The material in this book is an expansion of a lecture given at the Army Medical Center, Washington, D.C., on May 15, 1933. I have thought it best to leave it in the somewhat informal discourse of the lecture platform. References to the literature, and other annotations, are numbered consecutively and placed together at the end of the book. It should be pointed out here at the start, as it is in the text, that the author is not a medical man, but merely a biologist greatly interested in human biology; aware of his deficiencies in knowledge and experience consequent upon not having an equally lively sense of his inalienable right as a biologist to study man, the most interesting of all animals.