28 THE ORIGIN OF SPECIES 



With sterile neuter insects we have reason to believe that 

 modilications in their structure and fertility luivc been 

 slowly accumulated by natural selection, from an advan- 

 tage 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 genera in- 

 cluding 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 impossible 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, 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 



