The year is established by the relationship between this letter, the letter from Francis Galton, 2 December 1871 (Correspondence vol. 19), and the letter from Francis Galton, 28 March 1872.

In 1871, Galton had sent CD some transfused rabbits to breed and to take care of until they could be used in further experiments (see Correspondence vol. 19, letter from Francis Galton to Francis Darwin, 7 April 1871). Galton later recommended that all the young be killed except for a reserve of four or five (ibid., letter from Francis Galton, 13 September 1871), and sent for the old ones in November (letter from Francis Galton, 21 November 1871). CD seems to have kept six young rabbits (ibid., letter from Francis Galton, 24 November 1871), and may have bred from them. Galton’s experiments in rabbit blood transfusions were intended to test CD’s hypothesis of pangenesis (see Variation 2: 357–404) by producing different colours in the offspring of transfused rabbits.

Galton had evidently contacted the chemist William Crookes about his investigations of mediums; see letter from Francis Galton, 28 March 1872.