16 



Indiana University Studies 



seems to me, can deny this statement. Immediately, however, 

 the question arises : What has been the method of selection 

 and what is the nature of the variations on which selection 

 has acted? The analysis which follows answers this ques- 

 tion for this particular case. 



TABLE III 

 Back Selection Line 



All parents with normal bristle number except in first generation 





Number 



Mean 



Per 





Per Cent 



Generation 



of 



of 



Cent 



With Extra 





Offspring 



Offspring 



Normal 



Bristles 



1 



321 



5 



548 



21 



18 



78.80 



2 



542 



4 



933 



44 



46 



55.53 



3 



763 



5 



031 



41 



15 



58.78 



4 



770 



5 



205 



32 



07 



67.92 



5 



819 



5 



534 



25 



03 



74.98 







5 



486 



20 



47 



71 09 



7 



795 



5 



217 



31 



57 



68.42 



8 



593 



4 



715 



52 



61 



47.38 



9 



1583 



4 



728 



52 



87 



47.12 



10 



1846 



4 



833 



46 



74 



53.25 



11 



1418 



4 



748 



53 



10 



46.89 



12 



491 



4 



782 



50 



71 



49.28 



13 



258 



4 



830 



45 



45 



54.54 



14 



871 



4 



754 



48 



56 



51.43 



15 



1373 



4 



827 



45 



22 



54.77 



16 



1177 



4 



863 



46 



04 



53.95 



17 



1322 



4 



604 



56 



19 



43.80 



18 



921 



4 



741 



48 



20 



51.79 



19 



538 



4 



628 



56 



68 



43.31 



20 



80 



4 



675 



47 



49 



52.50 



21 



435 



4 



662 



51 



02 



48.97 



22 



605 



4 



791 



46 



93 



53.06 



23 



341 



4 



747 



48 



37 



51.62 



24 



151 



4 



993 



38 



40 



61.59 



25 



381 



4 



716 



48 



81 



51.18 



Table III summarizes the data for the back selection line generation 

 by generation. It gives the number of offspring (total number is 19,322), 

 the mean bristle number of the offspring, the percentage of flies with 

 the normal number of bristles (four), and the percentage of flies with 

 extra bristles. 



The Effect of the Environment 



Since considerable variation appears in the number of 

 extra bristles present in the offspring of a single pair of par- 

 ents, it seems probable that the environment plays some role 

 in their development. No critical attempt has been made to 



