THE MESOZOIC AGES. 231 



in the period of the old Roman Empire. The Arab 

 of the desert remains in the Patriarchal period, and 

 there are some tribes not yet beyond the primitive age 

 of stone. But the world moves, nevertheless, and the 

 era of Victoria is not that of the Plantagenets or of 

 Julius Caesar. So while we may admit that certain of 

 the conditions of the Cretaceous seas still prevail in 

 the bed of the present ocean, we must maintain that 

 nearly all else is changed, and that the very existence 

 of the partial similarity is of itself the most con- 

 clusive proof of the general want of resemblance, and 

 of the thorough character of the changes which have 

 occurred. 



The duration of the Cretaceous subsidence must 

 have been very great. We do not know the rate 

 at which the Foraminifera accumulate calcareous mud 

 In some places, where currents heap up their shells, 

 they may be gathered rapidly ; but on the average of 

 the ocean bed, a foot of such material must indicate the 

 lapse of ages very long when compared with those of 

 modern history. We need not wonder, therefore, that 

 while some forms of deep-sea Cretaceous life, especially 

 of the lower grades, seem to have continued to our 

 time, the inhabitants of the shallow waters and the 

 land have perished ; and that the Neozoic or Tertiary 

 period introduces us to a new world of living beings. 

 I say we need not wonder ; yet there is no reason 

 why we should expect this as a necessary consequence. 

 As the Cretaceous deluge rose over the continents of 

 the Mesozoic, the great sea saurians might have fol- 



