-Chap. IV. Results of Natural Selection. 85 



natural selection will generally act very slowly, .only at long intervals 

 of time, and only on a few of the inhabitants of the same region. I 

 further believe that these slow, intermittent results accord well 

 with what geology tells us of the rate and manner at which the in- 

 habitants of the world have changed. 



Slow though the process of selection may be, if feeble man can 

 do much by artificial selection, I can see no limit to the amount of 

 change, to the beauty and complexity of the coadaptations between 

 all organic beings, one with another and with their physical con- 

 ditions of life, which may have been effected in the long course of 

 time through nature's power of selection, that is by the survival 

 of the fittest. 



Extinction caused by Natural Selection. 



This subject will be more fully discussed in our chapter on 

 Geology ; but it must here be alluded to from being intimately con- 

 nected 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 inhabit- 

 ants ; 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 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 indefi- 

 nitely 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 indi- 

 viduals 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 

 recorded 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, 



