Geographic Distribution of Animals and Plants 597 



Asia. Today they are extinct except in those regions mentioned above. 

 There was a rather continuous distribution originally, but today they il- 

 lustrate discontinuous distribution. 



The principle of vertical distribution states that the organisms of higher 

 elevations of mountains simulate those of the polar regions of the world. 

 As we progress downward in altitude, the forms simulate those which 

 would be found in travelling from the poles toward the equator. In 

 general, the temperate zones not only extend laterally north and south 

 of the equator but also vertically in parallel succession from the somewhat 

 tropical conditions at sea level to the somewhat frigid conditions at the 

 mountain peaks (Fig. 311). 



IV. GEOGRAPHIC REGIONS OF THE WORLD 



The world has been divided into seven regions (Fig. 310) each with 

 its different characteristics and peculiar, yet typical, fauna and flora. 

 Each of these seven regions will be briefly described. 



1. Nearctic Region (Gr. neo, late or new; arctic). — This region in- 

 cludes North America down to the edge of the Mexican plateau, as well 

 as Greenland. Great groups of mammals are characteristic of this region. 

 Many types in this region are related to the Palaearctic region, although 

 they differ in minor details. Animals which are peculiar to this region 

 include the blue jays, rattlesnakes, raccoons, opossums, skunks, prairie 

 dogs, water dogs (Cryptobranchus is found in the Mississippi RiverVal- 

 ley), and the musk ox. The latter, which is peculiar to North America, 

 was until recently present in Siberia and originally lived in continental 

 Europe and England. What a difference in the distribution of the musk 

 ox in the past and present ! 



2. Palaearctic Region (Gr. palae, ancient; arctic). — This region in- 

 cludes Europe, Africa (north of the Tropic of Cancer), Asia (north of 

 the Himalayas), and Japan. Many types of mammals of this region are 

 closely related to those of the Nearctic region, differing in minor respects. 

 Many trees and plants are common to both regions. These two regions 

 are the most similar of all regions and are sometimes combined into one, 

 known as the Holarctic region (Gr. holo, whole; arctic). Animals com- 

 mon to both Nearctic and Palaearctic regions include beavers, deers, 

 hares, foxes, wildcats, and bears. Animals of the Palaearctic region in- 

 clude the nightingale, Megalobranchus (water dog of Japan which re- 

 sembles our own water dog), and the camel and dromedary of central 

 Asia and northern Africa. 



