314 ORIGIN OF SPECIES 



But it would be superfluous to discuss this question in de- 

 tail; for with plants we have conclusive evidence that the 

 sterility of crossed species must be due to some principle, 

 quite independent of natural selection. Both Gartner and 

 Kolreuter have proved that in genera including numerous 

 species, a series can be formed from species which when 

 crossed yield fewer and fewer seeds, to species which never 

 produce a single seed, but yet are affected by the pollen of 

 certain other species, for the germen swells. It is here mani- 

 festly impossible to select the more sterile individuals, which 

 have already ceased to yield seeds ; so that this acme of ster- 

 ility, when the germen alone is affected, cannot have been 

 gained through selection; and from the laws governing the 

 various grades of sterility being so uniform throughout the 

 animal and vegetable kingdoms, we may infer that the cause, 

 whatever it may be, is the same or nearly the same in all 

 cases. 



We will now look a little closer at the probable nature of 

 the differences between species which induce sterility in first 

 crosses and in hybrids. In the case of first crosses, the 

 greater or less difficulty in effecting an union and in obtain- 

 ing offspring apparently 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 the pollen of 

 one species is placed on the stigma of a distantly allied spe- 

 cies, though the pollen-tubes protrude, 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 Thu- 

 ret'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 developed, and then perish 

 at an early period. This latter alternative has not been suf- 

 ficiently attended to; but I believe, from observations com- 

 municated to me by Mr. Hewitt, who has had great experi- 

 ence in hybridising pheasants and fowls, that the early death 

 of the embryo is a very frequent cause of sterility in first 



