BRISTLE INHERITANCE IN DROSOPHILA 



69 



TABLE 6 



To compare the means and the variability of extra bristles that appear after a cross 

 with those in corresponding generations of the extra race. 



Extracted Extras (Gen. 2). 

 Inbred Extras (Gen. 2) 



Extracted Extras (Gen. 10). 

 Inbred Extras (Gen. 10) 



1.665 

 1.921 



2.335 

 3.229 



377 

 6418 



2499 

 981 



STANDARD 

 DEVIATION 



0.695±0.017 

 0.863±0.005 



1.329±0.018 

 1.402±0.021 



than when flies of the first selected generation are crossed. The 

 variabihty of the extracted extra bristles is slightly less than 

 that of the corresponding inbred generation, 



5. DISCUSSION 



The interpretation that seems most simple to apply to the 

 above facts is that there is a positive restricting factor present 

 in the wild fly that prevents more than four bristles from develop- 

 ing. Although the extra race has something added somatically 

 to it, genetically it has lost a restricting factor. Now this 

 condition of extra bristles seems not to be phylogenetically new, 

 as studies on the chaetotaxy of related flies suggest. The 

 mechanism that is most concerned must have appeared when 

 Drosophila ampelophila arose, or before, so the extra bristles 

 are not formed by the origin of a new mechanism, but rather 

 by the removal of the restriction of the more recent mechanism. 



THE SELECTION PROBLEM 



The main problem to which the preceding experiments have 

 been contributory, has been the attempt to throw some light on 

 the question of selection as a formative process in evolution. 

 The attempt has been made to increase thg numbers of thoracic 

 bristles as much as possible by selecting and inbreeding. Ac- 

 cordingly, after the first pair that came from wild stock, high 

 grade brothers have been mated to high grade sisters. In the 

 following account the first eleven generations of this process 

 will be described. 



