The date is established by the reference to CD and Emma Darwin’s visit to George Henry Lewes and his common-law wife Marian Evans, the novelist George Eliot, and by the reference to Francis’s paper on the snail’s heart (see n. 2, below). The Monday following the visit on 30 April was 1 May.

According to Emma Darwin’s diary (DAR 242), she and CD visited Lewes and Evans on 30 April 1876. Francis Darwin’s article ‘On the structure of the snail’s heart’ (F. Darwin 1876a) had been published in April 1876; his letter on the use of pycrotoxine (now more commonly picrotoxin) in vivisection experiments in Nature, 16 March 1876, pp. 384–5. In the latter, Francis wrote to complain of biased reporting by the Society for the Protection of Animals Liable to Vivisection. The society’s summary of the report of the Commission on vivisection stated that James Crichton-Browne had sacrificed forty-six animals when experimenting with pycrotoxine (a poison commonly applied to wheat that caused convulsions and often death in animals when ingested), but failed to mention that these experiments had resulted in finding that chloral was an effective antidote to this poison (Report of the Royal Commission on vivisection, pp. 166–7; Society for the Protection of Animals Liable to Vivisection 1876, p. 22).

James Sully had published a collection of essays in 1874 under the title Sensation and intuition (Sully 1874); his article on Wilhelm Maximilian Wundt was published in Mind in January 1876 (Sully 1876).