metabolic fluctuations, now in favor of maleness (katabolic), now in favor of femaleness (anabolic), the net result being an approximately equal proportion of males to females. In other words, there must be selective fertilization, that is, a female ovum must be fertilized by a male sperm, and vice versa. The various assumptions made are not pure speculations, but rest upon many facts. Some animals do possess two kinds of eggs, of which one (the larger) develops into females and the other kind (the smaller) into males; there are many instances of selective fertilization; and many animals do produce two kinds of spermatozoa.
If there is only one kind of egg, as Correns suggests—as may well be the case, since of the four potential ova, three (the polar bodies) degenerate during maturation and only one becomes capable of fertilization—and two kinds of spermatozoa, the explanation of sex becomes very much simpler. One sex (female) must then be pure in respect to sex (a homozygote) and the other must be a sex-hybrid (heterozygote). If the egg is female in tendency, in order that there should appear 50 per cent, males, maleness must dominate over femaleness. In any case, an interpretation of the facts involves the application of some phase of Mendelism. As will appear farther on, the case of the honey-bee, ants and plant-lice offer serious obstacles to the universal application of Correns's interpretation, and even of the whole Mendelian scheme.
Within the last decade three main positions have been advocated by various investigators in regard to the cause of sex. The position held by Beard and his school is to the effect that there are two kinds of eggs, male and female, and one kind of spermatozoa; and that sex is determined exclusively by the egg, the spermatozoon having no role in sex production. This position can be supported by various facts. A certain worm (Dinophilus apatris) carefully studied by Korschelt, is known to produce two kinds of eggs, large and small, the former developing into females, the latter into males. Of course it might be urged that in consequence of selective fertilization only the small eggs are impregnated by a male-producing spermatozoan and the large eggs by a female-producing, and an entirely different interpretation of the facts would be necessitated. But in the case of Hydatina senta, a rotifer or "wheel animalcule," where large and small eggs are also laid, the former without fertilization develop into females and the latter into males. However, both kinds of females, the large-egglaying kind and the small-egg-laying kind, came originally from fertilized eggs, and it may be that a difference in metabolic activity of the several females kept the eggs of the one small and allowed those of another to grow large and so gave the victory to the female determinant in the well-nourished egg. This assumption is supported by the fact that the amount of nourishment taken by the young female between