324 THE ORIGIN OF SPECIES 



out three faunas and floras more utterly dissimilar. Or, again, 

 we may compare the productions of South America south of lati- 

 tude 35 degrees with those north of 25 degrees, which conse- 

 quently 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 in- 

 habitants 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 re- 

 lated in a close and important manner to the differences between 

 the productions of various regions. We see this in the great differ- 

 ence 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 cHmate, 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 pos- 

 sible. 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 con- 

 tinents, the differences are very inferior in degree to those char- 

 acteristic of distinct continents. 



Turning to the sea, we find the same law. The marine inhabit- 

 ants 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 northward and southward 

 in parallel lines not far from each other, under corresponding cli- 

 mate; but from being separated from each other by impassable 



