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Darwin Correspondence Project

To A. R. Wallace   22 February [1868]1

Down. | Bromley. | Kent. S.E.

Feb. 22nd.

My dear Wallace

I am hard at work on sexual selection & am driven half mad by the number of collateral points which enquire investigation, such as relative number of the two sexes, & especially on polygamy. Can you aid me with respect to birds which have strongly marked secondary sexual characters, such as Birds of Paradise, Humming-birds, the Rupicola2 or Rock thrush, or any other such cases. Many Gallinaceous Birds certainly are polygamous. I suppose that birds may be known not to be polygamous if they are seen during the whole breeding season to associate in pairs, or if the male incubates, or aid in feeding the young.

Will you have the kindness to turn this in your mind; but it is a shame to trouble you now that, as I am heartily glad to hear, you are at work on your Malayan travels.3 I am fearfully puzzled how far to extend your protective views with respect to the females in various classes.4 The more I work the more important sexual selection apparently comes out.

Can butterflies be polygamous? ie will one male impregnate more than one female? Forgive me troubling you & I daresay I shall have to ask your forgiveness again, & believe me, | My dear Wallace, | yours most sincerely | Ch. Darwin

P.S. Bates has had the kindness to set the Entomolg. soc. discussing the relative numbers of the sexes in insects & has brought out some very curious results.5

Is Orang polygamous, but I daresay I shall find that in your papers in (I think Annals. & Mag. of Nat. Hist.—6


The year is established by the relationship between this letter and the letter from A. R. Wallace, 24 February 1868.
The display of plumage by the male Rupicola crocea (now R. rupicola, the Guianan cock-of-the-rock) is described in Descent 2: 87–8. For CD’s discussions of hummingbirds and birds of paradise, see ibid., pp. 86 and 88.
On Wallace’s theory of protective coloration, see the letter from Albany Hancock, 8 February 1868, n. 3.
Wallace had published two papers on the orang-utan in the Annals and Magazine of Natural History (A. R. Wallace 1856a and 1856b).


Descent: The descent of man, and selection in relation to sex. By Charles Darwin. 2 vols. London: John Murray. 1871.


Reports work on sexual selection. Problems with the relative numbers of the two sexes and polygamy. Asks ARW’s help with several questions on polygamous birds.

Letter details

Letter no.
Charles Robert Darwin
Alfred Russel Wallace
Sent from
Source of text
The British Library (Add MS 46434: 104–5)
Physical description
LS(A) 4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 5912,” accessed on 1 April 2023,

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