CHAPTER VII 



LIFE OF THE EAETH AS BELATED TO PHYSICAL 

 CONDITIONS 



112. Ancient Life History. As the rock layers of the 

 earth are explored, fossils of different kinds of plants 

 and animals are discovered. The fossils of the more 

 recent rock layers correspond very closely to the plants 

 and animals that are found upon the earth to-day, but 

 the older the layers, the less they correspond. There 

 seems to have been a gradual development in life .forms 

 through the past ages, a fragmentar}^ record of which 



is engraved upon cer- 

 tain of the sedimentary 

 rocks. Rocks which 

 were formed under dif- 

 erent conditions contain 

 different species of life 

 forms, showing that 

 throughout all time the 

 geographic condition has 

 had a marked influence 

 upon plants and animals. 

 The rocks and fossils 

 show that the geographical conditions of certain areas also 

 have varied greatly. Some regions have been below and 

 above the sea several times. Regions now cold have been 

 warm, and those now dry have been wet, and vice versa. 

 Thus the life in certain areas has suffered great changes 

 by the geographical accidents to which the region has 

 been subjected. The petrified forests near Holbrook, 



242 



PETRIFIED TREES. 

 Found near Holbrook, Arizona. 



