It is not easy to suggest how the reduction in size of the canines and front teeth and of the length of the jaw could be of such advantage to incipient man as to lead to the survival of those individuals in which these parts were least developed, and so gradually to the crowding of the teeth, reduced in size, into a jaw of reduced length, whilst at a late stage, long after man was man and no ape, the teeth became so reduced in volume as to leave the lower margin of the lower jaw—projecting far in front of them as the "chin," the eminently human chin. The nutrition of these parts placed in the head near the brain, the great canine having so vast a fang that it reaches up to the eye-socket, whence it is called the "eye-tooth," renders it probable that there is a relation depending on nutrition and blood supply between them and that all-important organ contained in the neighbouring bony box, the brain. As the great teeth and long jaw have dwindled, the brain has increased in volume, and, what is more important, in activity.
Other neighbouring bony structures have dwindled whilst the brain has increased. The great longitudinal and transverse crests of bone seen on the skull of the gorilla may never have existed in that form of ape from which man is derived, but a tendency to such ridge-like outgrowth and to a greater thickness of the bony wall of the brain-case characterizes apes as distinguished from men, and its disappearance is one of the changes which have accompanied the expansion of the brain-case and the increased size of brain in man. Lower races of existing men have frequently thicker skulls than the higher races. The bony development of the skull in the higher apes is especially remarkable in the region just above the eye. The upper border of the orbit is greatly thickened, and projects as a bony arch overhanging the eye. But the extent of this growth, as also of crests on the skull, varies in individuals, and is much smaller in females than in males. In the young these ridges and prominences are absent. It is accordingly no very great change that they should disappear altogether in man, even were they as large in the ape-like ancestor of man, which probably they were not. But the existence of a considerable thickening and forward growth of the eyebrow region of the skull is noticed in many human skulls. It is particularly large in some skulls of Australian "black-fellows," and is still larger in and characteristic of the ancient species of men of the Moustierian period in Europe, Homo Neanderthalensis.
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.