348 
GEOGEAPHICAL DISTEIBTJTION. 
Chap. XL 
lofty and continuous mountain-ranges, and of great 
deserts, and sometimes even of large rivers, we find 
different productions; tliougli as mountain-cliains, 
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, 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 
