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Although only dimorphism and trimorphism have been recognised in the books, these conditions do not exhaust the actual complexities of structure. Darwin himself pointed out that if small differences were taken into account, no less than five different situations of the anthers could be distinguished. Alongside such plain cases of discontinuity, of the separation of the different degrees of maleness and femaleness in plainly distinct individuals, there are also cases in which the different degrees grade into each other without breaks in the series. There are analogous cases of discontinuity in the animal kingdom, although they have always been thought of as unique and isolated phenomena, as the parallel with heterostylism had not been suggested. In several genera of insects, as, for instance, some Earwigs (Forficulæ) and Lamellicorn Beetles (Lucanus cervus), the Stag-beetle (Dynastes Hercules), and Xylotrupes gideon, there are some males in which the antennæ, the secondary sexual characters by which they differ most markedly from the females, are extremely long, and others in which they are very short. Bateson, who has written most on this subject, distinguishes the two forms as “high males” and “low males.” It is true that a continuous series of intermediate forms links the extreme types, but, none the less, the vast majority of the individuals are at one extreme or the other. Unfortunately, Bateson did not investigate the relations between these different types of males and the females, and so it is not known if there be female types with special sexual affinity for these male types. Thus these observations can be taken only as a morphological parallel to heterostylism and not as cases of the law of complementary sexual attraction.