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Darwin Correspondence Project

To Asa Gray   10 September [1866]1

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

Sep 10

My dear Gray

Perhaps you will remember when one of my boys was terribly ill you sent him some stamps which he looked at, & then after a long silence there came out the words “He is awfully kind”.2 He never said a truer thing. In the 1st place your note about Agassiz has interested & amused me much;3 for the day before I had been reading the Atlantic Monthly & the copy of a letter from Mme. Agassiz to Lyell & one from him all about the Amazonian glacier.4 We were both lost in astonishment at the nonsense which Agassiz writes & I cd not resist sending to Lyell a copy of part of your note, for his pre-determined wish partly explains what he fancies he observed. The evidence advanced by him is so weak that I do not think it wd be admitted for the former existence of glaciers even in a temperate region.5

With respect to the Origin, you speak of reading the sheets, but Murray promised me to send you a bound copy.6 After all as there is no chance of a new edition perhaps it wd be as well to let the Appletons have the sheets if they wd make any use of them; though on the other hand it is hardly worth while taking trouble about giving a few of the additions.7

With respect to my next book “on domestic animals” I am in perplexity, though most grateful to you for the capital bargain which you have made with Messrs Ticknor.8 My perplexity is this that I really have no idea whether it will be in the least degree popular; I am sure some chapters are curious, but then many others enter into far too minute details for the general reader: hence I do not quite like Messrs Ticknor to agree to publish until they have seen some of the sheets. And here comes the difficulty; there are about 42 wood-cuts & it wd save great expence if metal copies were procured of these, & to do this Messrs Ticknor wd have to make up their minds soon & enter into some arrangement with Murray, as I think I cd not ask Murray, who publishes at his own risk, to give copies; though as far as I am concerned I wd willingly do so. I feel pretty sure that I shall not even begin to print till the beginning of next year.

Many thanks for the specimens of the Rhamnus; my son & self have both looked at the pollen of both forms but alas! can make out no difference.9 The difference seems confined to the pistil & to the peduncles. We cannot even conjecture whether this species is reciprocally dimorphic like Primula or is merely tending to become dioecious.10 This is a great disappointment to us & the nature of the two forms cd only be made out by experiment or by observing their seed-production in their natural state.

I suppose this species cd not be purchased in your nursery gardens—

In my last letter I asked you whether you knew of any striking cases of endemic or naturalized plants which never flowered or which never seeded; if at no time I get an answer I shall understand that you know of no cases like the Acorus or horse-radish in Europe.11

The only point which I have made out this summer which cd possibly interest you is that the Common Oxlip found every where more or less commonly in England, is certainly a hybrid between the primrose & cowslip; whilst the P. elatior found only in the Eastern Counties (Jacq.) is a perfectly distinct & good species; hardly distinguishable from the common oxlip except by the length of the seed-capsule relatively to the calyx. This seems to me rather a horrid fact for all systematic botanists.12

I have just begun a large course of experiments on the germination of the seed & on the growth of the young plants when raised from a pistil fertilized by pollen from the same flower, & from pollen from a distinct plant of the same or of some other variety.13 I have not made sufficient experiments to judge certainly, but in some cases the difference in the growth of the young plants is highly remarkable.

I have taken every kind of precaution in getting seed from the same plant, in germinating the seed on my own chimney-piece, in planting the seedlings in the same flower pot, & under this similar treatment I have seen the young seedlings from the crossed seed exactly twice as tall as the seedlings from the self-fertilized seed; both seeds having germinated on same day. If I can establish this fact (but perhaps it will all go to the dogs) in some 50 cases, with plants of different orders, I think it will be very important, for then we shall positively know why the structure of every flower permits, or favours, or necessitates an occasional cross with a distinct individual.14 But all this is rather cooking my hare before I have caught it. But somehow it is a great pleasure to me to tell you what I am about.

Believe me my dear Gray | ever yours most truly | & with cordial thanks | Ch. Darwin


