These fields--biology, ethnology, and psychology--are so complicated and so far apart technically, although their social implications are so closely interwoven, that it has seemed best to divide the treatment between three different writers, each of whom has devoted much study to his special phase of the subject. This leads to a very simple arrangement of the material. The first part deals with the physical or biological basis of the sex problem, which all societies from the most primitive to the most advanced have had and still have to build upon. The second part deals with the various ideas man has developed in his quest for a satisfactory adaptation of this physical basis to his own requirements. Part three attempts to analyze the effect of this long history of social experimentation upon the human psyche in its modern social milieu. In the social evolution of the human mind, the deepest desires of the individual have been often necessarily sacrificed to the needs of the group. Sometimes they have been unnecessarily sacrificed, since human intelligence is, unfortunately, not omniscient. Nevertheless, the sum total of human knowledge has now become great enough so that it is at least well to pause and take account of its bearing on the age-old problem of family life, in order that our evolution henceforth may be guarded by rational control rather than trial and error in so far as is possible. Such a summarization of our actual knowledge of the biology, sociology and psychology of the foundations of the family institution this book aims to present, and if it can at the same time suggest a starting point for a more rationalized system of social control in this field, its purpose will have been accomplished. THE AUTHORS. CONTENTS PART I BY M. M. KNIGHT, PH.D. THE NEW BIOLOGY AND THE SEX PROBLEM IN SOCIETY CHAPTER I. THE PROBLEM DEFINED What is sex? A sexual and mixed reproduction. Origin of sexual reproduction. Advantage of sex in chance of survival. Germ and body cells. Limitations of biology in social problems. Sex always present in higher animals. Sex in mammals--the problem in the human species. Application of the laboratory method. II. SEX IN TERMS OF INTERNAL SECRETIONS Continuity of germ plasm. The sex chromosome. The internal secretions and the sex complex. The male and the female type of body. How removal of sex glands affects body type. Sex determination. Share of the egg and sperm in inheritance. The nature of sex--sexual selection of little importance. The four main types of secretory systems. Sex and sex instincts of rats modified by surgery. Dual basis for sex. Opposite sex basis in every individual. The Free-Martin cattle. Partial reversal of sex in human species. III. SEX AND SEX DIFFERENCES AS QUANTITATIVE