GEOGRAPHICAL DISTRIBUTION OP ANIMALS 299 



tains of other realms whose summits rise above the timber 

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

 polar bear or the musk-ox or the reindeer, are not found 

 there, because barriers shut them off. But the flora of the 

 mountain top, even under the equator, may be character- 

 istically arctic, and with the flowers of the north may be 

 found the northern insects on whose presence the flower 

 depends for its fertilization. So far as climate is concerned 

 high altitude is equivalent to high latitude. On certain 

 mountains the different zones of altitude and the corre- 

 sponding zones of plant and insect life are very sharply 

 defined (Fig. 178). 



The North Temperate realm comprises all the land be- 

 tween the northern limit of trees and the southern limit of 

 frost. It includes, therefore, nearly the whole of Europe, 

 most of Asia, and most of North America. While there 

 are large differences between the fauna of North America 

 and that of Europe and Asia, these differences are of minor 

 importance and are scarcely greater in any case than the 

 difference between the fauna of California and that of our 

 Atlantic coast. The close union of Alaska with Siberia 

 gives the arctic region an almost continuous land area from 

 Greenland to the westward around to Norway. To the 

 south everywhere in the temperate zone realm the species 

 increase in number and variety, and the differences between 

 the fauna of North America and that of Europe are due in 

 part to the northward extension into the one and the other 

 of types originating in the tropics. Especially is this true 

 of certain of the dominant types of singing birds. The 

 group of wood-warblers, tanagers, American orioles, vireos, 

 mocking-birds, with the fly-catchers and humming-birds so 

 characteristic of our forests, are unrepresented in Europe. 

 All of them are apparently immigrants from the neotropical 

 realm where nearly all of them spend the winter. In the 

 same way central Asia has many immigrants from the Indian 

 realm to the southward. With all these variations there 



