192 GENETICS IN RELATION TO AGRICULTURE 
and intergrading with the parental forms, escaped notice as mutations 
but were selected for continuing the experiment? Such an assumption 
would render intelligible the efficacy of return selection which would be 
difficult of interpretation on even a multiple factor theory of heredity. 
That such a system may exist in qualitative characters has been 
shown by Bridges for the relation between eosin eye color and its modifiers 
in Drosophila. One modifier called dark intensifies the eosin character. 
The other six modifiers are all diluters. Thus cream a changes eosin to 
pale yellow or cream color, cream b has a similar effect, but not so marked. 
Whiting changes the eosin color to white, so that eosin-whiting flies are 
indistinguishable from white-eyed flies in color. In these cases there is 
no question as to the operation of a multiple system of factors, for the 
specific factors have arisen singly by mutation and their linkage relations 
establish completely their identities. Nevertheless taken together they 
would give in a qualitative character a remarkably close imitation of 
the behavior of Castle’s hooded rats. 
If, however, we assume with Castle that factors like characters are 
variable and that allelomorphic contamination occurs, then we may offer 
an explanation based on a consideration of a single allelomorphic system. 
For such an explanation the hooded pattern may in general be represented 
by h, and its dominant allelomorph, the fully colored condition, by H. 
Self-color is dominant to hooded, but the hooded condition varies greatly 
in the amount of pigmentation present in the coat. These variations 
appear to be correlated with definite factor variations, consequently we 
may designate the factors determining the various degrees of pigmen- 
tation in the hooded pattern by fi, he, hz, ha, . . . hy. This series runs 
from individuals which show practically no color to those which display 
almost a self-colored coat. If we assume that the character expression 
of an animal of the genetic constitution hihi) be intermediate between 
that of an animal of the genetic contitution hihi, a very light type, and 
one of the constitution hiohio, a very dark type, then we may point out 
what would occur if selection were carried out in the progeny of such 
an individual. In the first place the genetic constitution hihip of such an 
animal represents merely the values of the gametes that united to form 
the zygote. They are assumed to interact immediately, so that perhaps, 
in addition to the factors hy and hyo, such a zygote will produce gametes 
bearing for the most part the factors hs and he, representing a sort of 
equilibrium for the interaction of the factors hy and hip. There would, 
_ therefore, be in the progeny of such an individual some individuals of 
the genetic constitution hjhs which would be lighter than the parents, and 
some of the genetic constitution h¢hio which would be considerably darker 
than the parents. If other products of this reaction, such as hg, ha, hz, he, 
etc., were also produced, and like the original reacting factors h; and hig 
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