ANCESTORS BEFORE THE APES
THE BRAIN FROM FISH TO MAN
_Practical Significance of Evolution_
There are many who still harbour resentment against the ape, especially
in explaining man’s origin. As a result, hostilities often flare up
against evolution. It cannot be denied that the unattractive ape is
at the root of these reactions. He is the bar sinister and the real
stumbling block in the evolutionary theory. He is also, to many people
at least, the entire gist of it. That we are descended from monkeys is
rather generally accepted as the meaning of evolution. This view, at
best, is a superficial explanation of what evolution really means. No
scientist to-day believes that any one of the living apes is ancestral
to man. These animals belong to families totally divergent from the
human family. They have ascended well up into the trees. Here doubtless
they will remain, quite as unconcerned in human origin as they are
innocent of participation in it. Our interest in evolution should not
centre upon the ape kind. The line of our ancestry reaches far back of
them through millions of years. We were in the making long before there
were any apes on earth. They, in their tree life, merely afforded the
last finishing touches which shaped our course toward humanity. If we
wish to acknowledge our hereditary indebtedness properly, we would be
compelled to recognize in our family tree that highly important line of
mammals which first introduced the custom of arboreal living. Back of
them are still older lines which deserve equal ancestral credit. Here
are found those animals without the existence of which we should never
have arrived. Among these is the vast assortment of reptiles, together
with mammal-like reptilians which appeared in the Age of Reptiles. All
of these reptilian forms were in their turn indebted for existence to
earlier amphibians and fish, their progenitors during the long Age of
Fish. Thus the true line of evolutionary descent leads us from fish
to man. Not until we appreciate the meaning of this long vertebrate
lineage through all its various phases does the vital significance of
evolution become clear. If we view it in this way it is possible to
sense the irresistible force that has carried animal life onward and
upward through the ages from the earliest times. This force may still
carry us onward. In its broader applications such a viewpoint should
make an urgent appeal for thoughtful consideration. It offers many
suggestions concerning further advances and readjustments in human
behaviour.
_Evidence of Evolution in Our Bodies_
The brain is one of the best witnesses testifying to this long
evolutionary development of man. It contains convincing evidence of
this process in three striking particulars. First, it gives numerous
signs indicating its primitive origin from the lowest of the
vertebrates, the fish. Second, it bears identifying marks of intimate
association with animals of its own class, the mammals. Third, it
has a large number of details in its special mechanisms possessed in
common with all of the primate order, to which man belongs together
with the lemurs, tarsiers, monkeys, and apes. This evidence is not
circumstantial. It is direct and unimpeachable. It leaves no point in
the line of man’s long descent to be decided by inference. It embodies
factors which led, step by step, to the upbuilding of the human brain.
Other tissues and organs of the body tell the same story of slow,
steady progress upward, from some low and simple phase of life, through
many graded stages of improvement until the human form at length came
in sight.
The blood has been an especially positive witness concerning this
progressive development. Tests with many different kinds of animals
show that the blood of man is much nearer to that of the great apes
than to the lower Old World monkeys. The relation between the human
blood and that of the New World monkeys is still more remote. In
general, these blood tests are among the most convincing proofs of
evolution.
The bony system of the body is another decisive witness. The skeleton
of the fore and hind limbs sheds much light on the changing adjustments
which have been made in the motor apparatus. The use of the limbs as
fins, paddles, wings, hoofs, paws, claws, hands, or feet, indicates
the broad family relations and kinship of various animals. The size and
shape of the skull and the character of the teeth reveal the manner in
which this evolutionary process has passed through its several stages.
The muscular system, the system for eliminating waste products of
the body, the heart and the lungs, all afford important evidence of
vertebrate kinship and evolution. The increase in the complexity of the
breathing apparatus, from the early gill stages of the fish to the lung
of the mammal, through all its many intermediate phases, discloses with
astonishing clearness the course of this progress.
