oodedness are due to one
factor. The result shows at most that one factor that gives the hooded
types is a simple Mendelian factor. The changes in this type may be caused
by modifying factors that can show an effect only when hoodedness is itself
present. That this is not an imaginary objection but a real one is shown by
an experiment that Castle himself made which furnishes the ground for the
second objection.

Second. If the factor has really changed its potency, then if a very dark
individual from one end of the series is crossed to a wild rat and the
second generation raised we should expect that the hooded F_2 rats would
all be dark like their dark grandparent. When Castle made this test he
found that there were many grades of hooded rats in the F_2 progeny. They
were darker, it is true, as a group than were the original hooded group at
the beginning of the selection experiment, but they gave many intermediate
grades. Castle attempts to explain this by the assumption that the factor
made pure by selection became contaminated by its normal allelomorph in the
F_1 parent, but not only does this assumption appear to beg the whole
question, but it is in flat contradiction with what we have observed in
hundreds of Mendelian cases where no evidence for such a contamination
exists.

Later Castle crossed some of the extracted rats of average grade (3.01)
from the plus series to the same wild race and got F_2 hooded rats from
this cross. These F_2 hooded rats did not further approach the ordinary
range but were nearer the extreme selected plus hooded rats (3.33) than
were the F_2's extracted from the first cross (2.59). Castle concludes from
this that multiple factors can not account for the result. As a matter of
fact, Castle's evidence _as published_ does not establish his conclusion
because the wild rats used in the second experiment may have carried plus
modifiers. This could only be determined by suitable tests which Castle
does not furnish. This is the crucial point, without which the evidence
carries no conviction.

Furthermore, from Castle's point of view, these latest results would seem
to increase the difficulty of interpretation of his first F_2 extracted
cross, and it is now the first result that calls for explanation if one
accepts his later conclusion.

These and other objections that might be taken up show, I think, that
Castle's experiment with hooded rats fails entirely to establish his
contention of cha