Chap. IX.] AND TRIMOEPHISM. 33 



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 in- 

 fertility of illegitimate unions with that of their illegiti- 

 mate 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, doubt- 

 ful whether this is really so; but I will not enlarge on 

 this obscure subject. 



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

 sideration 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 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 



