To George Henslow   15 [June 1866]1

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

15th

My dear Mr Henslow.

I do most strongly believe in Reversion. There can be no shadow of doubt both in uncrossed & in crossed varieties.— I have seen many instances.2

The evidence of Kölreuter & Gärtner is distinct for Hybrids, & some of Naudin’s cases leave no doubt whatever in my mind.—3 I have been writing a chapter on subject, & the cases of Reversion in ordinary Hybrids appeared so clear that I have not thought it worth while to give instances, only references.4 So that I have discussed only more curious cases. Think over that of Cytisus adami—5

I had forgotten whether Herbert gave cases.6 But I know that he believed to largest extent in Reversion, as I remember in conversation7 & as I think is stated in his remarks on stripes in Asses & Horses—8 but this latter case does not concern you, as it does not refer to Hybrids.— I fully believe that Gärtner is right when he says that Reversion occurs only rarely in hybrids made from plants, which have not been cultivated 9   Hence wide difference in Wichura’s & Naudin’s results.—10

Your’s very sincerely | Ch. Darwin

1
The date is established by the relationship between this letter and the letter from George Henslow, 19 June 1866.
2
See letter from George Henslow, [13 or 14 June 1866] and n. 3; see also n. 4, below.
3
CD discussed the works of Joseph Gottlieb Kölreuter, Karl Friedrich von Gärtner, and Charles Victor Naudin for their bearing on reversion in Variation 1: 392 and 2: 36–7, 48–50.
4
CD devoted chapter 13 in Variation to reversion, and gave numerous examples in other chapters. CD entered in his ‘Journal’ (Correspondence vol. 14, Appendix II) for 10 May 1866, ‘began going over Ch. XIII of Dom. Animals’; see letter to J. D. Hooker, 16 May [1866] and n. 14.
5
For CD’s interest in the graft hybrid Cytisus adami (now +Laburnocytisus adamii), some tissues of which often revert to those of one or other parent, see the letter to Robert Caspary, 21 February [1866] and n. 3, and the letter from Robert Caspary, 25 February 1866 and nn. 4 and 9.
6
William Herbert published many works on hybrid plants; CD had discussed Herbert 1837 and 1846 in detail in the manuscript for his ‘big book’ on species (see Natural selection). Heavily annotated copies of Herbert 1837 and 1846 are in the Darwin Library–CUL (see Marginalia 1: 372–6). CD cited Herbert on cases of reversion in Variation 1: 377, 388.
7
CD corresponded with Herbert on reversion in 1839 (see Correspondence vol. 2), and visited him in 1845 and 1847 (see Correspondence vol. 3, letter to Charles Lyell, 8 October [1845] and n. 5, and Correspondence vol. 4, letter to J. D. Hooker, [2 June 1847]).
8
The example of a dun pony with a black stripe down its back is mentioned in Herbert 1837, p. 340, as evidence that the horse may have originated from the wild ass. CD had referred to this case in his unpublished ‘big book’ on species (see Natural selection, p. 332, n. 2). In Origin, pp. 163–7, and Variation 1: 55–64 and 2: 41–3, CD discussed stripes in asses and horses as cases of partial reversion in colour to a common progenitor.
9
CD refers to Gärtner 1849, pp. 474, 582.
10
Naudin’s claim that all hybrids inevitably undergo reversion had been based exclusively on experiments with cultivated plants. However, Max Ernst Wichura’s experiments on uncultivated hybrid willows had led him to question whether hybrids ever reverted to their parent forms (Wichura 1865, p. 23). CD discussed Wichura’s and Naudin’s findings in Variation 2: 50, and explained the discrepancy in their results using Gärtner’s observation of the comparative frequency of reversions in hybrids whose parent species had long been cultivated. See also Origin 4th ed., pp. 332–3, and letter from George Henslow, [13 or 14 June 1866], n. 4.

Manuscript Alterations and Comments

2.3 Reversion in] interl
2.3 appeared] after del ‘are’
2.3 clear] above del ‘clear’
2.4 have not] ‘have’ above del ‘did’
2.4 thought] above del ‘think’
3.1 believed] ‘d’ added
3.2 & as] ‘as’ interl
3.2 is] interl

Please cite as “DCP-LETT-5123A,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-5123A