Double-Sexed Individuals
Another difficulty might be supposed to arise between animals which produce themselves other than by sexual reproduction. This has already been slightly touched upon; and it has been shown that numerous plants and animals propagate themselves through their double-sexed organs. It occurs in a great majority of plants, but only in a minority of animals; for example, the garden-snail, leeches, earth-worms, and many other worms. Every garden-snail produces in one part of its sexual gland eggs, and in another part sperm.
Parthenogenesis offers an interesting form of transition from sexual reproduction to the non-sexual formation of germ-cells (which most resembles it). It has been demonstrated to occur in many cases among insects, especially by Seebold’s excellent investigations. Among the common bees, a male individual (a drone) arises out of the eggs of the queen, if the eggs have not been fructified; a female (a queen or working bee), if the egg has been fructified.
Gonochorismus or sexual separation, which characterizes the more complicated of the two kinds of sexual reproduction, has evidently been developed from the condition of hermaphroditism at a late period of the organic history of the world. In this case the female individual in both animal and plant produces eggs or egg-cells. In animals, the male individual secretes the fructifying sperm (sperma); in plants, the corpuscles, which correspond to the sperm.