9 6 PROBLEMS OF GENETICS 



calyx are required to prevent that part of the plant from being 

 red. 



We may proceed further and extend this mode of reasoning to 

 all cases of genetic variation, and thus conceive of all alike as 

 due to loss of factors present in the original complex. Until we 

 can recognize factors by means more direct than are provided by 

 a perception of their effects, this doubt cannot be positively 

 removed. For all practical purposes of symbolic expression we 

 may still continue to use in our analyses the modes of representa- 

 tion hitherto adopted, but we must not, merely on the ground of 

 its apparent perversity, refuse to admit that the line of argument 

 here indicated may some day prove sound. 



