Chap. IV.J NATURAL 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^" and 2") or two species, according 

 to the amount of change supposed to be represented be- 

 tween the horizontal lines. After fourteen thousand 

 generations, six new species, marked by the letters n^* to 

 z", 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 dif- 

 ferent places in the polity of nature: hence in the dia- 

 gram 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 con- 

 tinue to transmit unaltered descendants; and this is 

 shown in the diagram by the dotted lines unequally pro- 

 longed 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 con- 

 stant tendency in the improved descendants of any one 

 species to supplant and exterminate in each stage of de- 

 scent their predecessors and their original progenitor. 

 For it should be remembered that the competition will 

 generally be most severe between those forms which are 



