The date range is established from the fact that the letter was written to Lyell at 73 Harley Street, London, on a Thursday, suggesting a meeting on Friday or Saturday. Lyell’s house on Harley Street was renumbered from 53 to 73 in September or October of 1866. CD’s first visit after September 1866 to the house of his brother, Erasmus Alvey Darwin, at 6 Queen Anne Street, London, began on Thursday 22 November 1866. His last visit to Queen Anne Street before Lyell’s death in 1875 took place from 10 January 1874, but CD arrived and left on a Saturday (10 and 17), which makes it unlikely that he would have suggested meeting at any time on Saturday. The visit before that began on 17 December 1872, but according to Emma Darwin’s diary (DAR 242), CD was in bed on the only possible Thursday, which makes it unlikely that he would have committed himself to a meeting so soon. The visit before that started on 14 December 1871, a Thursday. (CD’s ‘Journal’, Correspondence vols. 19, 20, 22, Appendix II.)