DAWN OF THE PRIMATE BRAIN
THE LOWEST OF THE MONKEY KIND
We are now approaching a critical period in the history of the brain.
It is a period that contains many incidents of the utmost importance.
Particularly noteworthy are the episodes which favoured the production
of human characteristics in the animal kingdom. These characteristics
showed many manlike tendencies that much later were to appear full
fledged in the human race. They were from the first limited to a
single, highly interesting order of mammals. And this seems especially
strange because from the beginning of the Age of Mammals (65,000,000
years ago) a great variety of new animals came into existence. The
fact that a single group out of all this vast number was picked out
to develop human resemblances must hold the secret of some potent
selective influence. Such an influence was definitely at work. Its
operations were slow but steady. Little by little it changed and
reshaped the structure of the body until at length there appeared a
race of animals so human in their organization that they might well
have been the forerunners of mankind.
It would be difficult to conceive the kind of modification in structure
that could produce the form of man from a horse, from a whale, or
even from a dog. But this difficulty becomes far less in the case of
the animals usually referred to as the monkey kind. In many features
of their structure these animals resemble men. Existing in a great
variety of forms, they manifest numerous modifications in the different
parts of their bodies and exhibit a wide range of behaviour in their
habits. Only a superficial acquaintance with them is necessary to
reveal their many progressive traits. But their progress, like all
other progress, had its humble beginnings. At first the apes were
very simple creatures. Their coming, however, marked the dawn of a
new day in animal life. We shall be interested to follow the advances
that occurred in their mental capacities as they slowly made their
progressive strides forward. We shall be particularly struck by those
changes which gradually led up to the development of a brain capable to
control all of the complex activities of human behaviour.
Naturally we may expect to find a simple controlling organ in the
lowest of the monkey kind. As we pass upward, however, into the higher
families of the apes, we shall not only observe a pronounced increase
in manlike tendencies but, as the great anthropoids at length become
human in miniature and then almost human, we shall recognize in these
animals a brain which very closely resembles that of man.
_Class Distinctions in the Monkey World_
In the ape world there are animals of high and low degree. Some are
so humble that it is hard to decide whether they actually belong to
the monkeys or not. With few exceptions they all prefer to live in the
jungles and tropical forests. We could not fail to be impressed by
the striking resemblance that many of them bear to man. Yet there are
such marked differences among them that they cannot all be regarded as
members of the same family. If we grouped them as we do human races,
we might most advantageously assign them to certain large classes
according to their nearness to man.
Monkeys of lowest degree include the lemurs, the tarsiers, and all of
the New World monkeys.
The intermediate monkeys in the next higher grade are those which live
in the Old World, with the exception of the three great manlike apes.
The higher anthropoids occupy the top rank and are the nearest to man
both in their appearance and in their habits.
These three ranks in apedom did not appear at the same time. One rank,
so to speak, successively developed from another. By a process of
selection and adjustment the higher forms arose from the lower. The
ranking great apes owe their superiority to many traits and characters
which they inherited from more humble forerunners and which they
improved by the process of progressive development. The lowest monkeys
likewise had their day of upward progress, during which they emerged
from some mammal still lower in the animal scale. These forerunners of
the earliest primates, the lemurs and tarsiers, had in all probability
been gradually specializing during the latter part of the Age of
Reptiles. Their ancestors came from that stock of mammal-like reptiles
which started from lowly beginnings and remained modestly in the
background during the reptilian period.
_The Lemurs_
In the endeavour to get some conception of these distant predecessors
of the monkeys and apes, it is believed that the tree shrews possess
those simple characters necessary for the proper starting point.
The shrew is an insect-eater and lives in the trees. It has many
specializations in its legs, in its head, and in its trunk. These
special adjustments might serve as the beginning of those important
changes in the body which later distinguished the monkey kind. In
the first place, the small size of the tree shrew was particularly
favourable for this purpose. Then, in the second place, its habit of
living in the trees foreshadowed advantages of great promise. Such an
epoch-making adjustment made its appearance when paws were replaced by
hands and when definite hand-like feet appeared. If an animal like the
tree shrew were the forerunner of the monkeys, it is not difficult to
appreciate how the lemurs arose from this stock. They and their kind
may be looked upon as the first chapter in the history of the ape world
and the ape brain. At present they live exclusively in Madagascar and
its small adjacent islands. They are not known in any other part of
the world, although fossils of them indicate that they were widespread
throughout the globe in earlier times. The reasons for their present
exclusiveness and their insular homes are not clear. Geologists claim
that the parts in which they live originally had land connection with
the continent thus permitting their wide dissemination. The later
disappearance of this connection accounts for their present isolation.
