12 AN ANALYSIS OF THE EFFECT OF SELECTION, 



The female was from culture 916, which contained a sepia, spineless, 



D' 



kidney, sooty, rough male, and a female . This female was the 



offspring of a Dichset from stock and of a fly from culture 869 {q. v. 

 below, in the pedigrees of 900 and crossbred minus lines) . No bristle 

 counts are available from culture 1002, except those of the pair (6X6) 

 selected to produce culture 1072, the Fi of this line. 



After this line had been inbred and selected for 11 generations, a 

 pair of 7-bristled flies were taken from 2389, and their descendants were 

 bred in mass cultures, unselected Dichsets being mated together, for 

 about 2 generations. The line was then re-established by selecting 

 pairs from this stock and was inbred for 8 generations more. 



The data and curves for this line are given in table 9 and figure 4. 



123456789 10 11 12345C7 



Fig. 4. — Means and standard deviations for 1002 inbred plus line. 



Here selection was perhaps effective for a few generations. Ref- 

 erence to the Appendix will indicate that this effectiveness was prob- 

 ably due in large part to the gradual elimination of the descendants 

 of one of the F2 pairs (1158), which were on the average of slightly 

 lower grade than those of the other F2 pair (1150). It is to be observed 

 that both of the apparently successful reversed-selection series were 

 made with descendants of the former branch of the family. 



The eighth to eleventh generations of this line and the contempo- 

 rary eleventh to fourteenth of the 864 line gave very similar results 

 as to the means and standard deviations. We shall see below (p. 19) 

 reason for believing that the two lines were of very similar constitution 

 at this period. The gradual rise of the means and fall of the standard 

 deviations is probably of environmental rather than genetic origin. 



