CHAPTER XXII 

 THE AGE OF MAMMALS 



THE Geological Record is not perfect. There are 

 breaks in it such as have not and may never be 

 filled up ; and it is because of these breaks that 

 some of the divisions are made in geologic time. At 

 present the earth's crust has only been scratched for 

 fossils. Great parts of Asia and of Africa and of South 

 America remain to be explored, and they may in some 

 future generation fill the gaps of our knowledge and 

 render superfluous some of the divisions which geologists 

 now place in the eras of the rocks and of the fossils. 

 But so far as we know at present there were real breaks 

 in the history of the continents, perhaps not swift or 

 sudden, but wholly changing the appearance and the 

 life, vegetable and animal, of half a world, perhaps the 

 whole world at a time. Many geologists believe that 

 the secret of these changes lies in the core of the earth ; 

 and that, to use our old simile of the golf ball, when the 

 tension and pressures inside the earth grow too much for 

 its strength something gives way, and the whole world 

 begins to change, the continents sinking under the oceans 



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