_The Embryo as a Witness_
Testimony from another source also stands undisputed. This
corroboration comes from the manner in which all vertebrates are
conceived and formed. The witness in this case is the embryo, which in
all animals begins in the same way. Embryonic existence starts from a
single cell. It holds true to the earliest beginnings of animal life
that first appeared in a single cell such as the amœba. In the higher
animals this cell is called the ovum. From it, after fertilization,
two cells are derived, then four, then eight, then sixteen, until it
has an appearance closely resembling some of the colonized protozoan
animals. Here again, even in man, is seen that decisive stage in which
a critical cellular distinction is made between outside and inside
cells. From this time specializing progress in the growing individual
goes forward. Each new phase repeats in a general way a stage of
development previously attained in the evolution of life. All embryos
of vertebrate animals pass through such phases. The fish embryo carries
the process up to the stage characterized by those improvements which
developed during the Age of Fish. The amphibian embryo takes the
process one step farther. It adds new features essential to living
on land. Embryos of reptiles and of birds introduce the progressive
advancements peculiar to their kinds. The mammal embryo takes the
final step, prior to which it passes successively through the several
phases of the lower grades of life. The human embryo follows the
mammalian plan and puts the finishing touches of development upon what
the mammal has gained from all the stages below it. Fish, amphibians,
reptiles--all have their beginning in a single cell. Regardless of the
differences in body form, in mode of life, and in behaviour, all are
cast in a mold of development based on a common design. Thus, while the
blood, the bony system, the muscles, the teeth, the eliminating system,
the heart, and the lungs tell the story of progressive development, the
embryo gives a summary of this process by disclosing the general plan
which underlies the manner in which every backboned animal is formed.
The brain contains a comprehensive record of this progress. There are
reasons why this is the case. Brain influences pervade and dominate all
other systems. This organ is the great transformer of energy, which so
assembles other parts in operation that the body as a whole becomes
a smoothly acting machine. It receives sensory impressions from its
environment. It controls the reactions incited by these impressions.
In this dual capacity the brain has been especially sensitive to
those influences of change and adjustment, of action, reaction, and
interaction that have affected animal life during its long existence.
It has responded to these changes and has retained the impression
of such responses. In many cases it has been structurally improved.
Gradually it became capable of sensing the world more effectively. It
acquired the capacity to react on a broader scale. Developing along
certain progressive lines it has served to transform impressions
received from the senses in such a way as to produce an increasingly
more effective turnover of nervous energy. For this reason it is
necessary for us to estimate the value of such senses as were utilized
in this way. Without going too extensively into detail, it may be said
that, with extremely few exceptions, vertebrate animals possess four
chief varieties of sense. Each of these supplies the brain with stimuli
necessary to its proper reaction.
_Value of Our Senses_
First, chemical sense, through special organs for smell and taste,
conveys information concerning certain chemical conditions in the
surroundings. The sense of smell derives its impressions from gaseous
or volatile substances which, among other things, may create a pleasant
or a disagreeable odour important in selecting food. The sense of taste
gathers its information from substances in solution. It depends upon
acid, sweet, bitter, salty, or other similar stimuli. The primitive
headquarters for taste are in the hindbrain, while the endbrain serves
in this capacity for the sense of smell.
Second, body sense furnishes information concerning what transpires
within the body, as in the heart and lungs, in the stomach and
intestines, and in other special organs. It also supplies equally
important information concerning what contraction is occurring in the
muscles, how the bones are being moved, what postures the different
parts are assuming, and how the body as a whole is being balanced.
Third, contact sense makes known what is going on immediately outside
the body. It depends upon many things which touch the body surfaces,
such as the touch and pressure of a handclasp, the temperature of
water upon the hand, the vibration of a heavy vehicle running over the
ground. Body and contact senses had their original headquarters in the
midbrain and interbrain.
