[40] The idea that alternation of generation is derived from polymorphism (not the reverse, as usually happens; i.e. polymorphism from alternation of generation) is not new, as I find whilst correcting the final proof. Semper has already expressed it at the conclusion of his interesting memoir, “Über Generationswechsel bei Steinkorallen,” &c. See “Zeitschrift f. wiss. Zool.” vol. xxii. 1872. [41] See my essay “Über den Einfluss der Isolirung auf die Artbildung.” Leipzig, 1872. [42] [In the case of monogoneutic species which, by artificial ‘forcing,’ have been made to give two generations in the year, it has generally been found that the reproductive system has been imperfectly developed in the second brood. A minute anatomical investigation of the sexual organs in the two broods of seasonally dimorphic insects would be of great interest, and might lead to important results. R.M.] [43] “Grundzüge der Zoologie.” 2nd ed. Leipzig, 1872. Introduction. [44] With reference to this subject, see the discussion by the Belgian Entomological Society, Brussels, 1873. [45] P. E. Müller, “Bidrag til Cladocerners Fortplantingshistorie,” 1868. [46] Sars, in “Förhandlinger i Videnskabs Selskabet i Christiania,” 1873, part i. [47] [Eng. ed. Recent researches on alternation of generation in the Daphniacea have convinced me that _direct_ action of external conditions does not in these cases come into consideration, but only _indirect_ action.] [48] See my memoir, “Über Bau und Lebenserscheinungen der _Leptodora hyalina_,” Zeitschrift f. wiss. Zool., vol. xxiv. part 3, 1874. [49] Stettin. entom. Zeit., vol. xviii. p. 83, 1857. [50] Compt. Rend., vol. lxxvii. p. 1164, 1873. [51] [“Accidental” in the sense of our being in ignorance of the laws of variation, as so frequently insisted upon by Darwin. R.M.]