The separation of the sexes exists even among the Protozoa--Conditions determining the occurrence of Hermaphroditism--Tape-worms, Cirrhipeds--Primordial males--Advantages of parthenogenesis--Alternation with bi-sexual generations--In Gall-wasps--In Aphides--Cross-fertilization secured in plants--Self-fertilization is avoided whenever possible--The mechanism of fertilization and the mingling of germ-plasms must be clearly distinguished from one another--Cases of persistent self-fertilization--The effects of inbreeding compared with those of parthenogenesis--The effect of purely asexual reproduction--In sea-wracks--In lichens and fungi--In cultivated plants--Degeneration of the sex-organs--Summary. We have seen that continued inbreeding must make the germ-plasm monotonous, and therefore unplastic as regards the requirements of adaptation. Accordingly, we found that the gametes of many unicellulars are so constituted that they only possess a power of attraction for gametes of a different lineage, not for those of their own stock. Among multicellular organisms the most intense mode of inbreeding is to be found in the uninterrupted self-fertilization of hermaphrodites: in such cases the monotony of the germ-plasm must reach extreme expression more readily than in the case of ordinary inbreeding. We can thus understand why, in the scale of organisms, there is such an early occurrence of gonochorism, the separation of the species into male and female individuals. Even among unicellular plants or Protophytes this occurs occasionally, as it does in the Vorticellids among Infusorians. In the Metazoa and Metaphyta the separation of the sexes finds emphatic expression; it is absent from no important group, and in many, such as, for instance, among the Vertebrates, it has become the absolutely normal condition, with hardly any exception. But in many divisions of the animal and plant kingdoms hermaphroditism also plays an important part, as, for instance, in terrestrial snails and in flowering plants.