238 RECIPROCAL DIMORPHISM [CHAP. IX. 



sterility is innately variable, so it is in a marked manner with 

 illegitimate plants. Lastly, many hybrids are profuse and per- 

 sistent 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 distinct 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 

 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, first, that the physiological 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 is some unknown bond which connects the 

 infertility of illegitimate unions with that of their illegitimate 

 offspring, and we are led to extend the same view to first crosses 

 and hybrids ; thirdly, because we find, and this seems to me of 

 especial importance, that two or three forms of the same species 

 may exist and may differ in no respect whatever, either in 

 structure or in constitution, relatively to external conditions, and 

 yet be sterile when united in certain ways. For we must re- 

 member that it is the union of the sexual elements of individuals 

 of the same form, for instance, of two long-styled forms, which 

 results in sterility ; whilst it is the union of the sexual elements 

 proper to two distinct forms which is fertile. Hence the case 

 appears at first sight exactly the reverse of what occurs, in the 



