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Darwin Correspondence Project

To Asa Gray   15 April [1867]1

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

April 15

My dear Gray

How good you have been to take so much trouble about the Expression-queries.— I wish I had thought earlier of having them printed, for in that case I might have sent a dozen to each of my few correspondents, as it is I can think of no one to send them to, so do not want any more.2

By the way I have just thought of Thwaites in Ceylon & will send him a couple.3

I have been lately getting up & looking over my old notes on Expression, & fear that I shall not make so much of my hobby-horse, as I thought I could: nevertheless it seems to me a curious subject, which has been strangely neglected.4

I have seen no one for months (but Hooker I rejoice to say will be here next Saturday)5 & have no news.— I am plodding on heavily correcting, & trying to make an atrociously bad style a little better, my book “on the Variation of Animals & Plants under Domestication”: I would offer to send you clean sheets, but I do not think you would care to receive them. There is not much about plants, & what there is, is almost all mere compilation; it will be a fearfully big book in two vols. & I shall be the next 5 or 6 months merely correcting the press: it is enough to make one curse one’s fate in being an author.—6

I manage to get a little amusement by some of my experiments.— I have proved that the trimorphic species of Oxalis behaves in exactly the same complicated manner in regard to their fertilisation as Lythrum.—7 I am going on with my trials of the growth of plants raised from self-fertilised & crossed seeds, & begin now to suspect that the wonderful difference in growth & conststitutional vigour occurs only with exotic plants which have been raised by seed during many generations in England, but which are not properly visited by insects & so have been rarely crossed.—8

I have just heard of a case which has interested me hugely, & which I am inclined to believe is true; namely that by cutting the tubers of differently coloured potatoes through the eye, & joining them, you can make a hybrid or mongrel. I am repeating this experiment on a large scale, for it seems to me, if true, a wonderful physiological fact.9

Here is a long prose all about my own doings.

Farewell with many thanks | Yours most sincerely | Ch Darwin


The year is established by the relationship between this letter and the letter from Asa Gray, 26 March 1867.
Gray had had copies of CD’s questionnaire on expression printed and had sent some to CD (see letter from Asa Gray, 26 March 1867). CD may have begun sending out a standard list of questions in December 1866 (see Correspondence vol. 14, letter to B. J. Sulivan, 31 December [1866]). CD had sent out a number of questionnaires in February 1867.
CD did not send questionnaires to George Henry Kendrick Thwaites, director of the Perediniya Botanic Gardens in Ceylon (Sri Lanka), until 1868, when he sent him a printed copy (see Correspondence vol. 16, letter to G. H. K. Thwaites, 31 January [1868]).
CD’s M and N notebooks (Notebooks), written between 1838 and 1840, and concerned in particular with human beings, contain notes on expression. See Barrett 1980 for additional notes. See also Browne 1985. Expression was published in 1872.
CD refers to Joseph Dalton Hooker; see letter from J. D. Hooker, 13 April 1867.
CD revised the page-proofs of Variation between March and November 1867 (CD’s ‘Journal’ (Appendix II)).
CD and Friedrich Hildebrand had worked simultaneously on the trimorphism of Oxalis; see Forms of flowers, pp. 169–83, and Correspondence vol. 14, letter from Friedrich Hildebrand, 11 May 1866, and letter to Friedrich Hildebrand, 16 May [1866]. CD’s notes on Oxalis are in DAR 109 and 111. For CD’s work on Lythrum, see ‘Three forms of Lythrum salicaria’ and Correspondence vols. 10–12.
For CD’s report of the start of these experiments, see Correspondence vol. 14, letter to Asa Gray, 10 September [1866]. In Cross and self fertilisation, CD gave the examples of Lathyrus odoratus (sweetpea) and Pisum sativum (common pea), which had been self-fertilised for many generations in Britain, and which insects rarely succeeded in pollinating, stating that their size and vigour could be increased by a cross with different stock (Cross and self fertilisation, pp. 157–63, 439).
See letter from Robert Trail, 5 April 1867. CD mentioned Trail’s information in Variation 1: 395–6; he said he had repeated Trail’s experiments without success.


Thanks AG for his trouble about expression queries; wishes he had thought earlier of having them printed.

Is "plodding on" correcting Variation

and getting "a little amusement" from plant experiments. Oxalis is trimorphic like Lythrum.

Is continuing his experiments on seedling vigour.

Has heard hybrid potatoes can be produced by joining halves of different tubers.

Letter details

Letter no.
Charles Robert Darwin
Asa Gray
Sent from
Source of text
Gray Herbarium of Harvard University (97)
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 5442,” accessed on 27 April 2017,

Also published in The Correspondence of Charles Darwin, vol. 15