Ɛpsilon

To August Weismann   6 December 1875

Down, Beckenham, Kent

Dec: 6th. 1875

My dear Sir

I have been profoundly interested by your essay on Amblystoma, and think that you have removed a great stumbling block in the way of evolution.1 I once thought of reversion in this case; but in a crude and imperfect manner. I write now to call your attention to the sterility of moths when hatched out of their proper season: I give references in Chap. 18 of my Variation under Domestication (Vol. 2 p. 157 of English Edit.) and these cases illustrate, I think, the sterility of Amblystoma.2 Would it not be worth while to examine the reproductive organs of those individuals of wingless Hemiptera which occasionally have wings, as in the case of the bed-bug.3 I think I have heard that the females of Mutilla sometimes have wings.4 These cases must be due to reversion. I dare say many anomalous cases will be hereafter explained on the same principle.

I hinted at this explanation in the extraordinary case of the black-shouldered peacock, the so-called Pavo nigripennis, given in my Var. under Domest.; and I might have been bolder, as the var. is in many respects intermediate between the two known species.5

With much respect | Yours sincerely, | Ch. Darwin

P.S.— There is a more appropriate case among Plants. Peloric flowers must be considered as cases of reversion to a symmetrical form; and many of these flowers are quite sterile and others quite fertile: see Chapt. 18 of Dom. under Var. in section Monstrosities as a cause of Sterility.6 I quote Godron on Corydalis solida; but he has since found other peloric flowers on this same species fertile; so sterility is variable in same species; so indeed with Pelargonium, as I show.—7

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1
In his essay ‘Ueber die Umwandlung des mexicanischen Axolotl in ein Amblystoma’ (On the transformation of the Mexican axolotl into an Amblystoma; Weismann 1875b), Weismann argued that the apparent metamorphosis of some axolotls (which were then classified as Siredon mexicanus) into the salamander Amblystoma (a synonym of Ambystoma) was due to reversion caused by adaptation to conditions different from their normal high-altitude, deep-water environment. Weismann stressed that all evidence showed that axolotls never transformed in their natural environment (Weismann 1875b, pp. 303–4). He further disagreed with the accepted view that the axolotl represented a lower phyletic stage and that transformed individuals had been induced to advance to a higher stage (ibid., p. 306). CD’s annotated copy of Weismann 1875b is in the Darwin Pamphlet Collection–CUL. For the impact of the axolotl on nineteenth-century zoology, see H. M. Smith 1989.
2
In Variation 2: 157–8 and n. 61, CD mentioned that the sterility of a number of moths, especially the Sphingidae, when bred out of season, was ‘still involved in some obscurity’. Weismann had pointed out an analogy between seasonal dimorphism in some butterflies and the changes in axolotls: just as the summer form of the butterflies could be induced to change into the winter form by exposing the pupae to cold temperatures, so the axolotl could be induced to metamorphose into an air-breathing land form by forcing it to breathe air. Weismann further noted that the so-called Amblystoma that developed from an axolotl was always sterile; he referred to Origin 5th ed., p. 325, for CD’s view that sterility was caused by the operation of widely varying circumstances of life. He concluded that the genus Siredon should be retained for species that reproduced at the gill stage while Amblystoma should be reserved for species that bred only when they reached the air-breathing stage (Weismann 1875b, pp. 327–8).
3
Hemiptera is a large order of insects that possess sucking mouthparts. Adult bedbugs (Cimex lectularius) usually have no wings but do possess small wing pads or vestigial wings (Harlan 2006, p. 99).
4
Mutilla is a genus of parasitoid wasps often known as velvet ants because the wingless females resemble hairy ants.
5
See Variation 1: 290–1. The two known species CD refers to are Pavo cristatus (the common or Indian peafowl) and P. muticus (the green peafowl). Pavo nigripennis is now considered to be a colour variation of P. cristatus.
6
See Variation 2: 166–7. Peloric flowers are aberrant forms in which a usually irregular floral structure appears regular or symmetrical.
7
In Variation 2: 167 and n. 91, CD referred to Dominique Alexandre Godron’s observations on Corydalis solida (fumewort; Godron 1864). In Variation 2d ed. 2: 150, CD mentioned Godron’s disovery that not all peloric flowers of C. solida were sterile, and noted that no seed had been obtained from the peloric central flower that often appeared on the trusses of pelargoniums raised in greenhouses.

Please cite as “DCP-LETT-10289,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-10289