Throughout the vegetable and animal worlds the sexual functions are periodic

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The Various Physiological and Psychological Rhythms—Menstruation—The Alleged Influence of the Moon—Frequent Suppression of Menstruation among Primitive Races—Mittelschmerz—Possible Tendency to a Future Intermenstrual Cycle—Menstruation among Animals—Menstruating Monkeys and Apes—What is Menstruation—Its Primary Cause Still Obscure—The Relation of Menstruation to Ovulation—The Occasional Absence of Menstruation in Health—The Relation of Menstruation to "Heat"—The Prohibition of Intercourse during Menstruation—The Predominance of Sexual Excitement at and around the Menstrual Period—Its Absence during the Period Frequently Apparent only. Throughout the vegetable and animal worlds the sexual functions are periodic. From the usually annual period of flowering in plants, with its play of sperm-cell and germ-cell and consequent seed-production, through the varying sexual energies of animals, up to the monthly effervescence of the generative organism in woman, seeking not without the shedding of blood for the gratification of its reproductive function, from first to last we find unfailing evidence of the periodicity of sex. At first the sun, and then, as some have thought, the moon, have marked throughout a rhythmic impress on the phenomena of sex. To understand these phenomena we have not only to recognize the bare existence of that periodic fact, but to realize its implications.