Chap. XIX. HYBRIDISM. 171 



Yet an advance of this kind, if the theory of natural selection 

 be brought to bear, must have incessantly occurred with 

 many species, for a multitude are mutually quite barren. 

 With sterile neuter insects we have reason to believe that 

 modifications in their structure and fertility have been slowly 

 accumulated by natural selection, from an advantage having 

 been thus indirectly given to the community to which they 

 belonged over other communities of the same species ; but an 

 individual animal not belonging to a social community, if 

 rendered slightly sterile when crossed with some other variety, 

 would not thus itself gain any advantage or indirectly give 

 any advantage to the other individuals of the same variety, 

 thus leading to their preservation. 



But it would be superfluous to discuss this question in 

 detail ; 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 general 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 manifestly im- 

 possible to select the more sterile individuals, which have 

 already ceased to yield seeds ; so that this acme of sterility, 

 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, what- 

 ever it may be, is the same or nearly the same in all cases. 



As species have not been rendered mutually infertile 

 through the accumulative action of natural selection, and as 

 we may safely conclude, from the previous as well as from 

 other and more general considerations, that they have not 

 been endowed through an act of creation with this quality, 

 we must infer that it has arisen incidentally during their slow 

 formation in connection with other and unknown changes in 

 their organisation. By a quality arising incidentally, I refer 

 to such cases as different species of animals and plants being 

 differently affected by poisons to which they are not naturally 



