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Darwin Correspondence Project

To George Bentham   15 December [1857]1

Down Bromley Kent

Dec. 15th.—

My dear Sir

I am in truth ashamed to trouble you so soon again; but since I sent you the list of Silene, I find that I very stupidly overlooked a list of Cucubalus, which I had forgotten that Gærtner ranked as quite distinct genus.—2

I have now copied a list in pairs of the crosses which he made in Cucubalus, & it would be a very great kindness, if you would look it over & mark whether in each pair, you consider the male & female as only varieties of the same species or as distinct species, & allow me to state that such is your opinion.—3

My object is to show that where the fertility of a cross makes a very close approach to perfect fertility there is often difference of opinion whether the forms are varieties or species. If any of the pairs are undoubtedly good & distinct species, the case does not concern me. If this would not cause you much trouble, it wd be a great kindness, for I have already got a curious parallel list of graduated evidence from fertility & ordinary evidence on what to call species and what varieties.—

My dear Sir | Yours very sincerely | C. Darwin


Dated by the relationship to the letter to George Bentham, 1 December [1857].
In a discussion of the difficulty of using fertility as a test for distinguishing species and varieties, CD cited Bentham’s view that the Cucubalus plants on which Karl Friedrich von Gärtner experimented were ‘only varieties of Silene inflata.’ (Natural selection, p. 404).


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.


For his studies on fertility of crosses, asks GB to mark a list of pairs of Cucubalus as to whether they are varieties of the same species, or distinct species.

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. 681
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 2184,” accessed on 15 January 2021,

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