214 MULTIPLE ALLELOMORPHS 
after the stock had been crossed to ebony, with which 
it is allelomorphic. Here too the mutant forms 
though both recessive to normal do not give normal 
gray color when crossed together, but a color inter- 
mediate between sooty and ebony. In both of these 
cases the complete linkage view would require that 
one of the mutant types had originated by a muta- 
tion in two factors at once. In the case of another set 
b 
B 
Fie. 53.—The abdomen of normal a,a, and spot, b,b’, males. The other 
allelomorph is yellow (not shown here). 
of triple allelomorphs known in Drosophila, namely, 
yellow and spot (Fig. 53) and their normal allelo- 
morph. The above argument does not apply; for 
although spot and yellow are both recessive to gray 
and give yellow when crossed to each other, spot 
originated in flies already containing the allelomorph 
for yellow. 
The reasons may now be given that incline us to 
think that the theory of identical loci is much more 
