CHAPTER XII. Implements of Human Success

IMPLEMENTS OF HUMAN SUCCESS

HOW THE HAND, FOOT, AND BRAIN LED THE WAY TO HUMANITY

It is not sufficient to know that the brain began as a simple organ and gradually became more complex. Sooner or later we must learn the reasons why it made this progress. At present we are able to identify some of the essential principles underlying brain development, yet with few exceptions the exact causes are still obscure. We may feel certain, however, that the progressive advances were due to the accumulation of slight changes which, modifying brain structure ever so little, ultimately made it more highly effective. Such changes in the different parts of the body are the result of a complex interplay of influences acting upon the animal as a whole. The brain has been particularly responsive to this interplay. It has at the same time been thoroughly conservative. Throughout all its wide range of variation it has maintained its basic designs. If readjustment of the body to certain conditions has resulted in the depreciation of a special part, such as the eye, the structure of the brain shows corresponding depreciations. The principle of compensation has also been at work. The power which may be depreciated or lost in one department is, to some degree at least, compensated for by others. An illustration of this compensatory power is afforded by the mole. This animal lives a burrowing life beneath the ground. Light rays do not reach it, and it therefore has no need for vision. In consequence, its eyes do not develop the function of sight. Its senses of touch and hearing, however, are greatly amplified, and the structure of its brain gives evidence of this compensatory readjustment.

Signs of the close relation between the brain and the parts which it controls may be found in many organs of the body. In some instances these signs are outspoken; in others they are less clear. It is much easier to find evidence of this correlation in those parts which play a conspicuous rôle in life. The arms and legs, the eyes and ears, are particularly good examples. Modifications which have affected these parts are distinctly reflected in the brain. If more brain power is required for their better operation, more ample provision is made for them in brain structure.

Relation of One Part of Body to Another

It is a debated question whether the brain or the external part of the body takes the lead in progressive modifications. Some authorities believe that all advances of this kind are dictated by development in the brain. Others ascribe the determining influence to the external part. For the present it seems wiser to consider these modifications as simultaneous, as affecting the external part and the brain together. Certain dangers arise from regarding the body as divided too strictly into definite parts. Such a division has advantages for purposes of description, but it may tend to obscure the important fact that life is carried on by the body acting as a whole. In this light the division between external part and the portion of the brain controlling it establishes an artificial distinction. Viewed in the light of purposeful life, one is of little use without the other. Both external part and the portion of the brain controlling it establish a special unit which, coöperating with all other special units, carry on the process of living.

This view is known as the organismal conception of life. It estimates the entire animal not as a collection of different parts but as a combination which makes life possible. According to this conception the external structure (arm, leg, eye, ear, etc.) and the portion of the brain controlling it form an operating part of the whole. Modifications in the one are reflected in the other. They cause mutual reactions. When eyes are developed for different kinds of vision, corresponding provisions are made for them in the brain. When legs are specialized for various kinds of locomotion, brain structure adapts itself accordingly.

It is important to realize what the eyes and the ears and the organ of smell have contributed to the progressive advance of the brain. In all of these organs there is a marked constancy and sameness among animals possessing them. Structures presenting a greater variety of form might have even greater pertinence. It therefore is a more leading question to ask what relation the brain bears to the extremities, to the fore and hind legs, to the hands and feet.

History of the Hand and Foot

There is a long history of progressive change back of the hand and foot. In their development they emerged from more simple structures connected with the ends of the limbs in certain four-legged animals. Because they are attached to the limbs in this way, they have played an important rôle in one of the chief activities of life--locomotion. The fore and hind legs act as a series of levers. They are moved by muscles and in this way make transportation possible. Consequently the modifications in the ends of the limbs in response to special types of locomotion have a most important bearing upon the life of the animal and thus upon the brain.

In animals living upon land such parts of the limbs as touch the ground are modified by many factors; thus the weight of the body, the speed of movement, and the kind of locomotion would all exert their modifying influence. Limbs of several different designs have thus been produced. Heavy animals, like horses and cattle, which require speed and endurance for long journeys, need hoofs. Still larger hoofs were developed by heavier animals, like the elephant and rhinoceros. The paw was the design utilized by animals like cats and dogs. Their bodies were not so heavy as those of horses and cattle. They were capable of great speed and needed sharp nails on their paws to hold the ground in running and springing. These talon-like nails they also used for defending themselves or in capturing their prey, as do the lion, tiger, leopard, and bear. The paw is a more flexible implement than the hoof. It provides a soft, elastic pad by means of which the animal touches the ground. In animals like the seal, walrus, and sea lion the flipper is the design utilized. Here the digits are connected by means of a web. The wing is the specialization in such animals as the bat whose transportation depends upon flight through the air.

