The increasing tendency to the upright position in Eoanthropus Dawsoni and Pithecanthropus Erectus to say nothing of the men of Cromagnon--led man to use as supports for his back the walls of his rough caves which he had adopted as dwellings instead of the branches of trees and the nests of the ape. He no longer affected entirely those hardy habits of sitting without support for his back that were _de rigueur_ in his ancestors, who probably looked upon him with as much disapproval as certain erect old ladies of the old school display towards the use of easy chairs by the rising generation. Wearied with the struggle for food, and against his savage rivals, he rested his back against the sides of his rude abode. When he slept in this attitude the relaxation of his voluntary muscles allowed mechanical forces to come into action which tended to oppose the downward trend of the hair. We know from our own experience that when sitting asleep with our backs supported there always occurs a certain amount of sinking down of the trunk. In this attitude are present, then, such conditions of the back and its hairy covering as give rise to mechanical forces which would interfere with the direction of the hair. These are, a heavy body, tending to slip downwards slightly while resting against a fixed surface, a growing tissue easily diverted from its normal course, and many hours spent in the attitude in question. The effects of these conditions increased with the increasing tendency of developing man to attend to his bodily comfort.