Chap. IV.] EXTINCTION BY NATURAL SELECTION. 133 



being intimately connected with natural selection. 

 Natural selection acts solely through the preservation of 

 variations in some way advantageous, which consequent- 

 ly endure. Owing to the high geometrical rate of in- 

 crease 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. Earity, 

 as geology tells us, is the precursor to extinction. We 

 can see that any form which is represented by few in- 

 dividuals 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 ene- 

 mies. 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 spe- 

 cific forms has not indefinitely increased, geology plainly 

 tells us; and we shall presently attempt to show why it 

 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 

 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 modi- 

 fied 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 



