So far, the study of this subject has rested on the assumption that man must be just as we find him, that man behaves just as he chooses, and that, if he does not choose to behave as he does, he can stop. Therefore, when we discovered that human behavior in the sex-relation was productive of evil, we exhorted the human creature to stop so behaving, and have continued so to exhort for many centuries. By law and religion, by education and custom, we have sought to enforce upon the human individual the kind of behavior which our social sense so clearly showed was right. But always there has remained the morbid action. Whatever the external form of sex-union to which we have given social sanction, however Bible and Koran and Vedas have offered instruction, some hidden cause has operated continuously against the true course of social evolution, to pervert the natural trend toward a higher and more advantageous sex-relation; and to maintain lower forms, and erratic phases, of a most disadvantageous character. Every other animal works out the kind of sex-union best adapted to the reproduction of his species, and peacefully practises it. We have worked out the kind that is best for us,—best for the individuals concerned, for the young resultant, and for society as a whole; but we do not peacefully practise it. So palpable is this fact that we have commonly accepted it, and taken it for granted that this relation must be a continuous source of trouble to humanity. “Marriage is a lottery,” is a common saying among us. “The course of true love never did run smooth.” And we quote with unction _Punch’s_ advice to those about to marry,—“Don’t!” That peculiar sub-relation which has dragged along with us all the time that monogamous marriage has been growing to be the accepted form of sex-union—prostitution—we have accepted, and called a “social necessity.” We also call it “the social evil.” We have tacitly admitted that this relation in the human race must be more or less uncomfortable and wrong, that it is part of our nature to have it so. Now let us examine the case fairly and calmly, and see whether it is as inscrutable and immutable as hitherto believed. What are the conditions? What are the natural and what the unnatural features of the case? To distinguish these involves a little study of the evolution of the processes of reproduction.