[Footnote 18: Maupas, _loc. cit._, p. 358. Later investigations have tended to discredit Maupas' experiments as a whole by showing that the Infusorians with which he experimented can be kept alive indefinitely by a change of diet, without the aid of sexual conjugation. This merely confirms the view, however, that abundant nutrition and crossing are alike favorable to health: "We must admire the skill of the investigator who was able to keep his colonies alive for months and years under such artificial conditions, but we may venture to doubt whether the fate of extinction which did ultimately overtake them was really due to the absence of conjugation, and not to the unnaturalness of the conditions." A. Weismann, _The Evolution of Theory_, Vol. I, p. 329. Since the above was written, Calkins has made a series of new experiments, the results of which differed in several respects from those yielded by Maupas' experiments. When his infusorian cultures began to grow weaker, as happened frequently and at irregular intervals, he was always able to restore them to more vigorous life by a change of diet, and especially by substituting grated meat, liver, and the like for infusions of hay. Certain salts too, had the same effect; the animals became perfectly vigorous again. Calkins believes that chemical agents, and especially salts, must be supplied to the protoplasm from time to time. He reared 620 generations of _Paramoecium_ without conjugation. But the 620th was weakly and without energy. The addition of an extract of sheep's brains made them perfectly fresh and vigorous again. Further experiments in this direction are to be desired, but, according to those of Calkins, it is probable that Infusorians can continue to live for an unlimited time even without conjugation. (Ibid., note.)] [Footnote 19: Westermarck, _loc. cit._, pp. 476-83, following a suggestion of Düsing, has brought together much of the evidence on this point, but the application of the facts here made has not, I believe, been suggested.] [Footnote 20: A. von Oettingen, _Die Moralstatistik_, 3. Aufl., p. 56.] [Footnote 21: Düsing, _Die Regulirung des Geschlechtsverhältnisses_, p. 237.] [Footnote 22: Westermarck, _loc. cit._, pp. 479 and 481 n.] [Footnote 23: Cf. _ibid._, pp. 476-83.] [Footnote 24: G. Delaunay, "De l'égalité et inégalité des deux sexes," _Revue scientifique_, September 3, 1881; C. Darwin, _Descent of Man_, chap. 10.] [Footnote 25: A. Weismann, _Essays on Heredity_, Vol. I, "The Duration of Life," has shown that size and longevity are determined by natural selection.] [Footnote 26: Darwin, _Descent of Man_, chap. 8.] [Footnote 27: Ibid.] [Footnote 28: A.R. Wallace, _Contributions to the Theory of Natural Selection_, chap. 3.]