I looked into this problem from many viewpoints and there seemed to be no solution until it occurred to me to seek the explanation in certain of the postulates which make up the doctrine of evolution. I realize fully the difficulty and the danger in attempting to reach the generalization which I shall make later and in the hypothesis I shall propose, for there is, of course, no direct final proof of the truth of even the doctrine of evolution. It is idle to consider any experimental research into the cause of phenomena that have developed by natural selection during millions of years. Nature herself has made the experiments on a world-wide scale and the data are before us for interpretation. Darwin could do no more than to collect all available facts and then to frame the hypothesis by which the facts were best harmonized. Sherrington, that masterly physiologist, in his volume entitled "The Integrative Action of the Nervous System," shows clearly how the central nervous system was built up in the process of evolution. Sherrington has made free use of Darwin's doctrine in explaining physiologic functions, just as anatomists have extensively utilized it in the explanation of the genesis of anatomic forms. I shall assume, therefore, that the discharge of nervous energy is accomplished by the application of the laws of inheritance and association, and I conclude that this hypothesis will explain many clinical phenomena. I shall now present such evidence in favor of this hypothesis as time and my limitations will admit, after which I shall point out certain clinical facts that may be explained by this hypothesis.