TYPES OF MENDELIAN HEREDITY 45 
speak of factors for eyes or for legs, we really mean 
factor-differences which can produce effects only in 
the eye, the leg, or other regions of the body. In 
other cases the expression of a factor-difference may 
not be limited to one region but may produce a 
different effect in different regions; for example, a 
gray white-bellied mouse, which differs from the 
yellow mouse by only a single factor, is lighter than 
yellow on the under side, but darker on the upper side. 
IIT. By the Influence of Other Factors 
Analogous also is the fact that certain factor- 
differences produce a visible effect only when they are 
in company with a particular complex of other heredi- 
tary factors. Thus, a fly with the factors for ver- 
milion eyes can not be distinguished from one with 
the factors for pink eyes if both contain, in addition, 
the factors for white eyes, for the factors for white 
allow no other color to develop. Again, it is obvious 
that without the factors necessary for the develop- 
ment of a given character, no factors merely deter- 
mining special modifications of that character can 
have any effect. In other cases, the effect of a given 
factor may not be entirely suppressed, but greatly 
changed, if certain other factors in the hereditary 
complex are changed. Thus, in flies which already 
have the factor for vermilion eyes, the factor for 
purple eyes produces an eye still lighter than ver- 
noilion, but in flies containing the normal allelomorph 
of the factor for vermilion, the factor for purple pro- 
