1856.] NATURAL SELECTION. 445 



questions on the causes of variation ought to have been sent 

 you. Whether I am right or wrong in these points is quite a 

 separate question, but the conclusion which I have come to, 

 quite independently of geographical distribution, is that ex- 

 ternal conditions (to which naturalists so often appeal) do by 

 themselves very little. How much they do is the point of all 

 others on which I feel myself very weak. I judge from the 

 facts of variation under domestication, and I may yet get 

 more light. But at present, after drawing up a rough copy 

 on this subject, my conclusion is that external conditions do 

 extremely little, except in causing mere variability. This 

 mere variability (causing the child not closely to resemble its 

 parent) I look at as vejy different from the formation of a 

 marked variety or new species. (No doubt the variability Is 

 governed by laws, some of which I am endeavouring very 

 obscurely to trace.) The formation of a strong variety or 

 species I look at as almost wholly due to the selection of what 

 may be incorrectly called chance variations or variability. 

 This power of selection stands in the most direct relation to 

 time, and in the state of nature can be only excessively slow. 

 Again, the slight differences selected, by which a race or spe- 

 cies is at last formed, stands, as I think can be shown (even 

 with plants, and obviously with animals), in a far more im- 

 portant relation to its associates than to external conditions. 

 Therefore, according to my principles, whether right or wrong, 

 I cannot agree with your proposition that time, and altered 

 conditions, and altered associates, are " convertible terms. * 

 I look at the first and the last as far more important : time 

 being important only so far as giving scope to selection. God 

 knows whether you will perceive at what I am driving. I 

 shall have to discuss and think more about your difficulty of 

 the temperate and sub-arctic forms in the S. hemisphere than 

 I have yet done. But I am inclined to think that I am right 

 (if my general principles are right), that there would be little 

 tendency to the formation of a new species, during the period 

 of migration, whether shorter or longer, though considerable 

 variability may have supervened. ... 



