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 



