To A. R. Wallace   29 January [1865]¹

The year is established by the relationship between this letter and the letter from A. R. Wallace, 20 January 1865.

Wallace 1863b and Wallace 1864b. See letter from A. R. Wallace, 20 January 1865 and n. 2.

Wallace 1864b discussed the distribution of parrots in the Malayan archipelago. Wallace presented his findings as evidence of a division of the region into two faunal territories (see Wallace 1864b, p. 277, and n. 4, below).

Wallace 1863b argued that the islands of the Malay Archipelago were divided into two faunal regions, one linked to the continent of Asia, the other to Australia. The map accompanying the paper showed a line, later known as ‘Wallace’s line’, separating Borneo, Java, and Sumatra from other islands to the south and east (see Camerini 1993). Following the observations of George Windsor Earl, Wallace noted that the boundary separating the two regions was a stretch of deep ocean, and that the islands on either side of the boundary were connected by shallow seas to the Asian or Australian mainland. Wallace added extensive zoological evidence of the distinctness of the two regions (Wallace 1863b, pp. 228–33). CD scored many passages in this section in his copy of Wallace’s paper (DAR 133: 10). CD had discussed Earl’s observations in his letter to Wallace of 9 August 1859 (Correspondence vol. 7). In Origin, p. 395, CD noted the relationship between the depth of the sea and faunal similarities on adjacent islands; in Origin 4th ed., p. 470, he added that the observations made by Earl of deep ocean separating two widely distinct mammalian faunas in the Malay region had been ‘fully confirmed by Mr. Wallace’s admirable researches’. CD had started making notes on the distribution of mammals in the Malay region in the late 1830s (Notebooks, Notebook B, 80, 82, and 241; DAR 205.3: 211). Observations on the depth of water between adjacent islands in the region appeared in the 1844 sketch of his species theory (see Foundations, p. 154). On the importance of the boundary between Asian and Australian faunal regions as a focal point for CD’s efforts to account for the distribution of organisms according to his theory of descent, see Camerini 1993.

Using a comparison of Borneo and New Guinea, which were similar in soil, climate, and elevation, yet zoologically divergent, Wallace emphasised that the line separating the two faunal regions was not based on differences of physical geography or climate (Wallace 1863b, pp. 231–2). He accounted for the separation of Java, Sumatra, and Borneo from the Asian continent by the elevatory action of volcanoes, accompanied by subsidence of the adjacent land (ibid., pp. 227–9). To illustrate further the contrast between these two regions, Wallace invited readers to imagine the continents of Africa and South America similarly connected by an archipelago of islands filling the channel of the Atlantic (ibid., pp. 232–3).

See letter from A. R. Wallace, 20 January 1865 and n. 1.

CD refers to the Aru pig collected on the 1826 to 1829 voyage of the French ships L’Astrolabe and La Zélée, and described by Henri de Blainville (see Blainville 1841–55, 2: 130–1, 224). The specimen was later discussed by Hermann Engelhard von Nathusius, who suggested that the pig was a feral Sus indicus (a synonym of Sus scrofa subsp. cristatus), a pig otherwise known only in a domestic state (Nathusius 1864, pp. 168–71). An annotated copy of Nathusius 1864 is in the Darwin Library–CUL (see Marginalia 1: 630–5, where Aru is given as ‘Arn’). CD discussed the Aru pig in Variation 1: 67. CD may have been interested in the Aru pig as a possible example of reversion. Feral pigs that had come to resemble wild boars were the principal evidence for the view that all domesticated animals, when they run wild, revert to the character of their parent stock (see Variation 1: 77–8, 2: 33). In January 1865, CD was revising the chapters on domestic animals for Variation (see ‘Journal’ (Correspondence vol. 13, Appendix II)); chapter 3 (Variation 1: 65–79) discusses pigs.

See letter from A. R. Wallace, 31 January [1865] and n. 5. The Aru Islands, located to the west of New Guinea, were included in Wallace’s study of the physical geography of the Malay Archipelago. Wallace’s paper contained accounts of the distribution of animals, including wild pigs, on various islands in the region (Wallace 1863b, p. 231).