Chap. XII]. GEOGRAPHICAL DISTRIBUTION. I31 



northern temperate forms, as there now is for the strict- 

 ly arctic productions. We see the same fact in the 

 great diflEerence between the inhabitants of Australia, 

 Africa, and South America under the same latitude; 

 for these countries are almost as much isolated from 

 each other as is possible. On each continent, also, we 

 see the same fact; for on the opposite sides of lofty 

 and continuous mountain-ranges, of great deserts and 

 even of large rivers, we find different productions; 

 though as mountain-chains, deserts, &c., are not as 

 impassable, or likely to have endured so long, as the 

 oceans separating continents, the differences are very 

 inferior in degree to those characteristic of distinct con- 

 tinents. 



Turning to the sea, we find the same law. The 

 marine inhabitants of the eastern and western shores 

 of South America are very distinct, with extremely few 

 shells, Crustacea, or echinodermata in common; but Dr. 

 Giinther has recently shown that about thirty per cent, 

 of the fishes are the same on the opposite sides of the 

 isthmus of Panama; and this fact has led naturalists 

 to believe that the isthmus was formerly open. West- 

 ward of the shores of America, a wide space of open 

 ocean extends, with not an island as a halting-place for 

 emigrants; here we have a barrier of another kind, and 

 as soon as this is passed we meet in the eastern islands 

 of the Pacific with another and totally distinct fauna. 

 So that three marine faunas range far northward and 

 southward in parallel lines not far from each other, 

 under corresponding climates; but from being sepa- 

 rated from each other by impassable barriers, either of 

 land or open sea, they are almost wholly distinct. On 

 the other hand, proceeding still farther westward from 

 34 



