GEOGRAPHICAL DISTRIBUTION 141 



longing to the same order of Kodents, but they plainly 

 display an American type of stracture. We ascend the 

 lofty peaks of the Cordillera, and we find an alpine spe- 

 cies of bizcacha; we look to the waters, and we do not 

 find the beaver or musk-rat, but the coypu and capy- 

 bara, rodents of the South 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 

 chapter, 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 vari- 

 eties, nearly alike. The dissimilarity 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 de- 

 pend 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 



