There are a good many slight variations among us, ranging from a three to a five weeks "month," but the majority of the women of our race have a moon-month of twenty-eight days, once during which comes the flow of menstruation. If we draw out a chart with succeeding periods of twenty-eight days each, looking on each period as a unit: When in this period is it that a normal healthy woman feels desire or any up-welling of her sex-tides? The few statements which are made in general medical and physiological literature on the subject of sex feeling in women are generally very guarded and vague. Marshall ("Physiology of Reproduction," p. 138), for instance, says: "The period of most acute sexual feeling is generally just after the close of the menstrual period." Ellis speaks of desire being stronger before and sometimes also after menstruation, and appears to lean to the view that it is natural for desire to coincide with the menstrual flow. After the most careful inquiries I have come to the conclusion that the general confusion regarding this subject is due partly to the great amount of variation which exists between different individuals, and partly to the fact that very few women have any idea of taking any scientific interest in life, and partly to the fact that the more profound, fundamental rhythm of sex desire which I have come to the conclusion exists or is potential in every normal woman, is covered over or masked by the more superficial and temporary influences due to a great variety of stimuli or inhibitions in modern life. For the present consideration I have tried to disentangle the profound and natural rhythm from the more irregular surface waves.