THE GEOLOGICAL HISTORY OF PLANTS. 



The evidence is similar to that obtained by Schlie- 



mann on the site of Troy, where, in digging through suc- 



I cessive layers of debris, he found the objects, deposited by 



IsuccessiTe occupants of the site, frona the time of the 



Eoman Empire back to the earliest tribes, whose flint 



weapons and the ashes of their fires rest on the original 



i surface of the ground. 



Let ns now tabulate the whole geological succession 

 with the history of animals and plants associated with it : 



It will be observed, since only the latest of the sys- 

 tems of formations in this table belongs to the period of 

 human history, that the whole lapse of time embraced in 

 the table must be enormous. If we suppose the modern 

 period to have continued for say ten thousand years, and 

 each of the others to have been equal to it, we shall re- 

 quire two hundred thousand years for the whole. There 

 is, however, reason to believe, from the great thickness of 

 the formations and the slowness of the deposition of many 



