But we may go further than this; for as new forms are produced, unless we admit that specific forms can go on indefinitely increasing in number, many old forms must become extinct.

That the number of specific forms has not indefinitely increased, geology plainly tells us; and we shall presently attempt to show why it is that the number of species throughout the world has not become immeasurably great.
We have seen that the species which are most numerous in individuals have the best chance of producing favourable variations within any given period.

We have evidence of this, in the facts stated in the second chapter, showing that it is the common and diffused or dominant species which offer the greatest number of recorded varieties.

Hence, rare species will be less quickly modified or improved within any given period; they will consequently be beaten in the race for life by the modified and improved descendants of the commoner species.
From these several considerations I think it inevitably follows, that as new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct.
The forms which stand in closest competition with those undergoing modification and improvement, will naturally suffer most.

And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms,--varieties of the same species, and species of the same genus or related genera,--which, from having nearly the same structure, constitution and habits, generally come into the severest competition with each other.