110 ON THE ORIGIN OF SPECIES 



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 

 to transmit unaltered descendants ; and this is shown 

 in the diagram by the dotted lines not prolonged far 

 upwards 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 neces- 

 sarily 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 

 parent. For it should be remembered that the com- 

 petition 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 

 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 modified offspring of 

 a species get into some distinct country, or become 

 quickly adapted to some quite new station, in which 

 child and parent do not come into competition, both 

 may continue to exist. 



If then our diagram be assumed to represent a con- 

 siderable amount of modification, species (A) and all 

 the earlier varieties will have become extinct, having 

 been replaced by eight new species (a 14 to m 14 ) ; and 

 (I) will have been replaced by six (n u to # 14 ) new 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, 



