It is biologically possible that similar conditions may have existed in the earliest human life, but that is pure speculation and has no bearing on the practical problems of sex in human life to-day. The fact is that the simple physiological stimuli which produce sexual excitement in both sexes of animals have practically no influence in determining human sexual union. On the contrary, memory associations consciously connected with the opposite sex, especially those associations that are centered in affection, may at any time in the normal individual of either human sex afford the basis for a chain of mental states leading to sexual excitement and union. There is not, as in the animals, instinctive dependence on the physiological conditions that are favorable for fertilization. In fact, spontaneous physiological demands play in civilized human life a minor part in initiating sexual excitement. The reason why some humans seem to have unusual sexual intensity is not so much a matter of exceptionally strong sexuality as of susceptibility to the numerous sexual stimuli with which modern life abounds. For this reason, a man who has formed lewd memory associations is more susceptible to sexual stimulations, _e.g._, by obscene pictures, vulgar words, unusual dress or actions of women, close physical association as in dancing, and certain forms of music. It is not at all uncommon that individuals who are hyper-sensitive to sexually suggestive stimuli are really functionally weak. [Sidenote: Intelligent control only.] It follows from the facts outlined above that instinctive control of sexual actions applies to animals but not to human life. On the contrary, human control must be on the basis of intelligent choice. This means the greatest task of human life, for it requires voluntary control of instinctive demands which are intensified by numerous stimuli or temptations that are exclusively human. No wonder that natural sex hunger left uncontrolled leads human beings to excesses and degradation that no species of animals with their guiding instincts could possibly reach. [Sidenote: Individual responsibility.] The absence from human life of any instinctive control of sexual actions leaves a great responsibility on each individual whose natural desires lead impulsively and insistently towards sexual union and must be restrained, controlled, and directed by voluntary choice. In short, all individuals who are intelligent beings are personally responsible for voluntary control of their sexual desires with reference to the ethical, social, and eugenic interests and rights of all other individuals now and in the future. [Sidenote: Sexual knowledge necessary.]