318 EVOLUTION AND ANIMAL LIFE 



of resemolance among individuals is in strict proportion to the 

 freedom of their movements. Variation within the hmits of 

 a species is again j^roportionate to the barriers which prevent 

 equal and free diffusion. 



The various divisions or realms into which the land surface 

 of the earth may be divided, on the basis of the character of the 

 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, Patagonian, Lemurian, 

 and Australian realms. Of these the Australian realm alone is 

 sharply defined. Most of the others are surrounded 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 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 by the cold. 

 Among the large animals are the polar bear, the walrus, and 

 certain species of "ice-riding" seals. There are a few species 

 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 

 migTate southward in the winter, finding in the Arctic their 

 breeding homes only. When we consider the distribution of 

 insects and other small animals of wide diffusion we must add 

 to the AiTtic realm all high mountains of other realms whose 

 summits rise above the timber line. The characteristic large 

 animals of the Ai^ctic, as the polar bear or the musk-ox or the 

 reindeer, are not found on the mountain tops because barriers 

 shut them off. But the Alpine flora, even under the equator, 

 may be characteristically arctic, and with the flowers of the 

 north may be found the northern insects on whose presence 

 the flowers depend for their fertilization and which in turn 

 depend on these for their food. So far as climate is concerned, 

 high altitude is equivalent to high latitude. On certain 

 mountains the different zone^ cf altitude and the corresponding 



