THE BRAIN OF APES AND OF MAN
A GREAT and undoubtedly very important difference between man and apes
is the much greater size of the brain in man. This difference is most
conveniently measured by filling the cavity of a skull, once occupied
by the brain, with shot or other such material, and then measuring the
bulk of the material required for that purpose. The unit which it is
convenient to use in all such measurements is the cubic centimetre,
because it is that used by scientific workers all over the world. A
cubic centimetre is a cube the side of which is a centimetre long, and
two and a half centimetres are equal to one inch. Moreover, if ever
one is doubtful as to just how much an inch is, one has only to get
hold of a halfpenny and mark off its breadth on a piece of paper. That
is an inch, and two-fifths of it are a centimetre. Using, then, cubic
centimetres as our units, we find that a good average European human
brain is of the bulk of 1500 units. The gorilla has a slightly larger
brain than the chimpanzee or the orang. Individual specimens differ a
good deal. This is noteworthy as showing a tendency of this important
organ to vary. One of good medium bulk measures 500 units, or a third
of that of the well-developed European. The size of European human
brains also varies within very large limits--about a third more and a
third less--that is, from about 1000 units to nearly 2000. Idiots have
abnormally small brains which are often deformed. We leave them aside
for the moment. Healthy European adults have been measured with a brain
of only 1000 units. Australian "black-fellows" have, it seems, in some
cases a brain which measures as little as 900 units, but in others it
reaches 1500. The skull of the fossil man from Pleistocene (possibly
Pliocene) gravels in Java (known as Pithecanthropus) had a capacity of
only 900 units.
If we suppose (as it is legitimate to do) that some specimens of
the gorilla may have a brain a third larger than the average we get
670 units for the biggest gorilla brain, and if we similarly assume
that the primitive human race of the Java gravel varied to the same
extent--namely, by a third more or less around 900 as the normal--we
find that the greatest size of the gorilla brain overlaps the smallest
of the Javan Pithecanthropus, whilst the largest of that race would
overlap not only the Australian but the smaller-sized brains of
Europeans. Hence, if we accept, as we must, the fact that the brain
of man and the man-like apes naturally varies greatly in volume in
different individuals, there is no absolute gap in regard to size
between the higher races of man and the apes. The difference is bridged
over by the lower races of man and the exceptional individuals of apes.
A remarkable feature in regard to man's brain is its growth. Since
it is contained in a bony box, which in the adult is firmly ossified
and incapable of expansion, it is obvious that the brain, too, must
cease growing when the bony box has closed in. In the apes this
occurs at an earlier age than in man. The brain-box has its sides
and roof constituted by a number of platelike pieces of bone, which
increase in area by addition to their margins, and finally meet each
other and grow into one another, forming an irregular notched line of
junction, which is called a "suture." The sutures themselves are often
obliterated by bony deposit in mature life. In man the bony plates of
the skull are separated by large membranous interspaces at birth--"the
fontanelles"--and by delay in the junction of the bony pieces the
expansion of the brain is permitted. About one-fourth of the cases
of idiocy reported upon by medical observers are accompanied by an
unusually small size of the brain-case (as small in some cases as 750
units), due to the premature closure of its bony walls at an unusually
early period of growth. It, indeed, seems (though this is a suggestion
rather than a demonstrated conclusion) that the increase of the size
of the brain in normal men, as compared with apes, and the consequent
development of increased mental capacity in man, may be directly set up
by a delay in the ossification of the walls of the brain-case in man,
as compared with his ape-like progenitors.
