Chap. XII.] GEOGRAPHICAL DISTRIBUTION. 133 



see the agouti and bizcacha, animals having nearly 

 the same habits as our hares and rabbits, and belonging 

 to the same order of Eodents, but they plainly display 

 an American type of structure. "We ascend the lofty 

 peaks of the Cordillera, and we find an alpine species 

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

 the 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 essentially 

 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, through- 

 out 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 in- 

 habitants 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 



