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 a 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.