Erotic chemotropism must be regarded as a general sense-function of the sexual cells in all amphigonous organisms, but in the higher organisms special forms of the sex-sense, connected with specific organs, are developed; as the source of sexual love they play a most important part in the life of many of the histona. In man and most of the higher animals these feelings of love are associated with the highest features of psychic life, and have led to the formation of some most remarkable customs, instincts, and passions. Wilhelm Bölsche has given us an admirable selection from this infinitely rich and attractive realm in his famous _Life of Love in Nature_ (1903). It is well known that this sexual sense as we have it in man has been developed from the nearest related mammals, the apes. But while it offers a shameless and repulsive spectacle in many of the apes, it has been greatly ennobled and refined in man in the development of civilization. However, the sexual sense-organs and their specific energy have remained the same. In the vertebrates and the articulates and many other metazoa the copulative organs are equipped with special cell-forms (voluptuous particles), which are the seat of intensely pleasurable feelings (see the _Anthropogeny_, chapter xxix., plate 30). The pubic hairs which clothe the _mons Veneris_ are also delicate organs of the sex-sense, and so are the tactile hairs about the mouth. In these cases the correlation between the sensitive forms of energy in the copulative organs and the psychic functions of the central nervous system has been remarkably developed. Moreover, a large part of the rest of the skin may co-operate as a secondary organ of the sex-sense, as is seen in the effect of caressing, stroking, embracing, kissing, etc. Goethe, at once the greatest lyric poet and the subtlest and profoundest monistic philosopher of Germany, has given unrivalled expression to this sensual, yet supersensual, basis of sexual love. Ontogeny teaches unmistakably that its elementary organs, the epidermic cells, develop entirely from the ectoderm.