Chap. IV.] NATUBAL SELECTION. 145 



In a large genus it is probable that more than one 

 species would vary. In the diagram I have assumed 

 that a second species (I) has produced, by analogous steps, 

 after ten thousand generations, either two well-marked 

 varieties (w 10 and s 10 ) or two species, according to the 

 amount of change supposed to be represented between 

 the horizontal lines. After fourteen thousand genera- 

 tions, six new species, marked by the letters n u to z u , 

 are supposed to have been produced. In any genus, 

 the species which are already very different in character 

 from each other, will generally tend to produce the 

 greatest number of modified descendants ; for these will 

 have the best chance of seizing on new and widely 

 different places in the polity of nature : hence in the 

 diagram I have chosen the extreme species (A), and the 

 nearly extreme species (I), as those which have largely 

 varied, and have given rise to new varieties and species. 

 The other nine species (marked by capital letters) of our 

 original genus, may for long but unequal periods continue 

 to transmit unaltered descendants; and this is shown 

 in the diagram by the dotted lines unequally prolonged 

 upwards. 



But during the process of modification, represented 

 in the diagram, another of our principles, namely that of 

 extinction, will have played an important part. As in 

 each fully stocked country natural selection necessarily 

 acts by the selected form having some advantage in the 

 struggle for life over other forms, there will be a constant 

 tendency in the improved descendants of any one species 

 to supplant and exterminate in each stage of descent 

 their predecessors and their original progenitor. For it 

 should be remembered that the competition will gene- 

 rally be most severe between those forms which are most 



