[Footnote 1:  Cf.  Geddes and Thomson, The Evolution of Sex passim.]

[Footnote 2:  Havelock Ellis, Man and Woman, has brought together a mass of very valuable material on the question of the somatic and psychic differences of man and woman, and H. Campbell, in a volume of much the same scope, Differences in the Nervous Organization of Man and Woman, has given a resume of the theory of Geddes and Thomson, and suggested its extension to the human species.]

[Footnote 3:  C. Duesing, (1) Die Regulirung des Geschlechtsverhaeltnisses bei der Vermehrung der Menschen, Thiere und Pflanzen. (2) Das Geschlechtsverhaeltniss der Geburten in Preussen.]

[Footnote 4:  H. Ploss, “Ueber die das Geschlechtsverhaeltniss der Kinder bedingenden Ursachen,” Monatsschrift fuer Geburtskunde und Frauenkrankheiten, Vol.  XII, pp. 321-60.]

[Footnote 5:  E. Westermarck, The History of Human Marriage, pp. 470-83.]

[Footnote 6:  Duesing, Das Geschlechtsverhaeltniss der Geburten in Preussen, pp. 29-33.]

[Footnote 7:  Duesing, loc. cit., pp. 14-19.]

[Footnote 8:  H. Ploss, Das Weib in der Natur- und Voelkerkunde, 3.  Aufl., Vol.  I, p. 419.]

[Footnote 9:  Axel Key, “Die Pubertaetsentwickelung und das Verhaeltniss 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 Verhaeltniss des maennlichen 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 Bluethenbildung,” Gesammelte Abhandlungen ueber 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.)]