113 RESULT OF THE ACTION 



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 tliose 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 upward. 



But during the process of modification, represented in 

 the diagram, another of our principles, namely that of ex- 

 tinction, 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 remem- 

 bered that the competition will generally be most severe 

 between those forms which are most nearly related to each 

 other in habits, constitution and structure. Hence all the 

 intermediate forms between the earlier and later states, 

 that is between the less and more improved states of 

 the same species, as well as the original parent species 

 itself, will generally tend to become extinct. So it proba- 

 bly will be with many whole collateral linesof descent, 

 which will be conquered by later and improved lines. If, 

 however, the modified offspring of a species get into 

 some distinct country, or become quickly adapted to 

 6ome quite new station, in which offspring and progen- 

 itor do not come into competition, both may continue to 

 exist. 



If, then, our diagram be assumed to represent a consid- 

 erable amount of modification, species (A) and all the 

 earlier varieties will have become extinct, being replaced by 

 eight new species (a" to to"), and species (I) will be re- 

 placed by six («" to z") new species. 



But we may go further than this. The original species of 

 our genus were supposed to resemble each other in unequal 

 degrees, as is so generally the case in nature; species (A) being 

 more nearly related to B, C and D than to the other spe- 

 cies; and species (I) more to G, H, K, L than to the others. 



