood. It caused the blood cells to become smaller and at the same time better conveyers of oxygen. These cells also began to lose their nuclei. As a result, certain animals passed from a cold-blooded, scaly reptilian condition to that of the warm-blooded, hair-covered mammal. The constant warm temperature of the blood in these mammalian forerunners must have been a decisive influence favouring the further development of the brain.

In many respects the reptilian brain is inferior to that of the mammals. All of its sense departments are fairly well represented. The senses of smell and taste have made slight advances over the amphibian stage. Body and contact senses have perhaps gained some slight advantage over the previous period. In sight and hearing there were some improvements. Collectively the reptilian mechanisms for managing impressions obtained through the senses are considerably better than those of such animals as the amphibious frog. At least one of the reptiles (Sphenodon) developed a third eye in the middle of the forehead. This is not, however, a highly efficient visual organ. The sense combiner in the reptile also shows some advantage, although in the main the reptilians appear to have acquired little more of practical value, except greater speed and more power, than their predecessors, the amphibians.

Even when reptile development took that bent which led to the appearance of birds, the brain received but a slight benefit from this adjustment to the air. Selective progress in the bird’s brain is unquestionably found in that marked expansion involving the department of sight. Body sense also expanded to meet the requirements of sensing and balancing the body in flight. But to offset these advances both the sense of smell and the sense of taste have undergone considerable recession. Adaptive progress here, as in many other instances, emphasized one department with some apparent loss of advantage in other parts. Consequently the sense combiner, which ultimately produces the most effective combinations of sense impressions, has shown no conspicuous advantage among the birds.

Disappearance of the Great Reptiles

The reptile stage of life, especially in its most imposing phases, witnessed but little advance in the progressive development of the brain. During this period all of the great departments of brain structure, such as the endbrain, the interbrain, the midbrain, and the hindbrain, were retained and somewhat expanded. But that highly important mechanism that was finally to act as the superbrain, technically known as the neopallium (new outer coating of the brain, the cortex), had not yet been acquired. It may be in part for this reason that, as the Mesozoic period advanced, catastrophe was rapidly overtaking many of the great reptilian groups. Of the eighteen orders of reptiles that once filled the world, all but five were mysteriously swept into oblivion. Why they passed is not yet clear. It may have been due to great changes in the surface and climate of the earth at different times. It may have been that the gigantic size of these reptiles made the struggle for existence too severe or the food supply too precarious. Whatever the cause, they all seem to have paid the penalty of excessive specialization. The five orders which have survived these destructive catastrophes include the snakes, the crocodiles, the lizards, the turtles, and the lizard-like tuateras of New Zealand.

Notwithstanding this wholesale destruction, there was a priceless heritage handed down from the Age of Reptiles. This heirloom was the beginning of the warm-blooded mammal, which slowly developed from the humble pro-mammalian reptiles. It endowed the animals that were to rule the next great period of the earth’s history with power to get about on four feet, with increased ability to withstand great changes of climate, with added capacities in preparing their food for digestion. This last advantage depended upon a new kind of teeth which the mammals inherited from their immediate reptilian ancestors. All of the teeth possessed by primitive reptiles were fang-like (laniary), used for seizing their prey or tearing their food. These reptiles had no grinding teeth, and this condition left the responsibility of digestion to the stomach and other organs. In most of the mammals digestion begins in the mouth with actual mastication. The early pro-mammalian reptiles (Cynodonts) were equipped with grinding teeth, and their dental apparatus, as in all mammals, included incisors, canines, pre-molars, and molars. Teeth such as these were important items in the legacy received by the mammals from their ancestors, the pro-mammalian reptiles.

When the Warm-blooded Mammal Appeared

In the fourth or mammalian stage, life entered upon the Age of Mammals with all of these new endowments. Almost at once it began to show signs of progress. It was in the brain that this progress became most apparent. A new mechanism long in the making now came into existence. This new structure may be rightly called the superbrain (neopallium), since it soon proved to be the most decisive step yet taken in the development of the sense combiner and in the further expansion of all the senses. At first it did not make its appearance in any preëminent manner. It came as an outer covering over the ancient parts of the endbrain. Within it, however, were possibilities of expansion such as were possessed by no other part of the brain. Ultimately it added about twelve billion cells to be used in many different kinds of brain activity. This addition was especially characterized by the orderly arrangement of the cells, layer upon layer, almost as if each successive layer imparted some new capacity for the management of life. In its fully developed form this structure constitutes the cortex of the hemispheres, and with its fibre connections makes up as much as eighty per cent. of the entire brain.

It could hardly be expected, even after the first arrival of the mammals, that this new brain addition would at once attain its fullest development. In fact, the first attempts along this line were feeble. A new and great production of weird mammals was in process. It might almost seem as if the imposing shadows of the previous Age of Reptiles still hung over these early mammalian experiments. Huge, ungainly proportions were still the fashion. In many instances the primitive mammals themselves developed gigantic and awkward bodies. They were strange, unsightly beasts as we know them now from their fossilized skeletons and from reconstructions of them. Were it possible to reassemble them, what a sensation they would create in our modern world. Even the best efforts of our foremost showmen would be ineffective to describe those strange monsters of most unfamiliar appearance, with their peculiar armours, their long unsightly horns and tusks, their strange hoofs and claws. The mammoth, the mastodon, the amblypod, the titanothere, the creodont, the sabre-toothed tiger, and many others would be among them to excite wonder.