Locomotor Devices

These various devices for moving the body about on the land, in the water, or through the air have been developed by mammals. By such contrivances they are enabled to subsist, each according to its own mode of living. Some of them have returned to a life in the water. The result of aquatic habits in mammals is extremely interesting. The flippers of seals, walruses, and sea lions equip these animals to swim with great ease and speed. They enable them to clamber about on the rocky coast by the edge of the sea, or upon the ice fields of the arctic regions. Because of its apparent limitations, such a life held little prospect for developing the powers of higher intelligence. A flipper is in no sense an efficient implement by means of which to acquire a superior position in the world. The seals and all of their kind, therefore, offer little promise of progress. They are capable of astonishing proficiency in the control of their neck muscles and movements of their heads, but this at best is a meagre advantage. They are somewhat better off than another group of mammals which took to the water, namely; porpoises and whales. Nothing in the equipment of these animals could serve as efficient instruments by which to gain a preëminent place in nature.

By developing wings in connection with their limbs the bats were also excluded from the lines of higher progress. However effective they are in flight, their wings could not be made to serve constructive purposes.

Animals with hoofs, such as horses and cattle, elephants and rhinoceroses, acquired solid and reliable feet for withstanding the heavy strain which their speed and weight imposed upon them. Hoofs, however, are far from ideal as universal instruments. Although sufficient for the work they have to do, they cannot be utilized for purposes other than those of transportation except, in a certain minor way, for offensive and defensive tactics. In these animals all of the digits are either bound together in one large supporting pad, as in the elephant, or are encased by a horny covering, as in cattle and deer. In the modern horse but one digit persists, and this is surrounded by a heavy, horny hoof. Such an implement would not require a highly specialized endowment of brain power for its control.

The daily programme of these animals, limited largely to transportation, calls for no constructive ability and no intentionally destructive one. The hoofed animals possess no means for accumulating or storing food in preparation against a day of need. They are forced to move from place to place in order to find their browsing and grazing lands. They cannot stand against great changes of climate or season. They must flee before the advance of winter as well as from their enemies. The hoof for this reason offered little promise for the development of a more efficient kind of instrument. Such hoofed animals as also possess a trunk developed an accessory organ of much value. It is doubtless an important factor in the high specialization of the elephant’s brain. Even this flexible instrument, however, has its decided limitations.

All of these mammals, whether hoofed, flippered, or winged, have failed to develop a brain of superior qualities. In no instance is it an organ capable of a high degree of learning or intricate control of life. The hoof of the horse, cattle, deer, elephant, rhinoceros, and the like set the stamp of the wild upon these animals. This is the keynote of their behaviour. Flipper and wing are equally indicative of inferior qualifications in so far as efficiency and brain power are concerned. There may be sufficient reasons for placing these mammals in the same bracket with man in the great classes of the animal kingdom. Their inferiorities are apparent, however, when their intelligence is estimated by human standards. It is then clear how far below the human level of brain power they are.

The Paw in Relation to Hand and Foot

In our search for animals capable of a greater range of adjustments we will find another group with a much more promising locomotor equipment. This group comprises those mammals possessing paws, such as dogs, cats, bears, rats, squirrels, and the like. In itself the paw is a most flexible implement susceptible to many modifications. It possesses five distinct finger-like processes or digits, each of which is capable of some degree of individual movement. The digits may be spread out or drawn together; they may be folded or extended. In every typical paw there are eighteen movable joints, each of which is capable of some independent motion. Twenty-five muscles make more than seventy separate movements possible. These figures afford some idea of what a complex structure the paw is. Attached to the extremity of each digit is a sharp claw-like nail, beneath which an enlargement in the skin forms a prominent “tip pad.” Over each of these pads the skin is arranged in ridges. The ridges roughen the surface and produce what is called “friction skin.” The roughened skin and the claws at the end of the digits give the animal better ground-gripping powers. In addition to the tip pad, each typical paw has four enlargements where the digits come together. These are the “palm” and “sole” pads. They are likewise covered with ridged friction skin. The paw terminates in the wrist or ankle, and at this junction there are two enlargements called respectively the “wrist” and “ankle” pads. They are also covered with friction skin.

This design of paw with its separate digits, its claw-like nails, and its eleven pads affords an especially adaptable structure from which to create many different kinds of useful implements. In the gnawing animals, like the rats and squirrels, the paw is developed particularly for running and climbing. The long sharp claws serve the purpose of spurs which, as in the case of the squirrel, may be driven into the bark of trees. All of the pads in the paw come in contact with the surface over which the animal is moving, thus giving information concerning its support and aiding its transportation.

In moles and burrowing animals the hind paw retains its usual features, while the fore paw is converted into something resembling a shovel. The paw becomes broad and flat, particularly in the moles, and there is no suggestion of any of its pads. Since this specialization is adapted principally for digging underground, little could be expected in the way of high attainment for animals of this kind. Their burrowing capacity is excellent, but this is the extent of their ability.

