No. 573] SHORTER ARTICLES AND CORRESPONDENCE 575 



occurred would be more likely to affect it than it would be to 

 affect most characters, ft is interesting to note that one such 

 mutation, of a very marked and unquestionable character, was 

 in fact observed. The mutant factor proved to be a strong 

 "plus" modifier, which was almost completely dominant, and 

 itself showed no contamination or variation, so far as could 

 be determined. It arose, as it happened, in the plus strain. 

 A part of the effectiveness of selection may therefore have been 

 due to the occurrence and sorting out of such occasional muta- 

 tions, but there is no way of telling how many of these took place, 

 or any need for assuming them at all in explaining the result. 

 These rare mutations, however, would form a very different phe- 

 nomenon from such fluctuating or frequent and progressive vari- 

 ation of a gene or genes concerned as ('astle postulates. Although 

 the academic possibility of variation of the latter type can not 

 be denied, there is no experimental evidence which can be used 

 to support it, and there is good evidence against it in many 

 individual cases. 



It is difficult to believe that this suggestion of Castle and 

 Phillips was not made in a spirit of mysticism, when we con- 

 sider also their suggestion that the genes may undergo contami- 

 nation, and especially when we consider the following passage, 

 with which their paper concludes : 



