348 GEOGRAPHICAL DISTRIBUTION. Chap. XI. 



lofty and continuous mountain-ranges, and of great 

 deserts, and sometimes 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 cha- 

 racteristic of distinct continents. 



Turning to the sea, we find the same law. No two 

 marine faunas are more distinct, with hardly a fish, 

 shell, or crab in common, than those of the eastern and 

 western shores of South and Central America; yet 

 these great faunas are separated only by the narrow, 

 but impassable, isthmus of Panama. Westward 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 here 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 wholly distinct. On the 

 other hand, proceeding still further westward from the 

 eastern islands of the tropical parts of the Pacific, we 

 encounter no impassable barriers, and we have innu- 

 merable islands as halting-places, until after travelling 

 over a hemisphere we come to the shores of Africa ; 

 and over this vast space we meet with no well-defined 

 and distinct marine faunas. Although hardly one shell, 

 crab or fish is common to the above-named three 

 approximate faunas of Eastern and Western America 

 and the eastern Pacific islands, yet many fish range 

 from the Pacific into the Indian Ocean, and many 

 shells are common to the eastern islands of the Pacific 



