326 THE ORIGIN OF SPECIES 



bond, throughout space and time, over the same areas of land 

 and water, independently of physical conditions. The naturalist 

 must be dull who is not led to inquire what this bond is. 



The bond is simply inheritance, that cause which alone, as far 

 as we positively know, produces organisms quite like each other, 

 or, as we see in the case of varieties, nearly alike. The dissimilarity 

 of the inhabitants of different regions may be attributed to modi- 

 fication through variation and natural selection, and probably in 

 a subordinate degree to the definite influence of different physical 

 conditions. The degrees of dissimilarity will depend on the migra- 

 tion of the more dominant forms of life from one region into an- 

 other having been more or less effectually prevented, at periods 

 more or less remote — on the nature and number of the former 

 immigrants — and on the action of the inhabitants on each other 

 in leading to the preservation of different modifications; the rela- 

 tion 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 

 individuals, which have already triumphed over many competitors 

 in their own widely extended homes, will have the best chance of 

 seizing on new places, when they spread out into new countries. 

 In their new homes they will be exposed to new conditions, and 

 will frequently 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 understand 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 vari- 

 ability of each species is an independent property, and will be 

 taken advantage of by natural selection, only so far as it profits 

 each individual 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 after- 

 ward 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 bringing organisms into 

 new relations with each other and in a lesser degree with the sur- 

 rounding physical conditions. As we have seen in the last chapter 



