Darzvms Selection Theory. 37 



intervals, give off variations in the same direction. In 

 this way it progresses *'step by step/'^ 



Let us look for a moment at Darwin's views on the 

 influence of external conditions. On this matter again 

 we find that his opinion is by no means fixed. Sometimes 

 he would appear to think that it has played very little 

 part in the origin of species, at other times he ascribes 

 great significance to it. And inasmuch as he was quite 

 familiar with the relation of individual variation to the 

 environment, it follows that he was chiefly concerned 

 here with single variations. In a letter to Hooker. 1856, 

 he says, ''My conclusion is, that external conditions do 

 extremely little, except in causing mere variability." "How 

 much they do is the point of all others on which I feel 

 myself very weak."- 



We are all familiar in the pages of Darwin's books 

 with the important role ascribed to changed conditions 

 of life. Especially in the case of the transport of a 

 plant from one climate to another and the effects of the 

 first years of cultivation on a wild species.^ Species 

 therefore with a wider geographical distribution are 

 more likely to produce new forms. 



In later years Darwin has again changed his views 

 on this point ; after reading Hoffmann's famous re- 

 searches he said : No doubt I originally attributed too 

 little weight to the direct action of conditions. Perhaps 

 hundreds of generations of exposure are necessary. It 

 is a most perplexing subject. (1881.)^ 



The strongest influence on Darwin in his relation 

 to this question was that produced by a criticism which 

 was published in 1869 by Fleeming Jen kin in the 



^ Ibid., p. 66. ^ Life and Letters, II, p. 87. 



^ Origin, p. 64, etc. * Life and Letters, III, p. 345. 



