Long and closely biologists have pondered these many and involved problems. How is it, they inquire, that an embryo bred of two parents of opposite sex develops the sex of one only of these? How is it that the mother, who belongs to one sex only, produces--and produces in about equal number--offspring of both? The phenomenon is expressed, biologically, in the term, "sex-limited factor"--an incalculable something in the embryo which limits its sex to the sex of one only of its parents. But the "something," and the method of this sex-limitation have remained enigmas. Sex is regarded by the new Mendelian school of biologists as that which is known as a "Mendelian factor." And to follow the argument to its conclusions, a few simple words about the Mendelian theory of Heredity are essential to those unacquainted therewith. * * * * * About forty years ago, a German monk, Mendel by name, was struck by the facts that in his bed of edible peas certain plants grew tall, while others remained dwarf; that the blossoms of certain plants were white always, while those of others were always coloured. He made a number of experiments in crossing the plants, with a view to discovering the law of inheritance by way of its operation in hybrid varieties. Briefly, the results of his experiments--which have since been repeated and confirmed by many later observers--were as follows: There are plants that are tall and can transmit only Tallness to offspring. There are plants that are dwarf and can transmit only Dwarfness to offspring. So too, there are plants of white blossom or of coloured blossom that can transmit, respectively, only White or Coloured blossoming to offspring. When a Tall is crossed with a Dwarf plant, however, or a Coloured with a White plant, strange to say, the hybrid offspring of this cross shows _one_ only of these opposite traits, to the exclusion of the other. No intermediate, or mixed, forms are produced. Thus, a Tall crossed with a Dwarf produces only Talls. Plants of Coloured flower crossed with those of White flower give only Coloured flowering varieties. A yellow and a green-seeded cross produce only yellow-seeded plants. In the cross between plants of opposite traits, _one_ set of traits appears thus, exclusively, in the hybrid offspring. These traits--because they _dominate_ growth and development--Mendel styled "Dominant." While those traits which are dominated by the other and opposite traits and do not appear in offspring, he styled "Recessive."