Thus the males of the Copepods possess on their anterior antennæ an arrangement which enables them to throw a long whiplike structure like a lasso round the head of the female as she rapidly swims away. The antennæ of the male Daphnids, too, are in one genus (_Moina_) developed into a grasping apparatus, instead of into smelling organs as in _Leptodora_. Fig. 57 shows the male, Fig. 58 the female of _Moina paradoxa_; the first antennæ of the male are not only much longer and stronger than those of the female (_at_^1), but they are also armed with claws at the end, so that the males can catch their mates as with a fork, and hold them fast. And even that was not enough, for, in addition, the males of most Daphnids possess a large sickle-shaped but blunt claw on the first pair of legs (Fig. 57, _fkr_), which enables them to cling to the smooth shell of the female, and to clamber up on it to get into the proper position for copulation. [Illustration: FIG. 57. _Moina paradoxa_, male. _at^1_, first antennæ, with claws at the tip for capturing the female. _at^2_, second antennæ. _fkr_, claws on the first pair of legs for clambering. _gh_, brain. _lbr_, upper lip. _md_, mandible. _md_, mid-gut, with the liver lobes (_lh_). _h_, heart. _sp_, testis. _aft_, anus. _sb_, caudal setæ. _skr_, caudal claws. _sch_, shell. _schr_, cavity of the shell. _kie_, gill-plates. Magnified 100 times.] If we inquire into the manner of the origin of secondary sexual characters of this kind, we shall find that both may have been increased by sexual selection, for a male with a better sickle will succeed more quickly in getting into the proper position for copulation than one with a less perfect mechanism. This assumption does not rest on mere theory, for I was once able, by a happy chance, to observe for a considerable time, under the microscope, a female to whose shell two males were clinging, each trying to push the other off. Nevertheless it seems to me very questionable whether the origin of this sickle-claw can be referred to sexual selection, for without this clamping-organ copulation in most Daphnids would not be possible. It was thus not as an advantage which one male had over another that the clamping-sickle evolved, but rather as a necessary acquisition of the whole family, which must have developed in all the species at the same time as the other peculiarities, and notably those of the shell. The competition of the males among themselves is thus in this case simply an expression of the struggle for existence on the part of the species as such, and it is not a question merely of a character which makes it easier for the males to gain possession of the females, but of one which had necessarily to arise lest the species should become extinct. In other words, in this case natural selection and sexual selection coincide.