The year is established by the relationship between this letter and the following letter.

The letter referred to has not been found. Moncure Daniel Conway was an American preacher and author with strong anti-slavery views; in 1863, he moved from Boston to London (DAB).

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In the fifth edition of John Stuart Mill’s System of logic, Mill discussed the proper use of scientific hypotheses, noting that ‘Mr. Darwin’s remarkable speculation on the origin of species is another unimpeachable example of a legitimate hypothesis’ (J. S. Mill 1862, 2: 18 n.). Arguing that CD had not violated the ‘rules of Induction’ since he did not pretend his doctrine was proven, Mill wrote (ibid.): He has opened a path of inquiry full of promise, the results of which none can foresee. And is it not a wonderful feat of scientific ingenuity to have rendered so bold a suggestion, which the first impulse of everyone was to reject at once, admissible and discussable, even as a conjecture? See also Correspondence vol. 9, letter from Henry Fawcett, 16 July [1861] and n. 3, and Hull 1973, pp. 27–8. CD was considered for the Royal Society’s Copley Medal in 1863, but on 5 November the medal was awarded to Adam Sedgwick; William Benjamin Carpenter was a member of the society’s council and seconded John Lubbock’s nomination of CD for the medal (Royal Society, Council minutes, 5 November 1863). The Copley Medal was awarded, generally annually, by the council of the Royal Society, for outstanding ‘philosophical research’ (Record of the Royal Society, Appendix IV).