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Letter 6153

Wallace, A. R. to Darwin, C. R.

1 May [1868]

    Summary Add

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    Answers CD's objection [see 6121 and 6146] about sexual differences and protective colouring. Summarises his theory of colour in nature.

Transcription

Hurstpierpoint,

May 1st.

Dear Darwin

I will confine myself to answering a few of your objections while the subject is on my mind.

Your explanation of the male seeking female is good, & your argument from it good too, as far as it goes. Has the free pairing of Pigeons been carefully watched?

I hardly see your difficulty or your objection to the case of the ♀ protected butterflies. You argued before (& have proved) that, ``characters appearing in one sex are sometimes transmitted to that sex exclusively''. The cases of these ♀ protected butterflies (by mimicry) are so few that we may well suppose the proper variations to have occurred sufficiently in that sex only. But there is also this important fact to be considered, that in most (perhaps all) these cases the ♀ has different habits, & the same variation would not be useful to the male, because he does not frequent the stations where the mimicked species abounds. He also has a strength of flight which does not accord with the slow weak flight of the Heliconidæ, while that of the ♀ does. In cases where this difference of habit & action does not exist both sexes are equally protected, as in the leaf butterfly (Kallima inachis) which is protected in repose.

Can you understand how it is that the female African Elephant has tusks the Indian not? Are they useful to the one female, useless to the other? Why have some female ruminants horns others not? We cannot explain every individual case, because we cannot know all the existing and past conditions of any case, but we must decide on the mass of cases explained by & the difficulties in the way of, each view.

There is a very important fact with regard to the male intensity of colour that puts it in quite a different class from sexually selected or protective colouring, & that is, that the more deeply coloured male is almost always smaller, so that it is really the same amount of colour concentrated on a smaller surface. This is the case in almost all butterflies & moths, and beetles,—also in Hawks. Where the female is equal or smaller in size the colour (if the same) is generally equally intense. I speak now from memory only, but I think you will find this a pretty correct statement of the facts. I do not maintain that this explains all the difference of depth & intensity of colour. Part may be due to greater vigour of male or be correlated with his sexual organs.

I do not understand what is your explanation of the female mimicking Pieris. Do you consider it inexplicable?

My theory of colour in nature is somewhat as follows:

1: Colour is ever varying, and is generally transmitted to both sexes.

2. It protects, by simple concealment.

" by mimicry.

" by making conspicuous.

4. It is useful also sexually, to the male by attracting the female;—perhaps to the female by attracting the male.

5. It is therefore selected & accumulated.

6. Owing to the special structure functions & habits of the female sex, this often requires more protection than the male, and is also of more importance in the preservation of the offspring. Protection by colour is therefore often acquired by this sex alone.

7. This occurs either by subduing or checking the colour as acquired by the male, or by the accumulation of entirely distinct colours & markings.

I really do not think we shall ultimately differ much on this point.

Yours very faithfully | Alfred R. Wallace

Many thanks for the Photo.

    Footnotes Add

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    f1 6153.f1
    The year is established by the relationship between this letter and the letter to A. R. Wallace, 30 April [1868].
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    f2 6153.f2
    See letter to A. R. Wallace, 30 April [1868].
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    f3 6153.f3
    See Variation 2: 71--5.
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    f4 6153.f4
    Wallace discussed Kallima inachis, which has leaf-like camouflage on the underside of its wings, in [Wallace] 1867b, p. 9; the wings are displayed when the butterfly is resting. Kallima inachis is now Kallima inachus, the orange oak-leaf butterfly.
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    f5 6153.f5
    Wallace's discussion of female Pieris butterflies that mimic Heliconidae species is in [Wallace] 1867b, p. 37. See also letter from A. R. Wallace, 28 April [1868] and n. 8.
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    f6 6153.f6
    See letter to A. R. Wallace, 30 April [1868] and n. 5.
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