Special Uses of the Paw

In the meat-eating animals, like the dog and the cat, the individual digits and the claws are somewhat shorter, but their most important modification is the fusion of the paw pads and the reduction in the first digit. This change is a specialization for their more springy type of locomotion. Such animals run on the tips of the digits, using especially the second, third, and fourth digits. The paw pads usually fuse to form one or two which serve to increase the spring of the animal. The fore limb of the rat may be accepted as the working model, because it has all of the general features that make up a typical paw. It provides for running, climbing, clinging, and clawing. When compared with the paw of a mole, the modifications necessary for a good digging implement are clearly seen. The pads are no longer needed and might, as a matter of fact, be in the way. The digits are shorter and the whole hand is broader and more scoop-like. The paw of the mole is modified for the work it has to do and has lost many of the structures necessary for ordinary locomotion over the ground. Long claws are no longer essential for climbing or clinging, and the nails have been converted into burrowing ground-breakers. The rabbit and the guinea pig show changes in the fore paw necessary for rapid transportation in a kind of jumping locomotion. They have lost the specializations in the paw necessary for climbing. The nails and the digits are less long and somewhat heavier. The squirrel, on the other hand, has a fore paw specialized for climbing trees. This modification has emphasized the length of the individual digit and particularly the length and sharpness of the claws. Often the squirrel may be seen sitting upon its haunches holding between its fore paws a nut, the shell of which it is attempting to crack with its teeth. Such grasping power is not found in the paws of animals specialized for running and jumping solely. The squirrel’s modification of the front paw is extremely important. It reveals how the animal’s life in the tree has lengthened the digits as well as the nails. Some degree of power for grasping small objects has come through this lengthening. The fore paw of a cat compared with that of a dog illustrates other important specializations. Both of these animals are strong runners. In running they travel along on the tips of the digits. For this reason the tip pads and the friction skin over them have become highly developed for ground-gripping purposes. The paw pads and the wrist pads have tended to fuse in order to give an elastic surface necessary for that springy gait determined by running on the tips of the digits. The individual digits are somewhat longer in the cat than in the dog. The claw-like nail of all the cat family is one of their distinguishing features. By means of these claws they are able to climb trees, which is a provision of great service in procuring food. Dogs, on the other hand, have short digits, with thick, heavy nails suited more as spikes in running but not adapted to climbing. In many of the great cats, like the leopard, climbing trees is an essential part of their hunting strategy. For this reason they require long, sharp claws, which may also be used as weapons in attacking their prey. The long claws of the bear likewise indicate a modification of the fore paw in adjustment to the animal’s climbing propensities. The great weight of the bear makes it necessary for it to have these long spur-like claws in order to get a proper grip on the bark of a tree when climbing.

Transformation from Paw to Hand

Illustrations of this kind might be multiplied to show that in all animals having paws these implements have been modified in one way or another to suit the kind of work they have to do. In the main, this work is transportation. But there are many special problems in the different kinds of transportation. There are also numerous other adjustments to life that are capable of producing profound modification in the paws. From such facts as these it must be clear that the paw has been serviceable as the basis for developing instruments suited to many special purposes. One prominent feature in the several modifications of the fore paw is the effect which climbing has had upon the length of the digits and upon the length of the claw-like nails. In the rat and particularly in the squirrel these effects of climbing are especially distinct. When climbing at length became a dominant factor in the life and livelihood of the animal, certain still more decisive modifications were produced in the paws. We may now endeavour to gain some idea of that important transformation which occurred when certain groups of animals took up more or less permanent life in the trees. These mammals were representative of the monkey kind. They did not resort to tree climbing as many others have done as an expedient in hunting or in escaping from their enemies. The trees became their abodes. Many changes were induced by this new adjustment to life, changes which affected the muscles and bones and even the skin. During the process of this adjustment certain ridges upon the skin in the palm of the hand and sole of the foot began to show marked changes, probably because they were in such immediate and constant contact with the branches of the trees. In their basic designs these ridges which form the friction skin may be traced back to the simplest of pawed animals. Their successive modifications offer one of the most certain guides in following the stages through which the hand emerged from the paw.

Each ridge upon the skin of the paw (chiridium) is an elevation of the superficial layer which contains, at regular intervals, the mouths of minute canals coming from sweat glands. In its simplest form each sweat gland in regions of the skin not covered by hair (sole of the foot and palm of the hand) consists of a mound-like elevation in the centre of which is the mouth of a sweat duct. With the higher development of the skin, numbers of these little mounds ran together in rows thus forming the friction ridges. Depending upon the pressure and the kind of contact made with the ground or other surface, the ridges of the skin are arranged either in concentric circles, in ellipses, or in parallel lines. They serve two useful purposes: First, they roughen the surface so that it can grip the ground more effectively; second, by the continuous secretion of fluid from the sweat glands, they keep the skin soft, pliable, and sensitive. In this last particular, namely, the sensitiveness of the skin, the ridges also serve in another capacity. They provide proper locations for nerve endings, necessary to the sense of touch in all of its various modifications. Thus the paws in the more minute architecture of their skin pads and friction ridges afford highly pliable and sensitive instruments by means of which different kinds of mammals are able to adjust themselves in a great variety of ways.

