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GEOGRAPHICAL DISTRIBUTION OF ANIMALS 297 
tainous country is full of barriers which obstruct the diffu- 
sion of life. Distinctness is in direct proportion to isola- 
tion. What is true in this regard of the fauna of any region 
is likewise true of its individual species. The degree of 
resemblance among individuals is in strict proportion to the 
freedom of their movements. Variation within the limits 
of a species is again proportionate to the barriers which 
prevent equal and free diffusion. 
159. Realms of animal life——The various divisions or 
-realms into which the land surface of the earth may be 
divided on the basis of the character of animal life have 
their boundary in the obstacles offered to the spread of the 
average animal. In spite of great inequalities in this regard, 
we may yet roughly divide the land of the globe into seven 
principal realms or areas of distribution, each limited by 
barriers, of which the chief are the presence of the sea and 
the occurrence of frost. : There are the Arctic, North Tem- 
perate, South American, Indo-African, Lemurian, Patago- 
nian, and Australian realms. Of these the Australian 
realm alone is sharply defined. Most of the others are sur- 
rounded by a broad fringe of debatable ground that forms 
a transition to some other zone. 
The Arctic realm includes all the land area north of the 
isotherm of 32°. Its southern boundary corresponds closely 
with the northern limit of trees. The fauna of this region 
is very homogeneous. It is not rich in species, most of the 
common types of life of warmer regions being excluded. 
Among the large animals are the polar bear, the walrus, and 
certain species of “ice-riding ” seals. There are a few spe- 
cies of fishes, mostly trout and sculpins, and a few insects. 
Some of these, as the mosquito, are excessively numerous 
in individuals. Reptiles are absent from this region and 
many of its birds migrate southward in the winter, finding 
in the arctic only their breeding homes. When we consider 
the distribution of insects and other small animals of wide 
diffusion we must add to the arctic realm all high moun- 
