[Illustration: FIG. 58. _Moina paradoxa_, female. The letters of Fig. 57 apply _mutatis mutandis_. _brr_, brood-pouch. _ov_, ovary. _sr_, margin of shell.] The case of the antennæ of _Moina_, which have been modified into grasping organs, is quite different; these owe their origin not to natural selection, but to sexual selection, for antennæ of that kind are not indispensable to the existence of the species, as we can see from the closely related genera, _Daphnia_ and _Simocephalus_, where the males have quite short stump-like antennæ, furnished with olfactory filaments not much more numerous than the females possess. Just as these supernumerary olfactory filaments were produced by sexual selection, and not by the ordinary natural selection, because those males with the more acute sense of smell had an advantage over those in which it was blunter, so the males of the genus _Moina_ which could grasp most securely had an advantage over those that gripped less firmly, and thus arose these two different kinds of male characteristics. Neither of them is of advantage to the species as such, but only to the males in their competition for the possession of the females. But, where the production of a novel character in the male is concerned, natural selection cannot proceed in a different manner from sexual selection; the process of selection is exactly the same: the better equipped males survive, the less well-equipped die without begetting offspring; the difference lies only in the fact that in the one case the improvement is in the species as such, in the other case only in one sex without the existence of the species being thereby made more secure. Such cases are instructive, because they make a denial of the process of sexual selection quite impossible if that of species-selection is admitted. If processes of selection are operative at all as factors in transformation, they must act even where the advantage is not to the species but only 'intra-sexual,' and the one process must often run into the other, so that it is often quite impossible to draw an exact line of demarcation between them.