MULTIPLE FACTORS 193 



truncate, though most were long. Males of these two 

 classes were then mated individually, again to black 

 pink females. From the result of these matings it 

 was clearly shown that the longs and the truncates 

 produced almost exactly the same proportion of 

 truncate, proving that they were alike genetically. 

 Moreover, continuous selection of males of this com- 

 position for many generations in an attempt to alter 

 this ratio was without effect. Since such an altera- 

 tion did not occur after many generations of out- 

 crossing (heterozygosis) there could not have been 

 any contamination or miscibility of the truncate 

 factors with their allelomorphs, nor any instability 

 of these factors. 



It will be recalled that in the truncate stock there 

 is a true genetic difference between the long-winged 

 and the truncate flies, but since it has been shown 

 that the truncate factors themselves do not vary, 

 this genetic variation that is continually occurring in 

 truncate stock must, therefore, be due to the fact that 

 flies homozygous for a large number of the factors 

 favoring the appearance of truncate are either not 

 viable or else infertile, and consequently a pure stock 

 cannot be maintained. 1 In support of the latter 

 explanation it is found that the greater the percentage 

 of truncate produced by a stock the lower its fertility. 



This case is of interest not only because the results 

 indicate that other non-conformable instances might 



1 Nevertheless stock can be maintained by the method of repeated 

 backcrossing to black pink, given above, from which individuals of a 

 definite, known composition can always be obtained. 



