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KIRTLEY F. MATHER 
which had come into being as a result of episode one and there- 
fore much more difficult of interpretation. All in all, the changes 
in the physical environment of the earth’s inhabitants were 
nearly or quite as sweeping as those which resulted in the strik- 
ing modifications of life as it passed from the Paleozoic to the 
Mesozoic types. And, as before, the living creatures responded 
to these changes so completely that another ^ devolution” is 
recorded in the older text-books. In a word, the host of rep- 
tiles which had dominated land and sea and sky were swept 
into oblivion and their places were promptly assumed by mam- 
mals, the most advanced of all creatures. As a consequence, 
• the Cenozoic Era is known as the Age of Mammals. 
Again, let us enquire more particularly into the history of 
the group of animals which seem to have profited most by 
this ^devolution. ” To state it briefly, the story of the mammals 
at this time is almost an exact parallel to that of the reptiles 
at the close of the Paleozoic Era. Instead of having been 
created subsequent to the devastating catastrophe which was 
believed to have left an uninhabited world for them to people, 
we now know that they had long been in existence. The first 
relics of mammalian life date far back in the Age of Reptiles. 
Before its close there' were countless hordes of small warm- 
blooded creatures, competing, in spite of their insignificant size 
and by virtue of their superior brain power, with the gigantic 
and powerful but clumsy and unintelligent reptilian masters 
of the world. This very competition, carried on during at 
least three geologic periods, gave them strength and cunning 
not only to outwit their opponents but also to adapt themselves 
even to such adverse environmental conditions as surrounded 
them during the geographic revolution inaugurated by the 
first birth-pains of the American Cordillera. Far more ready to 
profit by its changes, because of their mobility, their warmth- 
preserving pelts and their intelligence in securing new food 
supplies, than were the reptiles, they succeeded where their 
competitors failed. Coincident with the return of the balanced 
conditions which lead to long periods of comparative stability 
of the earth’s crust, the mammals promptly acceded to the 
