The hair-clad skin of primitive man provided ample raw material for the eventual differentiation of both end-organs and sensorial areas which is found to-day. Not only did he possess what is called Common Sensation in his skin but in the individual hairs lay a delicate tactile structure, which, though probably inferior in delicacy, serves a similar purpose to that of the vibrissæ on the muzzle of Felidæ. Each hair, being deeply inserted into the skin and supplied with fine nerve fibrils, when it is bent, acts as a lever communicating an impulse to an afferent nerve trunk In an animal covered with thick hair the sensory impulse conveyed might be exceedingly delicate, but, from the nature of the case, of much more limited range than in one like man in whom the hair is so greatly diminished in length and thickness. It would be fruitless to speculate as to which of these four forms of stimuli was the earliest to become effective in developing man. Cold and Pain. Two of them, cold and pain, may be termed _nocuous_; one, that of touch, _useful_, and one, that of warmth, _indifferent_. If it be true, as Professor Scott Elliott states,[86] that man’s earliest home had a climate which “lies between the regular tropical, with wet, steaming, impassable jungles, and the colder temperate zone, so affording chance of acclimatisation in both directions,” the stimuli of cold would even then not be wanting, however much they increased in severity when he passed through glacial periods; but wherever, whenever, and at whatever time he first became man he had to tread the Via Dolorosa in the course of his hard and eventful life, and must have been well accustomed in all regions of his skin to the stimuli of pain, working, as he did, for his living, and fighting for it and his mate, with varied and powerful enemies. Though it is correct to call both these fundamental stimuli “nocuous,” this is all a matter of degree, and both the stimulus of moderate cold, raising blood-pressure and activating metabolism, and that of minor pains, would do little else than good in his education for the higher terrestrial life to which he had descended. If he was to learn effectually to take care of himself the discipline of both moderate cold and pain would be as valuable to him then as in its measure it is to his descendant to-day. The triumphs of medicine and surgery could never have appeared if it were not for the beneficent warning voice of pain that so generally accompanies disease. [86] _Prehistoric Man and His Story_, p. 92.