GEOGRAPHICAL DISTRIBUTION. 351 



America, between latitudes 25 and 35 degrees, we shall 

 find parts extremely similar in all their conditions, yet it 

 would not be possible to point out three faunas and floras 

 more utterly dissimilar. Or, again, we may compare the 

 productions of South America south of latitude 35 degrees 

 with those north of 25 degrees, which consequently are sep- 

 arated by a space of ten degrees of latitude, and are 

 exposed to considerably different conditions ; yet they are 

 incomparably more closely related to each other than they 

 are to the productions of Australia or Africa under nearly 

 the same climate. Analogous facts could be given with 

 respect to the inhabitants of the sea. 



A second great fact which strikes us in our general 

 review is, that barriers of any kind, or obstacles to free 

 migration, are related in a close and important manner 

 to the differences between the productions of various 

 regions. We see this in the great difference in nearly 

 all the terrestrial productions of the New and Old Worlds, 

 excepting in the northern parts, where the land almost 

 joins, and where, under a slightly different climate, there 

 might have been free migration 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, 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, Crus- 

 tacea, 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 emigrants ; here we have a barrier of 

 another kind, and as soon as this is passed we meet in the 



