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Darwin Correspondence Project

From A. R. Wallace   18 September [1868]1

9, St. Marks’ Crescent | N.W.

Septr. 18th.

Dear Darwin

The more I think of your views as to the colours of females, the more difficulty I find in accepting them or even in clearly understanding them. As you are now working at the subject I hope it will not interrupt you to hear “Counsel on the other side”.2

1. You admit, I think, that the female in birds & insects requires more protection, & in the latter case, for a longer time.3

2. Now if the male & female were distinct species with different habits and organizations you would at once admit that a difference of colour, tending to make that one less conspicuous to which concealment was evidently most necessary, had been acquired by Nat. Select.

3. But you admit also & in fact maintain strongly that variations occurring in one sex may be and often are transmitted to that sex only.4 There is therefore nothing to prevent Nat. Select. acting on the two sexes as if they were two species.

4. But you admit that many species are protected by general or special tints or markings,—& that such protection has been acquired by Nat. selection.

5. When however the female only has exactly similar protective tints or markings, you deny that N. select. has given these tints to her alone. I cannot see why.

6. Your objection that the same protection would to a certain extent be useful to the male also seems to me quite unsound,5 and directly opposed to your own doctrines. For you urge strongly, that Nat. Select. can never improve an animal beyond its needs. So that even if we admit abundant variation of colour in the male, nat. select. will not bring him to resemble the female in protective tints (except where her variations are always transmitted to him) because the difference in their colours balances the difference in their organizations & habits. If it did more than this, Nat. Select. would produce something superfluous, wh. it cannot do.

7. The fact that in almost all protected groups the females are equally brilliant & conspicuous with the males, shews I think a tendency to transference of colour from one sex to the other when this tendency is not injurious. Or perhaps the protection may be acquired, because this tendency to uniform colour in the sexes exists. This will apply to the gay birds with nests in holes &c. where the gay colours may have been acquired by the males through sexl. selection & transferred to females by laws of inheritance. This I believe is your view; but it fails in cases of the mimicking Leptalis, as there the males certainly did not acquire their gay colours through sexual selection.6

Now for the more special case

8. In the weak- and slow flying Leptalis both sexes mimic Heliconidæ

9. In the more powerful and swifter Papilio Pieris and Diadema, the female alone generally mimics the protected group.

10. In these cases the female often has acquired more bright varied and conspicuous colours than the male,—sometimes as in Pieris pyrrha &c. remarkably so.7

11. Not a solitary case is known of any male insect alone mimicking a protected group.

12. Yet colour is more frequently developed in males, & variations of colour are always ready for purposes of sexual or other selection.

13. The reason of this apparent anomaly is evident, on the principle that each species and each sex can only be modified by selection just so much as is absolutely necessary,—not a step further.

One species indeed may be more perfectly adjusted than another, because that other may dwindle & die out without directly affecting it;—but one sex cannot be perfected by Nat. Select. much beyond the other, because directly it is so in the least degree, then the deficiency of the other pulls it back— the whole struggle for existence of the species falls upon the less perfect sex, and rigid selection soon brings that sex up to the same level as its partner.8

14. It follows, that a male insect or bird, being by structure or habits less exposed to danger than its partner, is, to that extent, better adjusted to conditions. The protective colouring of the female balances this inequality in many cases, & Nat. select. can do no more than balance the two; it cannot give the male the protection he does not absolutely require, although he may sometimes gain it by inheritance from his partner.

15. I cannot think that an objection derived from cases in which the protection of colour does not appear to exist (like that of the fish &c.) have any weight at all.9 Colour is only one of many modes of protection, and this kind of protection is never acquired when another already exists.

Exactly a similar objection is made to the whole doctrine of “mimicry”. Why, it is asked, are there any white and yellow, quite unprotected (apparently) Leptalis? We answer, we do not know, but we firmly believe that they have some protection which the others have not, & wh. exactly balances it. So, out of about 500 species of Papilionidæ, the females of about a dozen are undoubted “mimickers”.10 Are we to deny that the need of protection has produced this mimicry because we cannot in the immense balance of species point out the exact protection of another kind, which enables them to do without it?

