To Friedrich Hildebrand   16 May [1866]1

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

May 16

My dear Sir

The state of my health prevents my attending the Hort. Congress;2 but I forwarded yesterday your paper to the Secretary & if they are not overwhelmed with papers, yours will be gladly received.3 I have made many observations on the Fumariacæ & convinced myself that they were adapted for insect-agency;4 but I never observed anything nearly so curious as your most interesting facts.5 I hope you will repeat your experiments on the Corydalis on a larger scale, & especially on several distinct plants;6 for your plant might have been individually peculiar, like certain individual plants of Lobelia &c described by Gärtner,7 & of Passiflora & orchids described by Mr Scott.8

I hope you will send me your paper on Oxalis.9 Three years ago I received a large collection of Oxalis from the Cape of Good Hope, & the sender told me that some of the forms were trimorphic. Of the plants which I raised from the bulbs, none of the species presented more than one form, excepting two. And these 2 species exhibited only two forms, but from their structure & from getting plenty of seed when I fertilized the pistil with pollen from the stamens of equal height, I felt sure that they were trimorphic.10

Since writing to you before I have read your admirable memoir on Salvia & it has interested me almost as much as when I first investigated the structure of Orchids.11 Your paper illustrates several points in my Origin of Species, especially the transition of organs. Knowing only 2 or 3 species in the genus, I had often marvelled how one cell of the anther cd have been transformed into the moveable plate or spoon; & how well you show the gradations; but I am surprized that you did not more strongly insist on this point.12

I shall be still more surprized if you do not ultimately come to the same belief with me, as shown by so many beautiful contrivances, that all plants require, from some unknown cause, to be occasionally fertilized by pollen from a distinct individual.13

With sincere respect believe me my dear Sir | yours very faithfully | Ch. Darwin.

