On a later occasion he experienced once more the same sense of heaviness, and he lost no time in scratching the interior of his nose in order to provoke a return of the bleeding. The nosebleed thus excited again gave him entire relief from the unpleasant sensations, and upon the first convenient occasion he told his children and all his relatives about the successful results obtained from this curative procedure. Little by little this simple act, which was a first step in the healing art, developed into the intelligently and skilfully performed operation of venesection. Primitive man also increased his stock of knowledge in the healing art by reading attentively the book of nature,--_i.e._, by observing how animals, when ill, eat the leaves or stems of certain plants and thus obtain relief from their disorders. The virtues of a species of origanum, as an antidote for poisoning from the bite of a snake, were revealed, it is asserted, by the observation that turtles, when bitten by one of these reptiles, immediately seek for the plant in question and, after feeding upon it, experience no perceptible ill effects from the poisonous bite. The natives of India ascribe the discovery of the remarkable virtues of snakeroot (the bitter root of the ophiorrhiza Mungos) as an antidote for poisoning by the bite of a snake, to the ichneumon, a small animal of the rat species. The instinctive desire to escape pain taught man, as it does the lower animals, to keep a fractured limb at rest, thus giving the separated ends of the bone an opportunity to reunite; after which the limb eventually becomes as strong as it ever was. Simple as this mode of acquiring useful medical knowledge may appear to us moderns, there are good reasons for believing that hundreds of years must have elapsed before the accumulated stock of such experiences became really considerable. On the other hand, it is reasonable to suppose that this growth in medical knowledge took place more rapidly in certain tribes or races than in others, and that when, under the action of wars, the inferior men became tributary to those of greater intellectual powers, they acquired, through contact with their conquerors, additional knowledge at a much more rapid rate. One great hindrance, however, stood in the way of such progress. I refer to the deeply rooted belief, entertained by man in this primitive period of his existence, in the agency of malevolent spirits (demons) in the production of disease,--a belief which continued to exist for many thousands of years. Out of such a belief developed the necessity of discovering some practical method of appeasing the evil spirits and of thus obtaining the desired cure of the ills of the body. Usually some member of the tribe who had displayed special skill in the treatment of disease, and who at the same time was liberally endowed with the qualities which characterize