The widespread interest in the study of children has inspired in recent years a considerable literature bearing upon the original and inherited tendencies of human nature. The difficulty of distinguishing between what is original and what is acquired among the forms of behavior reported upon, and the further difficulty of obtaining accurate descriptions of the situations to which the behavior described was a response, has made much of this literature of doubtful value for scientific purposes. These studies have, nevertheless, contributed to a radical change in our conceptions of human nature. They have shown that the distinction between the mind of man and that of the lower animals is not so wide nor so profound as was once supposed. They have emphasized the fact that human nature rests on animal nature, and the transition from one to the other, in spite of the contrast in their separate achievements, has been made by imperceptible gradations. In the same way they have revealed, beneath differences in culture and individual achievement, the outlines of a pervasive and relatively unchanging human nature in which all races and individuals have a common share. The study of human nature begins with description, but it goes on from that point to explanation. If the descriptions which we have thus far had of human nature are imperfect and lacking in precision, it is equally true that the explanations thus far invented have, on the whole, been inadequate. One reason for this has been the difficulty of the task. The mechanisms which control human behavior are, as might be expected, tremendously complicated, and the problem of analyzing them into their elementary forms and reducing their varied manifestations to precise and lucid formulas is both intricate and perplexing. The foundation for the explanation of human nature has been laid, however, by the studies of behavior in animals and the comparative study of the physiology of the nervous system. Progress has been made, on the one hand, by seeking for the precise psycho-chemical process involved in the nervous reactions, and on the other, by reducing all higher mental processes to elementary forms represented by the tropisms and reflex actions.