umescence—the process by which the organism is brought into the physical and psychic state necessary to insure conjugation and detumescence—to some extent comes about through the spontaneous action of internal forces. To that extent it is analogous to the physical and psychic changes which accompany the gradual filling of the bladder and precede its evacuation. But even among animals who are by no means high in the zoölogical scale the process is more complicated than this. External stimuli act at every stage, arousing or heightening the process of tumescence, and in normal human beings it may be said that the process is never completed without the aid of such stimuli, for even in the auto-erotic sphere external stimuli are still active, either actually or in imagination. The chief stimuli which influence tumescence and thus direct sexual choice come chiefly—indeed, exclusively—through the four senses of touch, smell, hearing, and sight. All the phenomena of sexual selection, so far as they are based externally, act through these four senses.[1] The reality of the influence thus exerted may be demonstrated statistically even in civilized man, and it has been shown that, as regards, for instance, eye-color, conjugal partners differ sensibly from the unmarried persons by whom they are surrounded. When, therefore, we are exploring the nature of the influence which stimuli, acting through the sensory channels, exert on the strength and direction of the sexual impulse, we are intimately concerned with the process by which the actual form and color, not alone of living things generally, but of our own species, have been shaped and are still being shaped. At the same time, it is probable, we are exploring the mystery which underlies all the subtle appreciations, all the emotional undertones, which are woven in the web of the whole world as it appeals to us through those sensory passages by which alone it can reach us. We are here approaching, therefore, a fundamental subject of unsurpassable importance, a subject which has not yet been accurately explored save at a few isolated points and one which it is therefore impossible to deal with fully and adequately. Yet it cannot be passed over, for it enters into the whole psychology of the sexual instinct.