Classicly

Studies in the Psychology of Sex is a multi-volume work by Havelock Ellis. 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