This theory is strongly supported, and indeed elevated to the rank of a natural law, by the circumstance that in the higher animals the sense of smell, in the course of phylogenetic development, has attained a continually greater significance in relation to sexuality; and by the fact that, according to the discovery of Zwaardemaker, there exists widely diffused throughout Nature a =distinct group= of sexual odours, the so-called =capryl odours=, which have a natural biological connexion with the _vita sexualis._ These capryl odours, which already in plants play a sexual part, are in animals and in the human species localized in or near the genital organs (odoriferous glands of the beaver, the musk-ox, etc., the secretions of the male foreskin and the female vagina), or in other cases are found in the general secretions, such as the sweat. Recently Gustav Klein has succeeded in proving that a definite group of glands in the female genital organs (glandulæ vestibulares majores, or glands of Bartholin) must be regarded as a vestige from the time of periodic sexual excitement (rutting). At that time in the human species, as now in the lower animals, the sexual impulse was periodic in its activity, and the secretion of these odoriferous glands of the human female then served as a means of alluring members of the male sex. At the present time these glands have for the most part lost their significance as specific stimuli. Now it is rather the exhalation from the entire surface of the female body which exercises the erotic influence. Cases in which such stimuli proceed exclusively from the female genital organs are regarded by Klein as a phylogenetic vestige of the primitive relations between the rutting odours of the female and sexual excitement in the male. Friedrich S. Krauss, in his “Anthropophyteia” (1904, vol. i., p. 224), reproduces a Southern Slavonic story in which a man is described who obtained sexual gratification only by enjoying the =natural= smell of the female genital organs. The remarkable classification of Indian women according to the various odours proceeding from their genital organs must not be forgotten in this connexion. That this primitive phenomenon of love has even to-day a certain significance, although, in consequence of the enormous development of the brain and the predominance of purely psychical elements in man, its influence has been very notably diminished, is shown by the existing physiological connexion proved by Fliess to exist between the nose and the genital organs. On the inferior turbinate bones there exist certain “genital areas,” which, under the influence of sexual stimulus and excitement, as in coitus, during menstruation, etc., swell up. From these areas it is also possible to influence directly certain conditions of the genital organs.