MULTIPLE FACTORS 173 



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 different factors, not by their behavior 



