132 GEOGEAPHICAL DISTRIBUTION. [Chap. XII. 



farther -westward from the eastern islands of the tropical 

 parts of the Pacific, we enconnter no impassable barriers, 

 and we have innumerable islands as halting-places, or 

 continuous coasts, until, after travelling over a hemi- 

 sphere, 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 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 

 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 Pihea (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 



