I cannot say that I believe in centres of radiation for groups of species. From Darwin’s questions to me I think I perceive some of the grounds on which he would maintain it. One is attended to on page 77 of the January number [of “Silliman’s Journal”], but I am not clear that they are not just as susceptible of other interpretation.
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.