Fourth, distance sense supplies information concerning objects in the
world outside of the body more or less remote from it. The information
which this sense brings is news from abroad. It is gathered by the
sense of sight and the sense of hearing. Sight, in a way, is touch at a
distance. When an animal sees its enemy a long way off it, so to speak,
touches this enemy with its eyes and thus gives the brain the needed
information while there is yet time for escape. Sight depends upon
light waves, and hearing upon sound waves. By such means these two
highly specialized agents of distance sense gather their information.
The central offices of sight and hearing were at first situated in the
midbrain.
All impressions obtained from these senses were and still are the raw
materials utilized in the energy turnover produced by the brain.
Improvement was not always the result of the great struggle for
adjustment. There were many ups and downs, many trials, many failures.
Yet a certain insistent tendency toward progress was constantly in
evidence. By means not entirely clear, this tendency ultimately
succeeded in finding some way to become effective. It appears to
have exerted its influence by selecting definite parts of the animal
machinery for emphasis or repression.
Often some highly selective improvement was developed in the brain to
meet special conditions. Such is the expansion in the bird’s brain by
which the sense of sight is greatly amplified. This special increase
makes it possible for the bird to see its prey from great distances in
the air, as the hawk sees the fish in the water, or the vulture detects
the presence of carrion by its keen eyesight. The sense of smell in
birds is much less developed than vision.
In scenting animals, like the dog, the fox, and the cat, selective
improvement has affected the sense of smell. In a few instances the
addition of a relatively new sense was the means by which improvement
manifested itself. Such an addition is seen in that transition when
fish life first began to assume the characters of living upon land. At
that juncture the sense of hearing was added in some amphibious animals
belonging to the same class as the frog. These and other methods for
getting a better supply of raw materials through the senses contributed
to progressive development in the brain.
_The Sense Combiner_
Still more effective was the improvement which came as a new mechanism.
It provided a special apparatus that may for convenience be called the
“sense combiner.” The office of this mechanism was to assemble sense
impressions in the brain, to make composite pictures of sight, hearing,
taste, smell, and all other senses. This sense combiner served also
as an effective depository for impressions already received. It held
them in readiness for use as a background of experience that would be
needed for new or subsequent situations. At a glance it is evident that
the brain having the best sense combiner would outstrip all others
in its efficiency and output. In the earliest vertebrates this new
mechanism did not acquire a centralized headquarters. Its operations
were controlled from several scattered stations in the brain. Obviously
such division of responsibility could not be considered an efficient
method of control. Centralization was needed, and certain stages in
the development of the brain from fish to man illustrate how this
improvement was gradually brought about.
The first or fish stage, as might be expected, expresses the beginning
of this process of improvement in simplest terms. There are many who
do not credit the fish with such a thing as a brain. These animals,
however, are equipped with an effective organ of this kind. Its
efficiency is not high according to human standards, yet, as we shall
presently see, it has many characteristics of the human organ and
reacts to similar stimuli.
In the fish brain there are nearly all of the working departments
found in man. Much variation exists even among fish. Some of them have
very simple brains. This is true of the earliest forms, but the more
advanced types acquired brains thoroughly efficient for the special
complexities of existence in which they had to live. The several
departments in these brains are adjusted to their requirements. The
sense of smell in the fish is particularly well developed. It has
certain limitations, however, due to the fact that it must depend
upon substances borne by the water. The department of this sense,
nevertheless, occupies the major portion of what in these descriptions
will be called the endbrain. The sense of taste is also well organized
in fish. In certain of them, like the catfish, it has received special
emphasis, because in addition to taste organs in the mouth there are
organs of this kind scattered over the entire body from head to tail.
The primitive central office of the sense of taste in fish is located
in the hindbrain. Body sense is highly developed because most of the
fishes are able to control their muscles and joints in an amazing way
as they dart about in the water. Balancing of the body in swimming is
another important problem in the locomotion of the fish. It is solved
by means of certain highly specialized water levels (semicircular
canals). The body sense department occupies the interbrain. The sense
of sight in most fish is fairly well advanced, although it has distinct
limitations. Being placed on the side of the head, each eye acts more
or less independently of the other, and the fish, so to speak, gets a
two-eyed picture of its surroundings. It will subsequently become clear
that one of the most important events in the progress of the brain has
been the development of that kind of vision in which both eyes receive
the impression of an object at the same time. Then again, the medium
in which the fish lives is in many respects less favourable for the
passage of light rays than the air. The retina of the fish’s eye which
first receives the light rays also indicates a relative simplicity in
the organization of vision. For these and other reasons the fish’s
sense of sight cannot be as effective as in the higher forms of life.
