Seite - 121 - in The Origin of Species

EXTINCTION CAUSED BY NATURAL SELECTION 121 nitely increasing in number,many old forms must become ex- tinct. That the number of specific forms has not indefinitely increased, geology plainly tells us; and we shall presently at- tempt to show why it is that the number of species through- out 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 com- moner species. From these several considerations I think it inevitably fol- lows, 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 com- petition with those undergoing modification and improve- ment, 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 of related genera,—which, from having nearly the same structure, constitution, and habits, generally come into the severest competition with each other ; consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them. We see the same process of extermination amongst our domesticated pro- ductions, through the selection of improved forms by man. Many curious instances could be given showing how quickly new breeds of cattle, sheep, and other animals, and varieties of flowers, take the place of older and inferior kinds. In Yorkshire, it is historically known that the ancient black cattle were displaced by the long-horns, and that these "were swept away by the short-horns" (I quote the words of an agricultural writer) "as if by some murderous pestilence."