Chap. IX. and Trimorphism. 255 



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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 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 ; 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 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 consideration of 

 dimorphic and trimorphic plants, that the sterility of distinct 

 species when crossed and of their hybrid progeny, depends exclu- 

 sively on the nature of their sexual elements, and not on any differ- 

 ence in their structure or general constitution. We are also led 

 to this same conclusion by considering 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, whilst the 

 converse cross can be effected with perfect facility. That excellent 

 observer, Gartner, likewise concluded that species when crossed are 

 sterile owing to differences confined to their reproductive systems. 



Fertility of Varieties when Crossed, and of their Mongrel 



Offspring, not universal. 



It may be urged, as an overwhelming argument, that there must 



be some essential distinction between species and varieties, inasmuch 



as the latter, however much they may differ from each other in 



external appearance, cross with perfect facility, and yield perfectly 