There is much in the appearance of the lemurs that distinguishes them
from the monkeys and apes. Their most distinctive feature, the head,
is much like that of a fox and is drawn out in a long pointed muzzle.
Many characters appear in lemurs not seen in monkeys. They have no
cheek pouches. Their tails, never prehensile, are usually furry. They
develop no gluteal pads, which many apes possess in consequence of
their squatting postures. It is in their hands that they resemble
monkeys most strikingly. They have fingers and toes with finger nails
and toe nails. The thumb and great toe are always well developed, but
the second or third digit is often greatly modified. They also have
mammary glands like the monkeys. In the female these glands assume
certain definitely human characters. The lemur is a little smaller than
the domestic cat. Its fur is thick and woolly. Its large and prominent
eyes are more widely separated than in monkeys. The ears are long and
have tufts of hair on their upper portions. The arms are not quite so
long as the legs. The tail is long and often bushy. Fleshy pads appear
on the palms of the hands and soles of the feet, as well as upon the
palmar surface of the fingers. These enable the animal to grasp the
branch of a tree with great tenacity.
Little is known of the lemur’s habits in the wild state. It is not
strictly nocturnal, for some of these animals are known to seek their
food during the day. Often they travel about in troupes consisting of
many individuals. Most of them live in the forest. Their food consists
of fruits, insects, birds’ eggs, and birds themselves, which latter
they are most skillful in catching. During the heat of the day they
sleep with the head beneath the arm and the tail curled about the neck.
When walking they go upon the hands and feet, both when on the ground
and in the trees. The tail is used in the manner of a balancing or
steering organ. Sometimes they assume a semi-erect posture on the hind
legs, or sit in a half-crouching position. Both hands and feet are
employed primarily for climbing or running about on the ground.
The lemur has great ability in leaping from tree to tree. Its movements
are so rapid that it can only with difficulty be followed by the
eye. Hunters say that it is easier to kill a bird on the wing than a
lemur when leaping. If pursued and shot at it has a habit of dropping
suddenly from the topmost branches into the bushes, giving the hunter
the impression that he has succeeded in killing the animal. This
impression is soon dissipated upon seeing the lemur in another tree
at a considerable distance from the spot where it fell. When wild the
animals are said to subsist largely upon bananas. They also seem to be
fond of the brains of birds. After fracturing the bird’s skull with
their teeth, as they might puncture a nutshell, they suck the brain out
of the brain case. The lemur, however, does not eat the rest of the
bird. We may see from this description that this is an animal of great
agility. Not only does it possess much speed in its locomotion, but it
also manifests the utmost nicety in balancing and remarkable precision
in all movements.
_Tarsius_
Another of these lowest monkeys is a strange little animal called
tarsius, which has acquired a notable reputation. Several learned
authorities have singled it out as the standard bearer of human origin
from some lower mammal. The tarsius is about as large as a small
squirrel. Its appearance is peculiar because of its closely set bulging
eyes, its long tufted tail, its protruding ears, and the small circular
pads on the end of each finger and toe. Tarsiers have two curious
habits that attract attention at once. They can leap with astonishing
swiftness from tree to tree, often in pursuit of insects, and when they
turn their heads they seem at one instant to be looking forward and the
next directly backward. Their eyes, though very large and prominent,
do not seem to give them the best of eyesight. The animals can see
well at night, but during the day they appear to be almost stupid
because in the sunlight their vision is imperfect. Tarsius lives in
the jungle, usually in the low countries of the Malay Islands. During
the day it passes most of its time clinging in a vertical position to
the trunks of the smaller trees and underbrush. The way in which it
supports itself is interesting and peculiar. With its fingers and toes
it firmly grasps its slender support, at the same time pressing inward
with its long tail, which acts like a spring against the tree. If its
tail is pulled away from this support the tarsier at once tends to slip
backward. The tail, which has no grasping power, is used like a rudder
for balancing and steering when the animal is in motion.
