108 EXTIXCTIOX BY NATURAL SELECTION. CiiAP. IV. 



That natural sc^lcction acts "vvith extreme slowness I fully 

 iidniit. The result depends on there beings places in the polity 

 of Nature, wliich can be better filled through some of the in- 

 habitants of the country undergoinf^ modifications of some kind. 

 The existence of such places will often depend on physical 

 changes, which are generally very slow, and on the immigra- 

 tion of better-adapted forms being checked. But the effects 

 of natural selection will probably still oftener depend on some 

 few of the inhabitants becoming slowly modified ; the mutual 

 relations of the other inhabitants being thus disturbed. Al- 

 though all the individuals of the same species differ more or 

 less from each other, differences of the right nature, better 

 adapted to the then existing conditions, may not soon occur. 

 The results will often be greatly retarded by free intercrossing. 

 Many will exclaim that these several causes are amply suffi- 

 cient to neutralize the power of natural selection. I do not 

 believe so. But I do believe that natural selection generally 

 acts very slowly in effecting changes, 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 of natural 

 selection accord perfectly 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 infinite complexity of the 

 coadaptations between all organic beings, one with another and 

 with their physical conditions of life, "which may be effected in 

 the long course of time by Nature's power of selection, or 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 be here alluded to from being inti- 

 mately 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 ratio of increase of all organic beings, each area is 

 already stocked with the full number of its existing inhabit- 

 ants, and as most areas are already stocked with a great diver 

 sity of forms, it follows that, as each selected and favored form 

 increases in number, so generally will the less favored forms 



