Another theory which has been heard of frequently of late years is that in which it is held that the ova are expelled in alternating sex, each month. Thus, if a male ovum is expelled in January, the February ovum will be a female one, according to this theory. Under this theory if the date of conception of a child be ascertained, and the sex of the child noted at its birth, it is a simple matter to count forward from the menstrual period following which the child was conceived, and thus determine whether the ovum of any succeeding period is male or female. It should be noted, however, that the periods are regulated by the lunar months, and not the calendar months. The fact that twins of different sexes are sometimes born would seem to disturb this theory--but not more than any other theory of sex-determination voluntarily produced, for that matter. The several schools explain this apparent discrepancy by the familiar saying that "exceptions prove the rule." Another theory of sex-determination is that which holds that when conception occurs within a few days after the last day of menstruation, the child will be a girl; and that when conception occurs at a later period, the child will be a boy. Methods and systems based upon this theory are also reported as being reasonably successful in producing satisfactory results. But, inasmuch as there appears to be a great difference in individual women in this respect (even according to the claims of this school of sex-determination), it would seem that it would be difficult to proceed with certainty in the matter in most cases. One of the writers advocating this method, says: "Conception within five days after the end of the menstrual period is almost certain to produce a girl child; within five days to ten days, it may be either a boy or a girl; from ten to fifteen days, it is almost sure to be a boy; from eighteen to twenty-five days is the period of probable sterility, in which conception is extremely unlikely to occur." In conclusion, it may be said that Nature undoubtedly has certain rules of sex-determination which govern in these cases; and that it is possible if not indeed probable that these rules may some day be discovered by man, and turned to account; but that it is very doubtful whether the secret has as yet been solved by the investigators. The writer may be pardoned for suggesting that, in his opinion, if the discovery is ever made it will likely be found to be very simple--so simple that we have probably overlooked it because it was in too plain sight to attract our attention. Nature's methods are usually very simple, when once discovered. She hides her processes from man by making them simple, it would seem. LESSON X WHAT BIRTH CONTROL IS, AND IS NOT