THE NEOZOIC AGES. 239 



more recent. Such a mode of estimation is, no doubt, 

 to some extent arbitrary; but in the main, when it 

 can be tested by the superposition of deposits, it has 

 proved itself reliable. Further, it brings before us 

 this remarkable fact, that while in the older periods 

 all the animals whose remains we find are extinct as 

 species, so soon as we enter on the Neozoic we find 

 some which still continue to our time at first only a 

 very few, but in later and later beds in gradually 

 increasing percentage, till the fossil and extinct wholly 

 disappear in the recent and living. 



The Lyellian classification of the Tertiary will 

 therefore stand as in the following table, bearing in 

 mind that the percentage of fossils is taken from 

 marine forms, and mainly from mollusks, and that the 

 system has in some cases been modified by strati- 

 graphical evidence : 



POST-PLIOCENE, including that which immediate- 

 ly precedes the Modern. In this the shells, 

 etc., are recent, the Mammalia in part ex- 

 tinct. 



PLIOCENE, or more recent age. In this the 



Tertiary, or 



majority of shells found are recent in the 



Neozoic Time.\ upper beds. In the lower beds the extinct 



become predominant. 

 MIOCENE, or less recent. In this the large 



majority of shells found are extinct. 

 EOCENE, the dawn of the recent. In this only a 



few recent shells occur. 



If we attempt to divide the Tertiary time into ages 

 corresponding to those of the older times, we are met 



