For some years I have been making as scientific and detailed a study as possible of this extremely complex problem. Owing to the frank and scientific attitude of a number of women, and the ready and intimate confidence of many more, I have obtained a number of most interesting facts from which I think it is already possible to deduce a generalisation which is illuminating, and may be of great medical and sociological value. A detailed statement of this will be given in a scientific publication, but as it bears very intimately on the subject of the present chapter, a short and simple account of my conclusions must be given here. It is first necessary to consider several other features of woman's life, however. The obvious moon-month rhythm in woman, so obvious that it _cannot_ be overlooked, has been partially studied in its relation to some of the ordinary functions of her life. Experiments have been made to show its influence on the rate of breathing, the muscular strength, the temperature, the keenness of sight, etc., and these results have even been brought together and pictured in a single curved diagram supposed to show the variability in woman's capacities at the different times in her twenty-eight-day cycle. But it brings home to one how little original work even in this field has yet been done, that the same identical diagram is repeated from book to book, and in Marshall's Physiology it is "taken from Sellheim," in Havelock Ellis "from Von Ott," and in other books is re-copied and attributed to still other sources, but it is always the same old diagram. This diagram is reproduced by one learned authority after another, yet nearly every point on which this curve is based appears to have been disputed. According to this curve, woman's vitality rises during the few days before menstruation, sinks to its lowest ebb during menstruation and rises shortly after, and then runs nearly level till it begins to rise again before the next menstrual period. This simple curve may or may not be true for woman's temperature, muscular strength, and the other relatively simple things which have been investigated. My work and observations on a large number of women all go to show that this curve does _not_ represent the waves of woman's sex-vitality. The whole subject is so complex and so little studied that it is difficult to enter upon it at all without going into many details which may seem remote or dull to the general reader. Even a question which we must all have asked, and over which we have probably pondered in vain--namely, what is menstruation?--cannot yet be answered. To the lay mind it would seem that this question should be answerable at once by any doctor; but many medical men are still far from being able to reply to it even approximately correctly. (See also Appendix, note 2.)