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 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 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.