Let us glance over the animal kingdom with special reference to these questions: we find that hermaphroditism prevails chiefly among species which at maturity have lost their power of free locomotion, and have become sedentary, such as oysters, barnacles among Crustaceans, the Bryozoa, and the sea-squirts (Ascidians) which are fixed to the rocks at the bottom of the sea. For forms such as these it must often have been advantageous that each individual could function both as male and as female, especially when it was capable of self-fertilization, since individuals which settled down singly, or in very small numbers together, would not be lost as regards the persistence of the species. The continuance of the species is thus better secured than it would be by separation of the sexes, because in the latter case it might frequently have happened that the animals which had settled beside each other by chance were of the same sex, and would therefore remain unfertile. But many of these species do not fertilize themselves, but fertilize each other mutually; and this, too, carries a great advantage with it, because in sedentary animals the sperms will fertilize twice as many individuals, if each contains eggs, than if half were exclusively male. It is thus to some extent an economy of sperms, but at the same time also of ova, which is effected by hermaphroditism: the result is that these valuable products are wasted as little as possible. On this account we find that not only sedentary, but also sluggish, slow-moving animals are equipped with male and female organs of reproduction, as, for instance, all our terrestrial snails. They fertilize each other mutually: when two meet it is always as males and females, and notwithstanding the sluggishness of movement, it is not likely to happen that a snail does not attain to reproduction because it has not found a mate. The same is true of the earthworms, which are likewise not adapted for making long journeys in search of the opposite sex; they, and the leeches also, function as male and female simultaneously, while their nearest relatives, the marine Chætopods, are of separate sexes, which may be associated with their much greater power of free movement in the water.