[5] Recently, apart from sexual periodicity, a general periodicity of vital manifestations, more especially of the psychical phenomena associated with sexuality, has been proved to exist in both sexes. In a work that attracted much attention--“The Course of Life: Elements of Exact Biology” (Vienna, 1905)--Wilhelm Fliess proved the occurrence in the human species of a twenty-three day “masculine,” and a twenty-eight day “feminine” period. Not merely do physical phenomena recur quite spontaneously at intervals of twenty-three and twenty-eight days respectively, but the same is true of perceptions, feelings, and voluntary impulses. Hermann Swoboda, a thoughtful supporter of Fliess’s theory, has treated this question in two works--“The Periods of the Human Organism in their Psychological and Biological Significance” (Leipzig and Vienna, 1904), and “Studies in the Elements of Psychology” (Leipzig and Vienna, 1905). In these he has described also twenty-three-hour and eighteen-hour vital undulations in human beings, and has discussed the significance of this periodicity to psychology. These researches of Fliess and Swoboda need to be confirmed by other investigators before they can be regarded as definite additions to our scientific knowledge. In this connexion also the older work of Carl Reinl--“Undulatory Movements of the Vital Processes in Woman” (Leipzig, 1884)--may be consulted. See also Van de Velde’s “Ovarian Functions, Undulatory Movement, and Menstrual Hæmorrhage” (Jena, 1905). [6] Virey likewise explains the enduring nature of human love as dependent upon an excess of potent nutritive material, whereas the poor savages of Northern Europe and America, who must often go hungry, really experience no more than an instant of sexual pleasure, just like the wild animals, who rut only at certain distinct seasons. For the same reason, our domestic animals, which have a superfluous supply of nutriment, copulate far more frequently. And in our own case, the incessant intimate association of the sexes in our domestic life is a continued source of ever-renewed sexual needs, even contrary to our own will. The assumption of the =upright posture= by man, which is so intimately connected with the preponderance of the human brain, is also regarded by Virey as “an enduring cause of sexual excitement.” _Cf._ J. J. Virey, “Das Weib” (“Woman”), p. 301; Leipzig, 1827.