94 EXTINCTION BY NATURAL SELECTION. 



differ in some slight degree from each other, it would often 

 be long before differences of the right nature in various 

 parts of the organization might occur. The result would 

 often be greatly retarded by free intercrossing. Many will 

 exclaim that these several causes are amply sufficient to 

 neutralize the power of natural selection. I do not believe 

 so. But I do believe that 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 inhabitants 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 conditions 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 chap- 

 ter on Geology ; but it must here be alluded to from being 

 intimately connected with natural selection. Natural selec- 

 tion 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 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 extinc- 

 tion, 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 

 $reat 



