8o THE 'ORIGIN OF SPECIES.' [i860. 



It is funny how each man draws his own imaginary line at 

 which to halt. It reminds me so vividly what I was told* 

 about you when I first commenced geology — to believe a 

 little^ but on no account to believe all. 



Ever yours affectionately, 



C. Darwin. 



C. Darwin to Asa Gray. 



Down, February i8th [i860]. 



My dear Gray, — I received about a week ago two sheets 

 of your Review ; \ read them, and sent them to Hooker ; they 

 are now returned and re-read with care, and to-morrow I 

 send them to Lyell. Your Review seems to me admirable ; 

 by far the best which I have read. I thank you from my 

 heart both for myself, but far more for the subject's sake. 

 Your contrast between the views of Agassiz and such as mine 

 is very curious and instructive. X By the way, if Agassiz 

 writes anything on the subject, I hope you will tell me. I am 

 charmed with your metaphor of the streamlet never running 

 against the force of gravitation. Your distinction between 

 an hypothesis and theory seems to me very ingenious ; but I 

 do not think it is ever followed. Every one now speaks of 

 the undulatory theory of light ; yet the ether is itself hypotheti- 

 cal, and the undulations are inferred only from explaining the 

 phenomena of light. Even in the theory of gravitation is the 

 attractive power in any way known, except by explaining the 

 fall of the apple, and the movements of the Planets .'* It seems 

 to me that an hypothesis is developed into a theory solely by 

 explaining an ample lot of facts. Again and again I thank 



* By Professor Henslow. 



f The ' American Journal of Science and Arts,' March, i860. Re- 

 printed in ' Darvviniana,' 1876. 



X The contrast is briefly summed up thus : " The theory of Agassiz re- 

 gards the origin of species and their present general distribution over the 

 world as equally primordial, equally supernatural ; that of Darwin as 

 equally derivative, equally natural." — ' Darwiniana,' p. 14. 