After many intermediate stages of transition the fore paw assumed the appearance of a hand. Simultaneous with this change the hind paw also began to manifest many hand-like characters. Potent factors were at work determining this important transformation. Their influences were decisive not alone because they changed the paw into a hand but because they instituted equally profound changes in the structure of the brain. Such modifications as these brought about many adjustments to life destined to be the special determinants of human behaviour. One of the first changes to occur in transforming a front paw into a hand was the direct result of arboreal life. This modification consisted of a decisive lengthening of the digits, particularly the second, third, fourth, and fifth digits. In this way the fingers were formed. The first digit which ultimately became the thumb did not lengthen to the same degree as the other four. The chief influence in producing this lengthening to form fingers arose from the need of a firm grasp upon the branches. Its effects appear in the simplest monkeys, such as tarsius. The small hand of this animal has four long fingers and a diminutive thumb, all of which are well adapted to encircling and grasping a cylindrical branch. Another important transitional feature is the flattening in the ball of each digit. In tarsius each finger tip has a disk-like appearance. This is an extreme development. It produces what in effect is a suction pad on the tip of the finger not unlike that observed in some of the frogs (Hyladæ). Such suction pads enable the animal to strengthen its grasp upon the bark. The flattening of the finger tips due to the pressure required in grasping the limb of a tree produced a third great change. It caused a corresponding flattening of the back of the finger tip and thus developed a broad, flat finger nail to replace the sharp, claw-like nail of the cat, rat, and other similar mammals.

The Hand of Tarsius and Lemur

The three changes observed in the most primitive of the monkey kind (Tarsius) comprise the pronounced lengthening of the fingers, the flattening of the finger tips, and the flattening of the finger nails. These transformations are easily understood in connection with the necessity of grasping cylindrical branches. In other words, a prehensile hand came into existence as a result of living in the trees, and a new kind of instrument made its appearance in relation with the upper extremity. The need of a firm grasp on the branches was the fundamental cause of this modification of the paw. It had far-reaching effects because it created the facility to grasp many other objects and thus struck the keynote of those further developments which ultimately gave rise to the grasping hand of man.

All of the pads covered by friction skin which are characteristic of lower mammals like the rat and the squirrel may be identified in tarsius. The tip pads are somewhat changed to form the suction disks. The palm pads, four in number, occupy their usual position in the angle between the digits. The wrist pads, two in number, are well developed. By means of these elastic cushions the animal makes its contacts with the branches.

Transition from paw to hand is still more pronounced in the lemurs. These animals in many ways stand lower in the scale than tarsius. In them the lengthening of the digits to form real fingers, the marked development of the thumb, the appearance of friction pads, and broad, flat finger nails are all prominent. The index finger shows certain variations in its development. In other respects these lowly members of the monkey kind manifest definite progress in the change from paw to hand.

The Interesting Case of the Marmoset

At this point it is interesting to consider the case of the marmosets. Here the progress which the paw had made toward a more effective structural instrument encountered a serious setback. The hand of these little animals, in a general way, has much that resembles a paw. Although it has long fingers and a prominent thumb, there is an evident slipping backward. The claw-like finger nails suggest an actual retrogression in the process of developing a hand. If the marmosets were actual backsliders, other monkeys of the New World were particularly progressive. They developed hands which are extremely human in appearance. Their long, tapering fingers have broad, flat nails. Their thumbs are fairly well formed. Their finger and palm pads have characteristic appearances. This interesting group of South American monkeys show in a most striking manner those changes which life in the trees has brought about in the fore paw. Such modifications are especially significant because of their influence upon the behaviour of those animals which have taken up a permanent arboreal life. They have also made a deep impression upon the structure of the brain. The transition from a running, ground-living animal to the simpler arboreal forms is foreshadowed in the lemur’s hand. In many respects this transition stands just upon the border line. Its apparent indecisiveness is recorded in the brain, for the lemur retains many of the ancient brain features created by older ground-living habits. At the same time, it indicates certain adventurous attempts to break away from the earth and ascend into the trees. The grooves of the brain show this new departure particularly well. They retain their strong family resemblances inherited through long ages of four-legged ancestors. But added to this they manifest a tendency to assume the characters which in due course would lift their successors farther from the ground and into a more erect posture.