In some respects tarsiers are quite human. They go about in pairs and
are not gregarious like most of the monkeys. Furthermore, they give
birth to but a single offspring at a time. After the breeding season
the female and her young find a home by themselves. There are no
indications that these animals build nests or even live in holes of
trees. The tarsier often falls asleep in its characteristic clinging
position, and the head then sinks downward much as that of an old
man asleep in his chair. Often the young tarsius will perch upon the
mother’s head while she is asleep, and in this position fall asleep
itself. The general behaviour of the animal is extremely stereotyped
and limited. It learns but little under training. In captivity it is
able to make but few new adjustments. During the day its enormous
bulging eyes give it an almost ridiculous appearance as it gropes
awkwardly for food. This no doubt is due to the fact that its eyes are
constructed for hunting at night and do not contain the specialization
essential to the sharpest kind of vision. On the ground tarsius leaps
like a frog but is very awkward. In the trees, however, it is extremely
agile, and is probably the quickest jumper of all mammals. While
grasping a small branch it can turn its head so as to look directly
backward and jump more quickly than the human eye can follow. It seems
to be looking in one direction and jumping in another. This is due
to the great rapidity with which it turns its head. In captivity it
is pugnacious and cannot be tamed. It performs its toilet much as a
cat does and thus keeps itself scrupulously clean. It is not known
to make vocal sounds indicating fear or anger. On rare occasions,
and particularly when young, it has been heard to squeak. The infant
tarsius clings to the hair of the mother’s chest like other young
monkeys. The eyes are open at birth, and many reactions appear at once
that are long delayed in such animals as the rat, cat, dog, and higher
apes.
_The Marmosets_
Another lowly monkey is the marmoset. It has less renown than tarsius
but is nevertheless an interesting animal. It is often carried
around in the pocket of its owner and fits conveniently inside of
the old-fashioned fur muff. The marmosets belong to the group of the
New World monkeys. They inhabit South America and Central America.
Their chief interest arises from the fact that they represent one of
those moments of faltering experienced by the monkeys in their upward
strivings. These little animals have an almost pathetic expression
and features that are in many ways quite human. Yet in spite of this
human-like appearance they indicate an actual backsliding in the
attempts at progress. This backsliding is most apparent in their
fingers and toes. In fact, the entire hand and foot have lost most of
their human resemblance. The finger nails are now replaced by sharp,
talon-like claws, and the toes are equipped in the same way. The
marmosets, both because of their diminutive size and the imperfections
in their hands and feet, are now looked upon as monkeys that show signs
of retrogression.
The marmoset is as large as a small squirrel and covered with a thick,
silky fur. It is naturally very timid but soon becomes friendly to
those with whom it is familiar in captivity. The female produces
two or three young at a birth and in this respect is unlike most of
the monkeys. The marmoset’s facial appearance and shape of head are
certainly more ape-like than the lemur’s. The eyes are set much closer
together, and are separated by a flat, narrow nose suggesting that
vision now depends on the simultaneous operation of both eyes. The
animal has a long, bushy tail. It lives in the tree tops or small
underbrush and climbs the trees in a manner similar to the squirrel.
Although it has a cat-like agility, it does not make the long and
daring leaps characteristic both of lemur and tarsius. It often loses
its grip on the branches and falls from a considerable height to
the ground. In captivity it shows little inclination to develop new
actions. It is not easily trained, and to teach it to do tricks of any
kind is most difficult. It lives upon worms, insects, and fruit. It is
known also to invade birds’ nests and suck the eggs. Very rarely does
it prey upon bird life and then only when it is able to overpower one
of the smaller birds or unprotected young.
_South American Howling Monkeys_
In this group of lower monkeys we encounter one with a highly
interesting personality, known as the “red howling monkey of South
America.” He is a real monkey, noisy and disagreeable, often attaining
the size of a fox-terrier. He always seems to be in an unpleasant
mood, showing his teeth and howling on the slightest provocation. In
spite of all this ill temper, he belongs to the progressive party
of the monkeys. There is not the slightest doubt that he has made
definite advances along the lines of progress. If we should question
this progress we would soon have our doubts set at rest when we saw
the astonishing manner in which he uses his tail like a fifth hand.
Even more convincing in this respect is the almost human appearance of
his hands. Not long ago a young woman visiting the ape house in the
zoölogical gardens was struck by these human similarities. She was
still more impressed when a large howling monkey thrust his long tail
through the bars and deftly tossed her hat into the air.
The howling monkeys enjoy this gift of a capable, grasping tail in
common with most of their fellows who live in South America. The
prehensile tail is especially well developed in the spider monkeys
and in the woolly monkeys. At its end this tail looks like a long,
tapering finger. It is a highly developed sense organ and gives the
monkey a new instrument for locomotion and for exploring. These monkeys
are able to swing themselves from the branches by their tails and thus
leave the hands and feet free for other purposes.
In addition to this highly efficient tail, the howlers have developed
a larynx and vocal cords with which they produce awe-inspiring sounds.