LS

1
The year is established by the relationship between this letter and the letter from Friedrich Hildebrand, 11 May 1866.
2
Hildebrand had hoped to meet CD in London at the the International Horticultural Exhibition and Botanical Congress (see letter from Friedrich Hildebrand, 11 May 1866 and n. 4).
3
See letter from Friedrich Hildebrand, 11 May 1866 and n. 5. Hildebrand’s paper on Corydalis cava was published in the report of the congress (Hildebrand 1866d). Maxwell Tylden Masters was honorary secretary of the congress (see letter from M. T. Masters, March 1866).
4
CD began experiments on the pollination mechanisms of the Fumariaceae in 1858, following his correspondence with Asa Gray, who argued that self-fertilisation occurred in the family (see Correspondence vol. 6; see also Correspondence vols. 7 and 9–11). CD’s notes on Fumariaceae species, made between 1858 and 1863, are in DAR 76: B13–21. CD published a notice on pollination in Fumariaceae in Nature 9 (1874): 460 (Collected papers 2: 182–3). His experiments indicated that some species were self-sterile, and thus required insect agency for fertilisation, while others were fertile without insect aid (Cross and self fertilisation, pp. 329–59, 366).
5
In his paper on Corydalis cava (Hildebrand 1866d), Hildebrand reported that individual flowers of the species were self-sterile, even though the stigma was in contact with pollen from the same flower; he also described various degrees of fertility when different flowers from the same plant, from different plants, or from different varieties, were crossed. CD discussed Hildebrand’s experiments on C. cava in Variation 2: 132–3, remarking that the case was ‘highly instructive’, as it showed how widely different the action of the same pollen was, depending on whether it was placed on the stigma of the same flower, or on that of another flower on the same raceme, or on that of a distinct plant. CD added information on C. cava from Hildebrand to Origin 5th ed., p. 304. See also Cross and self fertilisation, pp. 331, 340–2. CD had experimented with C. solida and C. lutea, finding them to be self-sterile (see Cross and self fertilisation, pp. 358–9).
6
Most of Hildebrand’s crossing experiments involved as many as five plants; however, one experiment, which involved pollinating flowers on the same raceme with one another, and which resulted in some limited seed production, was performed with only one specimen (see Hildebrand 1866d, p. 157).
7
Karl Friedrich von Gärtner had observed two self-sterile plants of Lobelia fulgens (a synonym of L. cardinalis), a species generally self-fertile (see Gärtner 1849, pp. 64, 357). CD’s heavily annotated copy of Gärtner 1849 is in the Darwin Library–CUL (see Marginalia 1: 256–98). CD discussed Gärtner’s findings in Variation 2: 136. See also Correspondence vol. 11, letter to John Scott, 6 March 1863.
8
CD discussed cases in which individual plants within a species were self-sterile in Variation 2: 136–8, including examples of Passiflora observed by John Scott and Robertson Munro. Scott had also observed that one of several self-pollinated plants of the orchid Oncidium divaricatum produced seed capsules, but without any good seed; the other specimens were all self-sterile (Scott 1864, p. 548). CD’s annotated copy of Scott 1864 is in the Darwin Pamphlet Collection–CUL. CD reported the observation in Variation 2: 164. See also CD’s concluding remarks on why individual plants, or species, might be self-sterile in Variation 2: 141–2, and Cross and self fertilisation, pp. 341–5.
9
Hildebrand 1866c. See letter from Friedrich Hildebrand, 11 May 1866 and nn. 2 and 3.
10
Roland Trimen had sent CD bulbs of Oxalis from South Africa in 1864 (see Correspondence vol. 12, letters to Roland Trimen, 13 May 1864 and 25 November 1864). CD experimented on the genus from 1864 to 1868; his notes are in DAR 109: B3–27, B101, B108, B110, B115, and DAR 111: A44–8, A64, A67–8, B33–5, B43. He discussed Hildebrand’s findings and his own experimental work on Oxalis in Forms of flowers, pp. 169–83.
11
CD refers to Hildebrand 1866b. In his letter to CD of 21 June 1864 (Correspondence vol. 12), Hildebrand stated that his research on pollination mechanisms in Salvia and other plants had been inspired by CD’s Orchids. For CD’s interest in Salvia, see Correspondence vol. 12, letter to Friedrich Hildebrand, 25 June [1864], and this volume, letter to Friedrich Hildebrand, 20 April [1866]. CD cited Hildebrand’s work on Salvia in Cross and self fertilisation, p. 93 n. CD’s heavily annotated copy of Hildebrand 1866b is in the Darwin Pamphlet Collection–CUL.
12
CD discussed the development of organs by transitional gradations in Origin, pp. 179–94. He added a lengthy section on pollination mechanisms in Origin 4th ed., pp. 229–31. Hildebrand 1866b described gradations in the length of the connective (the portion of the filament connecting the two lobes of the anther) in Salvia; the connective allowed for varying degrees of movement. Hildebrand also described the flattened lobe or ‘cell’ characteristic of one pair of anthers in different species of the genus. CD’s experiments with Salvia coccinea and S. tenori, and his observations of S. grahami, are described in Cross and self fertilisation.
13
CD had first publicly stated his belief that it was a ‘law of nature that every organic being should occasionally be crossed with a distinct individual of the same species’ in a letter to the Gardeners’ Chronicle, [before 13 November 1858] (Correspondence vol. 7). He reiterated this view in Origin, p. 97, and Orchids, p. 359. However, CD continued to investigate possible cases of perpetual self-fertilisation (see Correspondence vols. 12 and 13, and ‘Three forms of Lythrum salicaria’, pp. 191–2 n. (Collected papers 2: 130–1)). See also letter to Robert Caspary, 4 March 1866 and n. 6. Hildebrand did not make any theoretical generalisation based on his results in Hildebrand 1866b. He later came to support CD’s views on crossing (see Hildebrand 1867, p. 5).

Please cite as “DCP-LETT-5092,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-5092