dimorphism to originate in such a way that the determinants of an id first became double, and subsequently the doubled determinants varied in different directions. Repeated consideration of the facts now leads me to hold the modified view of this text as more accurate. I now suppose that constitutional dimorphism of a species—even sexual dimorphism—is due to homologous determinants varying differently in different ids: so that, for example, one id contains 'male' and another 'female' determinants of the reproductive organs, of the genital ducts, or of the pattern of wings, and so on. In like manner I imagine that every other kind of dimorphism of polymorphism has originated by the different variation of corresponding determinants of the various ids. Seasonal dimorphism alone, so far as it depends only on the direct effects of external influences, implies no difference in the ids of the germ-plasm. Compare The Germ-Plasm, pp. 379, 380.
My present view seems to have the advantage of simplicity, as it does not require the hypothesis of a doubling of the determinants involved prior to the different variations—a complication that, as I was quite aware, could not well be accounted for. It permits, too, a perfectly gradual increase of the differentiation of the ids until a complete distinction arises in all the determinants of the id, so that two or more essentially different kinds of id constitute the germ-plasm. Moreover even a complete separation of these into two groups is possible, and must actually occur among species with male and female eggs.
This view of the facts also affords a deeper insight into the causes of the determination of sex. For if there are special 'male' and 'female' ids, the number of them in certain circumstances will give the decision as to whether the developing organism shall be male or female. For though in some cases sex is probably or even certainly determined by external influences which affect the egg, these are not to be regarded as the only or even principal cause of the decision in all cases: we are here concerned with an adaptation to special circumstances. In the case of human beings it appears very obvious that in a certain sense sex may be
