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Darwin Correspondence Project

From J. J. Weir   [before 3] March 18681

6 Haddo Villas | Blackheath SE

March 1868

My Dear Sir

Many thanks for the kind manner in which you were good enough to receive my last communication, and also for your book which you so liberally present.—2

Since my last I introduced an Emberiza Schœniclus to my aviary, the only bird which noticed the new arrival was a Bullfinch ♂, he drove the Bunting about so unmercifully that I feared it must be removed, this Bullfinch is a very good tempered bird & I never before saw him molest any other bird, not even a specimen of the same species (schœniclus) also a captive with him, but which only differed by not yet having a black crown to the head. I cannot myself but think the black cap was the cause of so much animosity.—3

I may observe in passing that Bullfinches pair for life, nor do I recollect ever to have seen them separate, even in a wild state.

It has been to me a great puzzle to account for the separation of the sexes in many gregarious birds; & after years of seeking but one little fact has come before me bearing on the subject.

A very old bird catcher informs me that Goldfinches are thus sexually divided, he has found that if the flock is caught feeding on Teazle (Dipsacus Fullonum) they are all males, or nearly so—but if on Betony (Scrofularia) they are nearly all females.—4

This he accounted for by the fact that the male has a beak a little longer than the female & was thus able to obtain the Teazle seed which the shorter beak of the female failed to reach.—5

This solution appears to me to be the true one & I can easily imagine the differentiation proceeding till the male & female become as different in the beak as the New Zealand Neomorpha6

With regard to sexual selection in Lepidoptera.—

I do not think Dr. Wallace’s observations are by any means conclusive, the Bombyx of which he speaks is probably that which feeds on the Ailanthus,7 this is not a bright colored species, and in common with the European Bombyx mori the ♂ is eager for copulation immediately after its wings are expanded, indeed all Bombyces are so, our own species Saturnia pavonia ♀ minor Lasiocampa Quercus ♀ and others,8 attract the ♂ when placed in an opaque box, I have known cases in which the male of the first must have come down the chimney into a room where the female had emerged from the cocoon in a breeding cage.— Moreover they do not feed & it is therefore essential that the procreative act should be performed without delay.—

The ♀ I am inclined to think in the case of all Bombyces is quite passive in the matter.—

I will however write further to you very soon.—

I am | My dear Sir | Yours most gratefully | J Jenner Weir

C Darwin Esqr | Down

CD annotations

1.1 Many … present.— 1.2] crossed pencil
2.1 Since … wild state. 3.2] crossed ink
5.3 Betony] ‘Betony’ added pencil
8.1 With … very soon.— 11.1] ‘Keep’ added and circled pencil; ‘A’ added ink
9.1 I do not … very soon.— 11.1] crossed blue crayon
Top of letter: ‘A’ blue crayon; ‘A’ ink; ‘Keep’ pencil circled pencil


The date is established by the relationship between this letter and the letter to J. J. Weir, [6 March 1868].
In his letter to Weir of 29 February [1868], CD wrote that he would have his publisher, John Murray, send a copy of Variation.
The male Emberiza schoeniclus (reed bunting) has a black head with white moustachial streaks. Both male and female adult bullfinches (Pyrrhula pyrrhula), have black-capped heads.
Weir refers to teasel (Dipsacus fullonum) and water betony (Scrophularia auriculata). The European goldfinch is Carduelis carduelis.
CD cited Weir’s description of differences in goldfinch beaks in Descent 2: 39–40.
The extinct New Zealand huia (Neomorpha gouldii, now Heteralocha acutirostris) was a highly sexually dimorphic species of the family Callaeidae (wattlebirds). It was originally classified as two different species (Neomorpha acutirostris and N. crassirostris; see J. Gould 1865, 2: 530–3). In Descent 2: 39, CD described the differences in beak size and shape in males and females of the species.
Alexander Wallace had described sex ratios and mating behaviour in Bombyx cynthia (now Samia cynthia; see letters from Alexander Wallace, 25 February 1868, and 28 February 1868). Caterpillars of the moth feed mostly on leaves of the tree Ailanthus altissima and the moth is commonly known as the ailanthus silk moth (Eastman 2003, p. 2).
In modern classification, Bombyx mori (the common silk moth) is in the family Bombycidae, Saturnia pavonia (the emperor moth) is in the family Saturniidae, and Lasiocampa quercus (the oak eggar) is in the family Lasiocampidae. These families belong to the superfamily Bombycoidea (Moths and butterflies of Great Britain and Ireland vol. 7(2)).


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

Eastman, John Andrew. 2003. The book of field and roadside: open-country weeds, trees, and wildflowers of eastern North America. Mechanicsburg, Pa.: Stackpole Books.

Gould, John. 1865. Handbook to the birds of Australia. 2 vols. London: the author.

Moths and butterflies of Great Britain and Ireland. Edited by A. Maitland Emmet et al. 10 vols. Vols. 5, 6, and 8 not yet published. London: Curwen Books. Colchester: Harley Books. 1976–


Aggressive behaviour of a bullfinch toward new arrival in JJW’s aviary.

Sexual differences in goldfinches: size of beaks.

Sexual selection in Lepidoptera.

Thinks Dr Alex Wallace’s observations on Bombyx not conclusive in proving that no preference is shown by females.

Letter details

Letter no.
John Jenner Weir
Charles Robert Darwin
Sent from
Source of text
DAR 84.1: 51–2 and DAR 82: A107–8
Physical description
7pp †

Please cite as

Darwin Correspondence Project, “Letter no. 5964,” accessed on 6 December 2021,

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