AN ANALYSIS OF THE EFFECT OF SELECTION. 9 



longer. Moreover, there is a chance of combining more of the desired 

 modifiers in the same individual when crossing is done, so that this 

 method might produce more extreme results than the inbreeding 

 method. However, each time a cross is made some of what has been 

 gained may be hidden by dominants in the other stock; therefore 

 progress might sometimes be slower. 



Accordingly, in these experiments parallel series have been carried 

 on. In one set selection has been accompanied by continuous brother- 

 sister matings; in the other, frequent crosses have been made between 

 individuals more or less closely related. The same method has been 

 followed in both the plus and the minus selected lines. The four 

 series will be considered in order: (1) inbred plus; (2) crossbred plus; 

 (3) inbred minus; (4) crossbred minus. 



INBRED PLUS SERIES. 



Two main lines of this series have been carried on. A few cultures 

 have been made from other sources, but none of these are sufficiently 

 extensive so that we need follow their histories here. 



864 LINE. 



Culture 864, from which this line arose, was produced by a female 



D' r 



of the constitution p 7 , from culture 847, and two males from 



p p s s ke r 



the sepia, spineless, kidney, sooty, rough stock ; 847 was the result of 

 mating four peach, spineless, kidney, sooty, rough males from stock to a 



D' T 

 female of the constitution : -. This female TABLE 7 ' 



p p s s e 



Dorso- 

 centrals. 





 1 

 2 

 3 



Total . . 



Offspring. 



12 

 8 

 9 

 1 



30 



was descended from the Dichset, ebony, peach, 

 spineless, kidney, sooty, rough, and other stocks. 

 Her pedigree is not now traceable in detail. 



At the time culture 864 was counted, the scu- 

 tellar bristles were not observed. The dorso- 

 central bristles were recorded on 30 flies, as 

 shown in table 7. 



The 3 (almost certainly a 7, according to the system later adopted), 

 a male, was mated to a 2 (6) female to produce culture 893. For the 

 details of the remainder of the pedigree see Appendix. 



In the accompanying tables and curves the offspring of culture 

 893, above, are considered FI. Table 8 gives the data for this line 

 summarized by generations. In this and the following tables, n is 

 the number of individuals in the generation, M is the mean bristle- 

 number of the generation, a is the standard deviation, r is the parent- 

 offspring correlation, and is recorded in the generation to which the 

 offspring belong. Diff. M. is the mean bristle-number of the off- 

 spring minus the mean bristle-number of their parents, weighted 



