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These embryological conclusions can be brought into relation with another set of facts. Haeckel has used the word “gonochorism” for the separation of the sexes, and in different classes and groups of creatures different degrees of gonochorism may be noted. Different kinds of animals and plants may be distinguished by the extent to which the characters of one sex are rudimentary in the other. The most extreme case of sexual differentiation, the sharpest gonochorism, occurs in sexual dimorphism, that is to say, in that condition of affairs in which (as for instance in some water-fleas) the males and females of the same species differ as much or even more from each other as the members of different species, or genera. There is not so sharply marked gonochorism amongst vertebrates as in the case of crustacea or insects. Amongst the former there does not exist a distinction between males and females so complete as to reach sexual dimorphism. A condition much more frequent amongst them is the occurrence of forms intermediate in regard to sex, what is called abnormal hermaphroditism; whilst in certain fishes hermaphroditism is the normal condition.