But as to a centre of radiation for each separate species, I must say I have a bias that way. You seem to have also, and you can best judge whether this, combined with geological considerations, would not involve centres of radiation for groups of species as well, to a certain extent. Would not the fact that the members of peculiar groups (in Vegetable Kingdom) are to a great extent localized favor that view? I am glad to hear that your idea of the unity of the human species is confirmed more and more. The evidence seems to me most strongly to favor it. And you well discriminate the separate questions of unity of birthplace and unity of parentage.... As to the physical question, surely you do not suppose that, in a fresh race, the one or two necessary close intermarriages would sensibly deteriorate the stock. Look at domestic animals of peculiar races,--how long you can breed in and in without much abatement of health or vigor! Did you ever consider the question of the cause of deterioration from interbreeding? I think I have somewhere in the “Journal” stated my notion about it, or hinted at it. If not, I will, some day; for I have a pretty decided opinion about it: that hereditary transmission of individual peculiarities involves also, among them, the transmission of disease, or tendency to disease,--a constantly increasing heritage of liability as interbreeding goes on; in plants well exemplified by maladies affecting old cultivated varieties long propagated by division. I should much enjoy a visit with you at New Haven, and so would my wife, no less. Hope we may some day.... Yours faithfully, A. GRAY. TO A. DE CANDOLLE. March 26, 1857. Fendler is back again in the country of Venezuela, and making fine collections. He will complete the sets of his former distribution, but not send the same things over again. He has found many more Filices. Will you and M. Dunant continue? On Wright’s return home he was troubled with rheumatism, and longed for a warm climate to pass the winter in. So I sent him to the east end of Cuba (where I wished the Huets to go). He is doing very well there. Oregon is still in a disturbed and unsafe state. But I should inform you that a commission has been raised to run our northwestern boundary with the British government; and it will probably be commenced this year. The party would have a sufficient escort, and this would give the Huets a safe opportunity for botanizing across the continent in a high latitude, if they are so disposed. I know not any details, but I could learn them, if need be, and there would be no difficulty in procuring needful protection for the Huets, they finding their own subsistence. I have published two statistical articles, based on my “Botany of the Northern States,” in “Silliman’s Journal,” and a third is now printing in that journal for May. I shall have extra copies to send you. There are other topics I mean to take up, if I can find time.... TO GEORGE BENTHAM. May 4, 1857.