Lyell refers to Joseph Dalton Hooker. See letter from J. D. Hooker, 21 February 1866. CD had forwarded part of that letter to Lyell at Hooker’s request (see letter from J. D. Hooker, [26 or 27 February 1866], and letter to J. D. Hooker, [28 February 1866] and n. 2).

James Croll had newly calculated that the glacial epoch ended 80,000 years before 1800, having begun 240,000 years earlier; he estimated that the coldest period occurred between 200,000 and 210,000 years before 1800 (Croll 1866a). In his letter of 21 February 1866, Hooker expressed the view that the evidence on which belief in the glacial period was founded could have been produced by various earlier global circumstances, including a slight shift in the earth’s axis. Hooker may have misinterpreted Croll 1865a as implying a change in the axis of the earth (see Correspondence vol. 13, letter from F. H. Hooker, 6 September [1865] and n. 6); however, Croll 1865a concerned the effect of icecaps on the centre of gravity, rather than the axis, of the earth (see also Fleming 1998, p. 166). Lyell did not believe that the earth’s centre of gravity had altered much either (see Reader, 13 January 1866, p. 44).