178 THEORY OF EVOLUTION 



As early as 1903 Cuenot had carried out 

 experiments with spotted mice similar to those 

 of Castle with rats. Cuenot found that spotted 

 crossed to uniform coat color gave in F 2 a ratio 

 of three uniform to one spotted, yet selection 

 of those spotted mice with more white in their 

 coat produced mice in successive generations 

 that had more and more white. Conversely 

 Cuenot showed that selection of those spotted 

 mice that had more color in their coat produced 

 mice with more and more color and less white. 

 Cuenot does not however bring up in this con- 

 nection the question as to how selection in these 

 spotted mice brings about its results. 



Without attempting to discuss these results 

 at the length that they deserve let me briefly 

 state why I think Castle's evidence fails to es- 

 tablish his conclusion. 



In the first place one of the premises may be 

 wrong. The three to one ratio in F 2 by no 

 means proves that all conditions of hoodedness 

 are due to one factor. The result shows at most 

 that one factor that gives the hooded types is 

 a simple Mendelian factor. The changes in 

 this type may be caused by modifying factors 