One of the most definite distinctions between present man and
the higher apes is the length of time during which the period of
growth--namely, "childhood"--and the subsequent adolescent stage of
development is prolonged. The chimpanzee "Sally" was full-grown and
adult at eight years of age. Savage races show maturity at an age which
seems to Europeans astonishing--sometimes as early as the eleventh
year. But even within the European area there is great variation
in this matter, the Southern people maturing more rapidly than the
Northern. There certainly is a tendency in modern civilization to
defer the recognition of emergence from childhood, though whether
the physical facts of growth and maturity of structure justify such
a delay is not obvious. The history of our schools and universities
and the records as to the age at which marriage takes place bear
evidence of this modern increase of the duration of adolescence. In
any case, whether the prolongation of the period of physical growth
and development is even now still being increased, it is certain that
the extension has taken place in former ages, and that the mental
development of man is directly related in the first place to this
increased period of growth, and in the second place to the prolongation
of the period of organized "education" directed by the elder
generation. The brain of the human child at four years of age may not
infrequently reach as much as 1300 units in volume--more than double
that of a full-grown gorilla--and it continues to increase in volume
for some eight years, though it is difficult to say precisely when the
interlocking of the bony pieces of the skull reaches a point when they
can no longer yield to the expansion of the brain. The increase of the
cavity of the skull practically ceases in childhood, and the increase
in the size of the head subsequently is due to the increased size of
muscles and fibrous structures on the outer surface of the brain-box.
True as it is that man's brain is much larger than that of the higher
apes, it is also true that the difference is far greater between the
higher apes and the lower monkeys both as to the size of the brain and
the complexity of the folds and furrows which mark the surface of the
cerebral hemispheres. In these respects, as in every other anatomical
feature, as was insisted by Huxley, there is less difference between
man and the higher apes than between the higher apes and the lower
monkeys, so that there is no pretext for placing man in a group apart
from the apes and monkeys or for suggesting the existence of any great
structural chasm between man and apes; on the contrary, their likeness
in all important details of structure is very close.
The comparison of the size of the brain in various cases which has just
been made is one of absolute size, leaving out of consideration the
size and weight of the body and limbs. Putting aside the exceptional
pygmy races of man (which there is no reason to regard as primitive),
the average adult man is larger and heavier than the chimpanzee, and
taller than, though not so powerful as, the orang. The gibbons are
quite small--rarely 3 feet in height--but the male gorilla is, when
adult, a much heavier animal than man, and often measures 5 feet 8
inches from the heel to the top of the head. Recently even larger
specimens have been measured, and 6 feet 6 inches is quoted (probably
an over-estimate) as the height attained by some specimens. This fact
removes any difficulty about comparing the absolute size of brain in
man and these apes. It also renders it unlikely that the primitive
ape-men or men-apes were smaller than modern men, whilst the large size
and weight of some of the earliest "shaped" flints (of Pliocene age)
attributed to primitive man, make it probable that the men who used
these flints were larger and more powerful, at any rate in the hands
and arms, than modern races of men. Size and strength are, then, not
points which offer any difficulty in the passage from ape to man.
What (it may well be asked) is the significance of man's greater brain?
What was the advantage to man's ape-like progenitors in an increased
volume of brain? It should be noted at once that the pattern of the
"convolutions" marked out on the surface of the brain by a great
series of winding "ditches" or "furrows" is based on one common plan
in the group of monkeys and man--a plan differing from that seen in
other groups which have a convoluted brain-surface--for instance from
that seen in the carnivora (dogs, bears, and cats) and again from
that seen in the ungulates (hoofed mammals). The convolutions of the
brain of the higher apes have been minutely compared with those of
man's brain. The two sets of convolutions agree very closely, but are
less extensive in the apes and certain small tracts of convolutions
present in man, are deficient in the apes, especially in the frontal
region and at the hinder or occipital region. We know very little of
the exact significance of each region of convolution in the brain.
