GEOGRAPHIC AI DISTRIBUTION 139 



ate forms, as there now is for the strictly arctic produc- 

 tions. We see the same fact in the great difference 

 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, etc., 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 

 continents. 



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. Westward 

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

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

 grants; 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 separated 

 from each other by impassable barriers, either of land or 

 open sea, they are almost wholly distinct. On the other 

 hand, proceeding still further westward from the eastern 



