From A. B. Farn   18 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