BRISTLE INHi:RITANCE IN DROSOPHILA 451 



general make-up of the females larvae affords a more favorable 

 set of conditions for the ontogenetic development of extra 

 bristles. 



DISCUSSION OF LITERATURE 



The experiments most closely parallel to these with extra 

 bristles are those of Castle et al. with piebald rats. A brief 

 mention of this similarity was made in the first number of this 

 series. In preparation for the discussion of the literature in the 

 light of the final results, further studies of Castle's data were 

 made; these grew to such proportions that their independent 

 publication seemed advisable (MacDowell, '16). This discus- 

 sion, both through its origin and content, should rightly appear 

 at this point. The conclusions reached at that time seem to be 

 substantiated by the conclusion of the subsequent discussions. 



New data were presented by Castle (Castle and Wright, '16) 

 after the writing of that paper (although published later, it was 

 obviously written many months earlier; MacDowell, '16, foot- 

 note p. 739). The new data are quoted in full (Castle, '17) as 

 finally breaking down the hypothesis of modifying factors (p. 

 113). 



I have now presented the evidence that has led me to reject the 

 hypothesis formerly held tentatively that modifying factors were 

 largely concerned in the changes produced in the hooded pattern of 

 rats under repeated selection. The evidence seems to me to admit of 

 only one consistent interpretation, that a single variable genetic factor 

 was concerned in the original hooded race, that a changed condition of 

 this factor was produced in the minus race, and another changed con- 

 dition in the plus race, and a third appeared in the mutant race. 



However, after this Little ('17) has reviewed all the rat crosses 

 and concludes that ''they offer distinct evidence of segregation." 

 Sturtevant ('18) has not been convinced that modifying factors 

 are eliminated by the new data; he emphasizes the need of 

 further information in regard to the rat pedigrees and questions 

 the crucial value of the second back-cross, so strongly stressed 

 by Castle. The questions that Sturtevant raises concerning the 

 second back-cross may very well lead to an explanation of the 

 results obtained, for a third back-cross to wild of the extracted 

 hooded rats has given such clear evidence that Castle ('19) now 

 concludes : 



