EARTH-LIFEGENERATION. 399 



be briefly noted: (1) Palaeozoic or old-life, in 

 which are several periods, such as Silurian, 

 Devonian, Carboniferous. (2) Mesozoic, or 

 middle-life, which has also several periods. 

 (3) Caenozoic, new-life, which is also sub- 

 divided. As already indicated, each of these 



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eras, and indeed each of their subordinate 

 epochs, had its own peculiar species which 

 can be identified over the Globe. Then again 

 each of these important, stages had its spe- 

 cial forms of Life ; thus the Silurian is called 

 the age of mollusks, and the Carboniferous 

 the age of coal -plants. It would seem that 

 the total life-stuff of the Earth was expended 

 in a colossal effort to produce one kind of 

 living thing, animal or vegetal, at a time. We 

 may conceive the Earth-life as a whole to be 

 a huge animal which has had to go through 

 various stages of evolution, at first a fish, 

 later a reptile, and finally a mammal, which 

 it is now, though once it turned plant mainly, 

 back in the coal measures. Of course it has 

 kept in general the living shapes which it 

 once evolved, still its stress seems to have 

 been upon one great typical form during a 

 given time. In this trait Earth-life appears 

 like an individual undergoing a series of meta- 

 morphoses through the geologic ages, each 

 of which is characterized by such a metamor- 

 phosis. While the Earth-life may be deemed 



