298 GEOGRAPHICAL DISTRIBUTION. [CHAP. XII. 



clermata 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 corre- 

 sponding 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 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 travelling over a hemisphere, we come to the shores of 

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

 and distinct marine faunas. Although so few 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 innumerable instances. Nevertheless the 

 naturalists, in travelling, 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 constructed, 

 but not quite alike, with eggs coloured 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 inhabiting Africa and Australia under 

 the same latitude. On these same plains of La Plata we see 

 the agouti and bizcacha, animals having nearly the same habits 

 as our hares and rabbits, and belonging to the same order of 

 Rodents, but they plainly display an American type of structure. 

 We ascend the lofty peaks of the Cordillera, and we nd an alpine 

 epecies of bizcacha ; we look to the waters, and we do not find the 



