NATURAL SELECTION 95 



Isolation, also, is an important element in the process 

 of natural selection. In a confined or isolated area, if" 

 not very large, the organic and inorganic conditions of 

 life will generally be in a great degree uniform ; so 

 that natural selection will tend to modify all the 

 individuals of a varying species throughout the area in 

 the same manner in relation to the same conditions. 

 Intercrosses, also, with the individuals of the same 

 species, which otherwise would have inhabited the 

 surrounding and differently circumstanced districts, 

 will be prevented. But isolation probably acts more 

 efficiently in checking the immigration of better 

 adapted organisms, after any physical change, such as 

 of climate or elevation of the land, etc. ; and thus new 

 places in the natural economy of the country are left 

 open for the old inhabitants to struggle for, and 

 become adapted to, through modifications in their 

 structure and constitution. Lastly, isolation, by 

 checking immigration and consequently competition, 

 will give time for any new variety to be slowly im- 

 proved ; and this may sometimes be of importance in 

 the production of new species. If, however, an 

 isolated area be very small, either from being sur- 

 rounded by barriers, or from having very peculiar 

 physical conditions, the total number of the individuals 

 supported on it will necessarily be very small ; and 

 fewness of individuals will greatly retard the produc- 

 tion 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, 



