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Darwin Correspondence Project

From George Henslow   1 November 1865

10 South Crescent | Bedford Sq: W.C

Nov 1/65

Dear Sir,

My Brother in-law, Joseph Hooker,1 has given me your address in order that I might be able to write to you on one or two little botanical matters about wh. I should like to ask permission to consult you.

While at Margate in the Summer,2 I was casually examining the fls of Medicago sativa which is largely cultivated there & discoverd a remarkable irritability in the stamens, viz: that when the alæ are slightly touched in a definite way the essential organs spring violently upwards, & by a simultaneous curving of the filaments, the anthers are dashed against the Vexillum, or any object (e.g. a pin) which may have caused the essential organs to rise.3

Have you ever observed it? 〈N〉either J. D. Hooker nr Prof Oliver4 were aware of the fact, & the former suggested my making a few notes upon that & a few other plants for the Linnean5 & I thought I should first like to hear if you had made any observns in the same direction.

I, of course, imagined that this uprising had something to do with cross fertilization, (following yr valuable generalization)6 & examined as many other Leguminosæ as I could meet with, to see if there was anything analogous in others. But I find that in other cases (Onobrychis, Lupinus, trifolium, Lotus, &c &c) the carina falls by the downward pressure of any body introduced which causes the protrusion of the essential organs.7

On returning home, & taking up yr Origin of Species, I opened at a sentence wh I had quite forgotten.— Where you say that “so necessary are the 〈visits〉 of bees to Papilionaceous flowers that … their fertility is greatly diminished if these visits be prevented”.8 Will you kindly tell me where you have published yr remarks as I should much like to read them.9

another little observation is Salvia which, I think, seems constructed to secure cross fertilization. by depressing the abortive anther (wh forms the short arm to the levers,) the fertile one is brought down, as I conceive on to the back of an insect, that would strike against (with its head) the abortive anth.

[DIAG HERE] pt of attachmt. [RIGHTBRACE] = fulcrum filament lip

The stigma is so situated as to strike the back of the insect when inserted into another fl.—

All this is Hypothetical as I have never seen an insect do it.

〈I shou〉ld much like to hear if 〈you〉 have ever made any observati[ons] on this.

Crocus is another as affords (by Bees) means of cross fertilization

If you would kindly give me any informn. or refer me to any work bearing on these, I should be much obliged.

In mean time, | I beg to remain | Yrs very faithfully | George Henslow

C Darwin Esq.


Joseph Dalton Hooker had married George Henslow’s elder sister, Frances Harriet Henslow, in 1851 (DNB). George Henslow’s father, John Stevens Henslow, had been CD’s mentor at Cambridge (see Browne 1995, pp. 117–18, 128–39, and Walters and Stow 2001, pp. 78–107).
Margate is a seaside resort on the Kent coast.
The alae are the lateral petals of a papilionaceous flower; the vexillum is the standard or large posterior petal (Jackson 1928).
Daniel Oliver.
Henslow’s observations on Medicago sativa (alfalfa) were read at the 16 November 1865 meeting of the Linnean Society; his observations on Indigofera (family Leguminosae) were read at the meeting of 19 April 1866. The papers were published in the Journal of the Linnean Society (Botany) 9 (1867): 327–9 and 355–8 (G. Henslow 1865 and 1866).
Henslow refers to the ‘general law of nature’ that CD formulated in Origin, p. 97: ‘no organic being self-fertilises itself for an eternity of generations; but … a cross with another individual is occasionally—perhaps at very long intervals—indispensable’. CD had first enunciated the principle in his letter to the Gardeners’ Chronicle, [before 13 November 1858] (Correspondence vol. 7; see n. 9, below). That nature ‘abhors perpetual self-fertilisation’ is also one of the main conclusions of CD’s work on orchids (see Orchids, p. 359). See also letter to J. T. Moggridge, 13 October [1865] and n. 3.
In his 1865 paper, Henslow noted that the stamens in Genista and Cytisus also sprang upward, as did those in Medicago sativa; however, he added that intercrossing between distinct flowers was more generally facilitated ‘by the depression of the carina with or without that of the alæ’ (G. Henslow 1865, p. 329).
The reference is to Origin, p. 97. The full sentence reads: ‘So necessary are the visits of bees to papilionaceous flowers, that I have found, by experiments published elsewhere, that their fertility is greatly diminished if these visits be prevented.’ CD began to consider the possibility of insect agency in the cross-pollination of papilionaceous flowers in Leguminosae in 1856 (see Correspondence vol. 6, letter to George Bentham, 26 November [1856] and n. 2). He experimented with leguminous plants between 1857 and 1859, recording the results in his Experimental notebook (DAR 157a: 33–8, 88–9), and in the manuscript of his ‘big species book’ (see Natural selection, pp. 68–71). For CD’s interest in the subject, see Correspondence vols. 6 and 7.
CD reported his research on cross-pollination in kidney beans (Phaseolus vulgaris) and other leguminous plants in a letter to the Gardeners’ Chronicle, [before 13 November 1858] (Correspondence vol. 7). The letter appeared in the 13 November 1858 issue of the journal under the title ‘On the agency of bees in the fertilization of papilionaceous flowers, and on the crossing of kidney beans’; it was also published in the Annals and Magazine of Natural History 2 (1858): 459–65 (Collected papers 2: 19–25). CD’s observations on Phaseolus are cited in G. Henslow 1865, p. 329.


Has made observations on pollination mechanism in Medicago sativa [J. Linn. Soc. Lond. (Bot.) 9 (1867): 327–9], which his brother-in-law [J. D. Hooker] would accept. Wants to check that CD has not already made them.

Also sends interpretation of Salvia.

His observations come from following CD’s generalisation in Origin [p. 79] on necessity of out-crossing.

Letter details

Letter no.
George Henslow
Charles Robert Darwin
Sent from
London, South Crescent, 10
Source of text
DAR 166: 150
Physical description
4pp damaged

Please cite as

Darwin Correspondence Project, “Letter no. 4928,” accessed on 25 May 2017,

Also published in The Correspondence of Charles Darwin, vol. 13