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.