^ET. 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 

 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 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 ? 



