MULTIPLE FACTORS 229 
Such flies were actually obtained, although they were 
rather dark in color, since both black and ebony 
produce some effects on flies heterozygous for them. 
In F, the expectation is 9 gray, 3 black, 3 ebony, 
and 1 black ebony (double recessive). When F, 
was actually obtained it was found to be impossible 
to make an accurate separation of the four classes. 
There was a practically complete series ranging from 
the normal gray to individuals darker than either 
black or ebony. The gradation is obviously due 
chiefly to the fact that dominance is not complete. 
There are nine different classes expected, instead of 
four, if heterozygous forms be counted. These nine 
classes form groups, each with its own mode, the 
outlying members of each group overlapping neigh- 
boring groups. To add to the difficulty, the colors 
change considerably with the age of the fly. There 
are at least seven other mutant factors known in 
Drosophila that make the flies darker. It will 
readily be seen that, if one had a population contain- 
ing a mixture of all these characters, analysis would be 
well-nigh impossible. 
Before making the above cross the inheritance of 
black and of ebony had been studied separately, and 
no difficulty in classification is encountered unless 
they are used in the same cross. This information 
made it possible for us to interpret the black ebony 
cross. In the experiments now to be described, we 
are dealing with factors which had not first been 
studied separately, so that the interpretation is not so 
obvious as in the preceding case. 
