Ɛpsilon

From A. R. Wallace   13 December 1876

Rosehill, Dorking

Decr. 13th. 1876

My dear Darwin

Many thanks for your new book on Crossing plants, which I have read with much interest.1 I hardly expected however that there would have been so many doubtful and exceptional cases. I fancy that the results would have come out better had you always taken weights instead of heights; & that would have obviated the objection that will I dare say be made, that height proves nothing, because a tall plant may be weaker, less bulky and less vigorous than a shorter one. Of course no one knows you or who takes a general view of your results will say this, but I dare say it will be said.2

I am afraid this book will not do much or anything to get rid of the one great objection, that the physiological characteristic of species, the infertility of hybrids, has not yet been produced.3

Have you ever tried experiments with plants (if any can be found) which for several centuries have been grown under very different conditions,—as for instance potatoes on the high Andes, & in Ireland? If any approach to sterility occurred in mongrels between these it would be a grand step.4 The most curious point you have brought out seems to me the slight superiority of self fertilization over fertilization with another flower of same plant,—& the most important result, that difference of constitution is the essence of the benefit of cross fertilization—5 All you now want is to find the neutral point, where the benefit is at its maximum, any greater difference being predjuidicial.

Hoping you may yet demonstrate this | Believe me | Yours very faithfully | Alfred R. Wallace

Charles Darwin Esq.

1
Wallace’s name is on CD’s presentation list for Cross and self fertilisation (see Appendix III).
2
The results of CD’s experiments in Cross and self fertilisation were expressed in terms of the relative heights, weights, and fertility of crossed and self-fertilised plants of a given species, often over several generations, with the height variable the one most frequently recorded.
3
CD’s experimental programme was developed to investigate the question of sterility as a test of a physiological species, but the frequent occurrence of self-sterility within a so-called ‘physiological species’ called into question the whole notion. Wallace approached the question differently in that he did not question that sterility inter se was a characteristic of species, but argued that it could be produced by natural selection. CD rejected this view in Origin 4th ed., pp. 310–12.
4
Wallace had argued that natural selection could produce sterility of hybrids; see Correspondence vol. 16, letter from A. R. Wallace, 1 March 1868. Thomas Henry Huxley had challenged CD to produce a new ‘physiological species’ through artificial selection (i.e. to produce varieties of a species that were cross-sterile). This had led CD to perform a series of experiments on hybrids from 1861 to 1863 (see Correspondence vol. 10, Appendix VI; Variation 2: 185–9). CD had received Eschscholzia seeds from Brazil and found that self-sterility lessened within a generation of their being grown in England (Cross and self fertilisation, pp. 111–12). He concluded: ‘The degree of sterility is much affected by the conditions to which the plants have been subjected’ (ibid., p. 466).
5
In Cross and self fertilisation, pp. 85–7, CD concluded that especially from the evidence of weight, seedlings from a cross between flowers on the same plant had ‘a decided, though not great, advantage’ over those from flowers fertilised with their own pollen, more especially in the case of the plants subjected to severe mutual competition. Wallace’s conclusion was based on the tables that compared height alone. In ibid., pp. 454–5, CD concluded that the advantages of crossing resulted from the differences in sexual elements rather than from a mere change in conditions.

Please cite as “DCP-LETT-10717,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-10717