NATURAL SELECTION 157 



variations in some way advantageous, which consequently 

 endure. Owing to the high geometrical rate of increase 

 of all organic beings, each area is already fully stocked 

 with inhabitants; and it follows from this, that as the 

 favored forms increase in number, so, generally, will the 

 less favored decrease and become rare. Earity, as geol- 

 ogy 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 

 fluctuations 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 presently 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 numer- 

 ous in individuals have the best chance of producing 

 favorable variations within any given period. We have 

 evidence of this, in the facts stated in the second chap- 

 ter, showing that it is the common and diffused or domi- 

 nant species which offer the greatest number of recorded 

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

 tied or improved within any given period; they will con- 

 sequently be beaten in the race for life by the modified 

 and improved descendants of the commoner species. 



From these several considerations I think it inevitably 

 follows, that as new species in the course of time are 

 formed through natural selection, others will beconie rarer 



