166 MULTIPLE ALLELOMORPHS 



might be interpreted to mean that a mutation in eye 

 color had appeared in the white-eyed stock in a 

 factor located near the factor for white ('^completely 

 linked" with it) and that the effect of this new factor, 

 combined with that of the factor for white, which 

 was already there, gave the color that we call eosin. 

 Eosin from this point of view would be due to two 

 consecutive mutations of completely linked, neigh- 

 boring loci. This interpretation of two consecutive 

 mutations can not be made in the case of cherry, 

 however, for cherry arose from red by one step, just 

 as did white; yet cherry, like eosin, when mated to 

 white, does not give rise to offspring that are red. 

 It would follow on the complete linkage view that 

 cherry and white differ from red by the same factor, 

 but since they are not alike, that one of them must differ 

 from red by still another factor. Since each arose 

 from red immediately, it would follow that one of 

 them must have arisen by a simultaneous mutation 

 in two factors completely linked and affecting the 

 same character. All these assumptions must be 

 made on the theory of complete linkage, but are 

 avoided on the alternative theory of multiple 

 allelomorphs. 



Exactly the same argument applies in the case of 

 two other triple allelomorph systems of Drosophila. 

 The recessive mutants pink and peach colored eyes 

 each arose independently from red eyed flies, yet 

 when crossed do not give red, but a color intermediate 

 between pink and peach. Secondly, sooty body 

 color arose in wild stock, although it w^as found only 



