GEOGRAPHICAL DISTRIBUTION. 35& 



other instances could be giveD II we look to the islands 

 off the American shore, however much they may differ in 

 geological structure, tk- inhabitants are essentially American, 

 though they mav L^ all peculiar species. We may look back 

 to past age? <*3 shown in the last chapter, and we find 

 America**. «ypes then prevailing on the American continent 

 and ii clie American seas. We see in these facts some deep 

 or 6 aiiic bond, throughout space and time, over the same 

 areas of land and water, independently of physical condi- 

 tions. 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 modification through variation and nat- 

 ural selection, and probably in a subordinate degree to the 

 definite influence of different physical conditions. The de- 

 grees of dissimilarity will depend on the migration of the 

 itore 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 for- 

 mer immigrants — and on the action of the inhabitants on 

 each other in leading to the preservation of different modi- 

 fications ; the relation of organism to organism in the strug- 

 gle 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 individ- 

 uals, 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 modifi- 

 cation 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 w r as remarked in the last chapter, 

 of the existence of any law of necessary development. As 

 the variability of each species is an independent property, 



