CHAP. XII.] GEOGRAPHICAL DISTRIBUTION. 299 



beaver or musk-rat, but the coypu and capybara, rodents of the 

 S. American type. Innumerable other instances could be given. 

 If we look to the islands off the American shore, however much 

 they may differ in geological structure, the inhabitants are essen- 

 tially American, though they may be all peculiar species. We 

 may look back to past ages, as shown in the last chaper, and we 

 find American types then prevailing on the American continent 

 and in the American seas. We see in these facts some deep 

 organic 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 dis- 

 similarity of the inhabitants of different regions may be attributed 

 to modification 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 migration of the more dominant forms of life from 

 one region into another 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 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 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 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 

 variability 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 



