220 MULTIPLE FACTORS 
color, involving only one organ, is, so far as its mode 
of inheritance is concerned, in no wise different in 
kind from a complex character like stature which, as 
Bateson pointed out in 1902, must depend on all 
factors affecting length of head, neck, trunk, or legs. 
In the case of eye color in Drosophila, more than 
25 factor-differences have arisen by mutation. Most 
of these factor-differences are dissimilar in their 
effects upon the eye color—thus, one differentiates 
a purple eyed fly from the red, another differentiates 
vermilion from red, another white from red, and so 
on. It so happens, however, that two mutations 
occurred, one in the sex-linked group, and one in the 
third, each of which changed the red eye to a pink 
color. It is to such cases only—where factor- 
differences produce the same or very similar effects, 
or effects that differ only in degree—that the term 
“multiple factors’ has come to be _ specifically 
applied. It should be recognized that this restric- 
tion of the term is arbitrary, but there is a practical 
advantage in grouping these particular cases to- 
gether under a common heading, because crosses 
involving several factor-differences that are similar 
in effect give peculiar ratios and present certain 
difficulties to a factorial analysis, not commonly met 
with elsewhere. 
In the above illustration of the sex-linked and 
third chromosome pinks the two factor-differences 
were not present in the same cross, and their in- 
heritance was worked out separately. They were 
shown to be in different loci, not by their behavior 
