32 TYPES OF MENDELIAN HEREDITY 



gan and Bridges have shown. 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 hj^brid form ; but whether this can or can not 

 be done in an}^ given case does not affect the funda- 

 mental principle of segregation which is the essential 

 feature of Mendel's discovery. 



Manifold Effects 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 



