THE MUTATED GENE 79 



usually increases with more intimate knowledge. We may 

 consider a few examples. The vestigial gene in Drosophila 

 produces as its main effect the typical reduced wing. But it 

 also causes the scutellar bristles to point forward and upward, 

 prolongs the time of development, decreases viability, makes 

 wings divergent, and causes rudimentation of balancers. The 

 gene for red eyes in the flour moth simultaneously makes the 

 testes colorless, the skin of caterpillars pale, the optic ganglia red 

 instead of brown, the ocelli of the larvae less pigmented, and 

 decreases the speed of development and the vitality. Numerous 

 examples of this type may be given, some of which we shall 

 discuss in relation to special problems. These effects are usually 

 described as a consequence of the fact that any character is 

 controlled by all of the genes and that only the relative effect of 

 one gene may be larger in a given case, thus making it possible to 

 attribute one major effect to one gene. Such a statement may 

 be correct, but it is not necessarily so. This depends completely 

 upon the developmental process which is changed by a mutant 

 gene and upon the time of incidence of this process. If, for 

 example, a mutant gene like vg ni produces a deficiency of some 

 growth substance toward the end of development of the Droso- 

 phila wing, no other effects may occur, or only such simultaneous 

 or later differentiations may be affected as are dependent upon 

 the quantity of the same substance (if any). If, however, 

 the allele vg Nw produces a similar deficiency at an early stage of 

 differentiation of the wing disk, it is very likely that other 

 processes of growth that are intimately correlated with imaginal 

 disk differentiation at that stage will be influenced, too, resulting 

 in slower development, diminished viability, and eventual 

 morphological consequences. One of the latter can actually be 

 shown to be a mechanical consequence of the disturbed growth, 

 viz., the upright position of the scutellar bristles. It frequently 

 occurs as a nonheritable abnormality that the two halves of the 

 notum (which are partly developed within the same disk as the 

 wings) do not concresce properly. In such cases, the scutellar 

 bristles may assume the same position as in vestigial, showing 

 that this position is a consequence of certain mechanical condi- 

 tions during concrescence. In vestigial flies, such disturbances 

 are more frequent than in Wild stock, and it may be regarded 

 as extremely probable that the bristle character is nothing but 



