instance.
You doubtless know already that the more highly developed the animal,
the longer it takes the young one to grow before it is born, and the
longer the period when it is helpless to provide its own food and
care.

Now we come to human beings, and see how different they are! They
have no regular mating season, and while there is a certain amount of
instinct in men and women which tends to bring them together, the sex
impulse among highly developed people is far more the result of their
feeling of love for each other than mere animal instinct alone. Many
of the animals make no choice at all in their mating. Any near-by
female will do for the male. But among some of the higher animals the
male has a special instinct for a certain female, and the female will
not tolerate any but a certain male. Most of the animals have
different mates every season, though there are a few kinds where the
male and female, once having mated, remain mates for years, sometimes
even for life. But it is _only human beings_ whose mating is what we
call "falling in love," and that is an experience far beyond anything
that the animals know.

It means that a man and a woman feel that they _belong_ to each other
in a way that they belong to no one else; it makes them wonderfully
happy to be together; they find they want to live together, work
together, play together, and to have children together, that is, to
marry each other; and their dream is to be happy together all their
lives. Sometimes the dream does not come true, and there is much
failure and unhappiness, but just the same people go right on trying
to make it a success, because it is what they care most for.

The sex attraction is the strongest feeling that human beings know,
and unlike the animals, it is far more than a mere sensation of the
body. It takes in the emotions and the mind and the soul, and that is
why our happiness is so dependent upon it.

When a man and a woman fall in love so that they really belong to
each other, the physical side of the relation is this: both of them
feel at intervals a peculiar thrill or glow, particularly in the
sexual organs, and it naturally culminates after they have gone to
bed at night. The man's special sex organ or penis, becomes enlarged
and stiffened, instead of soft and limp as ordinarily, and thus it
easily enters the passage in the woman's body called the vagina or
birth-canal, which leads to the uterus or womb, which as perhaps y