The date range is inferred from CD’s reference to having completed his study of cirripedes. The last volume of his four-volume monograph was finished late in 1854 and published in May 1855 (Correspondence vol. 5, Appendix I; Freeman 1977, p. 68). The reference to being occupied with ‘other subjects of Natural History’ suggests that CD was at work on his big book on species (Natural selection), not on his ‘abstract’ (Origin), which he began in the summer of 1858. See Correspondence vol. 6.