But you will naturally ask, "Is there not, after all, a vast difference between the brain of man and that of the ape?" Let us examine this question as fully as our very brief time will allow. Considerable emphasis used to be laid on the facial angle between a line drawn parallel to the base of the skull and one obliquely vertical touching the teeth and most prominent portion of the forehead. Now this angle is in man very large--from seventy-five to eighty-five degrees, or even more, and rarely falling below sixty-five degrees. But this angle depends largely on the protrusion of the jaws, and varies greatly in species of animals showing much the same grade of intelligence. In some not especially intelligent South American monkeys the facial angle amounts to about sixty-five degrees. In this respect the skull of a chimpanzee reminds us of a human skull of small cranial capacity and large jaws, in which the cranium has been pressed back and the jaws crowded forward and slightly upward.
The weight of the brain in proportion to that of the body has been considered as of great importance, and within certain limits this is undoubtedly correct. Thus, according to Leuret, the weight of the brain is to that of the whole body: In fish, 1:5,668; in reptiles, 1:1,320; in birds, 1:212; in mammals, 1:186. These figures give the averages of large numbers of observations and have a certain amount of value. But within the same class the ratio varies extraordinarily. Thus the weight of the brain is to that of the whole body: In the elephant, 1:500; in the largest dogs, 1:305; in the cat, 1:156; in the rat, 1:76; in the chimpanzee, 1:50; in man, 1:36; in the field-mouse, 1:31; in the goldfinch, 1:24.
From this series it is evident that the relative weight of the brain is no index of the intelligence of the animal. Indeed if the brain were purely an organ of mind, there is no reason that it should be any larger in an elephant than in a mouse, provided they had the same mental capacity. As animals grow larger the weight of the brain, relatively to that of the body, decreases, and considering the size of man it is remarkable that it should form so large a fraction of his weight. Still the fraction in the chimpanzee is not so much smaller. It is still possible that this fraction is above the normal for the chimpanzee, for some of the observations may have been taken on animals which had died of consumption or some other wasting disease. I have not been able to find whether this possibility of error has been scrupulously avoided.