Their mournful howlings are often audible for miles around, and it
is supposed that they employ their cries as a means of defense to
intimidate their enemies. The howling monkeys possess a slightly
opposable thumb and well-developed fingers. While they are described as
being the most ferocious of the South American monkeys, they are also
credited with a low degree of intelligence. The face of this monkey is
naked with the exception of a heavy beard that hangs beneath the chin.
In captivity they are practically untamable and soon die. Their fur is
usually black, but in some cases is brown or reddish brown. They live
largely upon fruit, although like other South American monkeys they
feed upon caterpillars and insects.
_Measuring the Mentality of New World Monkeys_
Professor Thorndike, of Columbia University, has made careful studies
concerning the behaviour of several South American monkeys. He was
chiefly interested in the manner in which monkeys differ from other
animals in the mental capacities and methods of learning. In making his
tests he devised certain experiments which utilized boxes with pegs,
bolts, bars, and hooks. The object of these tests was to find out how
the animal learned to release itself from confinement, or gain access
to a goal containing food. Professor Thorndike concluded that these
monkeys did not learn by reasoning. They do, however, form more and a
greater variety of associations than other mammals. Their combinations
of this kind are remarkably slow and ineffectual in providing any new
behavioural accomplishment. Concerning the general mental development
of the South American monkeys, Dr. Thorndike believes that they
represent a certain advance from the generalized type of mammals
toward man. This is particularly true of their sense equipment and
their localized vision. All of this, he believes, is in reality an
advance due to the brain acting with increased delicacy and bringing
into line those activities which distinguish human mental faculty from
that of all other animals. Here, at length, among the lower monkeys is
well-attested proof of some progress toward the development of human
capacity.
_Monkey Behaviour_
The way in which these lower members of the monkey kind behave deserves
particular attention. It gives us the opportunity to observe certain
striking resemblances to our own human behaviour. This question
is one of primary importance. It acquires especial interest as we
compare the brains of the monkeys and apes one with another. As the
brain continues to improve from one stage to the next, we should be
on the lookout for new developments in behaviour. It might perhaps
be impossible to appreciate all of these minute changes among the
monkey kind. It is even somewhat questionable whether such an exact
comparison at the present time is necessary or possible. Yet there
are certain outstanding traits of conduct that may be easily traced
from stage to stage. One of the most important of these traits depends
upon the development of the tail from the time when it first acted as
a rudder-like organ for steering and balancing the animal until it
acquired all of its great facilities as a fifth hand. After this it
began to recede in importance and finally disappeared. The tail thus
created a special cycle of behaviour which had important bearing upon
the final outcome of man’s adjustment.
Another group of reactions centre upon the manner in which the hand
made its appearance, including the progressive changes in behaviour
when the monkeys first became four-handed. All of these changes
were dependent upon living in the trees and gradually found their
culmination in an animal that could stand upon two feet and use its
hands. Such usage as this foretold the beginning of human skill, of
human right-handedness, and of human speech.
Very important were the changes in behaviour that made their appearance
as the eyes worked more in harmony with each other. They produced a
kind of vision better able to guide the movements of the hand and
give more complete information concerning distance, direction,
and perspective. But far exceeding all other changes for getting a
better control over the surroundings were those progressive advances
introduced for making the fullest combinations of sense impressions.
These advances favoured the development of better powers for learning
and for profiting from experience. Progress in all of these particulars
concerning the behaviour of the monkeys may be clearly traced in
corresponding expansions in their brains.
_Brains of the Lower Monkeys_
In the brains of these four very simple members of the monkey kind
we may readily see the expansions that promoted development in the
governing organ. It will be apparent at a glance that progress followed
no direct or easy path. It met many rebuffs and obstacles. Often it
faltered and even stumbled. But struggling on it finally reached solid
ground and then went forward to real advances.
Placing the brains of the lemur, tarsius, marmoset, and howling monkey
side by side we may see how this progress began. To guide our way in
following this advance, certain signposts and milestones will prove
serviceable. Three of these landmarks are deep grooves or clefts. They
appear in the superbrain and indicate the places in which progress has
been particularly active. Around these grooves the outer covering of
the superbrain has been folded to make room for more brain cells. This
folding produces convolutions with the result that the more convoluted
a brain is, the more cells it has for the development of brain power.
Each of these three grooves has its own special meaning as a landmark:
1. The “Sylvian groove” is a fissure that runs between the department
for the sense of hearing, called the “temporal lobe,” and the
department for body and contact sense, called the “parietal lobe.”
