Chap. IV. TO NATURAL SELECTION. 105 



tions in their structure and constitution. Lastly, isola- 

 tion, by checking immigration and consequently com- 

 petition, will give time for any new variety to be slowly 

 improved ; and this may sometimes be of importance in 

 the production of new species. If, however, an isolated 

 area be very small, either from being surrounded by 

 barriers, or from having very peculiar physical condi- 

 tions, the total number of the individuals supported on 

 it w T ill necessarily be very small ; and fewness of indivi- 

 duals will greatly retard the production of new species 

 through natural selection, by decreasing the chance of 

 the appearance of favourable variations. 



If we turn to nature to test the truth of these re- 

 marks, and look at any small isolated area, such as an 

 oceanic island, although the total number of the species 

 inhabiting it, will be found to be small, as we shall see 

 in our chapter on geographical distribution; yet of 

 these species a very large proportion are endemic, — that 

 is, have been produced there, and nowhere else. Hence 

 an oceanic island at first sight seems to have been 

 highly favourable for the production of new species. 

 But we may thus greatly deceive ourselves, for to ascer- 

 tain whether a small isolated area, or a large open area 

 like a continent, has been most favourable for the produc- 

 tion of new organic forms, we ought to make the compari- 

 son within equal times; and this we are incapable of doing. 



Although I do not doubt that isolation is of consider- 

 able importance in the production of new species, on 

 the whole I am inclined to believe that largeness of 

 area is of more importance, more especially in the 

 production of species, which will prove capable of 

 enduring for a long period, and of spreading widely. 

 Throughout a great and open area, not only will there 

 be a better chance of favourable variations arising from 

 the large number of individuals of the same species 



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