The date range is based on the date of CD’s visit to his brother’s house (see n. 2, below), and on the relationship between this letter and the letter to J. V. Carus, 17 February [1867].
Six Queen Anne Street, London, was the home of CD’s brother, Erasmus Alvey Darwin. CD was a visitor there between 13 and 21 February (see CD’s ‘Journal’ (Correspondence vol. 15, Appendix II)).
Julius Victor Carus asked CD for a specimen of the Eozoon fossils in his letter of 11 February 1867. Carpenter had described the supposed fossils following their recent discovery; see Carpenter 1864 and Carpenter 1866, and the letter from J. V. Carus, 11 February 1867, n. 5. See also Origin 4th ed., pp. 371–2, and Peckham ed. 1959, pp. 514–15.