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 zoölogists 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 the thorax with its appendages has changed into a segment like the second (fig. 29). It is smaller than the normal mesothorax and its wings are imperfectly developed, but the bristles on the upper surface may have the typical arrangement of the normal mesothorax. The mutant shows how great a change may result from a single factor difference. A factor that causes duplication in the legs has also been found. Here the interesting fact was discovered (Hoge) that duplication takes place only in the cold. At ordinary temperatures the legs are normal. [Illustration: FIG. 30. Mutant race of fruit fly, called eyeless; a, a' normal eye.] In contrast to the last case, where a character is doubled, is the next one in which the eyes are lost (fig. 30). This change also took place at a single step. All the flies of this stock however, cannot be said to be eyeless, since many of them show pieces of the eye--indeed the variation is so wide that the eye may even appear like a normal eye unless carefully examined. Formerly we were taught that eyeless animals arose in caves. This case shows that they may also arise suddenly in glass milk bottles, by a change in a single factor.