198 EXTINCT MONSTERS 



in the future by the co-operating labours of naturalists and 

 geologists. 



In the great onward and upward struggle for existence, higher 

 types have supplanted lower ones ; and, in accordance with this 

 biological truth, we find that in the next era (known as the 

 Tertiary or Cainozoic) the mammal held the field while the 

 reptile took a subordinate place. 



