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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


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

Please cite as

Darwin Correspondence Project, “Letter no. 11747,” accessed on 18 February 2019,