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).
Aside from the decrease in fertility that occurs in certain stocks (a question that need not be treated here), there are among the types described in the text two cases that call for special comment. When the mutant type called "rudimentary" was first discovered, it was found that the females were sterile but the males were fully fertile. Later work has revealed the nature of the sterility of the female. The ovaries are present and in the young flies appear normal, but while in the normal flies the eggs in the posterior portion enlarge rapidly during the first few days after hatching, in the rudimentary females only a very few (about 15) eggs enlarge. The other eggs in the ovary remain at a lower stage of their development. Rarely the female lays a few eggs; when she does so some of the eggs hatch, and if she has been mated to a rudimentary male, the offspring are rudimentary females and males. The rudimentary females mate in the normal time with rudimentary or with normal males, and their sexual behavior is normal. Their sterility is therefore due to the failure of the eggs to develop properly. Whether in addition to this there is some incompatibility between the sperm and the eggs of this type (as supposed to be the case at one time) is not conclusively disproved, but is not probable from the evidence now available.
In the mutant called "fused" the females are sterile both with wild males and with males from their own stock. An examination of the ovaries of these females, made by Mr. C. McEwen, shows clearly that there are fewer than the normal number of mature eggs, recalling the case of rudimentary.
It should be noticed that there is no apparent relation between the sterility of these two types and the occurrence of the mutation in the X chromosome, because other mutations in the X do not cause sterility, and there is sterility in other mutant types that are due to factors in other chromosomes. { 19}