It is true that females carrying two _different_ lethals might arise and not die, because the injurious effect of each lethal would be dominated by its allelomorph in the other X chromosome. Such females can not be obtained by combining two existing lethals, since lethal males do not survive. They can occur only through a new lethal arising through mutation in the homologous chromosome of a female that already carries one lethal. Rare as such an event must be, it has occurred in our cultures thrice. The presence of a female of this kind will be at once noticed by the fact that she produces no sons, or very rarely one, giving in consequence extraordinary sex-ratios. The rare appearance of a son from such a female can be accounted for in the following way: If crossing-over occurs between her X chromosomes the result will be that one X will sometimes contain two lethals, the other none. The latter, if it passes into a male, will lead to the development of a normal individual. The number of such males depends on the distance apart of the two lethals in the chromosome. There is a crucial test of this hypothesis of two lethals in females giving extraordinary ratios. This test has been applied to the cases in which such females were found, by Rawls (1913), by Morgan (1914_c_), and again by Stark (1915), and it has been found to confirm the explanation. The daughters of {16} such a female should all (excepting a rare one due to crossing-over) give 2:1 ratios, because each daughter must get one or the other X chromosome of her mother, that is, one or the other lethal. Although the mother was fertilized by a normal male, every daughter is heterozygous for one or the other of the lethal factors. The daughters of the two-lethal females differ from the daughters of the one-lethal female in that the former mother, as just stated, gives all lethal-bearing daughters; the latter transmits her lethal to only half of her daughters. INFLUENCE OF THE ENVIRONMENT ON THE REALIZATION OF TWO SEX-LINKED CHARACTERS. The need of a special environment in order that certain mutant characters may express themselves has been shown for abnormal abdomen (Morgan, 1912_d_, 1915_b_) and for reduplication of the legs (Hoge, 1915). In a third type, club, described here (page 69), the failure of the unfolding of the wing which occurs in about 20 per cent of the flies is also without much doubt an environmental effect, but as yet the particular influence that causes the change is unknown.