140 THE ORIGIN OF SPECIES 



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 travel- 

 ling 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 east- 

 ern 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 naturalist, in 

 travelling, for instance, from north to south, never fails 

 to be struck by the manner in which successive groups 

 of l')eings, specifically distinct, though nearly related, re- 

 place each other. He hears, from closely allied yet dis- 

 tinct kinds of birds, notes nearly similar, and sees their 

 nests similarly constructed, 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 inhabiting Africa and Aus- 

 tralia under the same latitude. In these same plains of 

 La Plata we see the agouti and bizcacha, animals having 

 nearly the same habits as our hares and rabbits, and be- 



