Since bisexual behavior is a recognized fact among all mammals, Claude Aron believes that it had to be the expression of an essential characteristic of the sexuality of these species. He shows how nature has created a unique sexual mechanism among mammals, the differenciation between males and females ocurring through the inhibition of the other sexuality. The development of a male sexuality, - anatomy, physiology and behaviour - is obtained by inhibiting the development of the female aspects. Endocrinian manipulations have shown that it is possible to change from one sexuality to another in a reversible manner. In this book, Claude Aron, who is a specialist in the physiology of reproduction, attempts to understand the mysteries of our sexual behavior.