EFFECT OF THE GLACIAL PERIOD 



525 



distribute themselves as easily as animals. Certain birds 

 which are strong of flight are found widely distributed over 

 regions separated by barriers impassable to other animals. 



Some of the present barriers to life distribution have come 

 into existence in comparatively recent geological time. 

 There is good reason to believe that the British Isles and 

 Europe were formerly connected, and that in very ancient 

 times Australia was joined to 

 Asia. It is also believed that 

 for long ages North and South 

 America were separated by a 

 water barrier and that even after 

 they were once connected, the 

 Isthmus of Panama was again 

 submerged. 



These are but a few illustra- 

 tions of the changes in the earth's 

 surface which have affected the 

 distribution of animals and 

 plants. Climatic changes like 

 that which brought about the 

 great ice advance of the Glacial 

 Period have affected in a marked 

 degree the distribution of life. 

 It is thus found that when a study is made of the present 

 distribution of life, careful attention must be given to the 

 present and past geographical conditions of the region. 



Effect of the Glacial Period upon Plants and Animals. 

 All plants and animals were forced either to migrate be- 

 fore the slowly advancing ice or to suffer extermination. 

 Individual plants, of course, could not move, but as the ice 



CANA-DA THISTLE 



One of the most widely dis- 

 tributed of plants. 



