XV.] THE OPERATION OF NATURAL SELECTION. 341 



NOTE B. 



At the time when the foregoing chapter was written, I was The 

 not aware of the fact mentioned by Mi-. Lewes in an article on '^o^^^ti^ 

 Darwin's theory in the Fortnightly Review, that the domestic Guinea-pig 

 and the wild Guinea pig are not fertile together. Mutual fer- ^reed* 

 tility is the usually accepted test of identity of species ; so that together. 

 by this test these two are distinct species. 



