To Max Ernst Wichura   3 February [1865]¹

The date 3 February is given in the sale catalogue. The year is established by the reference to Wichura 1865 (see n. 2, below), and by CD’s 1865 discussions of Wichura 1865 with other correspondents (see, for example, letter to B. D. Walsh, 27 March [1865]).

Wichura 1865. There is a heavily annotated presentation copy of Wichura’s Die Bastardbefruchtung im Pflanzenreich erläutert an den Bastarden der Weiden in the Darwin Library–CUL; it includes an abstract made by CD (see Marginalia 1: 871–3). See also letter to B. D. Walsh, 27 March [1865].

Though Wichura concluded that hybrid willows were generally rare in nature, he found instances where they outnumbered their parent species (Wichura 1865, pp. 64–5). CD wrote in the margin of this discussion: ‘more Hybrids than pure parents!!’ (see Marginalia 1: 873). Wichura also noted the rapid germination of hybrid seeds in bare spots (Wichura 1865, p. 65); CD wrote by this passage, ‘Explains how these numbers come from Willows vegetating only in bare places’. See also CD’s inserted abstract of the book; most of the abstract was written by Emma Darwin (Marginalia 1: 871).

For Wichura’s description of the imperfections and irregularities in the development of hybrid pollen-grains, see Wichura 1865, pp. 32–40, 79–80, 89. In CD’s abstract (see n. 3, above), Emma wrote for pages 38 and 39, ‘pollen gets worse & worse in offspring of Hybrids inter se & in … the more complex hybrid’; CD made a similar annotation in Wichura 1865, p. 38 (see Marginalia 1: 871–3). Wichura’s discussion of irregular pollen-grains included those in plants that were highly variable but not hybridised; see Wichura 1865, p. 89, where CD wrote in the margin: ‘Plants which vary have often irregular pollen— I think some connexion between sterility & variability’ (see also CD’s abstract, and Marginalia 1: 872–3). In CD’s discussion of the causes of variability in Variation 2: 268, he referred to Wichura’s observations of irregular pollen-grains.

Wichura suggested that the reversions Karl Friedrich von Gärtner observed in his crossing experiments were due to his not taking enough care to prevent pollination of a hybrid offspring by a parent plant (see Gärtner 1849, pp. 437–41, Wichura 1865, pp. 27–8, and Marginalia 1: 871–3). There are heavily annotated copies of Gärtner 1844 and 1849 in the Darwin Library–CUL (see Marginalia 1: 248–98).

CD probably refers to Charles Victor Naudin’s most recent conclusions about reversions published in Naudin 1863; however, he had also read much of Naudin’s relevant earlier work. See Correspondence vol. 12, letters to J. D. Hooker, 22 [May 1864] and n. 6, and 13 September [1864], and letter from C. V. Naudin, 6 December 1864 and n. 5. CD also cut out and annotated a note written by Asa Gray in the American Journal of Science and Arts (vol. 39, January 1865, pp. 107–8) about Naudin’s recent observations of Datura hybrids returning to their parent forms (see DAR 205.7: 5). For discussions of Naudin’s work and CD’s assessment of it, see Geison 1969, pp. 404–1, Rheinberger 1983, Olby 1985, pp. 47–53, and J. Harvey 1997. There is an annotated copy of Naudin 1863 in the Darwin Pamphlet Collection–CUL.

See Wichura 1865, p. 28, and Marginalia 1: 872. The abstract CD inserted in Wichura 1865 indicates that he remembered Gärtner’s stating that reversions occurred primarily in cultivated plants (see Marginalia 1: 871–2; see also Origin, p. 269). CD discussed this conclusion of Gärtner’s, and the light it threw on the difference between Naudin’s and Wichura’s results, in his chapter on hybridism in Origin 4th ed., pp. 332–3, and in Variation 2: 49–50.

CD refers to chapter 8 of Origin, ‘Hybridism’ (pp. 245–78); for his conclusions about hybrid sterility, see pp. 254–67. Regarding the sterility of hybrids, CD wrote: ‘it is scarcely possible that two organisations should be compounded into one, without some disturbance occurring in the development, or periodical action, or mutual relation of the different parts and organs one to another, or to the conditions of life’ (Origin, p. 266).

CD refers to his experimental work on the dimorphic Primula and Linum, and the trimorphic Lythrum; he started this work in 1861, and was continuing to make crosses with dimorphic forms (see Correspondence vols. 9–12 and this volume). His results had led him to consider whether sterility might have been selectively acquired; when discussing his crosses of Lythrum, he wrote: ‘I am now strongly inclined to believe that sterility is at first a selected quality to keep incipient species distinct’ (see Correspondence vol. 10, letter to J. D. Hooker, 12 [December 1862]). CD’s notes on hybridism in DAR 205.7: 155–65 state that he had nonetheless rejected natural selection as a cause of sterility by 1863 (see Correspondence vol. 10, Appendix VI). However, CD’s comment to Wichura, and additions made to Origin 4th ed., pp. 310–26, 336–8, and Variation 2: 185–9, indicate that he was still deliberating about the causes of hybrid sterility, particularly in plants. Regarding Wichura’s discussion of hybrid sterility, CD wrote in Origin 4th ed., p. 318: The above view of the sterility of hybrids being caused by two different constitutions having been confounded into one has lately been strongly maintained by Max Wichura; but it must be owned that the sterility, so like in every respect to that of hybrids, which affects the illegitimate offspring of dimorphic and trimorphic plants of the same species … makes this view rather doubtful. CD was comparing the sterility of hybrids, or the offspring of hybrids, with the sterility of same-form unions among dimorphic and trimorphic plants; he evidently still thought the latter might be caused by natural selection (see Origin 4th ed., pp. 323–6). See also CD’s annotations to Wichura 1865, p. 83, and CD’s inserted abstract (see Marginalia 1: 871–2).

In the concluding chapter of his book Wichura briefly mentions CD’s ‘scharfsinnig’ (sagacious) theory of natural selection (Wichura 1865, p. 82).