The month and year are established by the references to Romanes’s paper (see n. 5, below) and to CD’s being in London (see n. 4, below).

Romanes had carried out grafting experiments on plants to test CD’s hypothosis of pangenesis (see letter to G. J. Romanes, 24 September [1875] and nn. 5 and 6); CD evidently thought that he should extend his experiments to pigeons, believing that animal experiments would be more convincing (see letter to G. J. Romanes, 18 July 1875). Spots and black barbs are types of fancy pigeon (Variation 2d ed. 1: 151–3 and 163–4). The common rock pigeon (Columba livia) is a slatey-blue colour (ibid. 1: 192); CD argued in Variation 2d ed. 1: 382, that there was a latent tendency in all pigeons to become blue. In 1855, CD had built a pigeon house in the garden at Down, where he kept several varieties of fancy pigeon (Secord 1981, pp. 165–6).

CD had been drumming up support for Edwin Ray Lankester’s second attempt to be elected a fellow of the Linnean Society after he had been blackballed at the meeting of 2 December 1875 (see letter to W. T. Thiselton-Dyer, 16 December [1875]).

CD, Emma Darwin, and Elizabeth Darwin were in London from 10 December 1875; they returned to Down on Monday 20 December 1875 (see Emma Darwin’s diary (DAR 242)).

Romanes delivered the Croonian lecture on the locomotor system of medusae at the Royal Society of London on 16 December 1875 (Romanes 1875b). He outlined his discovery in his letter of 20 July 1875