THEORY OF EVOLUTION 169 



An experiment of this kind can only be car- 

 ried out in a case where the groups of linked 

 gens are known. At present Drosophila is 

 the only animal (or plant) sufficiently well 

 known to make this test possible, but this does 

 not prove that the method is of no value. On 

 the contrary it shows that any claim that factors 

 can themselves be changed can have no finality 

 until the claim can be tested out by means of 

 the linkage test. For instance, bar eye (fig. 

 31 ) arose as a mutation. All our stock has de- 

 scended from a single original mutant. But 

 Zeleny has shown that selection within our 

 stock will make the bar eye narrower or 

 broader according to the direction of selection. 

 It remains to be shown in this case how selec- 

 tion has produced its effects, and this can be 

 done by utilizing the same process that was 

 used in the case of truncate. 



Another mutant stock called beaded (fig. 

 84 ) , has been bred for five years and selected 

 for wings showing more beading. In extreme 

 cases the wings have been reduced to mere 

 stumps (see stumpy, fig. 5), but the stock 

 shows great variability. It is probable here 



