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Darwin Correspondence Project

To George Bentham   26 November [1856]1

Down Bromley Kent

Nov. 26th

My dear Mr. Bentham

I venture a beg a favour of you. I have rather a wild bit of speculation afloat on the crossing of plants; & the Leguminosæ are my determined enemies, & worst of all are any forest-trees of this order.2 Now the only book which I have as any sort of guide is Loudon Encyclop. of Plants;3 & as far as I can most imperfectly judge; the Leguminosæ in the two sub-orders Cæsalpinieæ & Mimoseæ are mostly Trees or bushes— is this so? But what concerns me most is to know whether there are many timber trees with papilionaceous flowers, ie with a keel enclosing the stamens & pistil, so as to shut them up as in a common Pea.— In Loudon to my joy the little woodcuts seem to show that the trees Dipterix, Parivoa & Erythrina have the stamens protruding, unlike common papilionaceous flowers:— on the other hand, to my sorrow the trees Dalbergia Pongamia, Pterocarpus, Butea & Piscidia have flowers shut up just like a Pea.— Do you know these trees, & is my inference right? And can you say, whether the papilionaceous division of the Leguminosæ have as many trees as the two other divisions; or (but this is a very loose question) as many trees as most orders which have any trees?

(I have forgotten Robinia, but this I can myself watch to see if the keel opens next summer.)

Will you humour me in giving me a little information on this head, which I am very curious about, though the notion which I am testing is very wild.— I ought to be ashamed of myself to ask you to take so much trouble, but a brief answer, if you will kindly give me one, will suffice.—

Pray believe me, | Your’s very sincerely | Ch. Darwin

P.S. | I have been comparing all the evidence which I can collect on the natural crossing of the varieties of cultivated Leguminosæ; & it is most conflicting; but preponderates against crossing ever taking place.4 Do you happen to know of any facts throwing light on this question?—

Are many of the Cæsalpinieæ polygamous like many of the Mimoseæ?


The year is based on the subject of the letter, which was discussed in CD’s chapter ‘On the possibility of all organic beings occasionally crossing’ (Natural selection, pp. 35–91). The chapter was completed on 16 December 1856 (‘Journal’; Appendix II).
CD described how the flowers of Leguminosae enclose the plant’s reproductive organs as tightly as ‘a bivalve shell’ which, to many naturalists, indicated the impossibility of any cross-fertilisation (Natural selection, p. 68). CD believed that insects were a prime factor in the transport of pollen from one flower to another in this family, but he came to the conclusion that Leguminosae only rarely cross (Natural selection, p. 71). Trees were a problem because they had so many flowers opening at the same time that any crossing would take place with another flower on the same tree (see Natural selection, pp. 61–2, 71). Bentham was an expert on Leguminosae.
Loudon 1842 was cited in Natural selection, p. 61.
See n. 2, above, and letter to Gardeners’ Chronicle, [before 6 December 1856].


Loudon, John Claudius. 1842. An encyclopaedia of trees and shrubs; being the arboretum et fruticetum Britannicum abridged. London.

Natural selection: Charles Darwin’s Natural selection: being the second part of his big species book written from 1856 to 1858. Edited by R. C. Stauffer. Cambridge: Cambridge University Press. 1975.


Asks GB for help in clearing up his problems about Leguminosae, in connection with his "wild bit of speculation on the crossing of plants" [see Natural selection, p. 71].

Letter details

Letter no.
Charles Robert Darwin
George Bentham
Sent from
Source of text
Royal Botanic Gardens, Kew (Bentham Correspondence, Vol. 3, Daintree–Dyer, 1830–1884, GEB/1/3: f. 684)
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 2003,” accessed on 22 January 2021,

Also published in The Correspondence of Charles Darwin, vol. 6