Ɛpsilon

To August Weismann   1 and 4 May 1875

Down, Beckenham, Kent

May 1st. 1875

My dear Sir

I did not receive your essay for some days after your very kind letter, and I read German so slowly that I have only just finished it.1 Your work has interested me greatly, and your conclusions seem well established. I have long felt much curiosity about Season-dimorphism, but never could form any theory on the subject. Undoubtedly your view is very important, as bearing on the general question of variability.2 When I wrote the Origin I could not find any facts which proved the direct action of climate and other external conditions.3 I long ago thought that the time would soon come when the causes of variation would be fully discussed, and no one has done so much as you in this important subject. The recent evidence of the difference between birds of the same species in the N. and S. United States well shows the power of climate.4 The two sexes of some few birds are there differently modified by climate, and I have introduced this fact in the last edition of my Descent of Man. I am, therefore, fully prepared to admit the justness of your criticism on sexual selection of Lepidoptera; but considering the display of their beauty, I am not yet inclined to think that I am altogether in error.5

What you say about reversion being excited by various causes, agrees with what I concluded with respect to the remarkable effects of crossing two breeds; namely that anything which disturbs the constitution leads to reversion, or, as I put the case under my hypothesis of pangenesis, gives a good chance of latent gemmules developing.6 Your essay, in my opinion is an admirable one, and I thank you for the interest which it has afforded me.

With much respect | I remain, my dear Sir | Yours very faithfully | Charles Darwin

P.S.— I find that there are several points, which I have forgotten. Mr. Jenner Weir has not published anything more about caterpillars, but I have written to him, asking him whether he has tried any more experiments, and will keep back this letter till I receive his answer.—7 Mr. Riley of the U. States supports Mr. Weir, and you will find reference to him and other papers at p. 426 of the new and much corrected Edit. of my Descent of Man.8 As I have a duplicate copy of Vol. I (I believe 2nd. Vol. is not yet published in German) I send it to you by this post.9 Mr. Belt in his “Travels in Nicaragua”, gives several striking cases of conspicuously coloured animals (but not caterpillars) which are distasteful to birds of prey: he is an excellent observer and his book, “The Naturalist in Nicaragua”, very interesting.10

I am very much obliged for your photograph which I am particularly glad to possess, and I send mine in return.11

I see you allude to Hilgendorf’s statements, which I was sorry to see disputed by some good German observer. Mr. Hyatt, an excellent palæontologist of the U. States, visited the place and likewise assured me that Hilgendorf was quite mistaken.12

I am grieved to hear that your eye-sight still continues bad, but anyhow it has forced your excellent work in your last essay.13

May 4th.

Here is what Mr. Weir says.

“In reply to your enquiry of Saturday I regret that I have little to add to my two communications to the Entomol. Soc. Transactions.

I repeated the experiments with gaudy caterpillars for years, and always with the same results, not on a single occasion did I find richly coloured, conspicuous larvæ eaten by birds. It was more remarkable to observe that the birds paid not the slightest attention to gaudy caterpillars, not even when in motion— the experiments so thoroughly satisfied my mind that I have now given up making them”.14

