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 lines 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 lines 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 plus dichaete race was crossed to a 
minus, the F, resembled the plus parent more nearly, 
