•376 aEOQRAPHIOAL DISTRIBTTTION. 



inhabiting Africa 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 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 bizcacha; we look to the 

 waters, and we do not find the beaver or muskrat, but the 

 coypu and capybara, 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 varieties, nearly 

 alike. The dissimilarity of the inhabitants of different 

 regions may be attributed to modification through variation 

 and tlatural 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 of the inhab- 

 itants on each other in leading to the preservation of differ- 

 ent modifications; the relation of organism to organism in 

 the struggle for life being, as I have already often re- 

 marked, 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 modifica- 

 tion through natural selection. Widely-ranging species, 

 abounding in individuals, which have already triumphed 

 over many competitors in their own widely-extended homes, 

 will have the best chance of seizing on new places, when 



