126 DYNAMICAL GEOLOGY. 



proach, and therefore the fauna of Africa and South 

 America remain very different even to Cape Colony and 

 Fuegia. 



Subdivisions. Continental faunas and floras are again 

 subdivided in longitude, more or less completely, by bar- 

 riers in the form of north and south mountain-chains. 

 Thus the fauna and flora of the United States are sub- 

 divided by the Rocky Mountain and Appalachian chain 

 into three sub-faunas and floras, an Atlantic slope, a 

 Mississippi basin, and a Pacific slope. The difference 

 between these is strictly in proportion to the impassable- 

 ness of the harriers. Thus, between the Atlantic slope 

 and the Mississippi basin the difference is very small, 

 because the Appalachian chain is low and may be over- 

 passed ; but the Pacific slope fauna and flora are almost 

 wholly peculiar. Almost the only exceptions are strong- 

 winged birds, like the turtle-dove, the turkey-vulture, the 

 large blue heron, etc. In many cases the species are very 

 similar and yet different. The meadow-lark and the yel- 

 low-hammer are examples. Similarly the Ural Mountains 

 separate a European from a northern Asiatic fauna and 

 flora. These subdivisions are perhaps more marked in 

 case of plants than animals. The spread of plants is pas- 

 sive (dispersal), the spread of higher animals also by 

 migration. 



Special Cases. Isolated islands, and in proportion to 

 the degree of their isolation, have peculiar species. We 

 shall mention only a few cases as examples of a general 

 law. 



Australia is undoubtedly the most striking case of all. 

 The trees of this isolated continent are so different from 

 those of the rest of the world that the whole aspect of 

 field and forest is peculiar and strange. The animals are 

 not only all different in species, but the genera and fami- 

 lies and even many orders are peculiar. Of two hundred 

 species of mammals, nearly all belong to a distinct sub- 



