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.
CHAPTER XXIX
THE MIND OF APES AND OF MAN
JUST as man's brain is enormously larger than that of the ordinary monkeys, although his general make and anatomy is closely similar to theirs, so we find that the rhinoceros has an enormous brain as compared with extinct rhinoceros-like animals, the predecessors and ancestors of those now living. The extinct Titanotherium of the lower Miocene period managed to carry on its life in an efficient way and to hold its own for a considerable period with a brain which was only one-eighth the bulk of that of a modern rhinoceros, as did other animals in the past with even greater bodies and smaller brains. To get some suggestion as to the significance of this fact we must, in however incomplete a way, distinguish some of the main features of the mental processes which go on in man and animals and have their "seat" in the brain.
Descartes and other philosophers have held that there is a great difference in the mental processes of animals as compared with those of man in this, namely, that man is "conscious," that is to say, conscious of himself as "I," and, as it were, looks on at himself acting on and being acted on by surrounding existences, whilst (so it is assumed) animals have not this consciousness, but are "automata," going through all the processes of life, and even behaving more or less as man does in similar circumstances, yet without being "conscious." It is, no doubt, true that many of the complicated actions of insects are carried on without consciousness of the purpose or significance of what they are doing. Such is the storing by certain wasps of smaller insects in carefully-cut chambers, to serve as food for the wasp's young, to be hatched from an egg to be laid in the "cold-storage chamber." The mother wasp will go on doing this when she has had the hind part of her body removed and has no eggs to lay. This mechanical unreasoning behaviour in insects is without exception, so that we must accept M. Fabre's conclusion that they are, in fact, unconscious "automata." I have already referred to this subject in an earlier chapter, p. 197.