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." 



