But the deeper meaning of this arrangement must doubtless be sought for in the individual variability which is thus continuously kept up and is always being forced into new combinations. Thus sexual reproduction is to be explained as an arrangement which ensures an ever-varying supply of individual differences.
Footnotes for Essay VI.
[234]. See Berichten der Naturforschenden Gesellschaft zu Freiburg i. B., Band III. (1887) Heft I, ‘Ueber die Bildung der Richtungskörper bei thierischen Eiern,’ by August Weismann and C. Ischikawa.
[235]. Vol. I. p. 60.
[236]. The most recent example of this kind is afforded by the excellent work of O. Schultze, ‘Ueber die Reifung und Befruchtung des Amphibieneies,’ Zeitschr. f. wiss. Zool., Bd. XLV. 1887. Schultze has proved that two polar bodies are expelled from the egg of the Axolotl and of the frog, although all previous observers, including O. Hertwig, had been unable to find them. Thus the latter authority states as the result of an investigation specially directed towards this point, that the nucleus is transformed in a peculiar manner (‘Befruchtung des thierischen Eies,’ III. p. 81).
[237]. O. Hertwig, ‘Beiträge zur Kenntniss der Bildung, Befruchtung, und Theilung des thierischen Eies,’ Morpholog. Jahrbuch, I, II, and III. 1875-77.
[238]. H. Fol, ‘Recherches sur la fécondation et le commencement de l’hénogénie chez divers animaux.’ Genève, Bâle, Lyon, 1879.
[239]. Bütschli, ‘Entwicklungsgeschichtliche Beiträge,’ Zeitschr. f. wiss. Zool. Bd. XXIX. p. 237. 1877.
[240]. C. S. Minot, ‘Account, etc.’ Proceedings Boston Soc. Nat. Hist., vol. xix. p. 165. 1877.