GEOGRAPHICAL DISTRIBUTION OF ANIMALS 441 



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. 



350. 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 denned. 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- 

 tains of other realms whose summits rise above the timber 

 line. The characteristic large animals of the arctic, as the 

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