BRISTLE INHERITANCE IN DROSOPHILA HI 



low selection). The success of this experiment would demon- 

 strate that the germ plasm in such a race had been altered by 

 the cross. 



HIGH SELECTED RACE 

 Frequency distributions by generations 



The most intimate view of the course of the main selection 

 experiment is given by the frequency distributions given in 

 figure 1. This figure presents two parallel series of curves; 

 those at the right represent the distributions of the totals of 

 each generation from the 12th to the 49th, on a percentage 

 basis; those at the left, the distributions, based on actual num- 

 bers, of an inbred line from a single pair of parents in each gen- 

 eration. The grades and numbers of the parents selected are 

 indicated by small frequency curves between the curves of the 

 totals of the generations ; the grades of the parents selected in 

 the line of single families are shown by small squares and 

 triangles. 



Besides showing the results of selecting high grade parents, 

 both in single families with few individuals and in total genera- 

 tions with many individuals, one of the most important points 

 to be brought out by these curves is, that the distributions from 

 single pairs of parents tell in general the same story as the total 

 generations that were raised from parents of different grades. 

 The comparison of the means in these two series will make 

 this point still more clear. This comparison is made in figure 

 2, A and B. 



Since the flies in the single families are also included in the 

 total generations, a certain amount of similarity would neces- 

 sarily be occasioned ; yet the differences in the numbers of fam- 

 ilies and of offspring (indicated in the figure) appear to give 

 the close similarity in the two sets of means a special sig- 

 nificance. That the curve of the means of the single families 

 follows that of the total generations, indicates that the latter 

 is not merely the chance result of a collection of families with 

 independently varying means, but fairly describes the condi- 

 tion of the race in different generations. Attention must be 



