GEOGRAPHICAL DISTRIBUTION 313 



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. 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, or continuous coasts, 

 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 

 and the eastern shores of Africa, on almost exactly 

 opposite meridians of longitude. 



A third great fact, partly included in the foregoing 

 statements, is the affinity of the productions of the 

 same continent or sea, though the species themselves 

 are distinct at different points and stations. It ia a 

 law of the widest generality, and every continent offer* 

 innumerable instances. Nevertheless the naturalist 



