244 ILLEGITIMATE OFFSPRING. CHAP. V. 



depend exclusively 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 detecting any 

 differences sufficient to account for certain species cross- 

 ing 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 conclusion 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 ele- 

 ments, the male element of the one species acting freely 

 on the female element of the other, but not so in a 

 reversed direction. And now we see that this same con- 

 clusion is independently and strongly fortified by the 

 consideration of the illegitimate unions of trimorphic 

 and dimorphic heterostyled plants. In so complex and 

 obscure a subject as hybridism it is no slight gain to 

 arrive at a definite conclusion, namely, that we must 

 look exclusively to functional differences in the sexual 

 elements, as the cause of the sterility of species when first 

 crossed and of their hybrid offspring. It was this con- 

 sideration which led me to make the many observations 

 recorded in this chapter, and which in my opinion make 

 them worthy of publication. 



