This is a book incorporating the results of ten years' research by a group of American investigators, carefully selected by the Committee on Research in Problems of Sex of the Division of Medical Sciences, National Research Council. Specialization in research has attained the point where any detailed authoritative survey requires a group effort. Consequently, the editor has gathered together a group of investigators whose work has established them in their respective fields. The body of the book is divided into nineteen chapters, and each contributor has developed his chapter in his own way, and assumes full responsibility for the content of his section, including his discussion of the work of other investigators.
At the onset it is admitted that a brief definition of the biologic conception of sex is impossible. Not "sex" but sexes are discussed, since there is no such biologic entity as sex. What exists in nature is