The year is established by the relationship between this letter and the letter from Asa Gray, 27 August 1866.
When Leonard Darwin was recovering from scarlet fever, Gray had sent a selection of American postage stamps for his collection (see Correspondence vol. 10, letters to Asa Gray, 10–20 June [1862] and 28 July [1862]).
Elizabeth Cabot Cary Agassiz’s letter to Charles Lyell has not been found. It is likely that she enclosed a copy of Louis Agassiz’s article from the Atlantic Monthly (J. L. R. Agassiz 1866a), which Lyell then passed on to CD along with her letter. Lyell’s letter to CD has not been found, but see the letter to Charles Lyell, 8 [–9] September [1866] and n. 5.
For the portion of Gray’s 27 August 1866 letter that CD copied and sent to Lyell, see the postscript to the letter to Charles Lyell, 8[–9] September [1866]. On the reasoning behind Agassiz’s theory, see the letter to Charles Lyell, 8[–9] September [1866] and n. 6.
See letter from Asa Gray, 27 August 1866. CD had asked his publisher, John Murray, to send copies of the fourth edition of Origin to a number of people (see letter to John Murray, 15 July [1866] and n. 3). Gray’s name appears on CD’s presentation list for the fourth edition of Origin (see Correspondence vol. 14, Appendix IV).
CD’s American publisher, D. Appleton & Co., printed from stereotypes. Since CD had made major revisions (see Origin 4th ed., pp. xi–xii), Appleton’s would have had to recast most of the book. See letter from Asa Gray, 18 July 1866 and n. 6.
CD refers to Variation; Gray had told CD that he thought he could make an arrangement with the Boston firm Ticknor & Fields to publish it (see letter from Asa Gray, 27 August 1866).
Gray had enclosed specimens of the flowers of Rhamnus lanceolatus with his letter of 7 August 1866. In CD’s notes on R. lanceolatus (DAR 109: A42 and 44), a table of pollen-grain sizes indicates a slight difference between those of long-styled and those of short-styled forms. In Forms of flowers, pp. 295–6, CD noted a ratio of 10 to 9 in the diameter of pollen-grains in the two forms. CD’s son William Erasmus Darwin had observed a difference in the size of the pollen-grains of the two male forms of a related species, R. cathartica (see letter from W. E. Darwin, 20 June [1866] and n. 5, and Forms of flowers, p. 295). William’s notes on the R. lanceolatus specimens received from Gray, dated 28 August 1866, are in DAR 186: 43.
In Forms of flowers, pp. 295–6, CD noted that the pistil of the short-styled form of Rhamnus lanceolatus was half to two-thirds the length of that of the long-styled form. Moreover, he noted the slightly larger size of pollen-grains in the long-styled form. He suggested that R. lanceolatus was a hermaphrodite plant that was becoming dioecious. In earlier work on various dimorphic species of Primula, CD had shown that heteromorphic unions resulted in complete fertility whereas homomorphic unions were incompletely fertile (‘Dimorphic condition in Primula, p. 91). CD later referred to this phenomenon as reciprocal dimorphism (see ‘Three forms of Lythrum salicaria, p. 191).
The letter to Gray requesting information on plants which never flowered or seeded has not been found. Acorus calamus (sweet flag) is a perennial herb propagated by rhizome division; horseradish is propagated by pieces of root. CD’s question about Acorus and horseradish was related to his work on sterility from the excessive development of the organs of growth or vegetation; he refers to the sterility of the two plants, and others, in Variation 2: 170.
In crosses of long-styled primrose (Primula vulgaris) with pollen of short-styled P. elatior, CD found only minimal fertility, compared with the high rate of fertility he had recorded in earlier experiments using pollen from the common oxlip (now P. veris x vulgaris) with P. vulgaris. CD’s interpretation of the results emphasised the necessity of going beyond structural criteria in classifying species (see ‘Specific difference in Primula, pp. 449–51). CD’s notes for his crossing experiments made in 1866 with the common oxlip and P. elatior (which he called the Bardfield oxlip), are in DAR 108: 8–10; the bulk of CD’s notes on Primula are in DAR 108–11. CD’s discussion of the common oxlip and P. elatior is in Forms of flowers, pp. 55–73.
This work marked a departure from many of CD’s previous crossing experiments, which were designed to assess the regulation of cross-fertility and self-fertility by variation in flower structure. Earlier experiments had focused on the number of seeds produced in controlled crosses while the new work concerned the relative growth of plants raised from such seeds (see Allan 1977, pp. 250–1). CD later published the results of these and further experiments in Cross and self fertilisation; see especially pp. 285–311.
In Cross and self fertilisation, p. 303, CD concluded that seedlings from a cross between distinct plants were generally superior in height, weight, and constitutional vigour to those that were self-pollinated.


Allan, Mea. 1977. Darwin and his flowers: the key to natural selection. London: Faber & Faber.

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

Cross and self fertilisation: The effects of cross and self fertilisation in the vegetable kingdom. By Charles Darwin. London: John Murray. 1876.

‘Dimorphic condition in Primula’: On the two forms, or dimorphic condition, in the species of Primula, and on their remarkable sexual relations. By Charles Darwin. [Read 21 November 1861.] Journal of the Proceedings of the Linnean Society (Botany) 6 (1862): 77–96. [Collected papers 2: 45–63.]

Forms of flowers: The different forms of flowers on plants of the same species. By Charles Darwin. London: John Murray. 1877.

Origin 4th ed.: On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. 4th edition, with additions and corrections. By Charles Darwin. London: John Murray. 1866.

Origin: On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. By Charles Darwin. London: John Murray. 1859.

‘Specific difference in Primula’: On the specific difference between Primula veris, Brit. Fl. (var. officinalis of Linn.), P. vulgaris, Brit. Fl. (var. acaulis, Linn.), and P. elatior, Jacq.; and on the hybrid nature of the common oxlip. With supplementary remarks on naturally produced hybrids in the genus Verbascum. By Charles Darwin. [Read 19 March 1868.] Journal of the Linnean Society (Botany) 10 (1869): 437–54.

‘Three forms of Lythrum salicaria’: On the sexual relations of the three forms of Lythrum salicaria. By Charles Darwin. [Read 16 June 1864.] Journal of the Linnean Society (Botany) 8 (1865): 169–96. [Collected papers 2: 106–31.]

Variation: The variation of animals and plants under domestication. By Charles Darwin. 2 vols. London: John Murray. 1868.


L. Agassiz’s evidence [for glaciation of America] is very weak.

Thanks AG for arranging for American edition of Variation, but doubts that the book will be successful.

Has found no differences in pollen of Rhamnus so cannot conjecture whether it is dimorphic.

The common oxlip of England is certainly a hybrid between the primrose and the cowslip whereas Primula elatior is a good species.

Reports experiments on the relative vigour of seedlings from cross- and self-fertilised plants.

Letter details

Letter no.
Charles Robert Darwin
Asa Gray
Sent from
Source of text
Gray Herbarium of Harvard University (92)
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 5210,” accessed on 18 January 2022,

Also published in The Correspondence of Charles Darwin, vol. 14