In the animal kingdom the methods of sexual reproduction are also varied and wonderful. In many of the lower forms of animals, such as the various sea-creatures, the methods of reproduction may be those of division, as first mentioned, or a method combining division with true sexual reproduction. In the case of fishes, the eggs of the female are deposited in the bottom of a stream and are later fertilized by the sperm-cells of the male fishes. This involves a tremendous waste of reproductive cells, scarcely less extravagant than the waste of pollen in plants, such as is seen in a corn-field when the ground becomes yellow, during the tasseling season, with the myriads of pollen grains that failed to secure lodgment upon the silks of the young ears of corn. [Sidenote: Reproduction in higher forms of life] In the types of animals that are of higher form than fishes, that is, reptiles, birds, and mammals, the fertilization of the germ-cell (egg) takes place within the body of the female. In the case of the latter group--mammals--the true egg is hatched within the body of the female, and the offspring, or embryo as it is known to scientists, grows there for a considerable period before birth. A RATIONAL VIEW OF SEXUAL HEALTH The anatomy and the physiology of reproduction will not be considered in detail in this work, as this would require a very lengthy and technical treatise. The remainder of the lesson will be devoted to the relation of the reproductive functions to general health and happiness. [Sidenote: Development of reproductive instincts] In the process of evolution this function of reproduction was vitally essential to the life of the race. As a result there developed in all animal life strong sexual or reproductive instincts. As is plainly evident, all animals, including man, with such instincts most strongly developed would be the most successful in producing young, and through these offsprings the race or species would inherit like reproductive desires. [Sidenote: Kinship of the sexual, paternal and social instinct] In the case of man and the higher form of animals, this general instinct, the purpose of which was to produce offspring, became diversified in to many instincts. Not only does the reproductive instinct in this broad sense include what is commonly known as sexual passion in man, but it may very truly be said to be the essence of sexual love and parental love. Broad-minded scientists are even inclined to believe that the so-called social instinct or love for our fellowmen is but a distant reflection or shadow, as it were, of the original or natural instinct to produce offspring. There has arisen among civilized man a tendency to separate and class as two distinct things the strictly physical element of sexual desire, and the associate emotion of intellectual love between the sexes. As a matter of fact there is no distinct line of demarcation. [Sidenote: Overindulgence, degenerating and destructive]