32 TYPES OF MENDELIAN HEREDITY 
gan and Bridges haveshown. ‘Therefore although the 
effect. of the white factor can not be detected in the 
single combination with red, it is reasonable to sup- 
pose that some effect is. really present. Similarly, 
conditions were found in which the effect of hetero- 
zygosis for eosin, vermilion, or pink could be demon- 
strated. While the question is one of only sub- 
sidiary importance, yet in the separation of classes 
it is often useful to be able to distinguish the pure 
from the hybrid form; but whether this can or can not 
be done in any given case does not affect the funda- 
mental principle of segregation which is the essential 
feature of Mendel’s discovery. 
Manirotp Errects oF SINGLE Factors 
It is customary to speak of a particular character 
as the product of a single factor, as though the factor 
affected only a particular color, or structure, or part 
of the organism. But everyone familiar at first hand 
with Mendelian inheritance knows that the so-called 
unit character is only the most obvious or most sig- 
nificant product of the postulated factor. Most 
students of Mendelian heredity will freely grant that 
the effects of a factor may be far-reaching and 
manifold. A few examples may make this plain. 
In Drosophila there is a mutant stock called 
“club,” in which the wing pads fail to unfold (Fig. 17) 
in about 20 per cent. of the flies. In the majority of 
club flies the wings expand fully, and are like those 
of the wild fly. Owing to this fact, that not all the 
