To Asa Gray 21 July 
Down Farnborough Kent
My dear Dr. Gray
I must write one line to thank you for your letter of the 30th ulto., & for the extremely kind manner in which you have acceded to my request (which I assure you I long hesitated whether I would make) about marking the close species. I do not quite understand from Hooker’s note, whether the sheets have arrived, but I fancy not; but I shall hear again in two or three days. Hooker read over your letter, & seems to have been as much struck with it as I have been,—as showing in a most striking manner the geographical affinities of species, & the difficulty of ascertaining what are species.— Your discussion on connecting & separating forms seems to me so philosophical, that I much hope that someday you will be as good as your word & write an “Essay on Species”.1
I hope, also, before end of year to hear that you have found time to write on the geographical distribution of the U.S. plants;2 & if my letter caused you to do this some year or two before you otherwise would have done it, I shall congratulate myself in private, at having done good Botanical work.
Pray believe me with cordial thanks | Yours very sincerely | Charles Darwin
Some of my immersed seeds have come up after 82 & 85 days immersion, viz Radishes Beet, Atriplex, Capsicum, Oats, Cucurbita, Rhubarb, Lettuce, Carrotts, Celery, & Onions.—3
Geographical distribution. "Close" species. Hopes AG will write an essay on species.