But whatever was the historical or evolutionary origin of sex, we may here be excused for attaching more importance--for it is of great practical consequence--to the origin or determination of sex in the individual. At what stage and under what influences did the child that is born a girl become female? To what extent can we control the determination of sex? Why are the numbers of the sexes approximately so equal? What determines the curious disproportions observed in many families, which may be composed only of girls or only of boys; and, as is asserted, also observed after wars and epidemics or during sieges, when an abnormally high proportion of boys is said to be born? These are some of the deeply interesting questions which men have always attempted to answer--with the beginnings of substantial success during the present century at last. In general it is true that, the more we learn of the characters and histories of living beings, the more importance we attach to nature or birth and the less to nurture or environment, vastly important though the latter be. Thus to the student of heredity nothing could well seem more improbable, at any rate amongst the higher animals, than that characters so profound as those of sex should be determined by nurture. He simply cannot but believe that the sex of the individual is as inborn as his backbone, and as incapable of being created by varying conditions of nurture. The causation of sex is therefore really a problem in heredity; and we may most confidently assert, in the first place, that the sex of every human being is already determined at the moment of conception when, indeed, the new individual is created: determined then by the nature and constitution of the living cells--or of one of them--which combine to form the new being. Subsequent attempts to affect the sex, as by means of the mother's diet and the like, are palpably hopeless from the outset and always will be. This is by no means to say that conditions affecting the mother--as, for instance, the semi-starvation of a prolonged siege--may not affect the construction of the germ-cells which she houses, and which are constantly being formed within her from the mother germ-cells, as they are called. But any given final germ-cell, such as will combine with another from an individual of the opposite sex to form a new being, is already determined, once for all, to be of one sex or the other. We naturally ask, then, how the two parents are concerned in this matter; and the first remarkable answer returned by the Mendelian workers during the last three or four years is that it is the mother who determines the sex of her children in the case of all the higher animals. Her contribution to the new being is called the ovum, and it is believed that ova are of two kinds, or, we are quite right in saying, of two sexes.