Chap. V HETEKOSTTLED PLANTS. 243 



manner. And as we have just seen that distinct species 

 when crossed resemble in a whole series of relations the 

 forms of the same species when illegitimately united, 

 we are led to conclude that the sterility of the former 

 must likewise depend exclusiyely on the incompatible 

 nature of their sexual elements, and not on any general 

 difference in constitution or structure. We are, indeed, 

 led to this same conclusion by the impossibility of de- 

 tecting any differences sufficient to account for certain 

 species crossing with the greatest ease, whilst other 

 closely allied species cannot be crossed, or can be crossed 

 only with extreme difficulty. We are led to this con- 

 clusion still more forcibly by considering the great 

 difference which often exists in the facility of crossing 

 reciprocally the same two species ; for it is manifest in 

 this case that the result must depend on the nature of 

 the sexual elements, the male element of the one 

 species acting freely on the female element of the 

 other, but not so in a rcTcrsed direction. And now we 

 see that this same conclusion is independently and 

 strongly fortified by the consideration of the illegiti- 

 mate unions of trimorphic and dimorphic heterostyled 

 plants. In so complex and obscure a subject as hybrid- 

 ism it is no slight gain to arrive at a definite conclu- 

 sion, namely, that we must look exclusively to func- 

 tional differences in the sexual elements, as the cause 

 of the sterility of species when first crossed and of 

 their hybrid offspring. It was this consideration which 

 led me to make the many observations recorded in this 

 chapter, and which in my opinion make them worthy 

 of publication. 



