The reference to correcting proof-sheets indicates that the letter was written after 24 November 1859, the day CD first heard from John Murray that a second edition of Origin would be required (see letter to John Murray, 24 November [1859]). The two remaining Mondays prior to CD leaving Ilkley were 28 November and 5 December; the first seems more probable, given that CD was intending to visit Lyell in London on Thursday, 8 December and could hardly have expected to receive a reply en route from Ilkley.

In Origin, pp. 303–4, CD discussed new fossil finds in early geological formations in order to demonstrate how erroneous previous beliefs of the sudden appearance of species in particular formations had been. In the second edition he added: ‘Had it not been for the rare accident of the preservation of footsteps in the new red sandstone of the United States, who would have ventured to suppose that, besides reptiles, no less than at least thirty kinds of birds, some of gigantic size, existed during that period?’ (Origin 2d ed., p. 304; Peckham ed. 1959, p. 509). The information was given in Lyell’s Manual of geology (C. Lyell 1855, p. 348). The discussion was revised in later editions of Origin.

At Lyell’s request, CD had deleted the sentences about the discovery of evidence of the pre-Secondary existence of whales (Origin, pp. 303–4). See letters to Charles Lyell,20 September [1859] and 24 [November 1859]. In Origin 2d ed., p. 304, CD inserted a sentence about the discovery of the bones of birds in the Upper Greensand, citing Lyell’s Manual. The information was actually drawn from Lyell’s Supplement (C. Lyell 1857, p. 40). CD’s copy of the latter is in the Darwin Library–CUL.