164 THEORY OF EVOLUTION 



carried by red eyed flies without their presence 

 being seen until eosin is used, the experimenter 

 must be continually on the lookout for such 

 factors which may lead to erroneous conclusions 



Red 



cctcreaml ee wwfwhitin 



eecc eeww 



FIG. 82. Scheme to indicate influence of the modifying fac- 

 tors, cream and whiting. Neither produces any effect alone but 

 they modify other eye colors such as eosin. 



unless detected. As yet breeders have not 

 realized the important role that modifiers 

 have played in their results, but there are indi- 

 cations at least that the heaping up of modify- 

 ing factors has been one of the ways in which 



