164 



THEORY OF EVOLUTION 



carried by red eyed flies without tlieir presence 

 l)eing' seen until eosin is used, tlie ex2>erinienter 

 must ])e continually on the lookout for sucli 

 factors which may lead to erroneous conclusions 



Red 



cc [cream] 



ee wwlwhTiin' 



[whitn 



eecc 



I'lG. 8;?. Scheme to irulieate influence of the modifying fac- 

 tors, cream and wliiting. Neitlier produces any effect alone but 

 they modify other eye colors such as eosin. 



unless detected. As yet ])reeders have not 

 realized the important role that modifiers 

 have played in their results, but there ai-e indi- 

 cations at least that the heaping up of modify- 

 ing factors lias been one of the ways in wliich 



