4 



Indiana University Studies 



extra bristle number in this experiment is due to two factors 

 and possibly more. One of these factors is located in the 

 X-chromosome and the other is probably in the third. 



1 wish to express my thanks to Professor Morgan and Drs. 

 Sturtevant and Bridges for stocks of their different mutations. 

 Without these it would have been impossible to analyze my 

 results. 



Material and Methods 



Drosophila ampelophila Loew was chosen after a search 

 thru the list of material which could be bred in the labora- 

 tory. It was chosen for two principal reasons : first, it is 

 easily bred; and secondly, because of the four sets of linked 

 genes described by Morgan and others, it is possible, if there 

 are multiple factors present, to link them with other genes. 

 In other words, it is possible to demonstrate the presence of 

 multiple factors, if they are present. So far as I know such 

 a clear-cut demonstration cannot be made in any other form. 

 I might have increased the number of spots on the back of a 

 beetle, and I might have hypothesized how the results were 

 brought about, but I could not have given demonstrative evi- 

 dence. I make this statement because a number of persons 

 haye asked me why I used Drosophila when so many others 

 are working with it. My problem was that of artificial selec- 

 tion. The choice made, my next problem was the selection of 

 material, and it was done regardless of the number of workers 

 using this fly. 



The character chosen for selection was bristle number on 

 the scutellum. There are four large bristles on it, but no 

 small hairs as on the thorax (see Figure 1, a). This character 

 was chosen, in part, because it is a definite, clear-cut character. 

 The bristles are easily counted with a low-power binocular. 

 It was chosen also, in part, because the work of MacDowell 

 ('15 and '17) did not seem to be an absolute demonstration 

 in favor of multiple factors. In fact, MacDowell ('15) admits 

 this when he says, 'Taken then on their own merits, the 

 results presented in this paper do not give critical evidence 

 in support of either the hypothesis of modification or of acces- 

 sory factors." It is a matter of interpretation then, which 

 side of the question MacDowell's data favor. In this connec- 

 tion it is interesting to quote from Pearl ('17). He says, 

 ''Any theory which has to depend for its sole support upon its 



