Similar results were found in the mutant type called reduplicated legs, which is a sex-linked recessive character that appears best when the cultures are kept at about 10deg C. As Miss M. A. Hoge has shown, this character then becomes realized in nearly all of the flies that have the proper constitution, but not in flies of normal constitution placed in the same environment. Here the effect is produced by cold. SEXUAL POLYMORPHISM. Outside the primary and secondary sexual differences between the male and the female, there is a considerable number of species of animals with more than one kind of female or male. Darwin and his followers have tried to explain such cases on the grounds that more than one kind of female (or male) might arise through natural selection, in consequence of some individuals mimicking a protected species. It is needless to point out here how involved and intricate such a process would be, because the mutation theory has cut the Gordian knot and given a simpler solution of the origin of such diandromorphic and digynomorphic conditions. In _Drosophila_ a mutant, eosin eye-color, appeared in which the female has darker eyes than the male. If such stock is crossed with cherry (another sex-linked recessive mutant, allelomorphic to eosin) the females in the F_2 generation are alike (for the pure eosin and the eosin-cherry compound are not separable), but the cherry males and the eosin males are quite different in appearance. Here we have a simulation, at least, of a diandromorphic species. Such a group perpetuates itself, giving one type of female (inasmuch as eosin and cherry females are very closely similar) and two types of males, only one of which is like the females. A population of this kind is very directly comparable to certain polymorphic types that occur in nature. In _Colias philodice_ there is one type of male, yellow, and two types of females, yellow and {18} white. In _Colias eurydice_ the male is orange and the females are orange or white. In _Papilio turnus_ the male is yellow and the females either yellow or black. Those cases are directly comparable to an eosin-cherry population, except that in Lepidoptera the female is heterozygous for the sex differential, in Diptera the male. Since in _Drosophila_ the results are explicable on a sex-linked basis, a similar explanation may apply to polymorphism in butterflies. By suitable combinations of eosin and cherry most of the cases of polymorphism in butterflies may be simulated. To simulate the more complex cases, such as that of _Papilio polytes_ and _memnon_, another allelomorph like eosin would have to be introduced. A population of mixed cherry and white would give three somatic types of females (cherry, cherry-white, and white) and two of males (cherry and white). FERTILITY AND STERILITY IN THE MUTANTS.