146 EDWIN CARLETON MACDOWELL 



impossible in 8 generations to establish a race that was distin- 

 guishable from the high race. This attempt was repeated, start- 

 ing from the 26th generation of the high race, and continued for 

 6 generations with similar results. Return selection does not 

 reverse the progress made by the advance selection. Flies with 

 high and low bristle grades appear to have very similar offspring. 



IV. By selecting low grade parents from the F2 of a cross 

 between normals and flies from the 16th generation of the high 

 race, a low race was established (extracted low) The following 

 points were derived from the study of this race: 



1. One selection was sufficient to establish this race as dis- 

 tinct from the high race. 



2. For 4 generations the curves of the parents and off- 

 spring are parallel. After this, the two curves are completely 

 independent. 



3. For 4 generations the low selection continued to lower the 

 means. 



4. Except in the first few generations, the curves of the prog- 

 eny rise and fall in harmony with the curves of the high race, 

 when families raised at similar times are compared. 



5. Besides being lower than the high race the variability of this 

 race is less than that of the high race; in response to the same 

 improvement in conditions this extracted race does not advance 

 as far. 



V. Comparing the different races it is found that no matter 

 what the parentage, they aU exhibit high points and low points 

 at the same times. Environment is accountable for the varia- 

 tions in most of the generations. The initial rise in the high 

 race however, was not due to environment, as this rise resulted 

 in a genetic change in the race. 



VI. The supposition of a single varying factor to explain the 

 above results can not be justified, as it would require num- 

 erous further assumptions. All these results are simply ex- 

 plained on the assumption that there were genetic differences 

 present among the original flies with extra bristles, that these 

 genetic differences, or genes, are entirely independent of the 

 main factor that occasions the monohybrid ratio in crosses with 

 normal flies. 



