Chap. VIII. CAUSES OF STERILITY. 263 



of crossing distinct species. And as we must look at 

 the curious and complex laws governing the facility with 

 which trees can be grafted on each other as incidental 

 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 dissimilarity 

 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 endow- 

 ment ; although in the case of crossing, the difficulty 

 is as important for the endurance and stability of spe- 

 cific 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 apparently 

 depends on several distinct causes. There must some- 

 times 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 distantly 

 allied species, though the pollen-tubes protrude, they 

 do not penetrate the stigmatic surface. Again, the 



