108 THE AMERICAN NATURALIST [Vol. XLIX 



The reciprocal cross gave : 



Bed Heterozygous ? X P™Tc S 



i 



Lived 9 



ed I Pi 

 * 9 











25 



5 



144 



72 



290 



IS 



210 

 5 



63 



2 OKI 

 5 

 152 



98 



518 



586 



185 



470 

 10 



101 



596 

 590 

 346 



1.1 : 1 

 59.0 : 1 



1.1+ : 1 



The pair with which we started {F, Table VII) gave 

 the ideal 3 : 1 ratio ; but in each of the three generations 

 which were bred from its offspring (Tables IX, X, XI) 

 there appeared again the same fluctuations which were 

 observed in the preceding experiments, and with even 

 more striking emphasis. Among the offspring of the 

 same pair are found some that give a 3 : 1 ratio and some 

 that give a 7.5 : 1 ; in a second pair we have some giving 

 1 : 1, and one giving 59 : 1. The latter especially suggests 

 the presence of a factor that actually inhibits the devel- 

 opment of the pink flies, and, moreover, that it is being 

 segregated in a mixed stock. 



If the presence of such factor is assumed, we should be 

 able by inbreeding to select stocks in which it is present 

 and in which it is absent. For this reason the experi- 

 ment recorded above (Table V) was here repeated with 

 some modifications. It will be remembered that in the 

 former, a fertilized red female and a fertilized pink male 

 were placed in each bottle and their offspring, the F t , 

 counted. In the following experiment, in order to secure 

 segregation, the F 2 were counted. Four virgin pairs, two 

 red and two pink, were taken out of the culture bottles and 

 mated separately. . Of the Fj of each of the four, six pairs 

 were taken out, 24 pairs in all, and bred for the F 2 . 

 Unlike the first experiment, the males in this case were 

 allowed to remain with the females throughout the ex- 

 periment. This insured sufficient sperms for the eggs. 

 Every second day the food was removed, together with the 



