228 MULTIPLE FACTORS 
one class may vary so much from each other as to 
overstep the small differences separating the classes. 
An accurate separation of the individuals into differ- 
ent classes and a count of the number in each class is 
then impossible, and .it becomes so difficult to de- 
termine the number of factors involved and the 
effect of each factor (or, rather, factor-difference) 
that such cases have at times been used in attempts 
to disprove the factorial hypothesis. The problem is 
likewise more difficult if more than two factor- 
differences occur. This is true especially in those 
cases where the effects of the different factors are 
cumulative, for then classes are produced showing 
characters intermediate in various degrees between 
the characters of the most extreme classes, just as in 
cases of incomplete dominance. It will be instructive 
to consider several instances of crosses of the above 
types, since, although definite ratios can not be 
obtained, there are various characteristic effects 
produced which show that multiple factors are re- 
‘sponsible for the peculiarities of the results. 
The inheritance of black color in Drosophila has 
already been described. Black is recessive to the 
normal (‘“gray”’) color, but the heterozygous forms 
are a little darker than the pure grays. Ebony is 
another body color, similar in appearance to black, 
but somewhat darker. It is similarly recessive to 
gray, but the factor responsible for it is located in a 
different chromosome (III) from that which carries 
the factor for black (II). When black and ebony 
are mated together we should expect gray flies in F,. 