The Paths to Extinction and Progress

But all of these have passed, in part at least, because, like the dinosaurs, they possessed inferior or unprogressive brains. Indeed, many of the earliest mammals had brains that in some particulars resembled those of the reptiles. They grew in size and power until they became repulsive brutes, although their brains improved but little. In many of them the superbrain developed only in a small way. It was notable not for its size but for the position it occupied above more ancient structures. In their struggle for life these huge beasts seemed to be unable to adjust themselves to changing environment; so probably when the conditions became too severe, not having the capacity to adapt themselves, they failed to survive. Many orders of these animals became extinct in the early part of the Age of Mammals (Oligocene and Eocene, thirty million to sixty-five million years ago). Others, showing more progressive tendencies, continued to advance, and their descendants have come down into modern times. One striking difference between these progressive and unprogressive mammals was certainly in the brain. Wherever this organ remained primitive, wherever the superbrain was only feebly developed, the fate of extinction seems to have been a foregone conclusion. Such animals soon reached the end of their line. But wherever the superbrain expanded, there the signs of progress were unmistakable. One extremely important factor in the survival of most of the mammals alive to-day was the progressive development in the most recently acquired portion of the brain. Great practical results were brought about by its expansion in the administration of brain power. It produced, so to speak, the final consolidation of all the sense departments under one roof. Reactions connected with the sense of smell and of taste, which had so long depended upon the primitive endbrain, marked this structure as the most advantageous location for centralization. Whatever may have been the influences that established this preference, here the departments of body and contact senses, of sight and hearing, were finally organized. The effects of this consolidation were immediately felt by the endbrain. It at once became a superbrain in the truest sense. Rapid expansions in the actual size of the hemispheres were the first signs of this new development. Then came the process of convolution and folding to obtain more brain room, and this for the same reason was followed by still more complex convoluting. These advantages especially favoured contact sense, the expansion of which was largely due to the fact that the mammal body was now covered with a highly sensitive skin equipped with hair. Such a skin was a new sensory device by which finer impressions of touch might be conveyed to the brain. In this manner the animal was able to form more complete judgments concerning objects with which it came in contact. Little by little, these judgments of touch became more critical and discriminating. A great range of understanding of the world through touch sense was made available. One critical impression of touch was added to another until complex judgments in this sense were constructed. Similar expansions in the powers of vision, hearing, and body sense led to their localization in this new part of the brain. Their most effective activity soon required still further extension, which ultimately, by the development of the frontal lobe, made provision for the highest faculties. The mammals have thus shown their progressive tendency in the acquisition of an efficient sense combiner. Through their better sense capacities they have been able to understand their surroundings more thoroughly than lower animals. Consequently their energy turnover in the brain has resulted in a better output by means of which they have made more ample adjustments to life. All of this they have been able to accomplish because they possessed a mechanism of incalculable value, the superbrain. Yet the mammals have not in all cases utilized this mechanism to its full extent. Its advantages have been applied in different ways and for different purposes. In some instances they have been utilized for the special adjustments of the hoofed animals, or in the hunting craft of the great meat-eaters, or in that furtiveness of the moles, which seek their protection by burrowing in the ground. The advantages of the superbrain were applied to many other diverse specializations, such as the adjustments of bats for flying, or of beavers, seals, whales, and porpoises for living in the water.

The Superior Brain of Mammals

The mammalian brain has made possible a wide range of behaviour and adjustment. This range exceeds that of the fish, amphibian, reptile, or bird. Concerning the increased capacity of the mammals as a class there seems to be no doubt. But this greater power of adaptability is also true of every mammal. The differences in this respect between the lower mammals, like the rat, the opossum, or the sloth, when compared with the bird, the snake, the frog, or even the fish, may not be striking. But when we contrast the actions and capabilities of such mammals as dogs, horses, elephants, or any of the cat family with those of the bird or snake the vast differences speak for themselves. A dog, for example, has by comparison with lower vertebrates a greatly increased capacity for getting on in life. He is capable of adapting himself to many complications incident to his associations with man. He has a much more ample repertoire of performances. He is capable of learning many intricate accomplishments. In general, such learning is also true of most of the higher mammals; it is particularly true of those having a highly developed superbrain. Even aquatic mammals like the seals show a remarkable degree of adaptability. They are among the most interesting of trained performers. A casual glance is sufficient to reveal what an excellent superbrain they possess. Elephants, in spite of their huge proportions and awkwardness, are capable of remarkable adjustments. Their brains are also highly developed.

Yet, however decisive the mammalian superiority in brain power may be over the lower vertebrates, most of the mammals are held down by many handicaps, restrictions, and limitations. They all possess a capacity for broad adjustments to strictly limited conditions. For life in the water, in the air, upon the plains, underground, or in the forest, they may be well adapted. But the specializations of their own bodies hold them to their specifically restricted adjustments. With trunk and head, with hoof and paw, with wing and flipper, they may do the things which these implements make possible. Here their opportunities cease. In this way even the progressive mammals are confronted by serious obstacles. These mammalian obstacles were difficult to overcome. Some of the mammals, however, became specialized for a more varied kind of life. They manifested a strong tendency to live chiefly in the trees. This fact influenced their further adjustments profoundly. It opened the way for new specializations in their limbs. It gave a new direction to progress, which finally called upon the brain for its supreme development. These important tree-living animals are the monkey kind and the manlike apes. All of the events in adjustment preceding this great epoch might be likened diagrammatically to a succession of plateaus. Each plateau, beginning with that of the fish, then rising to the level of the amphibians, of the reptiles, and finally of the mammals, contributed some important elements to progress. From these at length came the upper level of the apes, that plateau destined to give rise to many varieties of primates, and also to afford those footholds essential to the further upward climb of man.