NATURAL SELECTION 1503 



consequently endure. Owing to the high geometrical 

 rate of increase of all organic beings, each area is 

 already fully stocked with inhabitants; and it fol- 

 lows from this, that as the favored forms increase in 

 number, so, generally, will the less favored decrease 

 and become rare. Rarity, as geology tells us, is the 

 precursor to extinction. We can see that any form 

 which is represented by few individuals will run a 

 good chance of utter extinction, during great fluc- 

 tuations in the nature of the seasons, or from a tem- 

 porary increase in the number of its enemies. But we 

 may go further than this; for, as new forms are pro- 

 duced, 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 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 nu- 

 merous in individuals have the best chance of pro- 

 ducing favorable variations within any given period. 

 It is the common and diffused or dominant species 

 which offer the greatest number of recorded vari- 

 eties. Hence, rare species will be less quickly modi- 

 fied 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 inevi- 

 tably follows that as new species in the course of time 

 are formed through natural selection, others will be- 



