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. 
