Chap. XII.] GEOGRAPHICAL DISTRIBUTION. 131 



for the northern temperate forms, as there now is for 

 the strictly arctic productions. 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, &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 

 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. Gunther 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 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 separated from each other by impass- 

 able barriers, either of land or open sea, they are almost 

 wholly distinct. On the other hand, proceeding still 



