The date is established by the relationship between this memorandum, the memorandum from W. E. Darwin, [30 April 1864], and the letter from W. E. Darwin, [19 May 1864], in which he said he had received CD’s cowslip and would measure it. See n. 3, below.

In the letter to W. E. Darwin, [14–17 May 1864], and the letter from H. E. Darwin to W. E. Darwin, [18 May 1864], CD had asked that the cowslip pollen also be sketched dry; the pollen in this memorandum appears to be dry compared with the pollen in the memorandum from W. E. Darwin of [30 April 1864]. This sketch was probably filed by CD with the sketch of wet cowslip pollen on which he recorded measurements by William of both dry and wet cowslip pollen, ‘1864–1865’ (see memorandum from W. E. Darwin, [30 April 1864] and n. 3). Possibly these two sketches were also attached to others that have not been found. It is not known who struck through the sketch of red cowslip pollen.