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Darwin Correspondence Project

To Mary Treat   5 January 1872

Down, | Beckenham, Kent.

Jan 5. 1872

Dear Madam

Your observations & experiments on the sexes of butterflies are by far the best, as far as known to me, which have ever been made.1 They seem to me so important, that I earnestly hope you will repeat them & record the exact numbers of the larvæ which you tempt to continue feeding & deprive of food, & record the sexes of the mature insect.

Assuredly you ought then to publish the result in some well-known scientific journal.2 I am glad to hear that your observations on Drosera will be published.3

I have attended to this subject during several years, & have almost M.S enough to make a volume; but have never yet found time to publish it.

I am very much obliged for yr courteous letter & remain dear Madam | yours faithfully | Charles Darwin


In her letter of 20 December 1871 (Correspondence vol. 19), Treat wrote about her observations on Papilio asterias (now P. polyxenes asterius, the black swallowtail butterfly), which suggested that the sex of the adult was determined by amount of time the larva spent feeding.
Treat repeated her experiments in 1872 and published the results in American Naturalist (Treat 1873). See Gianquitto 2007, pp. 155–9.
See Correspondence vol. 19, letter from Mary Treat, 20 December 1871 and n. 5. Treat’s observations were published in a note submitted by Asa Gray to the American Journal of Science (Treat 1871), and in A. Gray 1872a, p. 44, where Gray wrote of Drosera, ‘in one of our species with longer leaves (D. longifolia) the blade of the leaf itself incurves (as an intelligent lady has observed), so as to fold around its victim!’ Drosera longifolia is now D. anglica (the English sundew).


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 29 vols to date. Cambridge: Cambridge University Press. 1985–.

Gianquitto, Tina. 2007. ‘Good observers of nature’. American women and the scientific study of the natural world, 1820–1885. Athens, Ga., and London: University of Georgia Press.

Treat, Mary. 1871. Drosera (sundew) as a fly-catcher. American Journal of Science and Arts 3d ser. 2: 463.


Praises MT’s observations and asks her to repeat experiments on the the relation of sexes of butterflies to the nutrition of the larvae.

Is glad she will publish her observations on Drosera.

Letter details

Letter no.
Charles Robert Darwin
Mary Lua Adelia (Mary) Davis/Mary Lua Adelia (Mary) Treat
Sent from
Source of text
Amy Nagashima (private collection)
Physical description
LS 3pp

Please cite as

Darwin Correspondence Project, “Letter no. 8146,” accessed on 14 April 2024,

Also published in The Correspondence of Charles Darwin, vol. 20