2. The “central groove” is a fissure between the department for body
and contact senses and the department of supreme brain activity, called
the “frontal lobe.” This lobe of the brain is situated immediately
above the eyes and behind the bone of the forehead (frontal bone). A
small frontal lobe means a low brow with a correspondingly inferior
mentality. As this lobe of the brain increases from ape to man, the
forehead gradually becomes higher and more prominent.
3. The “ape groove” separates the occipital lobe in the back of the
head from the parietal lobe. In the occipital lobe is situated the
department for sight.
The three grooves form the boundary lines between the four chief
departments of the superbrain, each of which is known as a lobe;
namely, (1) the parietal lobe, department of body and contact senses;
(2) the temporal lobe, department of hearing; (3) the occipital lobe,
department of sight; and (4) the frontal lobe, department of the high
mental faculties like judgment and reason.
Further advances from this point will occupy our attention in tracing
the brain of the monkey kind upward. Two other landmarks in the
brain have special value. One of them is the bridge (pons) which
connects the larger brain (cerebral hemispheres) with the lesser
brain (cerebellum). This lesser brain acts as the chief muscle timer
and adjuster. It balances one muscle’s action to that of another and
adjusts the force of such action. All of our most exact movements,
whether in walking or writing or speaking, depend upon the little
brain. If it is injured or destroyed the movements of our hands and
feet, head and trunk, become shaky, unsteady, and very irregular. For
an animal to become highly skillful requires high development in the
little brain. The animal having the highest intelligence also possesses
the greatest capacity for skill in its actions. The size of the bridge
reflecting the degree of this skill is a good index of the intelligence
possessed by the animal.
The pyramid is another important indicator of progress. Like the
bridge, it is found on the base of the brain. It is called pyramid
because of its somewhat pyramidal shape. It acts as the main trunk
line for getting the orders of the superbrain out to the muscles.
It transmits, so to speak, the highest commands of the brain in
controlling the motor machinery. By means of it we act according to the
dictates of our wills. If both of these great pyramidal trunk lines
are interrupted, we become completely paralyzed. The pyramids conduct
the highest output of the brain’s activity and increase in direct
proportion as the animal’s behaviour becomes more and more complex.
The brains of low monkeys are of small size: lemur, 18 grams; tarsius,
6 grams; marmoset, 6.2 grams; and howling monkey, 24.5 grams.
Size and weight of brain, we must bear in mind, vary to a considerable
degree with the size of the body, so that certain other signs of
expansion in the brain are more impressive. These signs clearly
indicate that progress is under way as follows: First, the large
superbrain begins to cover over the lesser brain. In lemur this
extension backward has only just begun. It is only slightly more
marked than in many of the lower animals, like the cat and the dog.
In tarsius the large brain has extended backward over the lesser
brain to a considerably greater degree. This is an important change
because the tarsier has transferred much of its business of sight to a
new department in the occipital lobe of the superbrain. The marmoset
shows this transfer carried a little farther, for the large brain now
overhangs the lesser brain. The great advance shown in the howling
monkey reveals the way in which the superbrain has taken complete
control of the situation. It now covers over the lesser brain entirely.
All of this change in the superbrain has been mainly in the interest of
making a better department for sight, but the departments for the sense
of hearing and for body and contact senses have not been behindhand in
expanding in these lower monkeys.
Another pronounced sign of progress is the gradual change in the
position of the groove of Sylvius. In lemur it is almost vertical,
as in the cat, in the dog, and other lower mammals. The arrangement
of other smaller grooves around it is also similar to that in lower
animals. In the tarsius this groove is equally primitive. It is
beginning to tip backward a little in marmoset. Finally, in the howling
monkey this groove has become quite oblique, as it is in most of the
apes and man.
All of this change has occurred as a direct result of perfecting the
organization in the department of hearing. The more tilted the Sylvian
groove becomes, the better developed is the temporal lobe which carries
on the business of hearing. The tilting backward of this groove also
results from an increase in that part of the superbrain which lies
immediately about the groove. This is the parietal lobe, the department
of contact and body sense. It is in this department that the especially
important information concerning the movements in the hands and feet
is registered. Thus the tilting backward of the Sylvian groove plainly
tells the story of improvements in the departments of hearing and of
body and contact sense.
