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.