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 F,, 
when it was found to have a somewhat lighter 
color— +3.17—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 differences 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 
