The year is established by the relationship between this letter and the letter from A. R. Wallace, 31 January [1865].

Wallace enclosed his photograph with his letter of 31 January [1865]. CD’s ‘Scientific Album’ has not been found.

Wallace 1862, 1863a, 1863c, and 1864c. See letter from A. R. Wallace, 31 January [1865].

The photograph, the first to show CD with a beard, was taken by William Erasmus Darwin in 1864 (see Correspondence vol. 12, letter from W. E. Darwin, [19 May 1864]). It is reproduced as the frontispiece to Correspondence vol. 12.

In his letter of 31 January [1865], Wallace asked CD to sit for his brother-in-law, the photographer Thomas Sims.

See letter from A. R. Wallace, 31 January [1865] and n. 5.

CD refers to the descriptions of pigs by Hermann Engelhard von Nathusius and Henri de Blainville (Nathusius 1864, pp. 168–71, and Blainville 1841–55, 2: 130–1, 224). Nathusius maintained that the Aru pig was Sus indicus (a synonym of Sus scrofa subsp. cristatus; known to Europeans as the Chinese breed), and that it had become feral after being introduced onto one or more of the Aru islands. The ‘Sanglier d’Arrou’ (or Aru pig) had been identified by Blainville as ‘Sus scrofa (ferus)’. Blainville also described ‘Sangier des Papous’ (the New Guinea pig, or Sus papuensis), as an introduced animal that resembled the pigs of Malabar. Blainville’s description was based on an adult specimen; however, Nathusius noted that Sus papuensis (a synonym of Sus scrofa) was first established on the basis of a young specimen. Nathusius reported Blainville’s findings, but added that the evidence establishing the New Guinea pig as a distinct species was open to question. CD did not mention Sus papuensis in Variation. He followed Nathusius in dividing all known pig breeds into two groups, the Sus scrofa group and the Sus indicus group (Variation 1: 65–79). CD believed the former had descended from the common wild boar; he noted that the wild species from which the latter had descended was unknown (Variation 1: 65).