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 the chief place among many animals, is of
comparatively less importance, though of considerable interest, in man; it
is only less intimate and final than touch. Sight occupies an intermediate
position, and on this account, and also on account of the very great part
played by vision in life generally as well as in art, it is the most
important of all the senses from the human sexual point of view. Hearing,
from the same point of view, is the most remote of all the senses in its
appeal to the sexual impulse, and on that account it is, when it
intervenes, among the first to make its influence felt.