The marked tendency toward physical degeneracy among the men and women of all civilised races has been the constant theme of physiologists, therapeutists and other specialists; endless explanations have been put forward to account for it, and countless remedies suggested to counteract it. In this question, as in the details of medicine and surgery, the general inclination of the human mind is always towards a treatment of epidemic symptoms, towards vague generalisations in the diagnosis and treatment of individual symptoms, whether the word “individual” in this case refers to a specific sufferer or a correlated class of diseases, towards a regard of effects rather than of causes. As a reaction against this long-accepted method of dealing with individual symptoms by differentiated treatment, there has arisen a great diversity of so-called “mind-healers,” whose _a priori_ methods and lack of any clearly conceived system have brought their efforts into disrepute. Such were the conditions which over twenty years ago I sought to understand, believing—as I still do—that the whole human race was at some great psycho-physical turning point in its history, and that if the true nature of this evolutionary stage could be understood, it might and should be possible to direct man’s physical and mental progression and so combat, and in time eliminate, a thousand evils which seem to have no counterpart in the world of the lower animals, save in very exceptional cases. In embarking upon this enquiry I realised from the outset that I was dealing not with a world-wide epidemic but with a stage of progress, and that it was essential therefore that I should at once discard all theories which advocated, implicitly or explicitly, a return to similar conditions. Evolution knows no such return to extinction. The species must go forward to a triumphant perfection, or give place to a more dominant, more complete, self-controlled type.