Altogether there is the closest identity in character and behaviour between illegitimate plants and hybrids. It is hardly an exaggeration to maintain that the former are hybrids, but produced within the limits of the same species by the improper union of certain forms, whilst ordinary hybrids are produced from an improper union between so-called distinct species. We have already seen that there is the closest similarity in all respects between first illegitimate unions, and first crosses between distinct species. This will perhaps be made more fully apparent by an illustration: we may suppose that a botanist found two well-marked varieties (and such occur) of the long-styled form of the trimorphic _Lythrum salicaria_, and that he determined to try by crossing whether they were specifically distinct. He would find that they yielded only about one-fifth of the proper number of seed, and that they behaved in all the other above-specified respects as if they had been two distinct species. But to make the case sure, he would raise plants from his supposed hybridised seed, and he would find that the seedlings were miserably dwarfed and utterly sterile, and that they behaved in all other respects like ordinary hybrids. He might then maintain that he had actually proved, in accordance with the common view, that his two varieties were as good and as distinct species as any in the world; but he would be completely mistaken. The facts now given on dimorphic and trimorphic plants are important, because they show us, firstly, that the physiological {184} test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of specific distinction; secondly, because we may conclude that there must be some unknown law or bond connecting the infertility of illegitimate unions with that of their illegitimate offspring, and we are thus led to extend this view to first crosses and hybrids; thirdly, because we find, and this seems to me of especial importance, that with trimorphic plants three forms of the same species exist, which when crossed in a particular manner are infertile, and yet these forms differ in no respect from each other, except in their reproductive organs,--as in the relative length of the stamens and pistils, in the size, form, and colour of the pollen-grains, in the structure of the stigma, and in, the number and size of the seeds. With these differences and no others, either in organisation or constitution, we find that the illegitimate unions and the illegitimate progeny of these three forms are more or less sterile, and closely resemble in a whole series of relations the first unions and hybrid offspring of distinct species. From this we may infer that the sterility of species when crossed and of their hybrid progeny is likewise in all probability exclusively due to differences confined to the reproductive system. We have indeed been brought to a similar conclusion by observing that the sterility