Page - 397 - in The Origin of Species

GEOGRAPHICAL DISTRIBUTION 397 and this fact has led naturaHsts to believe that the isthmus was formerly open. Westward of the shores of America, a wide space of open ocean extends, with not an island as a haltinj^-place for emigrants ; here we have a barrier of an- other kind, and as soon as this is passed we meet in the east- ern islands of the Pacific with another and totally distinct fauna. So that 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 almost wholly distinct. On the other hand, proceed- ing still farther 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, 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 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 state- ment, is the affinity of the productions of the same continent or of the same sea, though the species themselves are dis- tinct at dififerent 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, though nearly related, replace each other. Pie 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 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