Alfred Russel Wallace 



prodigality of original thought in it. But how unfortunate 

 it is that it seems scarcely ever possible to discriminate 

 between the direct effect of external influences and the 

 ^' survival of the fittest." 



9 St. Mark's Crescent, Regent's Park, N.W, Nov. 19, 1866. 



Dear Darwin, — Many thanks for the fourth edition of the 

 *^ Origin/' which I am glad to see grows so vigorously at 

 each moult, although it undergoes no metamorphosis. How 

 curious it is that Dr. Wells should so clearly have seen the 

 principle of Natural Selection fifty years ago, and that it 

 should have struck no one that it was a great principle of 

 universal application in nature! 



We are going to have a discussion on ^^ Mimicry, as pro- 

 ducing Abnormal Sexual Characters,'' at the Entomological 

 to-night. I have a butterfly (Diadema) of which the female 

 is metallic blue, the male dusky brown, contrary to the rule 

 in all other species of the genus, and in almost all insects; 

 but the explanation is easy — it mimics a metallic Euplwa, 

 and so gets a protection perhaps more efficient than its 

 allies derive from their sombre colours, and which females 

 require much more than males. I read a paper on this at 

 the British Association. Have you the report published 

 at Nottingham in a volume by Dr. Kobertson ? If so, you 

 can tell me if my paper is printed in full. 



I suppose you have read Agassiz's marvellous theory of 

 the Great Amazonian glacier, 2,000 miles long! I presume 

 that will be a little too much, even for you. I have been 

 writing a little popular paper on '* Glacial Theories " for 

 the Quarterly Journal of Science of January next, in which 

 I stick up for glaciers in North America and icebergs in the 

 Amazon ! 



I was very glad to hear from Lubbock that your health 

 is permanently improved. I hope therefore you will be 



176 



