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In other gall-flies and in aphids we find that a sexual generation alternates with one or with many virgin generations.

The spore of a fern or a moss has only half the number of chromosomes that the cells of its producer have, yet it proceeds by asexual multiplication or ordinary cell-division to build up the gametophyte or sexual generation.

The phenomena of inheritance in the sexual generation of hybrids show this clearly.

That is to say, only alternate generations attain sexual maturity, those that do so being produced as buds from a sexless generation, which itself arises from the fertilized eggs of a previous sexual generation.

V-VIII represents the sexual generation.