Where did Life Begin? 23 



sun, and have had more mass per square foot 

 to cool, in proportion to radiating surface, than 

 the polar zones, so, on the other hand, they 

 have always received less heat from the sun 

 and have had less mass to cool, in proportion 

 to radiating surface, than the torrid zone ; and 

 so when the arctic zones cooled from a tropi- 

 cal to what we now call a temperate climate, 

 the temperate zones had cooled down to that 

 temperature which we now call a torrid cli- 

 mate, while the equatorial belt was still too 

 hot for any form of life. Thus the lowering 

 of temperature, climatic change, and that life 

 which made its advent in these zones surround- 

 ing the poles, have crept thence slowly along, 

 pari passu, from these polar regions to the 

 equator. Doubtless, through all geologic 

 time, wave after wave of climatic change and 

 corresponding forms of life, including the re- 

 motest extinct species, from laurentian to allu- 

 vium, from eozoon to mammal, whose several 

 biographies in the rocks, are now called 

 epochs, have followed each other In succes- 

 sion from this originally prolific polar zone to 

 the equatorial belt. 



