236 ON THE ORIGIN OF SPECIES 



the curious and complex laws governing the facility 

 with which trees can be grafted on each other as in- 

 cidental on unknown differences in their vegetative 

 systems, so I believe that the still more complex laws 

 governing the facility of first crosses, are incidental on 

 unknown differences, chiefly in their reproductive 

 systems. These differences, in both cases, follow to a 

 certain extent, as might have been expected, systematic 

 affinity, by which every kind of resemblance and dis- 

 similarity between organic beings is attempted to be 

 expressed. The facts by no means seem to me to 

 indicate that the greater or lesser difficulty of either 

 grafting or crossing together various species has been a 

 special endowment ; although in the case of crossing, 

 the difficulty is as important for the endurance and 

 stability of specific forms, as in the case of grafting it is 

 unimportant for their welfare. 



Causes of the Sterility of first Crosses and of Hybrids. — 

 We may now look a little closer at the probable causes 

 of the sterility of first crosses and of hybrids. These 

 two cases are fundamentally different, for, as just 

 remarked, in the union of two pure species the male 

 and female sexual elements are perfect, whereas in 

 hybrids they are imperfect. Even in first crosses, the 

 greater or lesser difficulty in effecting a union ap- 

 parently depends on several distinct causes. There 

 must sometimes be a physical impossibility in the male 

 element reaching the ovule, as would be the case with a 

 plant having a pistil too long for the pollen-tubes to 

 reach the ovarium. It has also been observed that 

 when pollen of one species is placed on the stigma of a 

 distinctly allied species, though the pollen tubes pro- 

 trude, they do not penetrate the stigmatic surface. 

 Again, the male element may reach the female element, 

 but be incapable of causing an embryo to be developed, 

 as seems to have been the case with some of Thuret's 

 experiments on Fuci. No explanation can be given 

 of these facts, any more than why certain trees cannot 

 be grafted on others. Lastly, an embryo may be 



