MULTIPLE FACTORS 247 
knowledge the case would have been practically 
insoluble. 
Similar methods of attack have also been used 
by Dexter, and by Muller, in their experiments 
with the ‘‘beaded” wing of Drosophila (Fig. 60). 
Fig. 60.—Normal wing (to left) and beaded wing (to right) of 
Drosophila. ; 
The beaded character likewise is a variable one, 
some of the beaded individuals being very nearly 
normal in appearance. The degree of abnormality 
and the proportion of abnormal offspring are both 
capable of being altered, within limits, by selection 
or by crossing to normal stock. 
Dexter crossed beaded flies to flies carrying 
