342 Cell as Basis of Organic Activity 



this earth, but the earHest fossils date from a mere 900,000,000 years 

 ago ! No doubt the first hving forms appeared many ages before that, 

 but left no traces. 



For the sake of convenience in discussion, this extent of time is 

 divided into eras, which in turn are subdivided into divisions, periods, 

 and epochs. These divisions are based upon the characteristic plants 

 and animals and major geological changes. 



The best method for understanding the manner in which these 

 periods are divided is to examine a table in which they are characterized. 



Geological Time Scale 



