Most of the inmates of prisons convicted of sex crimes are the poor and wretched and the plainly defective. Nature, in her determination to preserve the species, has planted sex hunger very deep in the constitution of man. The fact that it is necessary for the preservation of life, and that Nature is always eliminating those whose sex hunger is not strong enough to preserve the race, has overweighted man and perhaps all animal life with this hunger. At least it has endowed many men with instincts too powerful for the conventions and the laws that hedge him about.  
Rape is almost always the crime of the poor, the hardworking, the uneducated and the abnormal. In the man of this type sex hunger is strong; he has little money, generally no family; he is poorly fed and clothed and possesses few if any attractions. He may be a sailor away from women and their society for months, or in some other remote occupation making his means of gratifying this hunger just as impossible. There is no opportunity for him except the one he adopts. It is a question of gratifying this deep and primal instinct as against the weakness of his mentality and the few barriers that a meagre education and picked-up habits can furnish; and when the instinct overbalances he is lost.
 
Incest, which is peculiarly the crime of the weak, the wretched and the poor, has a somewhat different origin. Westermarck in his "History of Human Marriage" shows that in the early tribe there was no inhibition against the marriage of blood relations; that the restriction then was against the members of the tribe that used one tent; these might or might not be blood relations. The traditions and folk-ways against the marriage of close relations grew from the familiarity that came from the living together of brother and sister, for instance, in one home. This feeling gradually worked itself into custom and habit and from that into folk-ways and laws. Sometimes we read accounts of the marriage of a man and woman who found, after years had gone by, that they were brother and sister who had been separated in infancy and grew up ............