480 DISTRIBUTION OF LAND ANIMALS 



tation. Accordingly, it is not favorable for a rich and 

 varied development of species, but the ones that do 

 exist are more distinctive than those of any other realm. 

 Deep water acts as a barrier, and for millions of years 

 its animals have evolved without entering into competi- 

 tion with more powerful and more specialized types. A 

 very few hogs, the dingo or native dog, and several species 

 of bats very nearly complete the list of higher mammals. 

 On the other hand, there are many unique species of 

 primitive mammals such as the kangaroos, wombats, and 

 numerous other related species that likewise are akin to 

 the opossum. This realm is also the home of the re- 

 markable egg-laying duck-bill and spiny anteater, as well 

 as of numerous species of birds, such as the brush turkey, 

 parrots and pigeons, that occur in no other realm. 



Realm Subdivisions. — In much the same manner 

 that the United States is composed of states, counties, 

 and townships, each of the zoological realms is sub- 

 divided into lesser districts, each with a characteristic 

 assemblage of animal forms. The Holarctic of North 

 America, for example, is divided into not less than eight 

 geographical regions (see map facing page 458), with 

 fairly well defined and characteristic conditions dependent 

 upon the nature of the climate, soil, and vegetation. 

 Each region also possesses a distinctive group of animal 

 species which are specially adapted for feeding upon a 

 particular kind of vegetation, for example, or for living 

 amid surroundings of a very definite character, although 

 a smaller number of less specialized types may roam from 

 one region to the next. 



As a rule, the distribution of a given species of ani- 

 mal is not as extensive as the region it occupies. It 

 may be confined, for example, to a forested area, a 

 desert, a marsh, or a definite type of soil. No two species 

 occupy exactly the same range, however, so the controll- 

 ing factors obviously differ with the species concerned. 

 Speakly broadly, these factors are suitable and available 



