70 Vermilion. 



cross-overs over singles. In any case, according to MuUer, it was 

 not due to a chromosomal reorganization, as was shown by genetic 

 test in cases where the "cross-over modifier" was eliminated. 



It is perhaps too early to draw general conclusions from this and 

 other cases involving cross-over modifiers,^ but it is to be observed 

 that Muller's case gives the appearance of involving a rearrangement 

 of genes, and is analogous in some respects to the case of vermilion 

 just cited and to those in D. simulans and D. obscura. Until further 

 evidence is obtained, therefore, it is necessary to consider the possi- 

 bility of an inverted sequence of loci being due to a disturbance in 

 crossing-over rather than to an actual rearrangement of genes. 



' Cf. especially MuUer (1916) and Sturtevant (1919). In Sturtevant's table 17 (p. 320) the 

 three cross-overs involving the region b— pr were all doubles. 



