ysical
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 zooelogical 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.

Of the four senses--touch, smell, hearing, and sight--with which we are
here concerned, touch is the most primitive, and it may be said to be the
most important, though it is usually the last to make its appeal felt.
Smell, which occupies