– Your observation on Primroses relates to a subject with the general facts

which I am very familiar, though I never thought of the accurate investigation by measurements &c you undertake. I have long ago convinced myself that the two forms are both fertile, but always looked at it as the first step toward bisexuality. When your notion about the probable necessity of cross-fertilization came to my knowledge, these diœcio-dimorphous flowers (See Torr. & Gray, Fl. N. Amer. 2. p. 38)² occurred to me as a pretty case of arrangement for it. The case is extremely common, in very various families,— tho‘ I did not know it in Primulaceæ. The Rubiaceæ, Borragineæ, Labiatæ, &c &c. are full of it. I think cases can be found of its gliding into structural unisexuality.

Adieu, dear Darwin, | Truly Yours | A. Gray