This sense department is located in the midbrain.
_Starting with the Fish_
The fish stage in the development of the brain shows a striking
deficiency in its lack of provision for a sense of hearing. Strictly
speaking, fish have no ears. It is believed that the ability to hear
which the human being possesses is denied to them. In still another
respect, however, a more obvious deficiency makes itself apparent.
The brain is poorly equipped in mechanisms that could specifically
be called sense combiners. Some slight degree of combination between
the senses does take place, but this at best is meagre and simple.
Consequently the brain’s output, that is to say, its productive
turnover, is limited. It confines itself to those reaction patterns
with which we are familiar in the habits and behaviour of fish. The
limitations by which these patterns are restricted are evident in the
fact that the animal’s entire life programme is carried on largely
under water. If an attempt were made to estimate the capabilities of
the fish as a machine compared with other animals, it would almost
certainly receive a low rating. The justification of this low estimate
is obvious. The reasons for it are twofold: first, the relatively low
degree of development in each of the sense departments including the
lack in one department (sense of hearing); second, the poorly developed
sense combiner.
Professor Gregory has devoted much time in the American Museum of
Natural History to the study of the progressive stages from fish to
man, and especially to those changes which appear in the head. He has
shown that in this fish stage the animal at first had no lower jaw and
no teeth. Its mouth served as a sucking organ, which thus obtained food
in the form of minute organisms and small particles of organic matter.
Certain new patterns were introduced with the appearance of primitive
sharks. These animals had a lower jaw impregnated with lime salts,
thus made effective for supporting many successive rows of formidable
teeth. Such sharks also had well-developed gills. Certain lobe-finned
fishes of a somewhat later period (_Crossopterygian_) began to live in
streams and swamps. By means of their peculiar fins they were able to
crawl over the surface of the land, and thus they were the forerunners
of the next more completely air-breathing stage determined by the
appearance of the amphibians.
_The Beginning of Life on Land_
The second or amphibian stage came after those steps had been taken
which led certain modified forms of fish life to attempt a partial
adjustment to living on land and to breathing air. True amphibians
then made their appearance. Animals called tetrapods, or four-footed
creatures, were the result of this change. They were the forerunners of
all higher animals. By the slow conversion of their fins and paddles
into legs they acquired a new kind of transportation machinery. With
the aid of these four legs the animal could now hop about on land and
also swim in the water much as do the frogs. Such a transformation
had a profound effect upon the entire body, which became greatly
shortened and in many instances no longer possessed a tail (except in
the polliwog stage). The head also changed. New devices were necessary
for the purposes of air-breathing, which replaced the old method of
getting oxygen out of the water. One of the most important changes,
however, was the addition of the new sense of hearing. The amphibians,
living partly on land, were now able to receive useful information by
means of air waves. The advent of this new sense was destined to have
momentous effects upon the further development of the brain. Each of
the several sense departments is well represented in the frog. The
sense of smell is highly organized. It contains some improvements over
the fish for the reason that the animal is now able to scent odours
borne by the air. The sense of taste shows little if any improvement.
Compared with many of the fish it has actually receded. Body sense is
well provided for and shows certain refinements due to the fact that
it has taken on the new responsibility of sensing four legs. It also
has the duty of supervising what is going on in the muscular machine
when the animal performs its new kind of motion, hopping about over
the ground, leaping into the water, or using the new frog-method of
swimming. The department of the sense of sight shows some improvements
when contrasted with that of the fish. The frog is able to adjust its
vision both to air and water. While on land it is able to see many
things that never come into the range of the fish’s field of vision.
