ephemeral males in hermaphrodite, self-fertilizing animals; it follows also from the theory, for with continued self-fertilization all the ids in the germ-plasm of an individual would tend to become identical, and the mingling of two germ-plasms which contained _identical_ ids would, at least according to the germ-plasm theory, have no meaning at all. [Illustration: FIG. 112 (repeated). Development of the parasitic Crustacean _Sacculina carcini_, after Delage. _A_, Nauplius stage. _Au_, eye. _I_, _II_, _III_, the three pairs of appendages. _B_, Cypris stage. _VI-XI_, the swimming appendages. _C_, mature animal (_Sacc_), attached to its host, the shore-crab (_Carcinus mænas_), with a feltwork of fine root-processes enveloping the crab's viscera. _S_, stalk. _Sacc_, body of the parasite. _oe_, aperture of the brood-cavity. _Abd_, abdomen of the crab with the anus (_a_).] Thus we see that in the animal kingdom hermaphroditism is always associated with cross-fertilization in some way or other, even though the latter may occur rarely, being usually periodically interpolated, and thus bringing new ids into the germ-plasm which is rapidly becoming monotonous or uniform. Adaptations quite analogous to these are found in relation to parthenogenesis, and it will repay us to give a brief summary of these. Parthenogenesis effects a very considerable increase in the fertility of a species, and in this increase the reason for its introduction among natural phenomena obviously lies. By the occurrence of parthenogenesis, the number of ova produced by a particular colony of animals may be doubled, because each individual is a female, and as the multiplication increases in geometrical ratio a few parthenogenetic generations result in a number of descendants enormously in excess of those produced by bi-sexual reproduction. We can therefore understand why parthenogenesis should obtain among animals whose conditions of life are favourable only for a short time, and are then uncertain and dangerous for a long period. This is the case with the water-fleas, the Daphnids (see Figs. 57 and 58), whose habitats--pools, ponds, and marshes--often dry up altogether in summer, or freeze in winter, so that it becomes almost if not quite impossible for the colonies to go on living, and the preservation of the species can only be secured by the production of hard-shelled 'lasting' eggs, which sink to the bottom, dry up in the mud, or become frozen, or at least remain latent in a sort of slumber. As soon as the favourable conditions reappear, young animals which emerge from the eggs are all females and reproduce parthenogenetically, so that after a few days there is a numerous progeny swimming freely about, which in their turn are all females, and reproduce after the same manner. In many Daphnids this goes on for a series of generations, and there thus arises an enormous number of animals, which may fill a marsh so densely that, by drawing a