GEOGRAPHICAL DISTRIBUTION 325 



barriers, either of land or open sea, they are almost wholly distinct. 

 On the other hand, proceeding still farther westward from the east- 

 ern islands of the tropical parts of the Pacific, we encounter no im- 

 passable barriers, and we have innumerable islands as halting- 

 places, or continuous coasts, until, after travelHng over a hemis- 

 phere, we come to the shores of Africa; and over this vast space 

 we meet with no well-defined and distinct marine faunas. Al- 

 though 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 nat- 

 uralist, 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 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 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 as- 

 cend the lofty peaks of the Cordillera, and we find an alpine 

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

 beaver or muskrat, but the coypu and capybara, rodents of the 

 South American type. Innumerable other instances could be given. 

 If we look to the islands off the American shore, however much 

 they may differ in geological structure, the inhabitants are essen- 

 tially American, though they may be all peculiar species. We may 

 look back to past ages, as shown in the last chapter, and we find 

 American types then prevailing on the American continent and 

 in the American seas. We see in these facts some deep organic 



