DISTRIBUTION. 403 



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 Bodents, 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 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, though 

 they may be all peculiar species, are essentially American." 



What is the generalization implied by these two groups of 

 facts? On the one hand, we have similarly-conditioned, and 

 sometimes nearly-adjacent, areas, occupied by quite different 

 Faunas. On the one hand, we have areas remote from one 

 another in latitude, and contrasted in soil as well as climate, 

 occupied by closely-allied Faunas. Clearly then, as like or- 

 ganisms are not universally, or even generally, found in like 

 habitats, nor very unlike organisms in very unlike habitats, 

 there is no manifest pre-determined adaptation of the organ- 

 isms to the habitats. The organisms do no occur in such 

 and such places solely because they are either specially fit for 

 those places, or more fit for them than all other organisms. 



The induction under which these facts come, and which 

 unites them with various other facts, is a totally-different one. 

 When we see that the similar areas peopled by dissimilar 

 forms, are those between which there are impassable barriers ; 

 while the dissimilar areas peopled by similar forms, are those 

 between which there are no such barriers ; we are at once re- 

 minded of the general truth exemplified in the last section 

 the truth that each species of organism tends ever to expand 

 its sphere of existence to intrude on other areas, other 

 modes of life, other media. And we are shown that through 

 these perpetually-recurring attempts to thrust itself into every 



