ARW's simple explanation of dimorphic forms is satisfactory.
On "non-blending" of certain varieties, CD thinks ARW has not understood him. He does not refer to fertility. He crossed two differently coloured varieties of peas and "got both varieties perfect, but none intermediate". Something like this must occur in ARW's butterflies.
Down Bromley SE
My dear Wallace
After I had despatched my last note, the simple explanation which you give had occurred to me, & seems satisfactory.
I do not think you understand what I mean by the non-blending of certain varieties. It does not refer to fertility; an instance will explain; I crossed the Painted Lady & Purple sweet-peas, which are very differently coloured vars, & got, even out of the same pod, both varieties perfect but none intermediate. Something of this kind I sh
I am heartily glad that you mean to go on preparing your journal.
Believe me yours | very sincerely | Ch. Darwin
- f1 4989.f1The date is established by the relationship between this letter and the letter from A. R. Wallace, 4 February 1866. In 1866, the first Tuesday after 4 February was 6 February.
- f2 4989.f2The reference is to the letter to A. R. Wallace, 22 January 1866, in which CD asked about the coexistence of different female forms in a Malayan species of butterfly (see letter to A. R. Wallace, 22 January 1866 and nn. 4--6). In his reply, Wallace emphasised that polymorphic forms of butterfly were consistent with natural selection, particularly when the survival of certain forms was secured by mimicry of protected species (see letter from A. R. Wallace, 4 February 1866).
- f3 4989.f3In his letter to Wallace of 22 January 1866, CD had mentioned that he knew of a good many varieties that would not blend or intermix; Wallace in his reply had asked whether such varieties were not new species (see letter from A. R. Wallace, 4 February 1866 and n. 5). For more on CD's examination of the relationship between hybrid sterility and the origination of new species, see Origin, chapter 8, and Correspondence vol. 10, Appendix VI.
- f4 4989.f4CD's results from crossing these two sweetpea varieties are described in Variation 2: 93--4 and Cross and self fertilisation, pp. 154--9. In those accounts, CD added that the early flowers resembling one or other parent were followed later in the season by others that were intermediate in colour between the parents. See also Correspondence vol. 8, letters to M. T. Masters, 7 April  and 13 April , and letter from William Masters, [after 7 April 1860]. For an interpretation of CD's experiment with sweetpeas and comparison with the work of Gregor Mendel, see Dawkins 2003. On CD's continuing interest in the cross-pollination of leguminous flowers, see Correspondence vol. 6, letters to Gardeners' Chronicle, [before 6 December 1856] and 18 October , Correspondence vol. 7, letter to Gardeners' Chronicle, [before 13 November 1858] and this volume, letter to Gardeners' Chronicle, [before 11 August 1866].
- f5 4989.f5CD refers to A. R. Wallace 1864b and to `Three forms of Lythrum salicaria'. In the exposition of his provisional hypothesis of pangenesis, CD wrote, `With those animals and plants which habitually produce several forms, as with certain butterflies described by Mr. Wallace, in which three female forms and the male exist, or as with the trimorphic species of Lythrum and Oxalis, gemmules capable of reproducing several widely-different forms must be latent in each individual' (Variation 2: 399--400). Wallace's work on dimorphic butterflies had been likened to CD's on dimorphism in Linum and Primula, owing to the lack of intermediate offspring (Reader, 16 April 1864, pp. 491--3).
- f6 4989.f6See letter from A. R. Wallace, 4 February 1866 and n. 7.