[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.
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