218 MULTIPLE ALLELOMORPHS 
is no more subject to mutation than are other factors, 
1.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. 
At present the series of white allelomorphs is: 
white, eosin, cherry, blood, tinged, buff, ecru, ivory, 
coral, and apricot. Each of these forms has appeared 
by direct mutation from the wild-type. Crossed to 
each other, the members of this series give com- 
pounds that are intermediate in color between the 
two types used as parents. In no case do such 
crosses give wild-type progeny, which would be the 
result expected if they were closely linked but non- 
allelomorphic mutants. The hypothesis of close 
linkage would require here the absurd supposition 
that each type was produced not simply by one 
change but by as many simultaneous changes as 
there are mutant types in the series. 