Appearance of the Hand-like Foot

Up to this point attention has been centred upon the important changes which attended the transition from paw to hand. Equally momentous were the modifications in the hind paws which resulted in hand-like feet. This transformation slowly altered the digits, the claw-like nails, and the friction pads. It modified all of these parts in such a way as to produce better limb-gripping instruments. A great change in transportation had taken place. Running over the ground in easy, secure fashion now gave place to the more hazardous method of climbing among the branches of trees. A dependable grip was the prime need. This capacity required long toes with which to encircle the branches, a powerful sole, and a great toe with strong grasping power. The four-legged animals that travel over the ground on various kinds of paws support the weight of the body on two main arches of the foot. One arch consists of an elastic span between the tip and the sole pads. The other arch extends between the sole and ankle pads. Generally speaking, those animals living on the ground first strike the surface at each step on the tip pads of the four outer toes. As the full weight of the body is accepted by the hind paw, the sole pads touch the ground. Last and most lightly, the ankle pads in the region of the heels rest on the supporting surface. In many running animals of this kind the heel touches the ground infrequently. Their running and walking in consequence have a springy quality that prepares them for a quick bounding start at an instant’s notice.

Strong Grasping Powers

Animals like the rabbit and kangaroo possess hind legs that work together, while the fore limbs are put forward first one and then the other. The most effective type of transportation in animals possessing paws has developed a gait in which the action of the hind leg of one side follows the action of the fore leg of the opposite side. This is the manner in which the dog runs. It is also true of all members belonging to the great cat family. The hind paw is put down in the footprint of the opposite fore paw. Apparently there is no deliberate supervision of this action which seems to be wholly automatic in its nature. To a great extent, however, this automatic regularity in the hind legs ceased when the four-handed animals came into existence and began to live in the trees. The problem then was a totally different one. It was not necessary for these animals to be on their toes every moment. They did not require the powerful spring formed by the two arches in the sole of the foot. Their chief necessity was a foot that would have the grasping powers of a strong hand. In this way they could make sure of seizing the branches securely.

The first digit of the foot, which in most pawed animals often fails to develop, became of greatest service to the monkeys. In most of them the great toe offers an added means for securing a firm grasp. It may be extended behind the branch while the other toes encircle it and all working together produce a firm grip not unlike a wrench on a pipe. The need of a long lever extending from the tip of the toes back to the heel, essential to the springy gait of the ordinary pawed animal, is not so strongly felt in arboreal life. In fact, a foot which is too long may be an actual disadvantage, while one facilitating the best kind of gripping power would necessarily require a shortening from toe to heel. This was the change which took place in the early beginning of tree life.

Under Direction of the Eye

It is difficult to appreciate all of the decisive modifications throughout the body which the development of such hand-like structures determined. Their influences operated in profound and subtle ways. They caused a great change in body posture. The animal was now able to reach for branches above its head. This was a long step in the direction of standing upright. It modified the relation of the head which in most four-legged animals is directed so that both the eyes and the nose are turned toward the ground. Reaching upward to grasp branches and drawing the body in this direction lifted the head. It has been shown that this action of pushing the head backward and stretching the neck causes the hind legs to straighten out automatically in exactly the position necessary for standing erect. Such a beginning of the upright posture also produced a change in the position of the internal organs of the body as well as in the position of the eyes. These modifications influenced the growth of the superbrain, which finally acquired that appearance seen only in animals possessing hands. Coincident with these modifying factors, still another important change was in process. In all four-legged animals the paws, and more especially the hind paws, operate out of sight of the eyes. The animal does not see their action. The eye does not watch and supervise the movements of the paws step by step, but allows them to shift more or less for themselves. With the appearance of hands connected both with the fore and hind limbs, this state of affairs ceased. Both the hand and the foot now came under the critical supervision of the eye. The eye was able to hold in plain view the performances of the hands and hand-like feet. It could see and direct their movements. It could single them out individually or watch them while they all worked together. It could even make critical discriminations in each hand and in each foot. It could select a thumb or a great toe, or each one of the other fingers and toes, and thus guide its movements. This selective discrimination in the hands and feet was an advantage never enjoyed by any of the pawed animals whose habit it is to use all of the digits together. In this manner both hand and foot profited by their new adjustments. As instruments they were capable of a far wider range of application, although it was not alone by this expansion in their utility that they became more effective. They were better agents for sensing the world and possessed a more ample sensory capacity which arose from their own multiplied movements.

