NATURAL SELECTION 171 



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 n cessarily 

 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 progenitor. 

 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, 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 be- 

 come extinct. So it probably will be with many whole 

 collateral lines of 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 some quite new station, 

 in which offspring and progenitor 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, being replaced 

 by eight new species (a'* to m'*): and species (I) will 

 be replaced by six (w'* to z'*) new species. 



Bat we may go further than this. The original spe- 

 cies 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 





