The year is conjectured from CD’s interest in research on cats (see n. 3, below), and from the day the Down carrier visited London. The Down carrier service, operated by George Snow, went to London early on Thursdays (Post Office directory of the six home counties 1851), so it is likely that CD wrote this note on a Wednesday. Between 1842, when the Darwins moved to Down, and end of 1856, when the library of the Royal Society commenced its move from Somerset House (Record of the Royal Society of London, p. 65), 8 October fell on a Wednesday in 1845 (when, however, CD was away from Down from 15 September to 26 October; see Correspondence vol. 3, Appendix II), 1851, and 1856. In 1851 CD was working on the proofs of Fossil Cirripedia between August and November (Correspondence vol. 5, Appendix I), and there are no references to cats in his extant correspondence. In mid-October 1856, CD finished the second chapter of the manuscript of his ‘big book’ on species (Correspondence vol. 6, Appendix III). According to CD’s table of contents for his manuscript of his ‘big book’, cats were discussed in this chapter (Natural selection, p. 25; the first two chapters of the manuscript are no longer extant). Cats are mentioned frequently in his 1856 correspondence (see Correspondence vol. 6).

Volume 21 of Isis was the volume for 1828; CD may have been interested in Christian Ludwig Brehm’s article ‘Ueber Columba domestica, livia et Amalia’, in part 2, pp. 135–41.

CD refers to Brehm’s article ‘Wo stammt die Hauskatze her?’ (Where does the domestic cat originate?), Isis 22 (1829): 639.