GEOGRAPHICAL DISTRIBUTION OF ANIMALS 3803 
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.— Hach 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- 
