NATURAL SELECTION 85 



forms; but all this will take place very slowly. Although all the 

 individuals of the same species 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 be- 

 tween 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 chapter on Geol- 

 ogy; 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 in- 

 crease 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 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 pres- 

 ently attempt to show why it is that the number of species through- 

 out 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 favorable variations 

 within any given period. We have evidence of this, in the facts 