Some of the frogs even go so far as to have what is called a third
eye in the middle of the forehead. This organ, however, is but poorly
developed and serves more for light perception than for actual seeing.
The introduction of the sense of hearing, by establishing certain
innovations in the frog brain, provides an advantage over the fish. It
is, however, in furthering the development of the sense combiner that
the frog’s brain shows its most distinctive advance. The two great
hemispheres are now clearly outlined. The endbrain, in consequence of
land-living and air-breathing, has taken an important step forward. In
all further advances this part will bear the chief burdens of progress
and improvement.
The frog and his kind represent a machine that in many respects is
not much better organized than the fish. But amphibians did serve to
introduce advantages that were utilized in new adjustments to life;
such, for example, as living on land, breathing air, getting about on
four legs, and being able to hear. Besides this, the way was now opened
for a better type of sense combiner. There was promise, if not actual
profit, in these new amphibian endowments. Professor Gregory has shown
that among the most important changes in the amphibian head were those
which ultimately led to the formation of the ear. The skin in this
region was already beginning to act as a tympanic member or eardrum.
_Epoch of Giant Reptiles_
The third or reptile stage witnessed that critical advance that came
with the fully established habit of living on land. The amphibians,
both those which retained and those which lost the tail, took the
first somewhat hesitating steps in this direction. They were, however,
essential predecessors to the next higher order, the reptiles, which
upon their arrival stepped out boldly. During the remarkable Mesozoic
period these reptiles covered the earth with their dominating and
often hideous presence. No period compares with this one for the
awe-inspiring inhabitants that peopled the world. It was then that
the gigantic dinosaurs were the overlords of creation. Some of these
monstrous creatures were composed of many tons of flesh and bone. They
became the most terrific fighting machines ever produced by nature.
Even the tail, which had disappeared in many of the amphibians, became
prominent as part of the offensive equipment in these reptile monsters.
Gigantic size was an outstanding structural feature. But these huge
dimensions carried their own penalties. They were extremely hazardous
and destined to bring catastrophe. Even if some of the great reptiles
might have been thoroughly efficient fighting machines, they lacked
the essential advantages of progressive brains and brain power. In
this respect they had improved but little. That tremendous monster
_Tyrannosaurus rex_, the most destructive engine ever created, had a
body weighing many tons, with a brain of less than a pound.
The prolific Mesozoic reptiles inhabited the land and infested the
waters of the earth, its oceans and inland seas, its lakes and rivers.
They also for the first time attempted to realize the advantages
of another mode of life. Having adjusted their weird bodies to
the water and to the land, they next took to the air. Late in the
Permian or Triassic times (150,000,000 years ago) some lizard-like
reptiles, partially biped in habit and distantly related to the great
two-legged dinosaurs, assumed habits of life adapted in part to the
trees. Specialization of their fore limbs led to wing-like structures
for purposes of volplaning to the ground. Such modified fore limbs
eventually acquired the character of wings, and thus, according to
some authorities, the most ancient of known birds had their origin in
the Age of Reptiles. Many students of this subject believe that bird
life may have begun at an even earlier period.
More conservative and also far less conspicuous was another tendency
which developed in this reptilian age. For a long time it remained most
unpretentious. The spectacular development of huge animals for land and
sea held the centre of the stage. Mere size, however, is not always
sufficient for success and progress. In any event, a certain number
of relatively small reptiles began to show changes along entirely
different lines. At first it was difficult to discern the signs of
progress in them. Slowly, however, significant modifications came about
in two important details: First, in the readjustment of the fore and
hind legs, so that acting together they began to lift the body of the
animal clear off the ground. The second great change was an alteration
in the teeth, which were gradually specialized until they assumed the
characters recognized in those later animals known as mammals. These
two new traits, developed by relatively inconspicuous reptiles, led in
time to animals that became the actual forerunners of the mammals. They
are known as the pro-mammalian reptiles (_Cynodont_, _Theriodont_).
