THEORY OF EVOLUTION 163 



factor for the original character has varied, 

 while the truth may be that another factor has 

 appeared that has modified a character already 

 present. In fact, many or all Mendelian fac- 

 tors that affect the same organ may be said 

 to be modifiers of each other's effects. Thus 

 the factor for vermilion causes the eye to be 

 one color, and the factor for eosin another 

 color, while eosin vermilion is different from 

 both. Eosin may be said to be a modifier of 

 vermilion or vermilion of eosin. In general, 

 however, it is convenient to use the term "modi- 

 fier" for cases in which the factor causes a 

 detectable change in a character already pres- 

 ent or conspicuous. 



One of the most interesting, and at the same 

 time most treacherous, kinds of modifying 

 factors is that which produces an effect only 

 when some other factor is present. Thus 

 Bridges has shown that there is a factor called 

 "cream" that does not affect the red color of 

 the eye of the wild fly, yet makes "eosin" much 

 paler (fig. 82). Another factor "whiting" 

 which produces no effect on red makes eosin en- 

 tirely white. Since cream or whiting may be 



