236 W H AT IS LIFE 



Schuchert, Osborn) allow about eighteen million 

 years to archeozoic time. During these millions of 

 years organic matter began to accumulate, even then 

 greatly increasing the complexity of physicochemical 

 conditions. 



Whether or not the early earth had direct light 

 from the sun, and whether or not the ocean was salt 

 from its beginning, both sunlight and salt have 

 played an almost inestimably great role in the later 

 history of life on the earth. 



Whatever the constitution of the early atmosphere 

 may have been, it has changed greatly during 

 geologic time. To realize the importance of this one 

 factor, one needs only to call to mind the fact that 

 any considerable modification of the present at- 

 mosphere would result in the death of the human 

 race. 



As students hold, it does not seem likely that the 

 average temperature has varied very greatly since 

 life first appeared on the earth, that is, when and 

 where life flourished; for the peculiar life-process 

 can take place only within a very narrow range of 

 temperatures. The presence of glaciers and glacial 

 action, prolonged and over wide areas, necessarily 

 has meant absence of life. 



The present adjustment between temperature and 

 atmosphere and the highest life-forms on the earth, is 



