I do not deny that my exposition of the law is somewhat dogmatical and lacks confirmation by exact detail. But I am not so anxious to claim finished results as to incite others to the study, the more so as the means for scientific investigations are lacking in my own case. But even if much remains theoretical, I hope that I shall have firmly riveted the chief beams in my edifice of theory by showing how it explains much that hitherto has found no explanation, and so shall have, in a fashion, proved it retrospectively by showing how much it would explain if it were true.
A most remarkable confirmation of my law may be found in the vegetable kingdom, in a group of facts hitherto regarded as isolated and to be so strange as to have no parallel. Every botanist must have guessed already that I have in mind the phenomena of heterostylism, first discovered by Persoon, then described by Darwin and named by Hildebrand. Many Dicotyledons, and a few Monocotyledons, for instance, species of Primulaceæ and Geraneaceæ and many Rubiaceæ, phanerogams in the flowers of which both the pollen and the stigma are functional, although only in cross-fertilisation, so that the flowers are hermaphrodite in structure but unisexual physiologically, display the peculiarity that in different individuals the stamens and the stigma have different lengths. The individuals, all the flowers of which have long styles and therefore high stigmas and short anthers, are, in my judgment, the more female, whilst the individuals with short styles and long anthers are more male. In addition to such dimorphic plants, there are also trimorphic plants, such as _Lythrum salicaria_, in which the sexual organs display three forms differing in length. There are not only long-styled and short-styled forms, but flowers with styles of a medium length.
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.