easily understood, therefore, why sexual intercourse should be
more liable to be followed by pregnancy when it occurs about the
menstrual epoch than at other times.... Before its discharge, the egg
is immature, and unprepared for impregnation; and after the menstrual
period has passed, it gradually loses its freshness and vitality."

[Footnote 11: Dalton.]

The law of periodicity, as it affects the sexual activity of males of
the human species, is indicated in the following remarks by the same
author:--

"The same correspondence between the periods of sexual excitement in
the male and female, is visible in many of the animals [higher mammals],
as well as in fish and reptiles. This is the case in most species which
produce young but once a year, and at a fixed period, as the deer and
the wild hog. In other species, on the contrary, such as the dog, the
rabbit, the guinea-pig, etc., where several broods of young are
produced during the year, or where, as in the human subject, the
generative epochs of the female recur at short intervals, so that the
particular period of impregnation is comparatively indefinite, the
generative apparatus of the male is almost always in a state of full
development; and is excited to action at particular periods, apparently
by some influence derived from the condition of the female."

The facts presented in the foregoing quotations from Dr. Dalton may
be summarized as follows:--

1. The sexual function is for the purpose of producing new individuals
to take the place of those who die, and thus preserve the species from
becoming extinct.

2. In the animal kingdom generally, the reproductive function is
_necessarily_ a periodical act, dependent upon the development of the
reproductive organs of both the male and the female at stated periods.

3. In those exceptional cases in which the organs of the male are in
a state of constant development, sexual congress occurs, in lower
animals, only at those periods when the periodical development occurs
in the female.

4. Fecundation of the female element can only take place about the time
of periodical development in the female.

5. The desire for sexual congress naturally exists in the female only
at or immediately after the time of periodical development.

6. The constant development of the sexual organs in human males is a
condition common to all animals in which development occurs in the
female at short intervals, and is a provision of na