X32 GEOGRAPHICAL DISTRIBUTION. TChap. XII. 



the eastern islands of the tropical parts of the Pacific, 

 we encounter no impassable barriers, and we have in- 

 numerable 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 fau- 

 nas. Although so few marine animals are common 

 to the above-named three approximate faunas of East- 

 ern and Western America and the Eastern Pacific is- 

 lands, yet many fishes range from the Pacific into the 

 Indian Ocean, and many shells are common to the east- 

 ern islands of the Pacific and the eastern shores of 

 Africa on almost exactly opposite meridians of longi- 

 tude. 



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 them- 

 selves are distinct at different points and stations. It 

 is a law of the widest generality, and every continent 

 offers innumerable instances. Nevertheless the natural- 

 ist, in travelling, for instance, from north to south, 

 never fails to be struck by the manner in which suc- 

 cessive groups of beings, specifically distinct, though 

 nearly related, replace each other. He hears from close- 

 ly 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 Ehea (American ostrich) and north- 

 ward 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 lati- 

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



