When the scale of animal life is reached where egg-cell and sperm-cell have their habitats in different individuals, the attraction between ovum and spermatozoön, which is based upon a kind of erotic chemotropismus, is transferred to the two hosts who harbor the two different sex-cells. Thus the erotic chemotropismus between two cells has now grown to sex-attraction between two animals. But even at this stage the attraction sometimes exists only between the sex-cells, namely when fertilization takes place outside of the mother’s body. The best example of outside fertilization are the fishes. The female fish contains the roe, which is a mass of small eggs. At the proper time the female lets the roe fall on the ground of rivers or the dark bottom of lakes, etc., at a favorable place called spawning bed, secure against enemies. The male fishes swim over the spawn and pour their semen or milt over it. The two kinds of cells attract each other like iron and magnet. When the milt has reached the spawn, the eggs are fertilized and develop into new fishes. In the next higher class, in birds, fertilization is internal, and only the development is external. The female bird has an ovary containing a large number of eggs or ova, within its body. The male animal possesses an organ serving for the introduction of the semen into the body of the female. The fertilized egg is laid and develops outside, either by the sun-heat or hatched by the mother’s warmth. The last step in generation is reached in the mammals. Here not only fertilization is accomplished within the body of the female, but also the entire development of the fertilized ovum is carried out in the uterus, inside the female body. Hence mammals give birth to living mature young. In these higher animals the attraction of the sex cells has been definitely transferred to, or rather changed into, the attraction of the hosts of these cells or into the sexual instinct. The sex-instinct is, therefore, if not identical with the instinct of propagation, thus an offshoot of the latter instinct. In the more advanced species of the higher animals the instinct relating to the preservation of the kind is composed of three definite impulses. There is first the impulse relating to the act of conjugation, universal in all organic life. The average individual grows up surrounded by others of the opposite sex and normally unexcited by this difference until the age of puberty approaches. Then some day, the boy or girl find arising within an incipient impulse which, naturally, were there no artificial restrictions, would lead them to exercise their sexual powers, just as found among animals.