_Reptile Forerunners of the Mammals_
It is probable that while these momentous changes were in process an
equally important modification had begun. This change affected the
blood. It caused the blood cells to become smaller and at the same
time better conveyers of oxygen. These cells also began to lose their
nuclei. As a result, certain animals passed from a cold-blooded,
scaly reptilian condition to that of the warm-blooded, hair-covered
mammal. The constant warm temperature of the blood in these mammalian
forerunners must have been a decisive influence favouring the further
development of the brain.
In many respects the reptilian brain is inferior to that of the
mammals. All of its sense departments are fairly well represented.
The senses of smell and taste have made slight advances over the
amphibian stage. Body and contact senses have perhaps gained some
slight advantage over the previous period. In sight and hearing there
were some improvements. Collectively the reptilian mechanisms for
managing impressions obtained through the senses are considerably
better than those of such animals as the amphibious frog. At least one
of the reptiles (_Sphenodon_) developed a third eye in the middle of
the forehead. This is not, however, a highly efficient visual organ.
The sense combiner in the reptile also shows some advantage, although
in the main the reptilians appear to have acquired little more of
practical value, except greater speed and more power, than their
predecessors, the amphibians.
Even when reptile development took that bent which led to the
appearance of birds, the brain received but a slight benefit from this
adjustment to the air. Selective progress in the bird’s brain is
unquestionably found in that marked expansion involving the department
of sight. Body sense also expanded to meet the requirements of sensing
and balancing the body in flight. But to offset these advances both
the sense of smell and the sense of taste have undergone considerable
recession. Adaptive progress here, as in many other instances,
emphasized one department with some apparent loss of advantage in other
parts. Consequently the sense combiner, which ultimately produces
the most effective combinations of sense impressions, has shown no
conspicuous advantage among the birds.
_Disappearance of the Great Reptiles_
The reptile stage of life, especially in its most imposing phases,
witnessed but little advance in the progressive development of the
brain. During this period all of the great departments of brain
structure, such as the endbrain, the interbrain, the midbrain, and
the hindbrain, were retained and somewhat expanded. But that highly
important mechanism that was finally to act as the superbrain,
technically known as the neopallium (new outer coating of the brain,
the cortex), had not yet been acquired. It may be in part for this
reason that, as the Mesozoic period advanced, catastrophe was rapidly
overtaking many of the great reptilian groups. Of the eighteen orders
of reptiles that once filled the world, all but five were mysteriously
swept into oblivion. Why they passed is not yet clear. It may have
been due to great changes in the surface and climate of the earth
at different times. It may have been that the gigantic size of these
reptiles made the struggle for existence too severe or the food
supply too precarious. Whatever the cause, they all seem to have
paid the penalty of excessive specialization. The five orders which
have survived these destructive catastrophes include the snakes, the
crocodiles, the lizards, the turtles, and the lizard-like tuateras of
New Zealand.
Notwithstanding this wholesale destruction, there was a priceless
heritage handed down from the Age of Reptiles. This heirloom was the
beginning of the warm-blooded mammal, which slowly developed from
the humble pro-mammalian reptiles. It endowed the animals that were
to rule the next great period of the earth’s history with power to
get about on four feet, with increased ability to withstand great
changes of climate, with added capacities in preparing their food for
digestion. This last advantage depended upon a new kind of teeth which
the mammals inherited from their immediate reptilian ancestors. All of
the teeth possessed by primitive reptiles were fang-like (laniary),
used for seizing their prey or tearing their food. These reptiles
had no grinding teeth, and this condition left the responsibility of
digestion to the stomach and other organs. In most of the mammals
digestion begins in the mouth with actual mastication. The early
pro-mammalian reptiles (_Cynodonts_) were equipped with grinding teeth,
and their dental apparatus, as in all mammals, included incisors,
canines, pre-molars, and molars. Teeth such as these were important
items in the legacy received by the mammals from their ancestors, the
pro-mammalian reptiles.
