the brain, the erect position has been assumed gradually and naturally, and to maintain it has necessitated many other changes in skeleton and muscles; for example, the pelvis has broadened to support the intestines, which bear downwards instead of upon the abdominal walls; a double curve has arisen in the axis of the vertebral column, giving an easier balance to the upper part of the body and the head.  Countless structures of the human frame testify to an originally four-footed position and to a rotation of the longer axis through an angle of ninety degrees, as evolution has produced the human type.

The conclusion that the human brain has made mankind is thus established as one of fundamental importance.  Proceeding further, we learn that this organ proves to be essentially the same as the brain of lower primates; it does not gain its greater size and efficiency by the origination of wholly new and unique parts, but solely by the further elaboration of the ones present in lower forms.  In a word, it is only a difference in degree and not in essential kind that separates man from the apes and other primates.  Human nature is animal nature, and human structure is animal structure, for nowhere can final and absolute differences be found.  This does not mean that no differences appear, for it would be absurd to contend that man and the apes are identical in every respect; but it does mean that the resemblances are fundamental and comprehensive, and any details of dissimilarity are in the degree of complexity only.  The supreme place in nature attained by man is therefore due to progressive evolution in the nervous system.  The other systems have degenerated to a greater or less degree, but such regressive changes are more than compensated for by the superior control exerted by the improved brain.  In purely physical and mechanical respects, the human body is a degenerate as compared with a gorilla; the arm of the latter is more powerful than the lower limb of the former, while the gorilla’s chest is more than twice as broad as the human, and more than four times as capacious.  It is not through superior physique, but by superior ability to direct the activities of his body, that man excels in the struggle for existence with the lower animals.

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Moreover, the human body is a veritable museum of rare and interesting relics of antiquity.  This characterization is justified by those vestigial and rudimentary structures that represent organs of value to human relatives among the lower animals, though they play a less active part at the present time in human economy.  There is scarcely a single system that does not exhibit many or fewer of these rudimentary structures, but only a few need be specified.  As compared with those of the apes, the human wisdom teeth are degenerate; in the gorilla they are cut at the same time as the other molars; and in the lower human races they come through the gums in early youth,