color of the eye of the wild fly by a single mutant
factor. Here then at a single step a type appeared that was sexually
dimorphic.

Zoologists know that sexual dimorphism is not uncommon in wild species of
animals, and Darwin proposed the theory of sexual selection to account for
the difference between the sexes. He assumed that the male preferred
certain kinds of females differing from himself in a particular character,
and thus in time through sexual selection, the sexes came to differ from
each other.

[Illustration: FIG. 26. Clover butterfly (Colias philodice) with two types
of females, above; and one type of male, below.]

In the case of eosin eye color no such process as that postulated by Darwin
to account for the differences between the sexes was involved; for the
single mutation that brought about the change also brought in the
dimorphism with it.

In recent years zoologists have carefully studied several cases in which
two types of female are found in the same species. In the common clover
butterfly, there is a yellow and a white type of female, while the male is
yellow (fig. 26). It has been shown that a single factor difference
determines whether the female is yellow or white. The inheritance is,
according to Gerould, strictly Mendelian.

[Illustration: FIG. 27. Papilio turnus with two types of females above and
one type of male below.]

In Papilio turnus there exist, in the southern states, two kinds of
females, one yellow like the male, one black (fig. 27). The evidence here
is not so certain, but it seems probable that a single factor difference
determines whether the female shall be yellow or black.

Finally in Papilio polytes of Ceylon and India three different types of
females appear, (fig. 28 to right) only one of which is like the male. Here
the analysis of the breeding data shows the possibility of explaining this
case as due to two pairs Mendelian factors which give in combination the
three types of female.

[Illustration: FIG. 28. Papilio polytes, with three types of female to
right and one type of male above to left.]

Taking these cases together, they furnish a much simpler explanation than
the one proposed by Darwin. They show also that characters like these shown
by wild species may follow Mendel's law.

[Illustration: FIG. 29. Mutant race of fruit fly with intercalated
duplicate mesothorax on dorsal side.]

There has appeared in our cultures a fly in which the third division of