134 GEOGRAPHICAL DISTRIBUTION. [Chap. XIL 



of the inhabitants on 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 remarked, the most important of 

 all relations. Thus the high importance of barriers 

 comes into play by checking migration ; as does time 

 for the slow process of modification through natural 

 selection. Widely-ranging species, abounding in in- 

 dividuals, which have already triumphed over many 

 competitors in their own widely-extended homes, will 

 have the best chanee of seizing on new places, when 

 they spread into new countries. In their new homes 

 they will be exposed to new conditions, and will fre- 

 quently undergo further modification 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 under- 

 stand how it is that sections of genera, whole genera, 

 and even families, are confined to the same areas, as is 

 so commonly and notoriously the case. 



There is no evidence, as was remarked in the last 

 chapter, of the existence of any law of necessary 

 development. 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 

 anything. These principles come into play only by 



