A long time will doubtless be required to work out the details of differences in metabolism in the two sexes. Some of the main facts are known, however, and the general effects of the two diverse chemical systems upon the life cycles of the sexes are quite obvious. What we call the "quantitative theory of sex" has, besides a place in exact science, an interesting relation to the history of biological thought, especially as applied to society. It is thus in order to state as clearly as possible what it now is; then, so that no one may confuse it with what it is not, to run over some of the old ideas which resemble it. Experiments with transplanted sex glands, with sex-gland extracts (testicular and ovarian) and the observation of infusions of a male-type blood-stream into a female body, as occurs in nature in some cattle and in the so-called human "hermaphrodites," indicate a gross chemical difference between the respective determiners for femaleness and for maleness. So the chemicals involved, though not yet isolated, must be presumed to be _qualitatively_ different, since they produce such different results. But such experiments also indicate that both determiners must be present in some proportions in every individual of either sex. The basis for both sexes being present, the one which shall predominate or be expressed in the individual must depend upon the _quantitative_ relation between the determiners which come together at fertilization. The quantitative theory merely means that this predominance of one factor or the other (maleness or femaleness--Gynase or Andrase) is more pronounced in some cases than in others. In brief, then, the quantitative theory of sex is merely the most reasonable explanation of the known fact that intersexes exist--that is, females with some male characteristics, or with all their characters more like the female type than the average, and _vice versa_. Laboratory biology has established the phenomena of intersexuality beyond question, and the word "inter-sex" has become a scientific term. But the fact that this word and the idea it represents are new to _exact science_ does not mean that it is new in the world.