eferred to let
general and more or less abstruse rules govern conduct in sexual lines.
Until recent years there have been few sermons in which common sexual
problems have been presented so that the preacher's meaning has been
clear to all. On the contrary, there has been universal mystery and
evasion concerning the greatest facts of life.

[Sidenote: Sexual instincts offer no guidance.]

Many people have justified the mystery thrown around sexual processes
on the theory that the reproductive instincts of mature people are
sufficient guides for conduct. This involves a misunderstanding of
sexual instincts of the higher mammals which are often unscientifically
cited as models for human imitation. In these animals sexual union is
instinctively determined, because normally the sexual hunger or
excitement of both sexes is stimulated and controlled by the
physiological condition of the female at the times favorable for
fertilization (_i.e._, at the oestrual periods). For example, a pair of
dogs living in close companionship show signs of mutual sexual desires
only for a few days at the semi-annual oestrual or fertile periods of
the female. It occasionally happens that the males of various wild and
domesticated mammals exhibit signs of automatic sexual excitement
(_i.e._, not caused by the stimulus arising from the physiological
condition of the female); but in such cases of male excitement outside
of the mating or oestrual periods, the normal females invariably offer
instinctive opposition to attempted union by abnormally or
automatically excited males. Thus, directly and indirectly, there is
instinctive control and limitation of sexual union among the animals
that are most closely related to the human race.

It is biologically possible that similar conditions may have existed in
the earliest human life, but that is pure speculation and has no
bearing on the practical problems of sex in human life to-day. The fact
is that the simple physiological stimuli which produce sexual
excitement in both sexes of animals have practically no influence in
determining human sexual union. On the contrary, memory associations
consciously connected with the opposite sex, especially those
associations that are centered in affection, may at any time in the
normal individual of either human sex afford the basis for a chain of
mental states leading to sexual excitement and union. There is not, as
in the animals, instinctive dependence on the p