CORRESPONDENCE WITH DR. GRAY 



" I am glad to hear that your idea of the unity of the 

 human species is confirmed more and more. The evi- 

 dence seems to me most strongly to favor it. And you 

 will discriminate the separate questions of unity of birth- 

 place 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 

 can you breed in and in without much abatement of 

 health or vigor ! 



l< 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 heredi- 

 tary 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." 



ASA GRAY TO DANA 



11 CAMBRIDGE, November 7, 1857. 



" If you have plenty, please send me two copies of 

 your Thoughts on Species. I first read it carefully a week 

 ago, and I meant to write you at once how I like it, and 

 a few remarks, but something prevented at the time, and 

 I have been very busy and preoccupied ever since. 



" For the reason that I like the general doctrine, and 

 wish to see it established, so much the more I am bound 

 to try all the steps of the reasoning, and all the facts it 

 rests on, impartially, and even to suggest all the adverse 

 criticism I can think of. When I read the pamphlet I 

 jotted down on the margin some notes of what struck me 

 at the time. I will glance at them again and see if, on 

 reflection, they appear likely to be of the least use to 

 you, and if so will send them, taking it for granted that 

 you rather like to be criticised, as I am sure I do, when 

 the object is the surer establishment of the truth. 



299 



