OEOGBAPHICAL DISTRIBlfTION. 375 



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 

 wasformerly 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 dis- 

 tinct fauna. So that three marine faunas range northward 

 and southward in parallel lines not far from each other, 

 under corresponding climate; 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 further westward from the eastern 

 islands of the tropical parts of the Pacific, we encounter 

 no impassable barriers^ and we have innumerable islands as 

 halting-places, or continuous coasts, until, after traveling 

 over a hemisphere., we come to the shores of Africa; and 

 over this vast space we meet with no well-defined and dis- 

 tinct marine faunas. Although so fevv marine animals are 

 common to the above-named three approximate faunas of 

 Eastern and "Western America and the eastern Pacific 

 islands, yet many fishes range from the Pacific into the 

 Indian Ocean, and many shells are common to the eastern 

 islands of the Pacific and the eastern shores of Africa on 

 almost exactly opposite meridians of longitude. 



A third great fact, partly included in the foregoing 

 statement, is the affinity of the productions of the same 

 continent or of the same sea, though the species themselves 

 are distinct at different points and stations. It is a law of 

 the widest generality, and every continent offers innum- 

 erable instances. Nevertheless, the naturalist, in travel- 

 ing, for instance^ from north to south, never fails to be 

 struck by the manner in which successive groups of beings, 

 specifically distinct, though nearly related, replace each 

 other. He hears from closely allied, yet distinct kinds of 

 birds, notes nearly similar, and sees their nests similarly con- 

 structed, but not quite alike, with eggs colored in nearly the 

 same manner. The plains near the Straits of Magellan are 

 inhabited by one species of Rhea (American ostrich), and 

 northward the plains of La Plata by another species of the 

 same genus; and not by a true ostrich or emu, like those 



