04 - Natural Selection
04-11 - Divergence of Character
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.
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 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.