MULTIPLE ALLELOMORPHS 171 



is no more subject to mutation than are other factors, 

 i.e., the factor has lost its unusual instability. 



There is no a priori answer possible to the question 

 as to whether a mutation having occurred, a further 

 mutation of the mutated factor is more likely to 

 occur, for it is conceivable that while in one case 

 the new factor might be unstable, in another case it 

 might be even more stable than the original one. 

 In regard to the other question, as to whether a par- 

 ticular locus is more liable to mutate, the work on 

 Drosophila shows that certain loci do mutate more 

 often than do others, and this is shown not only in 

 the recurrence of the same mutation, but also in the 

 occurrence of multiple allelomorphs. 