The existence of convolutions separated by furrows clearly enough
increases the amount of surface of the brain, which consists of a
grey substance called "the cortex of the brain," and is known to be
a peculiar and specially active material. The mere comparison of the
size and height of the frontal region in different animals and in man
justifies the conclusion that an increase of this part of the brain
is more especially related to increased intelligence. Further, the
facts derived from observation of the consequences of disease or of
mechanical injury in man have led to the conclusion that the "faculty
of language" (the significant use of words, not the mere production
of them as sounds) is especially connected with one of the frontal
convolutions, which is feebly represented in the apes. The convolutions
of the brain of lower races of men have not been very fully studied,
but the brain of a Hottentot woman was long ago carefully described
and illustrated, showing less complexity of the convolutions than is
usual in European man, and making a distinct approach in this respect
to the apes; but still possessing in fair proportion the convolutions
characteristic of the human brain.
Abundance of convolutions and their increase at this or that part of
the brain must, it is obvious, increase the active brain substance.
But there is some evidence of a special kind as to the significance
of increased bulk of the entire brain, apart from the folding of its
surface. This is afforded by the brain cavities of the skulls observed
in the series of vertebrate animals. The older groups--those "lower,"
that is farthest removed from man and the animals most like him--have
in proportion to the bulk of their bodies much smaller brains than
the later-developed groups. Thus fishes have smaller brains than
reptiles, and these have much smaller brains than mammals. A cod-fish
has in proportion to its bulk of living material a smaller brain than
a crocodile or a turtle, and these have a much smaller brain than a
pig. Not only so, but earlier kinds of mammals than the pig have a
smaller brain proportionately than that animal has, and pigs have a
smaller brain in proportion to their bulk than monkeys, and monkeys
(as we have seen) a smaller brain than man. This increase of size is,
in general, proportionate to an increase in the variety and complexity
of the control of the movements of the body and their relation to the
activities of the great organs of sense, such as the eyes, and the
organs of smell and hearing.
But there is something more involved in the increase of the brain
than this. We now know that the brain of very many kinds of animals
has been increasing in size in the later geological periods. Huge
reptiles as big as elephants existed on the land surface of the
globe before the hairy, warm-blooded mammals which now dominate the
situation had developed in number or in size--namely, in the period of
and before the chalk which geologists call the Mesozoic or secondary
period, to distinguish it both from the tertiary period, when mammals
were abundant and large, and from the Palæozoic or primary period,
at the end of which terrestrial vertebrates first began to make
their appearance. These huge reptiles--such as the Iguanodon, the
Triceratops, and the Diplodocus (all to be seen in skeleton, though
not in the flesh, at the Natural History Museum)--had brains of an
incredibly small size, much smaller in proportion to their bulk than
those of living reptiles, such as lizards and crocodiles. The same
extraordinary difference of size of brain is seen when we compare the
large living mammals with their equally large extinct forerunners in
the early tertiary strata. The skulls and whole skeletons of great
rhinoceros-like animals--some of them ancestrally related to our living
rhinoceroses--are dug up in early tertiary sands and clays, which have
absurdly small brains. We can take a mould of the interior of the brain
cases of these extinct animals and compare them with that of the recent
rhinoceros. We find that the extinct animal's brain was in many cases
only one-eighth the bulk of that of its modern representative!
The same disproportion in the size of the more ancient animal's brain
is found when we compare the brain of the modern horse with that of
its early tertiary ancestors. The modern animal has, as a rule, a
very greatly increased size of brain when compared with its Miocene
forefather. In fact, it seems that the brain has had, as it were, an
independent development in several lines of descent, and whilst the
rest of the structure of the ancestral form has been only slightly
modified in its proportions, the brain cavity and the brain within it
has enormously increased. It is therefore not so exceptional a thing
as it at first appears--but only an instance of a change more or less
widely exhibited among later animals, as compared with their near
relatives in the past--when we establish the fact that the brain of
the man-like apes is much bigger than that of lower monkeys, and that
the brain of man, who is so closely similar in all structural details
to those apes, has attained to a bulk three times that of the ape. The
vast increase in the size of the brain in recent animals, as compared
with their closely related representatives of an earlier period, is a
frequent and regular thing. It is possible to make a suggestion, of
some plausibility, as to the meaning and value of this increased size
of brain, which will be found in the next chapter.