GEOGRAPHICAL DISTRIBUTION 319 



zones of plant and animal life are very sharply defined. Ex- 

 cellent illustrations are found in the San Francisco peaks of 

 Arizona and Mt. Orizaba in Mexico. 



The North Temperate or holarctic realm comprises all the 

 land between the northern limit of trees and the southern limit 

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

 most of Asia, and the 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 in 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 hum- 

 ming 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 which lies to the southward. With all these 

 variations there is an essential unity of life over this vast area, 

 and the recognition of North America as a separate (nearctic) 

 realm, which some writers have attempted, seems hardly 

 necessary. 



Alfred Russell Wallace refers to this unity of northern life 

 in these words: 



"When an Englishman travels on the nearest sea route from Great 

 Britain to Northern Japan, he passes countries very unlike his own 

 both in aspect and in natural productions. The sunny isles of the 

 Mediterranean, the sands and date palms of Egypt, the arid rocks of 

 Aden, the cocoa groves of Ceylon, the tiger-haunted jungles of Malacca 

 and Singapore, the fertile plains and volcanic peaks of Luzon, the 

 forest-clad mountains of Formosa, the bare hills of China pass suc- 

 cessively in review, until after a circuitous journey of thirteen thousand 



