206 



MULTIPLE FACTORS 



on the thorax (Fig. 63). In a few generations a 

 minus stock with no trident was established that bred 

 true. The plus stock went up and down, the selec- 

 tion being not always thorough. A stock that always 

 had the trident present to some degree was obtained 



FIG. 63. Thorax of mutant stocks of Drosophila ampelophila. a, 

 race "without" trident; b, race "with" trident; c, race called streak; 

 d, race called trefoil; e, race called band. 



after a time. Later several other mutations appeared, 

 some of which greatly increased the black on the 

 thorax ; some even swamped the trident, making it a 

 broad band. Three such mutant stocks were readily 

 isolated. It might have been concluded that these 

 mutations had occurred in the direction of selection, 



