Wallace comments on Darwin's work
Full transcription
9, St. Mark's Crescent N.W.
Septr. 18th. 1869[1868]
Dear Darwin
The more I think of your views as to the colours of females, the more difficulty I find in accepting them, and as you are now working at the subject I hope it will not interrupt you to hear "counsel on the other side".
I have a "general" and a "special"
argument to submit
1. Female birds & insects are generally exposed to more danger than the male, and in the case of insects their existence is necessary for a longer period.
2. They therefore require in some way or other a special balance of protection.
3. Now if the males & female were distinct species, with different habits & organizations, you would I think at once admit, that a difference of colour serving to make that one less conspicuous which evidently required more protection than the other, had been acquired by nat.[ural] selct[io]n.
4. But you admit that varieties appearing in one
sex are transmitted (often) to that sex only.
there is therefore nothing to prevent nat.[ural] select.[tion]
acting on the two sexes as if they were two
species.
6. Your objection that the same protection would to a be useful to the male, seems to me utterly unsound, & directly opposed to your own doctrine so convincingly argued in the "Origin", "that N. Seln. never can improve an animal beyond its needs". So that admitting abundant variation of colour in the male, it is impossible that he can be brought by nat.[ural] select.[tion] to resemble the female (unless her variations are always transmitted to him) because this difference of their colours is to balance the difference in their organizations & habits,- and nat.[ural] select.[tion] can <only balance> not give to the male <different uses, balancing different degrees of> more than is needed to affect that balance. <……… in selection, can not produce the same result>
7. The fact that in almost all protected groups the females perfectly resemble the males, shows I think a tendency to transference of colour from one sex to the other when this tendency is not injurious. Or perhaps the protection is acquired because this tendency exists. I admit that if there is the case of concealed visits they habits? every have been acquired for protection
Now for the special case.
8. In the very weak flying Leptalis both sexes mimic Heliconidae.
9. In the much more powerful Papilio Pieris and Diadema it is generally the female only that mimics Danaida.
10. In these cases the females often acquire more bright & varied colours than the male. sometimes as in Pieris pyrrha conspicuous so.
11. No single case is known of a male Papilio Pieris Diadema or any other alone mimicking a Danais &c.
12. But colour is more frequent in males, and variations always seem ready for purposes of sexual or other selection.
13. The <plain explanation> fair inference seems to be that given in pp[proposition]. 5. [6] of the general argument, - viz. that each species & each sex can only be modified by selection just as far as is absolutely necessary, not a step farther. A male, being by structure & habits less exposed to danger & less requiring protection than the female cannot have more protection given to it by nat.[ural] select[io]n.., but a female must have some extra protection to balance the greater danger, & she rapidly acquires it in one way or another.
15. An objection derived from cases like male fish which seem to require protection, yet having brighter colours, seems to me of no more weight than is that of the existence of many white & unprotected species of Leptalis; to Bates' theory of mimicry;- that only one or two species of butterfly perfectly resemble leaves, or, that the instincts or habits or colours that seem essential to the preservation of one animal <seem> are often totally absent in an allied species.
[unsigned part-letter]
[WP1/8/139]
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