_When the Warm-blooded Mammal Appeared_
In the fourth or mammalian stage, life entered upon the Age of Mammals
with all of these new endowments. Almost at once it began to show
signs of progress. It was in the brain that this progress became most
apparent. A new mechanism long in the making now came into existence.
This new structure may be rightly called the superbrain (neopallium),
since it soon proved to be the most decisive step yet taken in the
development of the sense combiner and in the further expansion of all
the senses. At first it did not make its appearance in any preëminent
manner. It came as an outer covering over the ancient parts of the
endbrain. Within it, however, were possibilities of expansion such
as were possessed by no other part of the brain. Ultimately it added
about twelve billion cells to be used in many different kinds of
brain activity. This addition was especially characterized by the
orderly arrangement of the cells, layer upon layer, almost as if each
successive layer imparted some new capacity for the management of life.
In its fully developed form this structure constitutes the cortex of
the hemispheres, and with its fibre connections makes up as much as
eighty per cent. of the entire brain.
It could hardly be expected, even after the first arrival of the
mammals, that this new brain addition would at once attain its
fullest development. In fact, the first attempts along this line were
feeble. A new and great production of weird mammals was in process.
It might almost seem as if the imposing shadows of the previous Age
of Reptiles still hung over these early mammalian experiments. Huge,
ungainly proportions were still the fashion. In many instances the
primitive mammals themselves developed gigantic and awkward bodies.
They were strange, unsightly beasts as we know them now from their
fossilized skeletons and from reconstructions of them. Were it
possible to reassemble them, what a sensation they would create in
our modern world. Even the best efforts of our foremost showmen would
be ineffective to describe those strange monsters of most unfamiliar
appearance, with their peculiar armours, their long unsightly horns and
tusks, their strange hoofs and claws. The mammoth, the mastodon, the
amblypod, the titanothere, the creodont, the sabre-toothed tiger, and
many others would be among them to excite wonder.
_The Paths to Extinction and Progress_
But all of these have passed, in part at least, because, like the
dinosaurs, they possessed inferior or unprogressive brains. Indeed,
many of the earliest mammals had brains that in some particulars
resembled those of the reptiles. They grew in size and power until they
became repulsive brutes, although their brains improved but little.
In many of them the superbrain developed only in a small way. It was
notable not for its size but for the position it occupied above more
ancient structures. In their struggle for life these huge beasts seemed
to be unable to adjust themselves to changing environment; so probably
when the conditions became too severe, not having the capacity to adapt
themselves, they failed to survive. Many orders of these animals became
extinct in the early part of the Age of Mammals (Oligocene and Eocene,
thirty million to sixty-five million years ago). Others, showing more
progressive tendencies, continued to advance, and their descendants
have come down into modern times. One striking difference between these
progressive and unprogressive mammals was certainly in the brain.
Wherever this organ remained primitive, wherever the superbrain was
only feebly developed, the fate of extinction seems to have been a
foregone conclusion. Such animals soon reached the end of their line.
But wherever the superbrain expanded, there the signs of progress were
unmistakable. One extremely important factor in the survival of most
of the mammals alive to-day was the progressive development in the
most recently acquired portion of the brain. Great practical results
were brought about by its expansion in the administration of brain
power. It produced, so to speak, the final consolidation of all the
sense departments under one roof. Reactions connected with the sense
of smell and of taste, which had so long depended upon the primitive
endbrain, marked this structure as the most advantageous location
for centralization. Whatever may have been the influences that
established this preference, here the departments of body and contact
senses, of sight and hearing, were finally organized. The effects of
this consolidation were immediately felt by the endbrain. It at once
became a superbrain in the truest sense. Rapid expansions in the actual
size of the hemispheres were the first signs of this new development.
