The year is established by the reference to Gray’s congratulations, which were probably for the Copley Medal. CD had been awarded the medal at the 3 November 1864 meeting of the council of the Royal Society (see Correspondence vol. 12).
CD may have seen Gray while he was visiting his cousin Sarah Elizabeth Wedgwood in London in August 1864 (see ‘Journal’ (Correspondence vol. 13, Appendix II)). Joseph Dalton Hooker remarked in his letter of [23 November 1864] (Correspondence vol. 12) that Gray was suffering from disease of the prostate.