282 RECIPROCAL DIMORPHISM AND TRIMORPHISM. 



try by crossing whether they were specifically distinct. Hf» 

 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 dis- 

 tinct species. But to make the case sure, he would raise 

 plants from his supposed hybridized 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 physio- 

 logical 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 con- 

 stitution, relatively to external conditions, and yet be sterile 

 when united in certain ways. For we must remember 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 ; while it is the union of the sexual ele- 

 ments proper to two distinct forms which is fertile. Hence 

 the case appears at first sight exactly the reverse of what 

 occurs, in the ordinary unions of the individuals of the same 

 species and with crosses between distinct species. It is, 

 however, doubtful whether this is really so ; but I will not 

 enlarge on this obscure subject. 



We may, however, infer as probable from the considera- 

 tion of dimorphic and trimorphic plants, that the sterility 

 of distinct species when crossed and of their hybrid progeny, 

 depends exclusively on the nature of their sexual elements, 

 and not on any difference in their structure or general con- 

 stitution. We are also led to this same conclusion by. con- 

 sidering reciprocal crosses, in which the male of one species 

 cannot be united, or can be united with great difficulty, with 

 the female of a second species, while the converse cross can 

 b<* effected with perfect facility. That excellent observe^ 



