No doubt, this is a very striking difference, but it loses much of its
apparent systematic value, when viewed by the light of certain other
equally indubitable facts respecting cranial capacities.

The first of these is, that the difference in the volume of the cranial
cavity of different races of mankind is far greater, absolutely, than
that between the lowest Man and the highest Ape, while, relatively, it
is about the same. For the largest human skull measured by Morton
contained 114 cubic inches, that is to say, had very nearly double the
capacity of the smallest; while its absolute preponderance, of 52 cubic
inches--is far greater than that by which the lowest adult male human
cranium surpasses the largest of the Gorillas (62 - 34-1/2 = 27-1/2).
Secondly, the adult crania of Gorillas which have as yet been measured
differ among themselves by nearly one-third, the maximum capacity being
34.5 cubic inches, the minimum 24 cubic inches; and, thirdly, after
making all due allowance for difference of size, the cranial capacities
of some of the lower Apes fall nearly as much, relatively, below those
of the higher Apes as the latter fall below Man.

Thus, even in the important matter of cranial capacity, Men differ more
widely from one another than they do from the Apes; while the lowest
Apes differ as much, in proportion, from the highest, as the latter
does from Man. The last proposition is still better illustrated by the
study of the modifications which other parts of the cranium undergo in
the Simian series.

It is the large proportional size of the facial bones and the great
projection of the jaws which confers upon the Gorilla's skull its small
facial angle and brutal character.