Page - 321 - in The Origin of Species

DIMORPHISM AND TRIMORPHISM 321 ner as the sterility of hybrids does not always run parallel with the difficulty of making the first cross between the two parent-species, so the sterility of certain illegitimate plants was unusually great, whilst the sterility of the union from which they were derived was by no means great. With hy- brids raised from the same seed-capsule the degree of ster- ility is innately variable, so it is in a marked manner with illegitimate plants. Lastly, many hybrids are profuse and persistent flowerers, whilst other and more sterile hybrids produce few flowers, and are weak, miserable dwarfs; exactly similar cases occur with the illegitimate offspring of various dimorphic and trimorphic plants. Altogether there is the closest identity in character and behaviour between illegitimate plants and hybrids. It is hardly an exaggeration to maintain that illegitimate plants are hybrids, 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 dis- tinct species. We have also 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 cross- ing 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 hy- bridized seed, and he would find that the seedlings were mis- erably dwarfed and utterly sterile, and that they behaved in all other respects like ordinary hybrids. He might then main- tain 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 com- pletely mistaken. The facts now given on dimorphic and trimorphic plants are important, because they show us, first, that the physio- logical test of lessened fertility, both in first crosses and in K—HC XI