10 FROM FISH TO PHILOSOPHER 



temperature of the oceanic water and withdrawal of 

 marine life toward the equator and the extinction of 

 many forms of plant and animal life. Such periods have 

 also been frequently accompanied by the spread of des- 

 erts over large continental areas. On the other hand, 

 periods of low relief between mountain-building episodes 

 have seen the spread of warm, shallow seas across the 

 continents and the extension of tropical oceanic currents 

 into the polar regions, with such ameHoration of climate 

 that marine and terrestrial forms of Hfe, now limited to 

 the equatorial regions, have grown abundantly close to 

 the Arctic zone. 



The history of the earth and the history of life are 

 related as a die and a mold: as geologic revolution has 

 succeeded revolution, each has left its imprint in the pat- 

 tern of hfe as well as in the pattern of the rocks. Living 

 organisms of ever-increasing complexity, and with ever- 

 increasing capacity to lead a free and independent life, 

 are a product of earth's troubled history. 



