Chap. XII.] GEOGRAPHICAL DISTRIBUTION. I33 



agouti and bizcacha, animals having nearly the same 

 habits as our hares and rabbits, and belonging to the 

 same order of Rodents, but they plainly display an 

 American type of structure. "We ascend the lofty peaks 

 of the Cordillera, and we find an alpine species of bizca- 

 cha; we look to the waters, and we do not find the 

 beaver or musk-rat, but the coypu and capybara, ro- 

 dents of the S. American type. Innumerable other 

 instances could be given. If we look to the islands ofE 

 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 natu- 

 raUst must be dull who is not led to enquire 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 varie- 

 ties, 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 dif- 

 ferent physical conditions. The degrees of dissimilar- 

 ity 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 



