20 



Through the study of geographical ecology we at- 

 tempt to understand how organisms are distributed 

 over the world, and what forces brought about this 

 distribution. Most organisms are of restricted dis- 

 tribution — only a few groups of animals, notably 

 cyprinid fish, frogs of the genus Rana, colubrid 

 snakes, passerine birds, rodents, and man, can be 

 called even nearly cosmopolitan. In order to under- 

 stand existing distributions, we must consider the 

 histories of species, where they originated, how they 

 got where they are now, and why they are not found 

 in other parts of the world. This takes us into cli- 

 matology, zoogeography, and palaeontology. In this 

 section we will be dealing with the broader problems 

 of the composition, characterics, and origin of geo- 

 graphic units in the distribution of organisms, rather 

 than with the local units of communities and habitats 

 discussed in Section II. These distributional units, 

 like communities, are distinctive groups of organisms 

 spatially distinct from other groups, but they do not 

 always show functional integration as do communities 

 and are sometimes characterized by genera, families, 

 and orders rather than species. We will begin our 

 discussion with zoological realms, areas defined 

 largely by the past and present relations of the larger 

 land masses to each other. Then we will proceed to 

 a consideration of how the physiographic peculiarities 

 of those land masses affect the origin and dispersal 

 of organisms. Finally, we will consider the major 

 ecological units, bionies, determined by climate and 

 vegetation. 



ZOOLOGICAL REALMS AND REGIONS 



Geographic 



Distribution of 



Communities: 



Distributional Units 



Geographic faunal divisions are character- 

 ized by the distinctiveness and uniformity of the 

 taxonomic groups represented. They are relatively 

 self-contained units isolated from other similar units 

 so that there is no free and easy intermingling of 

 species. There have been many attempts during the 

 last hundred or more years to divide the world into 

 such units (Kendeigh 1954). Probably the best sys- 

 tem was that devised by Sclater (1858), modified by 

 Huxley (1868), and extended by Wallace (1876). 

 Although present-day students of special animal 

 groups are inclined to modify the system further to 

 fit best the distributions of those organisms with 

 which each particularly deals, it commonly takes the 

 form demonstrated in Fig. 20-1. 



The regional divisions approximately coincide 

 with continents. That the continental land masses 

 should serve as the basis for the first major subdi- 

 vision of the fauna of the world is to be expected, 

 since they are mostly separated from each other by 

 large bodies of water that serve as effective barriers. 

 However, different continents have become isolated 



268 



