constantly undergoes regular rhythmic contractions at brief intervals.  The vascular system, down to the smallest capillaries, is acted on by three series of vibrations, and every separate fragment of muscular tissue possesses rhythmic contractility.  Growth itself is rhythmic, and, as Malling-Hansen and subsequent observers have found, follows a regular annual course as well as a larger cycle.  On the psychic sides attention is rhythmic.  We are always irresistibly compelled to impart a rhythm to every succession of sounds, however uniform and monotonous.  A familiar example of this is the rhythm we can seldom refrain from hearing in the puffing of an engine.  A series of experiments, by Bolton, on thirty subjects showed that the clicks of an electric telephone connected in an induction-apparatus nearly always fell into rhythmic groups, usually of two or four, rarely of three or five, the rhythmic perception being accompanied by a strong impulse to make corresponding muscular movements.[75]

It is, however, with the influence—­to some extent real, to some extent, perhaps, only apparent—­of cosmic rhythm that we are here concerned.  The general tendency, physical and psychic, of nervous action to fall into rhythm is merely interesting from the present point of view as showing a biological predisposition to accept any periodicity that is habitually imposed upon the organism.[76] Menstruation has always been associated with the lunar revolutions.[77] Darwin, without specifically mentioning menstruation, has suggested that the explanation of the allied cycle of gestation in mammals, as well as incubation in birds, may be found in the condition under which ascidians live at high and low water in consequence of the phenomena of tidal change.[78] It must, however, be remembered that the ascidian origin of the vertebrates has since been contested from many sides, and, even if we admit that at all events some such allied conditions in the early history of vertebrates and their ancestors tended to impress a lunar cycle on the race, it must still be remembered that the monthly periodicity of menstruation only becomes well marked in the human species.[79] Bearing in mind the influence exerted on both the habits and the emotions even of animals by the brightness of moonlight nights, it is perhaps not extravagant to suppose that, on organisms already ancestrally predisposed to the influence of rhythm in general and of cosmic rhythm in particular, the periodically recurring full moon, not merely by its stimulation of the nervous system, but possibly by the special opportunities which it gave for the exercise of the sexual functions, served to implant a lunar rhythm on menstruation.  How important such a factor may be we have evidence in the fact that the daily life of even the most civilized peoples is still regulated by a weekly cycle which is apparently a segment of the cosmic lunar cycle.