524 LIFE AS RELATED TO PHYSICAL CONDITIONS 



Distribution of Life. Plants and animals are found 

 wherever the conditions are suitable for their existence. 

 The surface of the earth is a universal battlefield of plants and 



animals struggling to 

 exist and to in- 

 crease. They extend 

 themselves wherever 

 attainable space is 

 opened. But barriers 

 may oppose their 

 spread and geo- 

 graphical accidents 

 may drive them from 

 areas which they had 

 heretofore held. The 

 retreat of the sea may 

 cause a change in the 

 position of shore life. 

 In the water a land 

 barrier or an expanse 

 of deep water may 

 prevent the spread 

 of shore forms. On 

 the land a mountain 

 uplift, a desert area, 



or a water barrier may limit the space occupied by animal 

 and vegetable species. 



Certain plants and animals are much more widely dis- 

 tributed than others. Plants like the dandelion and thistle, 

 whose seeds are easily blown about by the wind, spread 

 rapidly, while trees like the oak and chestnut spread slowly. 

 As plants have not the power to move about, they cannot 



GILA MONSTERS 



These are very poisonous reptiles of the 

 southwestern American desert. 



