110 NATURAL SELECTION. Chap. IV. 



have any means of knowing that any one region has 

 as yet got its maximum of species. Probably no region 

 is as yet 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 nume- 

 rous in individuals will have the best chance of pro- 

 ducing 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 gene- 

 rally press hardest on its nearest kindred, and tend to 

 exterminate them. We see the same process of exter- 

 mination amongst our domesticated productions, through 

 the selection of improved forms by man. Many curious 



