110 EDWIN CARLETON MACDOWELL 



sive character in crosses with normal wild flies; that, in the 

 second generations of such crosses, the distribution of the num- 

 bers of extra bristles as a whole is lowered in comparison with 

 the distributions in the uncrossed race; that, in general, small 

 flies have fewer extra bristles than, large ones; that the selec- 

 tion of high grade parents effected an immediate increase in 

 the means of the offspring; and that the continuous selection of 

 flies with the highest numbers of extra bristles as parents, failed 

 to be accompanied by a continuous increase in the means of 

 the offspring, after the 8th generation. The following hy- 

 pothesis was found to be in accord with all the facts discovered 

 at that time: 



There are units of inheritance that influence the numbers of 

 extra bristles; these units are effective only in the absence of 

 a certain other unit that prevents the appearance of all extra 

 bristles. 



In the present paper experiments are presented that give 

 unquestionable support to this hypothesis. Four main ex- 

 periments have been carried out as tests for its correctness. 



1. The continuation of the selection of high grade parents 

 till there could" be no possible doubt that the means were no 

 longer being raised as they were in the early generations of 

 selection. 



2. The selection of low grade flies at the beginning of the extra 

 bristled race, (low selection) to establish a low race. The suc- 

 cess of this experiment would show that the germ plasm of the 

 original extra flies was not all uniform. 



8. The establishment of a low grade race from the high se- 

 lected race by selecting the lowest instead of the highest grade 

 flies as parents (return selection). The success of such an ex- 

 periment would demonstrate that there were genetic differ- 

 ences between the high and low grade flies in the high selected 

 race; the failure of this experiment would show that there are 

 no longer present such genetic differences as were present in 

 the earlier generations, causing the rapid rise in the means. 



4. The establishment of a low grade race from flies with extra 

 bi-istlcs that appear in the F. of a cross with normals (extracted 



