396 ORIGIN OF SPECIES 



and Australia under the same latitude. On these same plains 

 of La Plata we see the agouti and bizcacha, animals having 

 nearly the same habits as our hares and rabbits, and belong- 

 ing 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 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 dn 

 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 dissimilarity of the inhabitants of different re- 

 gions 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 for- 

 mer immigrants; — and on the action of the inhabitants on 

 each other in leading to the preservation of different modifi- 

 cations; 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 selec- 

 tion. Widely-ranging species, abounding in individuals, 

 which have already triumphed over many competitors in 

 their own widely-extended homes, will have the best chance 



