CHAP. IV.] RESULTS OF NATURAL SELECTION. 79 



modification of some of its existing inhabitants. The occurrence 

 of such places will often depend on physical changes, which 

 generally take place very slowly, and on the immigration of better 

 adapted forms being prevented. As some few of the old inhabi- 

 tants become modified, the mutual relations of others will often 

 be disturbed ; and this will create new places, ready to be filled 

 up by better adapted 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 organisation 

 might occur. The result would often be greatly retarded by free 

 intercrossing. Many will exclaim that these several causes are 

 amply sufficient to neutralise 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 affected 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 

 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 rate 

 of increase 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. Barity, 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 increas- 

 ing 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 uresently attemot to show why it 



