Yet the calamity would not be so threatening if the effect of sexual instruction were really confined to the putrid influence on the young imagination. The real outcome is not only such a revolution in the thoughts, but the power which it gains over action. We have only to consider the mechanism which nature has provided. The sexual desire belongs to the same group of human instincts as the desire for food or the desire for sleep, all of which aim toward a certain biological end, which must be fulfilled in order to secure life. The desire for food and sleep serves the individual himself, the desire for the sexual act serves the race. In every one of these cases nature has furnished the body with a wonderful psychophysical mechanism which enforces the outcome automatically. In every case we have a kind of circulatory process into which mental excitements and physiological changes enter, and these are so subtly related to each other that one always increases the other, until the maximum desire is reached, to which the will must surrender. Nature needs this automatic function; otherwise the vital needs of individual and race might be suppressed by other interests, and neglected. In the case of the sexual instinct, the mutual relations between the various parts of this circulatory process are especially complicated. Here it must be sufficient to say that the idea of sexual processes produces dilation of blood vessels in the sexual sphere, and that this physiological change itself becomes the source and stimulus for more vivid sexual feelings, which associate themselves with more complex sexual thoughts. These in their turn reinforce again the physiological effect on the sexual organ, and so the play goes on until the irritation of the whole sexual apparatus and the corresponding sexual mental emotions reach a height at which the desire for satisfaction becomes stronger than any ordinary motives of sober reason.