Threshold of a Great Change

In the animal kingdom it would be difficult to find more provocative influences than those which determined the transformation of paws into four hands. Considered casually, the appearance of the quadrumanous monkeys in all their varieties seems little more than the addition of many interesting forms of life. This addition, however, had a far greater significance. The four-handed stage of animal existence led to the highest development of the brain. Without this stage the ultimate advances in life, the supreme achievements in progress, would have been impossible. Numerous factors contributed to the acquisition of hands and hand-like feet, but no one of them was more potent in the final outcome than the effects of tree-living. Almost every other combination of habitat and adjustment had exerted its influence upon the form of the mammalian body, yet in no other instance has there been achieved a success comparable to the development of hands. Most mammals are equipped with highly efficient eyes, keen ears, and a serviceable sense of smell. These endowments have had opportunity to contribute to the efficiency of life. But neither sight nor hearing nor smell was sufficient of itself to determine those advantages capable of giving the animal a supreme position. It was the hand which opened the door to give the senses those opportunities never enjoyed before. It called upon the brain for further expansions to direct new ranges of movement. It required additional brain extensions for a greatly amplified sense of touch in the fingers and palms, in the toes and soles of the feet. It was the hand, in a word, that afforded an entirely new grasp upon life and in the end created not only a new order of mammals but almost a new kingdom of life. The transition from paws to the hands of the quadrumana is the threshold of an epochal change. As the paw was the basic pattern for the hand, the hand was the indispensable stepping-stone to the development of man. This formula may perhaps seem altogether too simple and graphic. It would be such, in fact, if many of the important intermediate stages in the process of development were overlooked. These stages may now be considered.

The consequences of the transition produced under the influence of tree-living appear conspicuously in the lengthening of the digits to form fingers, in the appearance of an opposable thumb, in the acquisition of a grasping hand. All of these are definitely adaptive changes. They are applied directly to meet the conditions of locomotion through the trees. But if these modifications conferred upon the animals many real advantages, they also introduced certain imposing hazards to further progress. They were adequate for the mastery of arboreal life, yet at the same time they permitted the forest to become master of these four-handed animals. This is true in exactly the same way that the sea imposes its laws upon aquatic mammals, the plains dictate to the ungulates, and the air exerts its control over the bats.

Possession of too Many Hands

So far as the monkeys are concerned, an obstacle lies squarely across the path of further progress. They are possessed of too many hands. Hand and hand-like foot both serve the purposes of locomotion. Neither the one nor the other is afforded those opportunities of exclusive use which are essential to the highest development. This is true even of most of the monkeys of the Old World, like the macaques. Their locomotion requires the use of all four extremities. They run along on the top of the branches, grasping firmly as they go. They leap from one branch to another, employing all four hands in this mode of transportation. As a result of these activities the hands are long and slender, the fingers long and tapering, and the thumb short but opposable. The foot has much the appearance of the hand.

One group of the ape world offers a striking departure from this more general rule of development. This exception is particularly interesting. It appears in the baboon and more especially those members of their family which have taken up a life upon the ground. With the baboons the resumption of terrestrial life came long before any of the monkeys had made pronounced advances toward the erect posture. It is for this reason that when these animals adopted habits of ground life they readjusted themselves after the fashion of other four-legged animals. They travel about much like the dog or cat, with their muzzles directed to the earth. In fact, many of their features, both in head and body, take on a definite canine appearance. A feature of special significance is the manner in which their fore and hind limbs have reacted to the influences of ground-living. The great lengthening in the hands, fingers, feet, and toes, conspicuous in monkeys that live in the trees, has actually been reversed in the baboon. It is still proper to speak of hands and feet, but both hand and foot have shown striking tendency to revert to paws. This specialization illustrates a remarkable disgression in the development of the monkey kind. It means, if it means anything at all, that the adaptations necessary for carrying on life in the trees have withdrawn their influence and permitted the habits of adjustment to the ground to modify the character of the extremities. In four particulars the hand of the baboon shows distinct tendencies to revert to a paw:

1. All of the fingers are shortened.

2. The thumb has been reduced if not to the state of a vestigial tubercle as in the dog, at least until it has become extremely rudimentary.

3. The nails have become much longer and more slender, as if they were tending to form claws.

4. Both the tip pads and the palm pads have become more prominent, the latter actually fusing to form a single palmar cushion.

In the foot similar tendencies toward a paw are present. The lesser toes and the great toe are much shortened, and there is a distinct fusion of the plantar pads. This reversion in the hands and feet of the baboon shows clearly how readjustment occurred when the influences of tree living were withdrawn. It also demonstrates the strong tendency for the chirideal structures to assume the ancient patterns of the paw in response to the habits of four-footed living upon the ground. The baboons, therefore, cannot be considered in the direct line of progress. They not only failed to advance the cause of developing the hand but they did nothing to further the erect posture or the progressive expansion of the brain. It was perhaps the large size of their body that made it necessary for them to desert the tree and seek more secure support upon the ground. This increase of body size, however, came at an early period, long before the primates had begun to feel those decisive influences which favoured standing erect.

