GEOGRAPHICAL DISTRIBUTION OP ANIMALS 303 



but the presence of frost is a barrier which vast numbers of 

 species can not cross. Beyond the Patagonian realm lies 

 the Antarctic continent. The scanty fauna of this region 

 is little known, and it probably differs from the Patagonian 

 fauna chiefly in the absence of all but the ice-riding species. 



The Australian realm comprises Australia and the 

 neighboring islands. It is more isolated than any of the 

 others, having been protected by the sea from the invasions 

 of the characteristic animals of the Indo-African and tem- 

 perate realms. It shows a singular persistence of low or 

 primitive types of vertebrate life, as though in the process 

 of evolution the region had been left a whole geological 

 age behind the others. It is certain that if the closely 

 competing fauna of Africa and India could have been able 

 to invade Australia, the dominant mammals and birds of 

 that region would not have been left as they are now mar- 

 supials and parrots. 



It is only when barriers have shut out competition that 

 simple or unspecialized types abound. The larger the land 

 area and the more varied its surface, the greater is the 

 stress of competition and the more specialized are its char- 

 acteristic forms. As part of this specialization is in the 

 direction of hardiness and power to persist, the species from 

 the large areas, as a whole, are least easy of extermination. 

 The rapid multiplication of rabbits and foxes in Australia, 

 when introduced by the hand of man, shows what might 

 have taken place in this country had not impassable barriers 

 of ocean shut them out. 



160. Subordinate realms or provinces. Each of these great 

 realms may be indefinitely subdivided into provinces and 

 sections, for there is no end to the possibility of analy- 

 sis. No school district has exactly the same animals or 

 plants as any other, as finally in ultimate analysis we find 

 that no two animals or plants are exactly alike. Shut off 

 one pair of animals from the others of its species, and its 

 descendants will differ from the parent stock. This differ- 



