The chart given opposite may assist in making graphically clear what has been said in these last few pages. It is compounded from a number of individual records, and shows a fair average chart of the rhythmic sequence of superabundance and flagging in woman's sex-vitality. The tops of the wave-crests come with remarkable regularity, so that there are two wave-crests in each twenty-eight-day month. The one comes on the two or three days just _before_ menstruation, the other after; but after menstruation has ceased there is a nearly level interval, bringing the next wave-crest to the two or three days which come about eight or nine days after the close of menstruation--that is, just round the fourteen days, or half the moon month, since the last wave-crest. If this is put in its simplest way, one may say that there are fortnightly periods of desire, arranged so that one period comes always just _before_ each menstrual flow. According to her vitality at the time, and the general health of the woman, the length of each desire-period, or, as we might say, the size and complexity of each wave-crest, depends. Sometimes for the whole of as much as, or even more than three days, she may be ardently and quite naturally stimulated, while at another time the same woman, if she is tired and over-worked, may be conscious of desire for only a few hours, or even less. [Illustration: CHART I. Curve showing the Periodicity of Recurrence of natural desire in healthy women. Various causes make slight irregularities in the position, size and duration of the "wave crests," but the general rhythmic sequence is apparent. (_to face page 32_)] [Illustration: CHART II. Curve showing the depressing effects on the "wave-crests" of fatigue and over-work. Crest _a_ represented only by a feeble and transient up-welling. Shortly before and during the time of the crest _d_ Alpine air restored the vitality of the subject. The increased vitality is shown by the height and number of the apices of this wave-crest. (_to face page 33_)] The effects of fatigue, city life, bad feeding, and, indeed, of most outward circumstances may be very marked, and may for years, or all her life, so reduce her vitality that a woman may never have experienced any spontaneous sex-impulse at all. The effects of fatigue, which reduces the vital energy, even in a normal, strongly sexed woman, can be seen in the second curve opposite, where at _a_ the intermediate wave-crest is very much reduced. This is not a generalised chart, but a detailed record of an actual individual case.