134 GEOGRAPHICAL DISTRIBUTION. [Chap. XII. 



each other in leading to the preservation of different 

 modifications; the relation of organism to organism in 

 the struggle for life being, as I have already often re- 

 marked, the most important of all relations. Thus the 

 high importance of barriers comes into play by check- 

 ing migration; as does time for the slow process of 

 modification through natural selection. Widely-rang- 

 ing species, abounding in individuals, which have al- 

 ready triumphed over many competitors in their own 

 widely-extended homes, will have the best chance of 

 seizing on new places, when they spread into new coun- 

 tries. In their new homes they will be exposed to new 

 conditions, and will frequently undergo ■ further modi- 

 fication and improvement; and thus they will become still 

 further victorious, and will produce groups of modified 

 descendants. On this principle of inheritance with 

 modification we can understand how it is that sec- 

 tions of genera, whole genera, and even families, are 

 confined to the same areas, as is so commonly and notori- 

 ously the case. 



There is no evidence, as was remarked in the last 

 chapter, of the existence of any law of necessary de- 

 velopment. As the variability of each species is an 

 independent property, and will be taken advantage of 

 by natural selection, only so far as it profits each in- 

 dividual in its complex struggle for life, so the amount 

 of modification in different species will be no uniform 

 quantity. If a number of species, after having long 

 competed with each other in their old home, were to 

 migrate in a body into a new and afterwards isolated 

 country, they would be little liable to modification; for 

 neither migration nor isolation in themselves effect any- 

 thing. These principles come into play only by bring- 