I am afraid I have not put my argument very clearly or consecutively, but I am at present utterly unable to see my way to any other conclusions than those here indicated.

Your view appears to me to be opposed to your own laws of Nat. Selectn. & to deny its power & wide range of action. Unless you deny that the general dull hues of female birds and insects are of any use to them, I do not see how you can deny that Nat. Select. must tend to increase such hues, and to eliminate brighter ones. I could almost as soon believe that the structural adaptations of animals & plants were produced by, “laws of variation & inheritance” alone, as that what seem to me equally beautiful & varied adaptations of colour shd. be so produced.

There is a difficulty I have on another subject. Admitting sexual selection in its fullest extent as having developed male weapons and ornaments, and in birds colour: how is it that there is so little sexual colour or marking in Mammals? Have they no taste for colour? Again about insects. Is there any evidence to show that female insects ever reject or choose males by colour, or at all? It seems difficult to believe that they do.

I am sorry you troubled yourself to return the box.11 It was not worth it. Do not trouble yourself to answer this at length, unless you like to do so.

Just say whether it produces any effect or no.

Yours very faithfully | Alfred R. Wallace—

CD annotations

1.1 The more … sexual selection. 8.6] crossed pencil
7.5 (except … to him) 7.6] scored blue crayon
20.1 I am … or no. 24.2] crossed pencil

Footnotes

The year is established by Wallace’s reference to the returned insect box (see n. 12, below). A partial draft of this letter, mistakenly dated 1869 by Wallace, was published in A. R. Wallace 1905, 2: 18–20.
For CD’s and Wallace’s discussion of the derivation of colour in female birds and insects, see the letter to A. R. Wallace, 16 September [1868] and n. 4.
See, for example, letter to A. R. Wallace, 30 April [1868]; see also Origin 4th ed., p. 241.
See, for example, letters to A. R. Wallace, 15 April [1868] and 5 May [1868]. For CD’s recently published discussion on this subject, see Variation 2: 71–5.
In his letters to A. R. Wallace of 15 April [1868] and 30 April [1868], CD asked why male butterflies had not developed the same protective colours as female butterflies of the same species, given that such protection would be useful.
‘Protected groups’ are species that mimic another species that is distasteful to predators. On the mimicking Leptalis, see the letter from A. R. Wallace, 15 March [1868] and n. 2, Bates 1861, A. R. Wallace 1867b, pp. 21–2, and A. R. Wallace 1867f.
Wallace discussed Papilio species in which only the female mimicked another species in A. R. Wallace 1864, pp. 21–2. Wallace and CD had discussed mimicking females of Pieris and Diadema (now Hypolimnas) (see letters from A. R. Wallace, 28 April [1868] and 1 May [1868], and letter to A. R. Wallace, 5 May [1868]). Wallace referred to a Malayan Diadema and to Pieris pyrrha in [A. R. Wallace] 1867b, p. 37. Pieris pyrrha is now probably Perrhybris pyrrha, from South America.
In Origin, pp. 156–7, CD wrote: ‘sexual selection … is less rigid in its action than ordinary selection’.
See Correspondence vol. 15, letter to A. R. Wallace, [24 June 1867] and n. 5; see also CD’s annotations to the letter from Albert Günther, [late December 1867 or early January 1868] (ibid.). For CD’s argument regarding bright colours in fish, see also Descent 2: 17–18.
See Bates 1861 for an analysis of Leptalis species that were found to mimic; see also Correspondence vol. 15, letter from H. W. Bates, 29 March 1867, and [A. R. Wallace] 1867b, pp. 21–2.
For CD’s returning a box in which Wallace sent insect specimens, see the letter to A. R. Wallace, 16 September [1868].

Summary

Submits a 15–point argument against CD’s views on the coloration of female birds and insects.

Letter details

Letter no.
DCP-LETT-6375
From
Alfred Russel Wallace
To
Charles Robert Darwin
Sent from
London, St Mark’s Crescent, 9
Source of text
DAR 82: A14–17
Physical description
8pp †

Please cite as

Darwin Correspondence Project, “Letter no. 6375,” accessed on 26 August 2019, http://www.darwinproject.ac.uk/DCP-LETT-6375

Also published in The Correspondence of Charles Darwin, vol. 16

letter