GEOGRAPHICAL DISTRIBUTION 397 



and this fact has led naturaHsts 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 

 haltinj^-place for emigrants ; here we have a barrier of an- 

 other kind, and as soon as this is passed we meet in the east- 

 ern 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, proceed- 

 ing 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 state- 

 ment, is the affinity of the productions of the same continent 

 or of the same sea, though the species themselves are dis- 

 tinct at dififerent points and stations. It is a law of the 

 widest generality, and every continent offers innumerable 

 instances. Nevertheless, the naturalist, 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. Pie 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 



