Numerous secondary sexual differences probably depend purely on species selection, that is to say, they include an improvement of the species in relation to the struggle for existence. We may find a case in point in the dwarf-like smallness of the males in many parasitic crustaceans, in some worms, in many Rotifers, and in the Cirripedes. It can hardly have been of advantage for the individual male to be smaller than his fellows, but it was of advantage for the species to produce as many males as possible in order to ensure a meeting with the females, and, as the enormous production of males made it advantageous for the species that as little material as possible should be used in their individual production, we can readily understand the minuteness of the males, and in some cases, as in the Rotifers and _Bonellia_, their poor equipment, lack of nutritive organs, and ephemeral existence. The marine worm, _Bonellia viridis_, whose female may be a foot in length, is not the only case in which a microscopically small male lives like a parasite inside the female. Among the round-worms, too, there is a species called _Trichosomum crassicauda_, discovered by Leuckart in the rat, the dwarf males of which live in the reproductive organs of the female. All these are arrangements for securing the propagation of the species, which might have been endangered if the males had had to seek out the females, which, in the case of _Bonellia_, live in holes in the rocks on the sea-floor, and, in the case of _Trichosomum_, are concealed in the urinary bladder of the rat. Obviously, this is the reason which, in addition to the one already mentioned, has conditioned and produced, or helped to produce, the remarkable minuteness of certain males.