MULTIPLE FACTORS 249 



of Drosophila, which involves a reduction in the 

 number of dorso-central and scutellar bristles. 

 Pure dichaete stock cannot be maintained, since 

 dichaete, like the chief factors in both the preceding 

 cases, is lethal when homozygous, but the heter- 

 ozygous dichaetes show the wing and bristle 

 characters. Several different hnes were maintained, 

 some being selected in a plus direction, for a larger 

 number of bristles, others in a minus direction. 

 Quantitative studies were made of the bristle 

 number of the dichaetes in each generation, and 

 mean, standard deviation, and parent-offspring corre- 

 lation were determined. Selection was apparently 

 effective in changing the lines, and reversed selection 

 seemed effective in certain instances. Whatever 

 the source of the result may have been, there could 

 be no doubt that the two sets of lines — plus and 

 minus — showed significant differences from each 

 other after the more than eleven generations of 

 selection. Crosses made between these hnes and 

 races with known mutant factors in the second 

 and third chromosomes proved that at least one 

 pair of modifying factors for bristle number in each 

 of these chromosomes was involved in causing the 

 difference between the plus and the minus lines. 

 The plus modifier in each chromosome was dominant 

 to its minus allelomorph, when in a fly having the 

 chief factor for dichaete. In flies without dichaete 

 the modifiers had much less, if any, effect. 



When a i:)lus dichac^te rac(^ was crossed to a 

 minus, the Fi resembled the plus ]);u-ent more nearly, 



