castle: role of selection in evolution 387 



organism? I am not ready to say that natural selection is proved 

 as the method par excellence of evolution, but I am not ready to 

 abandon it as the most reasonable explanation of evolution 

 until something better supported than the mutation theory is 

 offered as a substitute for it. At the same time the fact should 

 be emphasized that biology has benefited greatly from the 

 investigation and the discussion initiated by the mutation 

 theory. Even though the mutation theory cannot be accepted 

 as a general theory of evolution it has done us great good in 

 dispelling or clarifying the hazy notions which formerly existed 

 as to what natural selection could accomplish. Selection, whether 

 natural or artificial, is, as the mutation theory rightly holds, 

 primarily an agency for the elimination of variations, not for 

 their production. It can only act on variations actually exist- 

 ing, and while it can, I believe, continue and extend variation 

 already initiated by shifting in the direction of selection the 

 center of gravity of variation, it cannot initiate new lines of 

 variation. It cannot change a vertebrate into something else 

 nor something else into a vertebrate. It is limited to the modi- 

 fication of existing types of organisms, and to their modification 

 in directions in which they show a tendency spontaneously to 

 vary. 



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