The date is conjectured on the basis of an endorsement in an unknown hand and by the reference to William Robert Grove’s presidential address to the British Association (see n. 2, below).

In his presidential address to the British Association for the Advancement of Science in August 1866 (W. R. Grove 1866, p. lxxiv), Grove referred to Benjamin Dann Walsh’s research on the effect of food on the formation of insect races (Walsh 1864–5) and to Fritz Müller’s observations on organs of different structure acquiring similar functions (F. Müller 1864c; both works are cited on p. lxxxii). While composing the address, Grove had asked for references to recent work supporting CD’s theory (see letter from J. D. Hooker, 29 May 1866, and letter to J. D. Hooker, 31 May [1866]).