Still another sign of progress appears in the central groove, which
has an equally interesting history. In the lemur this groove is just
discernible as a faint dent. In position it resembles a corresponding
groove in animals like the cat and dog. Lemur in this respect suggests
that in its striving to part company with the lower animals, to break
away from ancient contacts, and to get on an independent new line of
its own, it has not been entirely successful. This central groove
shows where the chief department of the superbrain begins, that is,
the frontal lobe. In the lemur this department is poorly developed.
In tarsius it is impossible to find anything that looks like a central
groove. This animal’s brain is an example of some of that hesitation
which was encountered in the path of progress. The same faltering
is also seen in the brain of marmoset, which has no central groove
whatsoever. These little South American animals, it must be remembered,
are thought to be backsliders, and this particular defect in their
brain strongly supports that conclusion.
In the brain of the howling monkey we find the central groove now
well developed. The superbrain shows that it is at length pursuing
some definite policy of expansion in its most responsible department.
Emphasis in growth is now obviously given to the frontal lobe for
advancing the capacity to transact all higher mental faculties. In the
howling monkey this department may not have attained any high degree of
development, but its presence is undoubted, and from this relatively
simple beginning it is only a matter of further expansion to bring
into existence the most productive mechanism of the brain. The howling
monkey shows its superiority over all lower monkeys in another respect.
It has developed the ape groove, and by it the boundary between the
department of sight and the department for body and contact sense is
fully established.
Viewed as a whole, the brains of these four lower monkeys show distinct
progress in the interests of developing a more efficient superbrain.
Each of the sense departments has gradually become better defined in
its boundaries, and doubtless correspondingly better organized for
the administration of its duties. Rising supreme above them all there
finally appeared the controlling department of the chief executive
in the frontal lobe. We see this in its earliest stage in lemur. It
assumes still more importance in the howling monkey. The departments of
sight (occipital lobe), of hearing (temporal lobe), of body and contact
sense (parietal lobe), show the effects of steady improvement from
lemur up to the howling monkey. If there have been some hesitations,
even some slipping back in the organization of efficiency, it is
because some of these animals were rather uncertain disciples of
progress. They may have been, as is probably true of tarsius, too
close to the starting point where the real advances of the monkey kind
began; or perhaps, like the marmosets, they ran into early difficulties
along the upward climb. It seems probable that they were not able to
extricate themselves with credit from these hazards or to overcome
the obstacles that confronted them. For this reason their brain shows
some actual backwardness. With these exceptions, however, the evidence
of progress is undisputed. It seems sufficient to convince the most
sceptical. The purpose of the progress is also sufficiently plain. It
clearly appears to be that effort toward promoting organization in
the superbrain so that the offices of the supreme executive might be
established in the permanent quarters of the frontal lobe.
_Measurable Improvements_
Any doubts due to lack of measurable proofs may be easily overcome
by several comparative measurements of the bridge and the pyramid.
The size of these structures, both of which reveal the behavioural
capacities of animals, has been carefully estimated. Accordingly the
bridge has been assigned the following values: lemur, .055; tarsius,
.057; marmoset, .095; howling monkey, .103. Thus the bridge, called
by some authorities an index of intelligence, shows distinctly the
advances made among these simple monkeys.
Quite as striking are the figures for the pyramid, which indicate the
degree of voluntary control that the superbrain has over all actions:
lemur, .110; tarsius, .032; marmoset, .064; howling monkey, .137.
From these figures the howling monkey stands in advance of his monkey
associates in the index of his voluntary control. Doubtless much of
this advantage is due to the high degree of hand-like specialization
in this animal’s hands and feet. But the grasping tail of the howling
monkey should not be overlooked. If tarsius and the marmoset appear to
stand lower than the lemur, it is because one of them is a primitive
type of animal with a much restricted repertoire of reactions, and the
other, the marmoset, is a backslider less richly endowed in the more
effective motor capacities.
All of these features in the brain seem to coincide with progress
in the behaviour of the lower monkeys. They show the path which
progressive advance has pursued. In the beginning, emerging from those
strivings of lower mammals and with much of the mammalian heritage
handed down by them, the lemurs took the first step of the monkey kind
toward a new type of brain. There was prophecy in these early attempts
made by the lemur. In some degree at least they foretold what this new
kind of brain was to be. Obviously they had as their distant mark the
ultimate upbuilding of the superbrain until an adequate department for
the supreme executive of life was produced. If tarsius hesitated in
reaching out toward this objective, it was none the less travelling in
the right direction. The destination of this course was clearly visible
in the brain of the howling monkey and other similar monkeys of the
New World. In this manner the first primate steps toward a more highly
efficient type of brain were taken. The conditions of tree life both
incited and successfully urged them onward.