100 ON THE ORIGIN OF SPECIES 



fully stocked, for at the Cape of Good Hope, where more 

 species of plants are crowded together than in any other 

 quarter of the world, some foreign plants have become 

 naturalised, without causing, as far as we know, the 

 extinction of any natives. 



Furthermore, the species which are most numerous 

 in individuals will have the best chance of producing 

 within any given period favourable variations. We 

 have evidence of this, in the facts given in the second 

 chapter, showing that it is the common species which 

 afford the greatest number of recorded varieties, or 

 incipient species. Hence, rare species will be less 

 quickly modified or improved within any given period, 

 and they will consequently be beaten in the race for 

 life by the modified descendants of the commoner 

 species. 



From these several considerations I think it in- 

 evitably 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 under- 

 going 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 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 gener- 

 ally press hardest on its nearest kindred, and tend to 

 exterminate them. We see the same process of ex- 

 termination amongst our domesticated productions, 

 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 ' 



