Chap. IV. DIVERGENCE OF CHARACTER. 121 



tween the horizontal lines. After fourteen thousand 

 generations, six new species, marked by the letters n li 

 to z 1 4 , are supposed to have been produced. In each 

 genus, the species, which are already extremely dif- 

 ferent in character, will generally tend to produce the 

 greatest number of modified descendants ; for these will 

 have the best chance of filling 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 a long period continue trans- 

 mitting unaltered descendants; and this is shown in 

 the diagram by the dotted lines not prolonged far up- 

 wards from want of space. 



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 

 descent their predecessors and their original parent. 

 For it should be remembered that the competition will 

 generally be most severe between those forms which 

 are most nearly related to each other in habits, constitu- 

 tion, and structure. Hence all the intermediate forms 

 between the earlier and later states, that is between the 

 less and more improved state of a species, as well as 

 the original parent-species itself, will generally tend to 

 become extinct. So it probably will be with many whole 

 collateral lines of descent, which will be conquered by 

 later and improved lines of descent. If, however, the 



G 



