BRISTLE INHERITANCE IN DROSOPHILA 135 



The families in the 31 generations of this low selection of ex- 

 tracted extras are grouped by half month periods — the same 

 method as used for the return selections. The last nine groups 

 include total generations; all the matings for a generation in 

 both the extracted and the high race in the same period were 

 made on the same day. The means of the extracted low race 

 are compared in figure 9 with the means of the high selected 

 families grouped in the corresponding half month periods. 

 The dotted lines give the means of the flies selected as parents 

 in both races, these have been weighted according to the num- 

 bers of sons 'A' and daughters 'B' from each grade of parents. 



Means 



The first fact shown by the curves of the means is, that the 

 extracted race is unquestionably lower than the high selected 

 race. The difference is less marked in the males than in the 

 females, but the range of fluctuations is considerably less for the 

 males in both curves. It may be suggested that the result of 

 selecting low grade flies has been to weaken this race, as was 

 supposed to be the case in the return race. But in this race 

 there were so many low grade flies that were not small, that the 

 same sort of selecting did not have the same tendency to weaken, 

 and thus lower, the race. 



The first selection of low grade extracted extras reduced the 

 means at once. There was no series of preliminary selections 

 that made little or no progress. However, after the initial dif- 

 ferentiation, the means were lowered still further, for a few gen- 

 erations. Beyond this, selection seems to have had no power. 

 During these early generations the curve of the parents is 

 closely parallel with, those of their sons and daughters, but this 

 close agreement is not found in any other part of the whole 

 series. Although the means of the parents and their offspring 

 do not form parallel curves, one finds that the means of the 

 extracted low race and the means of the high race do form 

 parallel curves with coincident high and low points. 



