To James Croll   19 September 1868

Down | Bromley | Kent.

Sept. 19, 1868

Dear Sir,

I hope that you will allow me to thank you for sending me your papers in the Phil. Magazine.1 I have never, I think, in my life been so deeply interested by any geological discussion. I now first begin to see what a million means, and I feel quite ashamed of myself at the silly way in which I have spoken of millions of years.2 I was formerly a great believer in the power of the sea in denudation and this was perhaps natural, as most of my geological work was done near sea coasts, and on islands.3 But it is a consolation to me to reflect that as soon as I read Mr. Whittaker’s paper on the escarpments of England, and Ramsay and Juke’s papers, I gave up in my own mind the case;4 but I never fully realised the truth until reading your paper just received.5 How often I have speculated in vain on the origin of the vallies in the chalk platform round this place, but now all is clear. I thank you cordially for having cleared so much mist from before my eyes. With sincere respect

I remain, | dear Sir, | Yours very faithfully, | Charles Darwin.

Typewritten copy

1
Croll evidently sent the first two parts of Croll 1868, ‘On geological time and the probable date of the glacial and the upper Miocene period’; the last part was published in the November 1868 issue of the Philosophical Magazine. All three parts are in the Darwin Pamphlet Collection–CUL and are annotated.
2
In support of his argument that there had been enough time to allow for change through natural selection, CD had originally estimated that the denudation of the Weald of south-eastern England had taken place over a period of 300 million years (Origin, p. 287). Owing to criticism, he halved this period of time in Origin 2d ed., p. 287, and eliminated the entire discussion in Origin 3d ed. (see Correspondence vol. 8, letter to J. D. Hooker, 3 January [1860] and n. 16, and Peckham ed. 1959, p. 485). In the fifth edition of Origin, CD included Croll’s representation of a million years: take a narrow strip of paper, 83 feet 4 inches in length, and stretch it along the wall of a large hall; then mark off at one end the tenth of an inch. This tenth of an inch will represent one hundred years, and the entire strip a million years. (Origin 5th ed. p. 353; see also Croll 1868, p. 375.) CD also cited Croll’s calculation that 1000 feet of rock might be removed by subaerial denudation over six million years. See Herbert 2005, pp. 350–4.
3
CD considered the action of sea currents in excavating coastal land forms in South America, chapters 1–3. See also Origin, pp. 283–7, for CD’s earlier views on marine denudation, including his observations on volcanic islands and the coast of southern England. See Herbert 2005, pp. 260–1.
4
CD refers to Jukes 1862a and 1862b, Ramsay 1863 and 1864, and Whitaker 1867 (William Whitaker). CD had corresponded with Andrew Crombie Ramsay and Joseph Beete Jukes about their theories (see Correspondence vol. 10). However, he had remained sceptical about the emphasis that they placed on the role of subaerial denudation (fluvial and glacial forces), as opposed to marine action. See Correspondence vol. 13, letter to J. D. Hooker, [29 July 1865] and n. 11. For a detailed contemporary account of subaerial denudation, see Greenwood 1866. The topic continued to be controversial in 1867 (see Correspondence vol. 15, letter from Daniel Mackintosh, 8 December [1867]). See Davies [1969], pp. 317–55, for a discussion of the revival of fluvialism.
5
CD’s annotations to Croll 1868, pp. 379, 382–4, suggest the extent to which he was influenced by Croll’s calculations of the erosive power of rivers and streams. He added to Origin 5th ed., pp. 349–50, his conclusion, following the observations of Croll, Ramsay, Jukes, and Archibald Geikie, that subaerial degradation was ‘a much more important agency than coast-action, or the power of the waves’ (see also Peckham ed. 1959, p. 479).

Please cite as “DCP-LETT-6380,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-6380