Chap. IV.] EXTINCTION BY NATUEAL SELECTION. 133 



being intimately connected with natural selection. 

 Natural selection acts solely through the preservation of 

 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 

 favoured forms increase in number, so, generally, will 

 the less favoured 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 fluctuations in the nature of the seasons, 

 or from a temporary increase in the number of its 

 enemies. But we may go further than this ; for, as new 

 forms are produced, 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 

 numerous in individuals have the best chance of pro- 

 ducing 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 re- 

 corded 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 commoner species 



From these several considerations I think it inevitably 

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



