No. 636] 



GAMETIC RATIOS IN DROSOrillLA 



61 



that are not satisfactorily represented. By far the longest of 

 these in the 25.8 unit interval from roughoid to sepia in the left 

 end-region of the third chromosome. Because of uncertainties 

 in classification, it is not ordinarily possible to use more than two 

 eye-color mutants together in an experiment, and such masking 

 of one character by others affecting the same organ leads to a 

 continued search for combinations of characters that can be 

 handled simultaneously. In general, the slight mutant char- 

 acters mask each other less than do extreme ones, and these are, 

 usually the least inviable. There is a continual improvement 

 of the working material by the substitution of better mutants. 



