22 CAUSES OF THE STERILITY [Chap. IX. 



the survival of those indi\ddnals which happened to 

 be endowed in a slightly higher degree with mutual 

 infertility, and which thus approached by one small 

 step towards absolute sterility ? Yet an advance of 

 this kind, if the theory of natural selection be brought to 

 bear, must have incessantly occurred ^^ith many species, 

 for a multitude are mutually quite barren. With 

 sterile neuter insects we have reason to believe that 

 modifications in their structure cind 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 com- 

 munities 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 

 felms 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 Ivolreuter have proved that in genera 

 including numerous species, a series can be formed from 

 species which wdien 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 gernien swells. It is here manifestly impossible to 

 select tlie 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 through- 



