Chap. xil. Geographical Distribution. 317 



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 lat. 35° with those north of 

 25°, which consequently are separated 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 inha- 

 bitants 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, &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 inha- 

 bitants of the eastern and western shores of South America are 

 very distinct, with extremely few shells, Crustacea or echinoder- 

 mata 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 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 impassable barriers ^ 

 either of land or open sea, they are almost wholly distinct. On the 

 other hand, proceeding still farther westward from the eastern 



