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.

Mantegazza has suggested that the sexual period became established with
relation to the lunar period because moonlight nights were favorable to
courting,[80] and Nelson remarks that in his experience young and robust
persons are subject to recurrent periods of wakefulness at night which
they attribute to the action of the full moon. One may perhaps refer also
to the tendency of bright moonlight to stir the emotions of the young,
especially at puberty, a tendency which in neurotic persons may become
almost morbid.[81]

It is interesting to point out that, the farther back we are able to trace
the beginnings of culture, the more important we find the part played by
the moon. Next to the alteration of day and night, the moon's changes are
the most conspicuous and startling phenomena of Nature; they first suggest
a basis for reckoning time; they are of the greatest use in primitive
agriculture; and everywhere the moon is held to have vast i