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.

A fair idea of the size of the brain may be obtained by measuring the cranial capacity.  This varies in man from almost one-hundred cubic inches to less than seventy.  In the gorilla its average is perhaps thirty, in the orang and chimpanzee rather less, about twenty-eight.  This is certainly a vast difference, especially when we remember that the gorilla far exceeds man in weight.

Le Bon tells us that of a series of skulls forty-five per cent, of the Australian had a cranial capacity of 1,200 to 1,300 c.c., while 46.7 per cent. of modern Parisian skulls showed a capacity of between 1,500 and 1,600 c.c.  The skull of the gorilla contains about five hundred and seventy cubic centimetres.  Broca found that the cranial capacity of 115 Parisian skulls, of probably the higher classes from the twelfth century, averaged about 1,426 cubic centimetres, while ninety of those of the poorer classes of the nineteenth century averaged about 1,484.  His observations seemed to prove that there has been a steady increase in Parisian cranial capacity from the twelfth to the nineteenth century.

Turning to the actual weight of the brain, that of Cuvier weighed 64.5 ounces, and a few cases of weights exceeding 65 ounces have been recorded.  The lowest limit of weight in a normal human brain has not yet been accurately determined.  From 34 to 31 ounces have been assigned by different writers.  The brain of a Bush woman was computed by Marshall at 31.5 ounces, and weights of even 31 ounces have been recorded without any note to show that the possessors were especially lacking in intelligence.  As Professor Huxley says in his “Man’s Place in Nature,” a little book which I cannot too highly recommend to you all, “It may be doubted whether a healthy human adult brain ever weighed less than 31 or 32 ounces, or that the heaviest gorilla brain has ever exceeded 20 ounces.  The difference in weight of brain between the highest and the lowest men is far greater, both relatively and absolutely, than that between the lowest man and the highest ape.  The latter, as has been seen, is represented by 12 ounces of cerebral substance absolutely, or by 32:20 relatively.  But as the largest recorded human brain weighed between 65 and 66 ounces, the former difference is represented by 33 ounces absolutely, or by 65:32 relatively.”