318 GEOGRAPHICAL DISTRIBUTION. [Chap. XII. 



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 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 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 con- 

 tinent 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. 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 Ehea (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 ascend 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 musk-rat, but the 'coypu and capybara, rodents 

 of the S. 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 

 essentially 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 

 bond, throughout space and time, over the same areas of land and 

 water, independently of physical conditions. The naturalist must 

 be dull, who is not led to inquire what this bond is. 



The bond is simply inheritance, that cause which alone, as fcr as 



