£7. 46.) TO JAMES D. DANA. 425 
inferences are slowly settling themselves in my mind, 
or taking shape ; but on some of the most vexed ques- 
tions I have as yet no opinion whatever, and no very 
strong bias, thanks, partly, to the fact that I can think 
of and investigate such matters only now and then, 
and in a very desultory way. 
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 
TT 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 ang of unity 
of birthplace and unity of parentage. 
As to the physical question, surely mer r* not sup- 
pose 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 abate- 
ment of health or vigor ! 
Did you ever consider the question of the cause of 
deterioration from interbreeding ? 
