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 you already know is the sac in which the egg or embryo grows into a baby. The penis and the vagina are about the same size, as Nature intended them to fit each other. By a rhythmic movement of the penis in and out, the sex act reaches an exciting climax or orgasm, when there is for the woman a peculiarly satisfying contraction of the muscles of the passage and for the man, the expulsion of the semen, the liquid which contains the germs of life. This is followed by a sensation of peaceful happiness and sleepy relaxation. It is the very greatest physical pleasure to be had in all human experience, and it helps very much to increase all other kinds of pleasure also. It is at this time that married people not only are closest to each other physically, but they feel closer to each other in every other way too. It is then most of all that they feel sure they belong to each other.

The sex act is called by various names, such as coitus, coition, copulation, cohabitation, sex-intercourse, the sex-embrace, etc. But all these terms refer to the same thing. The first coitus is apt to be somewhat painful for the woman, as there is usually a thin membrane, called the hymen, partly closing the vagina which has to be broken through, but all women do not have it and it varies in size and thickness with different people.

Without the sex act, no babies could be created, for it is by this means only that the semen which contains the male part of the germ of life can meet the ovum or the female part of the germ of life. When the two parts come together in the woman's body under just the right conditions, a baby begins to grow—at first so tiny that it could hardly be seen without a microscope, and finally, after nine months' growth in the uterus or womb of the mother till it weighs about seven or eight pounds, it is born, a live human being. The birth process is called labor, and it is indeed labor, for it usually means much pain and struggle for the mother, although the baby's journey from the uterus to the world is only a few inches. It takes anywhere from an