Then came the process of convolution and folding to obtain more brain
room, and this for the same reason was followed by still more complex
convoluting. These advantages especially favoured contact sense, the
expansion of which was largely due to the fact that the mammal body
was now covered with a highly sensitive skin equipped with hair. Such
a skin was a new sensory device by which finer impressions of touch
might be conveyed to the brain. In this manner the animal was able to
form more complete judgments concerning objects with which it came
in contact. Little by little, these judgments of touch became more
critical and discriminating. A great range of understanding of the
world through touch sense was made available. One critical impression
of touch was added to another until complex judgments in this sense
were constructed. Similar expansions in the powers of vision, hearing,
and body sense led to their localization in this new part of the
brain. Their most effective activity soon required still further
extension, which ultimately, by the development of the frontal lobe,
made provision for the highest faculties. The mammals have thus shown
their progressive tendency in the acquisition of an efficient sense
combiner. Through their better sense capacities they have been able
to understand their surroundings more thoroughly than lower animals.
Consequently their energy turnover in the brain has resulted in a
better output by means of which they have made more ample adjustments
to life. All of this they have been able to accomplish because they
possessed a mechanism of incalculable value, the superbrain. Yet the
mammals have not in all cases utilized this mechanism to its full
extent. Its advantages have been applied in different ways and for
different purposes. In some instances they have been utilized for the
special adjustments of the hoofed animals, or in the hunting craft of
the great meat-eaters, or in that furtiveness of the moles, which seek
their protection by burrowing in the ground. The advantages of the
superbrain were applied to many other diverse specializations, such as
the adjustments of bats for flying, or of beavers, seals, whales, and
porpoises for living in the water.
_The Superior Brain of Mammals_
The mammalian brain has made possible a wide range of behaviour and
adjustment. This range exceeds that of the fish, amphibian, reptile, or
bird. Concerning the increased capacity of the mammals as a class there
seems to be no doubt. But this greater power of adaptability is also
true of every mammal. The differences in this respect between the lower
mammals, like the rat, the opossum, or the sloth, when compared with
the bird, the snake, the frog, or even the fish, may not be striking.
But when we contrast the actions and capabilities of such mammals as
dogs, horses, elephants, or any of the cat family with those of the
bird or snake the vast differences speak for themselves. A dog, for
example, has by comparison with lower vertebrates a greatly increased
capacity for getting on in life. He is capable of adapting himself to
many complications incident to his associations with man. He has a much
more ample repertoire of performances. He is capable of learning many
intricate accomplishments. In general, such learning is also true of
most of the higher mammals; it is particularly true of those having a
highly developed superbrain. Even aquatic mammals like the seals show a
remarkable degree of adaptability. They are among the most interesting
of trained performers. A casual glance is sufficient to reveal what an
excellent superbrain they possess. Elephants, in spite of their huge
proportions and awkwardness, are capable of remarkable adjustments.
Their brains are also highly developed.
Yet, however decisive the mammalian superiority in brain power may be
over the lower vertebrates, most of the mammals are held down by many
handicaps, restrictions, and limitations. They all possess a capacity
for broad adjustments to strictly limited conditions. For life in the
water, in the air, upon the plains, underground, or in the forest,
they may be well adapted. But the specializations of their own bodies
hold them to their specifically restricted adjustments. With trunk and
head, with hoof and paw, with wing and flipper, they may do the things
which these implements make possible. Here their opportunities cease.
In this way even the progressive mammals are confronted by serious
obstacles. These mammalian obstacles were difficult to overcome. Some
of the mammals, however, became specialized for a more varied kind
of life. They manifested a strong tendency to live chiefly in the
trees. This fact influenced their further adjustments profoundly. It
opened the way for new specializations in their limbs. It gave a new
direction to progress, which finally called upon the brain for its
supreme development. These important tree-living animals are the monkey
kind and the manlike apes. All of the events in adjustment preceding
this great epoch might be likened diagrammatically to a succession of
plateaus. Each plateau, beginning with that of the fish, then rising
to the level of the amphibians, of the reptiles, and finally of the
mammals, contributed some important elements to progress. From these at
length came the upper level of the apes, that plateau destined to give
rise to many varieties of primates, and also to afford those footholds
essential to the further upward climb of man.