BRISTLE INHERITANCE IN DROSOPHILA 



127 



the means of the sons in the successive groups, as well as the 

 means of the daughters, follow along closely together. The 

 one exception to this close similarity of the two sets of groups 

 lies in group 18 in the second return series. Yet this is probably 



MEANS 



ALES 



EXTRA BRISTLES 



MEANS 



FEMALES 



EXTRA BRISTLES 



J \ \ \ L J \ L 



2 4 6 8 10 14 16 18 



CROUPS 



_J \ L 



B 



Fig. 7 Return race, selected for low bristle grades, starting from the 15th 

 generation of high selection. Means of offspring and their parents are compared 

 with means of offspring and their parents in the high selected race. All the 

 flies have been grouped according to the half month in which the parents were 

 mated. All the parents are weighted according to the numbers of their sons or 

 daughters. 'A' based on son, 'B,' on daughters. The offspring from high and low 

 grade parents average very nearly the same. The means of the return race 

 fall slightly lower than those of the main high race, but this difference does not 

 increase with continued selection and is probably due to the weakening effect 

 of breeding the lowest grade flies, since these are the smallest and the weakest. 



not significant, as there happen to be only a few flies included in 

 this mean. The low productivity in these groups is one of the 

 signs of the weakened condition of the return race at this time. 

 Even though the means of the return and the high race are close 

 together, there appears a tendency for the return means to hang 

 just a little lower than those of the high race. Such a difTer- 