Copy

1
Weismann’s letter has not been found. He had sent CD a copy of his essay on seasonal dimorphism in butterflies (Weismann 1875b).
2
Butterflies of a single species sometimes exhibit morphological differences depending on the time of year they emerge from the pupa: this is known as seasonal dimorphism. In his experiments, Weismann found that some seasonal forms were more variable than others and concluded that while the summer form could be induced to change into the winter form, the reverse was not the case (Weismann 1875b, p. 28). Weismann stressed the importance of the difference between seasonal variation and local variation, noting that the former was due to the direct action of climate, while the latter resulted from indirect changes in the conditions of life, that is from natural selection (ibid., pp. 33–4, 65).
3
See Origin 6th ed., p. 54.
4
CD cited work on the birds of Florida and of Kansas by Joel Asaph Allen (Allen 1871 and Allen 1872) when making this point in Descent 2d ed., p. 422 and n. 34.
5
Weismann noted that although CD’s theory of sexual selection could explain some morphological diferences, such as different male and female colour patterns, it could not explain seasonal differences (Weismann 1875b, pp. 74–5). In Descent 2d ed., p. 423, CD had stated that the changes in the colours of birds owing to climate were not incompatible with the belief that the colours of birds were mainly due to the accumulation of successive variations through sexual selection.
6
Weismann had noted other causes of reversion to the primary form besides the principal one of temperature change (see n. 2, above). He found that pupae of the summer form of Pieris napi (the green-veined white butterfly), despite having been kept at a high temperature during a seven-hour train journey, emerged as winter forms. He concluded that the shaking of the pupae during the journey had resulted in reversion (Weismann 1875b, p. 28). CD noted that any change in the conditions of life favoured a tendency, inherent or latent in the species, to return to the ‘primitive state’, and that crossbreeding was one of the most powerful ways of affecting an organism’s constitution and producing reversion (Variation 2d ed. 2: 22, 368–9, 382). His hypothesis of pangenesis was based on the notion that every separate part of the whole organism reproduces itself, and that the reproductive organs included gemmules (or granules) thrown off from each separate part; reversion was the awakening of latent characters present in each generation (ibid., pp. 350, 369).
7
See letter to J. J. Weir, 1 May 1875. John Jenner Weir’s reply has not been found. See also n. 14, below.
8
Either CD or the copyist wrote p. 426 in error; the references to the work of Weir (Weir 1869 and Weir 1870) and Charles Valentine Riley (Riley 1869–77, third report (1871)) are given in Descent 2d ed., p. 326 n. 34. Riley and Weir agreed that birds did not eat gaudy caterpillars.
9
Descent 2d ed. was translated into German by Julius Victor Carus; in February 1874, Carus told CD that the printing of the new, two-volume, German edition (Carus trans. 1875a) would soon be finished (see letter from J. V. Carus, 5 February 1875).
10
Thomas Belt, in The naturalist in Nicaragua, gave the example of a duck spitting out a brightly coloured frog and then ‘jerking its head as if to throw off some unpleasant taste’ (Belt 1874a, p. 321). He also mentioned that many insects were preserved by being distasteful to insectivorous birds (ibid., p. 74), noting that this included all species of the genus to which fireflies belonged even though only some were phosphorescent (ibid., p. 317). These cases led him to conclude that any animal endowed with special means of protection from its enemies was always either conspicuously coloured, or in other ways attracted attention, and did not seek concealment (ibid., p. 341).
11
Weismann’s photograph has not been found.
12
Weismann had proposed a law that in every species a period of variability alternated with one of relative stability, and supported his statement with evidence from Franz Hilgendorf’s phyletic developmental history of fossil snails from Steinheim (Weismann 1875a, p. 79; see also Hilgendorf 1866 and Weismann 1872). Alpheus Hyatt may have discussed Hilgendorf’s conclusions about the fossil snails of Steinheim when he visited CD (see letter from Alpheus Hyatt, 8 January [1875] and n. 4); no letter disputing Hilgendorf’s conclusions about the snails has been found. Hyatt later published his interpretation of the Steinheim fossils in Hyatt 1880. For more on the differences between Hilgendorf’s and Hyatt’s reconstruction of the phylogeny of the Steinheim snails, see S. J. Gould 2002, pp. 373–82.
13
Weismann suffered from a rare disease in the retina of his left eye; the condition led to periods of blindness in that eye, making work at the microscope impossible (Petrunkevitch 1963, pp. 21–2).
14
Weir’s letter to CD, from which the extract was copied, has not been found. It was a reply to the letter to J. J. Weir, 1 May 1875. Weir’s articles on the colour and edibility (to birds) of Lepidoptera and their larvae were published in the Transactions of the Entomological Society of London (Weir 1869 and Weir 1870).

Please cite as “DCP-LETT-9965,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-9965