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Show 3±8 GEOGRAPHICAL DISTHIBUTION. CHAP. XI. lofty and continuous mountain-ranges, ~nd of great deserts and sometimes even of large nvers, we find differe~t productions; though as mountain-chains, deserts, &c., are not as impassable, or likely to have endured so long as the oceans separating con tin en ts, the differences are very inferior in degree to those characteristic of distinct continents. Turning to the sea, we find the same law. No two 1narine faunas are more distinct, with hardly a fish, shell, or crab in common, than those of the eastern and western shores of South and Central America; yet these great faunas are separated only by the narrow, but impassable, isthmus of Panama. Westward of the shores of America, a wide space of open ocean extends, with not an island as a halting-place for emigrants; here we have a barrier of another kind, and as soon as this is passed we n1eet in the eastern islands of the Pacific, with another and totally distinct fauna. So that here 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 wholly distinct. On the other hand, proceeding still further 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, until after travelling over a hemisphere we come to the shores of .A.frica ; a,nd over this vast space we meet with no well-defined and distinct marine faunas. Although hardly one shell, crab or fish is common to the above-named three approximate faunas of Eastern and Western America and the eastern Pacific islands, yet many fish range from the Pacific into the Indian Ocean, and many shells are common to the eastern islands of the Pacific \ CHAP. XI. GEOGRAPHICAL DISTRIBUTION. 349 and the eastern shores of Africa, on almost exactly opposite meridians of longitude. A third great fact, partly included in the foregoinO' statements, is the affinity of the productions of th: same continent or 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 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, yet clearly rel~ted, repl.ac~ each other. lie hears from closely allied, yet distmct kinds of birds, notes nearly similar, a~d sees their nests similarly constructed, but not quite ahke, 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 Pia ta by another species of the same genus ; and not by a true ostrich or emeu like those found in Africa and Australia under the 'same latitude. On these same plains of La Plata we see the agou~i and bizcacha, animals having ne~rly the same habits as our hares and rabbits and belonging to the sam~ order of Rodents, but they plainly display an Amerwan type. of structure. We ascend the lofty pea~s of the Cordillera and we find an alpine species of b1zcacha; we look to the waters, and we do not find the beaver or musk-rat, but the coypu and capybara, rodents of the American type. Innumerable other instances could be given. If vve look to the islands off the A~erican shore, however much they may differ in geologwal st~·ucture, .the inhabitants, though they may be all pecuhar speCies, are essentially American. We may look back to past ages, as shown in the last chapter, and we find American types then prevalent on |