CHAPTER XXVIII

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.