Xo. 583] ROLE OF ENVIRONMENT IN DROSOPHILA 407 



females are gray. The F 2 male classes alone can be used 

 for testing the extent of crossing over. 



CHANGE OF TYPE AS THE CULTURE GROWS OLDER 



The preceding tables do not bring out the change that 

 takes place as the culture gets older— a change by which 

 the abnormal classes become replaced by the normal 

 classes. A few results will therefore be given in detail to 

 illustrate this relation. 



In none of the relatively few counts in Tables I, and II, 

 involving two pairs of factors (EN and WAb), was any 

 change in type during the time of the experiment noticed ; 

 but in other cases a very marked change was observed as 

 the cultures grew older. 



In the two following tables consecutive counts of the F 2 

 flies (from YEAb $ by GWN ?) emerging from day to day 

 from the same culture are given. The change of the 

 YEAb to the YEN and of GRAb to GRN is very striking. 



TABLE XIV 



In the next table two F 2 counts are given derived from 

 GWN $ by BRAb c? grandparents. The GRAb changes 

 to GRN and BRAb to BRN. 