Brachiation and the Erect Posture

Considerably later in geologic times another class of apes made its appearance, which felt the full power of this determining influence. These animals were the gibbons. They introduced a new type of transportation. Their locomotion no longer depended upon running along on the tops of the branches, or leaping from one support to the next. They introduced the novel method of swinging by the hands. Reaching for a branch over the head with the right hand, the gibbon swings its body forward to grasp the next branch in advance with the left hand. Swinging in this manner, step by step, first with the right hand, then with the left, these animals walk through the trees. The results of this arm-swinging locomotion (brachiation) are apparent in the development of the hand. The fingers, tip pads, the palm, and the palm pads are greatly elongated. Similar lengthening is also apparent in the forearm. The acrobatic manœuvring requisite to such locomotion has developed a high degree of skill in using the hands and arms. It also requires a close coöperation between the movements of the upper extremities, eyes, and head. The influence of these several modifications has impressed itself upon the brain. But the most decisive effect of the gibbon mode of locomotion is seen in the posture of the body. The swinging by the hands well above the head produces an almost constant erect posture. The muzzle no longer points, as in the great majority of monkeys, toward the ground. It, as well as the eyes, is now directed toward the horizon, and thus those factors which have contributed most to an upstanding, forward-looking primate were first introduced by the gibbon. The foot of these animals, while it retains many features and markings of a hand, affords a fairly satisfactory support for bipedal locomotion in the erect posture. Obviously the effects of tree life are responsible for these changes in the gibbon. All other monkeys up to this stage have been embarrassed by an over-endowment of hands. But the gibbon, by over-emphasizing the upper extremity, has to some degree nullified the importance of hand-like feet. It has begun the solution of that perplexing problem which was imposed upon the monkeys by their almost exclusive tree life and which must be solved in order to provide for the manlike specializations essential to bipedal locomotion.

In this gibbon level of the ape world such specializations began to manifest themselves. From some gibbon-like progenitor, early in the Age of Mammals, there arose a common stock capable of producing all of the modern gibbons, the great anthropoid apes, and man himself. This gibbon stage of development contained the potential material from which to evolve the erect posture, bipedal locomotion, hands freed for the purposes of the greatest utility, and a brain adequate to the needs of the highest primates.

A New Grasp on Life

In the three great anthropoids, orang-outang, chimpanzee, and gorilla, the hand is approaching more closely to the human pattern. In all three the leading advance is due to the development of a more effective opposable thumb. The result of this change has caused the disappearance of the two wrist pads so characteristic of the mammalian paw and so prominent in the great majority of monkeys. Power to oppose the thumb against each one of the fingers separately has increased to a great extent. The opponens muscle of the thumb has become more prominent and caused the appearance of a conspicuous muscular swelling in the palm of the hand, the thenar eminence. The palm muscles developed in connection with the little finger have likewise occasioned the appearance of the hypothenar eminence and at the same time the disappearance of the second wrist pad. These developments, all clearly seen in the anthropoid apes, and most prominent in the gorilla, reach their greatest proportions in man. They are evidence not of the further adaptation of the hand to locomotion but of its liberation for other and more constructive purposes.

The effects of this advance in the hand from one primarily intended to provide a firm grip upon the limbs of trees to one of almost universal application are revealed by alterations in the palmar lines. These lines are three in number, namely, the anterior, middle, and posterior groove. In the gibbon they extend across the palm almost parallel to each other. They are creases which represent the lines of palmar flexion resulting from grasping cylindrical branches. In the orang-outang these lines are still essentially parallel, indicating a hand designed to grasp a cylinder. In the chimpanzee and gorilla the palmar grooves begin to converge toward the space between the index finger and thumb. In man this convergence is complete, due to the development of the powerful hand muscle which permits the opposable thumb to reach the other fingers. This progressive convergence of the palmar lines indicates the development of a hand no longer intended for the simple purpose of grasping a cylinder, but not constructed to take firm hold upon a sphere. Figuratively this change in hand from cylinder- to sphere-holding capacity is illustrative of actual development in the intellectual grasping powers that became the distinguishing feature of mankind.

A Firm Foundation for Humanity

Thus far we have been able to trace the stages by which the hand developed in consequence of tree life. It is now necessary to follow the modifications which terminated this arboreal domination and consequently liberated the animal from the forest. This transition determined an adjustment to life that was finally productive of the most effective behaviour. The outcome of this modification was the freeing of the hand for purposes other than locomotion. The immediate agent that made such a result possible was the development of a foot capable of supporting the upright posture. This foot, as it made its appearance in man, passed through a long series of transitional phases. It had its beginning in a definitely prehensile stage when in the earliest of the monkey kind it was hand-like in its appearance. The structure that was the forerunner of the human foot had the same bones, the same muscles, the same ligaments. The only substantial difference was in the form and arrangement of these parts. Even in such a minute particular as the three contravehent muscles in the sole of the monkey’s foot, which draw together the heads of the metatarsal bones, the correspondence is complete. These muscles are present and active in the gibbon. They are much diminished in the chimpanzee. In the orang and the gorilla they are still further reduced and closely resemble the atrophic fibrous strands found in man. A similar correspondence involves the muscles which separate and draw the toes together (the interossei). They are deeply situated in the plantar surface of the foot in most monkeys. In the orang and gorilla they have exactly the same position and relations as in man. The human embryo affords the final connecting link, for in this stage of development the muscles correspond to those of the lower monkeys.

