22 CAUSES OF THE STERILITY [Chap. IX 



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 with many species, 

 for a multitude are mutually quite barren. With sterile 

 neuter insects we have reason to believe that modifica- 

 tions 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 so- 

 cial community, if rendered slightly sterile when crossed 

 with some other variety, would not thus itself gain anj 

 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 through- 

 out the animal and vegetable kingdoms, we may infer 



