256 MULTIPLE FACTORS 



modifies the amount of pigmentation when the 

 chief factor is present. Similarly the truncate and 

 the beaded intensifiers produced an effect upon the 

 wing only if their respective chief factor was present. 

 When it was recognized that the 3 : 1 ratio did 

 not establish the view that only one pair of factors 

 was being dealt with in the hooded rats, a new 

 experiment was devised, by Wright, which was 

 intended to discriminate between the effect of a 

 single factor-difference, combined with fluctuating 

 potency, and the effect of multiple factor-differences. 

 It will be remembered that the plus race — which 

 had an average grade at the time of +3.75 — had 

 been crossed to normal and then extracted in F2, 

 when it was found to have a somewhat lighter 



color h3.17 — than before. On the other hand 



the minus race 2.54, — when crossed to normal 



and extracted, averaged in F. -0.38. If multiple 

 factor difl'erences were present, then if either ex- 

 tracted race were again crossed to normal and 

 extracted it should be changed still more in the 

 same direction, for it would come to contain an 

 assortment of independent modifying factors more 

 nearly like that in the normal race, even though it 

 had the original factor for hoodedness. With each 

 crossing and extraction the resulting hooded in- 

 dividuals should thus tend to approach more nearly 

 to this same normal composition as a limit, and 

 therefore they should also approach each other. 

 On the other hand, if the differences between the 

 hooded races lay in the potencies of their chief 