The human foot is foreshadowed by that of the great anthropoids. It is, in fact, the culminating stage in that series which had almost reached the human goal in the orang, chimpanzee, and gorilla. The plantar grooves in the feet of the anthropoid apes clearly indicate the lines of flexion adapting the foot for purposes of grasping the limbs of the trees. In passing from the gibbon to the orang and the chimpanzee, with the slow development of semiterrestrial life, there is a progressive disappearance of the plantar grooves. This change illustrates the manner in which the foot became adapted to the purposes of bipedal locomotion. Of all the great apes, the gorilla makes the nearest approach to the human foot. The toes have become shorter and have lost their finger-like resemblances. The great toe has become larger and is partially assuming an axis in parallel with the other toes. It has also migrated toward the end of the foot and, in older adults, has lost much of its prehensile character. Another modification is the gradual broadening of the heel and the appearance of the plantar arch. All of these changes have been developed for the purposes of bipedal locomotion and the erect posture. In consequence of these new functions the simple grasping foot of the monkey is altered to serve as a powerful stepping lever. In its simian form the foot is a Y-shaped prehensile organ. The stem of the Y is represented by the long heel. The two branches are formed by the great toe and the lesser digits respectively. In the higher primates, such as the orang, chimpanzee, and gorilla, the simple Y foot has undergone a striking change. The sole of the foot, including the ball and the heel, has greatly increased, while the toes or grasping elements have become shorter. In gorilla this is particularly true of all the toes except the great toe, which has not only become somewhat longer but now tends to be in the main axis of the foot.

The most important features in the development of the foot are the increase in the supporting surface of the heel and the appearance of the plantar arch. In the lower monkeys the arch of the foot is double. In the great apes, more especially in gorilla, the plantar arch is single and corresponds practically to that of the human foot. The sole pads have become fused to form the ball of the foot, while the development of the heel has caused the disappearance of the ankle pads.

Whatever may have been the influences which caused certain members of the prehuman stock to desert the trees and live upon the ground, it is clear that one most important result of this change was the formation of the human foot. This structure was a solid foundation for the highest achievements of organic evolution. It ultimately produced an animal capable of dominating the world. It was responsible for all of the extensive changes incident to the erect posture--for the rearrangement in the shape of the body, for the squaring of the shoulders and the broadening of the pelvis, for readjustments in the position of the heart and lungs, for new provisions in supporting the abdominal organs, for a reordering in the relation of the eyes to provide for binocular, stereoscopic vision, for the modifications in the neck to suit the purposes of the most effective head movements, for the freeing of the hands so that they might become constructive agents, and, above all, for impressing upon brain structure the effects of these many progressive advantages. If there could be any doubt that the hand and the foot contributed in this decisive manner to the development of the brain, we might test this supposition by a pertinent question: What, for example, would the brain have been if neither hand nor foot had made its appearance? It is clear to us what limited advantages were acquired by animals equipped with hoofs or paws or flippers or wings. The brain responded to the requirements of these specialized organs. None the less, such response was always and unmistakably the brain of an ungulate or of a meat-eater, of a flying or of a swimming mammal. It was the brain of a creature of restricted behaviour, as limited in the development of its intelligence as it was in the amplitude of its adjustment to life. It was particularly deficient in one great department which is the hallmark of all animals possessing hands. Summarized as briefly as possible, it may be said that what the brain owes to the hand and foot is the frontal lobe. Through all the stages of progress, from the time when the monkeys first began to live in the trees until their successors, through graded intermediate phases, developed the hand and foot of man, this lobe has been the outstanding feature of the brain.

It is perhaps unwise and also unwarranted to speak of the debt that one organ owes to others, especially when the activities of all represent a unified process. Brain, hand, and foot are in the strict sense a single functional unit. Each is indispensable to the others. Yet it may be assumed that it was the new opportunities for action provided by the hand and foot which at length gave the brain its human capacities. These ultimate instruments of man’s success amplified brain power and increased its sphere of influence. The hand in particular was the instigator, if not the originator, of human speech. Herbert Spencer, in his essay on “The Philosophy of Style,” clearly points out the fundamental relation of the hand to speech, in the following words: “To say ‘leave the room,’ is less expressive than to point to the door. Placing a finger on the lips is more forcible than whispering ‘Do not speak.’ A beck of the hand is better than ‘Come here.’” As the creator of indicative gesture the hand laid the foundations for the use of symbols, which, when vocalized, became established as language. This attainment was the most important single step in the ascent leading to humanity.