The date is conjectured from the address; CD stayed at 6 Queen Anne Street, London, between 4 and 14 February 1863 (see ‘Journal’ (Correspondence vol. 11, Appendix II)). In 1863, 5 and 12 February were Thursdays. The wording of the letter suggests that Stokes’s letter was forwarded from Down, making 12 February the most likely date.

Stokes was a mathematical physicist and Lucasian professor at Cambridge University (DNB). CD had apparently asked Stokes to determine the statistical chances of the same peculiarity recurring in a family. In Variation 2: 5, CD stated on Stokes’s authority that, given a peculiarity occurring on average in one individual in a million, and a population of sixty million, consisting of ten million families of six members each, ‘the odds will be no less than 8333 millions to 1 that in the ten million families there will not be even a single family in which one parent and two children will be affected by the peculiarity in question’. See also the manuscript of CD’s ‘big book’ on species (Natural selection, pp. 480–1 n.), in which Stokes is cited with regard to the extreme improbability of the same rare peculiarity appearing in father and child without some genetic connection.

The letters from Stokes have not been found.