This volume has been thoroughly revised for the present edition and
considerably enlarged throughout, in order to render it more accurate and
more illustrative, while bringing it fairly up to date with reference to
scientific investigation. Numerous histories have also been added to the
Appendix.
It has not been found necessary to modify the main doctrines set forth ten
years ago. At the same time, however, it may be mentioned, as regards the
first study in the volume, that our knowledge of the physiological
mechanism of the sexual instinct has been revolutionized during recent
years. This is due to the investigations that have been made, and the
deductions that have been built up, concerning the part played by
hormones, or internal secretions of the ductless glands, in the physical
production of the sexual instinct and the secondary sexual characters. The
conception of the psychology of the sexual impulse here set forth, while
correlated to terms of a physical process of tumescence and detumescence,
may be said to be independent of the ultimate physiological origins of
that process. But we cannot fail to realize the bearing of physiological
chemistry in this field; and the doctrine of internal secretions, since it
may throw light on many complex problems presented by the sexual instinct,
is full of interest for us.