GEOGRAPHICAL DISTRIBUTION OF ANIMALS 999 
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 
