Turning for a very brief space to animals

on the land there are some hermaphrodites, as land-mollusca and earthworms; but these all pair ... we can understand this remarkable fact ... for we know of no other means, analogous to the action of insects and of the wind in the case of plants, ... by which an occasional cross might blow ... here the currents offer an obvious means for an occasional cross ... and in the case of flowers, I have failed