NATURAL SELECTION 151 



not possibly intercross, uniformity of character can be 

 retained by them under the same conditions of life, only 

 through the principle of inheritance, and through natural 

 selection which will destroy any individuals departing 

 from the proper type. If jthe conditions of life change 

 and the form undergoes modification, uniformity of char- 

 acter can be given to the modified offspring, solely by 

 natural selection preserving similar favorable variations,^. 

 Isolation, also, is an important elemert in the modifi- 

 cation of species through natural selection. In a confined ' 

 or isolated area, if not very large, the organic and in- 

 organic conditions of life will generally be almost uni- 

 form; so that natural selection will tend to modify all 

 the varying individuals of the same species in the same 

 manner. Intercrossing with the inhabitants of the sur- 

 rounding districts will, also, be thus prevented. Moritz 

 Wagner has lately published an interesting essay on this 

 subject, and has shown that the service rendered by 

 isolation in preventing crosses between newly-formed 

 varieties is probably greater even than I supposed. But 

 from reasons already assigned I can by no means agree 

 with this naturalist, that migration and isolation are 

 necessary elements for the formation of new species. 

 The importance of isolation is likewise great in prevent- 

 ing, after any physical change in the conditions, such as 

 of climate, elevation of the land, etc., the immigration of 

 better adapted organisms; and thus new places in the 

 natural economy of the district will be left open to be 

 filled up by the modification of the old inhabitants. 

 Lastly, isolation will give time for a new variety to be 

 improved at a slow rate; and this may sometimes be of 

 much importance. If, however, an isolated area be very 



