Looks forward to reading Variation.
Explains how two or more female forms occur in one species through selection. The physiological problem remains of how each produces offspring like the other without intermediates. Is not CD's case of varieties that will not blend the physiological test of a species needed for "complete proof of the origin of species"?
Feb. 4th. 1866
My dear Darwin
I am very glad to hear you are a little better, & hope we shall soon have the pleasure of seeing your volume on ``Variation under domestication''.
I do not see the difficulty you seem to feel about 2 or more female forms of one species. The most common or typical female form must have certain characters or qualities which are sufficiently advantageous to it, to enable it to maintain its existence;—in general such as vary much from it, die out. But occasionally a variation may occur which has special advantageous characters of its own, (such as mimicking a protected species) & then this variation will maintain itself by selection. In no less than 3 of my polymorphic species of Papilio, one of the female forms mimics the Polydorus group, which like the Æneas group in America seem to have some special protection. In two or three other cases one of the female forms is confined to a restricted locality to the conditions of which it is probably specially adapted. In other cases one of the female forms resembles the male, and perhaps receives a protection from the abundance of the males, in the crowd of which it is passed over.
I think these considerations render the production of two or three forms of female very conceivable. The physiological difficulty is to me greater, of how each of two forms of female, produces offspring like the other female as well as like itself, but no intermediates.
If you ``know varieties that will not blend or intermix, but produce offspring quite like either parent'',—is not that the very physiological test of a species which is wanting for the complete proof of the ``origin of species:''?
I have by no means given up the idea of writing my travels, but I think I shall be able to do it better for the delay, as I can introduce chapters giving popular sketches of the subjects treated of in my various papers.
I hope, if things go as I wish this summer, to begin work at it next winter. But I feel myself incorrigibly lazy, & have no such system of collecting & arranging facts or of making the most of my materials, as you, & many of our hard working naturalists possess in perfection.
With best wishes | Believe me Dear Darwin | Yours most sincerely | Alfred
C. Darwin Esq.
- f1 4997.f1In his letter to Wallace of 22 January 1866, CD noted that his health had improved to the extent that he was able to work one or two hours per day. Variation was published in 1868.
- f2 4997.f2After reading Wallace's report of three different forms of female in Papilio memnon (A. R. Wallace 1864b, p. 22), CD had expressed difficulty in understanding how two different forms of female could persist on one island (see letter to A. R. Wallace, 22 January 1866 and n. 4). Wallace later wrote a fuller explanation of how the different forms of female had arisen within the species (ML 1: 265--6, n. 3).
- f3 4997.f3By protected species, Wallace means species possessing `some hidden means of protection' (A. R. Wallace 1864b, p. 21).
- f4 4997.f4See A. R. Wallace 1864b, pp. 24--7.
- f5 4997.f5The quotation is from the letter to A. R. Wallace, 22 January 1866, in which CD considered Wallace's criterion for distinguishing between varieties and dimorphic forms. For CD's example of the red cowslip as a new `physiological' species, exhibiting a high degree of sterility when crossed with the common cowslip, and for Thomas Henry Huxley's origination of the term `physiological species' as part of his appeal for proof of the theory of natural selection, see Correspondence vol. 12, letter to Asa Gray, 13 September  and n. 13. See also T. H. Huxley 1863a, pp. 107--8.
- f6 4997.f6Wallace had travelled throughout the Malay Archipelago from 1854 until 1862, and had started `a small book' about his travels by January 1864 (Correspondence vol. 12, letter from A. R. Wallace, 2 January 1864 and nn. 7 and 10). CD enquired about the book's progress in his letters to Wallace of 22 September  (Correspondence vol. 13) and 22 January 1866. The two-volume narrative, The Malay Archipelago, was published in 1869 (A. R. Wallace 1869); in the preface, p. viii, Wallace explained, `I could, indeed, at once have printed my notes and journals, leaving all reference to questions of natural history for a future work; but I felt that this would be as unsatisfactory to myself, as it would be disappointing to my friends, and uninstructive to the public.' Between 1862 and 1869, Wallace published over thirty papers based on his collections (A. R. Wallace 1869, 1: viii--xiv). For a bibliography of Wallace's work, see C. H. Smith ed. 1991, pp. 372--437.
- f7 4997.f7Wallace spent much of 1867 and 1868 writing A. R. Wallace 1869 (A. R. Wallace 1905, 1: 405--6). See also letter to A. R. Wallace, 22 January 1866 and n. 7.