DISTRIBUTION OF LIFE 



243 



Arizona, show some of the most remarkable tree fossils 

 ever found and indicate that the region has been sub- 

 jected to remarkable geographical changes. 



113. Distribution of Life. Plants and animals are found 

 wherever the conditions are suitable for their existence. 

 In ice-covered areas, like the interior of Greenland, and 

 in exceedingly dry regions, like the Sahara and certain 

 parts of southwestern United States, there is little life of 

 any kind. With a few 

 such exceptions, how- 

 ever, the surface of the 

 earth is a universal bat- 

 tlefield of plants and 

 animals struggling to 

 exist and to increase. 

 They extend themselves 

 wherever attainable 

 space is opened. But 

 barriers may oppose their 

 spread and geographical 

 accidents may drive them 

 from areas which they 



GILA MONSTERS. 



The most poisonous reptiles of the south- 

 western American desert. 



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 



