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Darwin Correspondence Project

From William Baird   7 February 1868

British Museum

7th. Febry. 1868

My dear Sir

I do not know that the sexes in Annelids (proper) differ in colour— It is so seldom that I see them except in spirits, when generally their colour is pretty well discharged— I should say however that as far as I know they do not— In the Entozoa, the colour does often shew a difference— I have now before me specimens of a species of Sclerostoma1—the females of which are abundant—the males very few in number— The former are red or reddish, the latter (taken at the same time from the same animal) is quite white—2

Believe me | Your’s very truly | W Baird


Sclerostoma is now Strongylus, a genus of nematodes in the family Strongylidae.
CD cited Baird in Descent 1: 321, reporting that males of certain entozoa differ slightly in colour from the females. CD added that there was no reason to suppose that such differences had arisen through sexual selection, and remarked that ‘true sexual characters’ did not occur in the lowest classes or subkingdoms of animals.


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


Colour differences in annelids and entozoa.

Letter details

Letter no.
William Baird
Charles Robert Darwin
Sent from
British Museum
Source of text
DAR 82: 50
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 5849,” accessed on 18 February 2020,

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