[Footnote 3: C. Düsing, (1) _Die Regulirung des Geschlechtsverhältnisses bei der Vermehrung der Menschen, Thiere und Pflanzen_. (2) _Das Geschlechtsverhältniss der Geburten in Preussen_.] [Footnote 4: H. Ploss, "Ueber die das Geschlechtsverhältniss der Kinder bedingenden Ursachen," _Monatsschrift für Geburtskunde und Frauenkrankheiten_, Vol. XII, pp. 321-60.] [Footnote 5: E. Westermarck, _The History of Human Marriage_, pp. 470-83.] [Footnote 6: Düsing, _Das Geschlechtsverhältniss der Geburten in Preussen_, pp. 29-33.] [Footnote 7: Düsing, _loc. cit._, pp. 14-19.] [Footnote 8: H. Ploss, _Das Weib in der Natur- und Völkerkunde_, 3. Aufl., Vol. I, p. 419.] [Footnote 9: Axel Key, "Die Pubertätsentwickelung und das Verhältniss derselben zu den Krankheitserscheinungen der Schuljugend," _Verhandlungen des X. Internationalen Medicinischen Congresses_, 1890, Vol. I, p. 91.] [Footnote 10: Ibid., pp. 84-90.] [Footnote 11: Geddes and Thompson, _loc. cit._, Book I, chap. 4.] [Footnote 12: Rolph, quoted by Geddes and Thompson, _loc. cit._, Book I, chap. 4.] [Footnote 13: Geddes and Thompson, _ibid._] [Footnote 14: G. Klebs, _Ueber das Verhältniss des männlichen und weiblichen Geschlechts in der Natur_, p. 19.] [Footnote 15: Food affords the basis for metabolic changes in the parent organism, but it is probable that food is less _directly_ related than heat and light to the determination of sex. Sachs, whose experiments must be given the greatest possible weight, has determined that the ultra-violet rays of light are necessary to the chemical changes essential to the formation of the reproductive organs. (J. Sachs, "Ueber die Wirkung der ultravioletten Strahlen auf die Blüthenbildung," _Gesammelte Abhandlungen über Pflanzen-Physiologie_, Vol. I, pp. 293ff.) More recently, Klebs has shown that by diminishing the intensity of light the development of female sex organs in ferns can be interrupted, so that, in spite of the presence of male organs, fertilization is impossible; at the same time, the prothallia are enabled in weak light to grow feebly and to put out small asexual processes, which in the presence of bright light become normal prothallia. Similarly, the development of sexual organs in algae is dependent on a certain intensity of light, and the plant remains sterile if the light is diminished below a certain point. (G. Klebs, _Ueber einige Probleme der Physiologie der Fortpflanzung_, pp. 13-16.)] [Footnote 16: E. Maupas, "Théorie de la sexualité des Infusoires ciliés," _Comptes rendus_, Vol. CV, pp. 356ff.] [Footnote 17: The extinction took place at about the 330th generation in _Onychodromus grandis_, at about the 320th generation in _Stylonichia mytilis_, at about the 330th generation in _Leucophrys patula_, and at about the 660th generation in _Oxytricha_ (indeterminate). (Maupas, _loc. cit._, p. 358.)]