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Darwin Correspondence Project

From A. B. Farn   18 November 1878

The Dartons, Dartford, Kent.

18th. November, 1878.

My dear Sir,

The belief that I am about to relate something which may be of interest to you, must be my excuse for troubling you with a letter.

Perhaps among the whole of the British Lepidoptera, no species varies more, according to the locality in which it is found, than does that Geometer, Gnophos obscurata.1 They are almost black on the New Forest peat; grey on limestone; almost white on the chalk near Lewes;2 and brown on clay, and on the red soil of Herefordshire.

Do these variations point to the “survival of the fittest”?3 I think so.

It was, therefore, with some surprise that I took specimens as dark as any of those in the New Forest on a chalk slope; and I have pondered for a solution. Can this be it?

It is a curious fact, in connexion with these dark specimens, that for the last quarter of a century the chalk slope, on which they occur, has been swept by volumes of black smoke from some lime-kilns situated at the bottom: the herbage, although growing luxuriantly, is blackened by it.

I am told, too, that the very light specimens are now much less common at Lewes than formerly, and that, for some few years, lime-kilns have been in use there.

These are the facts I desire to bring to your notice.

I am, Dear Sir, | Yours very faithfully, | A. B. Farn

C. R. Darwin Esq. F.R.S. | ⁠⟨⁠&⁠⟩⁠c &c &c


Gnophos obscurata is a synonym of Charissa obscurata (Scotch annulet), a moth of the family Geometridae. For a discussion of colour polymorphism in the moth, and of this letter in relation to the later literature on industrial melanism in the peppered moth (Biston betularia), see Hart et al. 2010.
The New Forest is in Hampshire; Lewes is a town in East Sussex.
The expression ‘survival of the fittest’ was introduced by Herbert Spencer in Principles of biology (Spencer 1864–7, 1: 444–5). It was first used by CD in Variation (see Correspondence, vol. 14, letter to A. R. Wallace, 5 July [1866]).


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 29 vols to date. Cambridge: Cambridge University Press. 1985–.

Spencer, Herbert. 1864–7. The principles of biology. 2 vols. London: Williams & Norgate.

Variation: The variation of animals and plants under domestication. By Charles Darwin. 2 vols. London: John Murray. 1868.


Discusses colour variations in the geometer moth, Gnophos obscurata. Concludes that the increasing proportion of the darker form is related to the effect of smoke that is blackening the chalk slopes on which they live.

Letter details

Letter no.
Albert Brydges Farn
Charles Robert Darwin
Sent from
Source of text
DAR 164: 26
Physical description
ALS 2pp

Please cite as

Darwin Correspondence Project, “Letter no. 11747,” accessed on 8 February 2023,