244 



FIRST YEAR SCIENCE 



V 



spread slowly. As plants have not the power to move 

 about, they cannot distribute themselves as easily as ani- 

 mals. Certain birds 

 which are strong of 

 flight are found widely 

 distributed over regions 

 separated by barriers 

 impassable to other ani- 

 mals. 



Some of the present 

 barriers to life distribu- 

 tion have come into ex- 

 istence in comparatively 

 recent geological time. 

 There is good reason to 

 believe that the British 



^^^ Isles and Europe were 



4* \4 li^ ^k~ formerly connected, and 



Tf.A(L >II ^E^ 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 illustrations 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. 



CANADA THISTLE. 



One of the most widely distributed of 

 plants. 



