ORIGIN OF SPECIES 237 



of very recent establishment — very recent, that is, 

 compared with the total of the many millions of 

 years that geologic history covers. 



The increase in complexity (physicochemical com- 

 plexity) of conditions on the earth, broadly viewed, 

 has been a gradual process; though great upheavals 

 and cataclysmic changes indeed have occurred at 

 intervals, some have fancied, with almost rhythmic 

 regularity. 



The increased physicochemical complexity that 

 obtained at each succeeding time in the earth's 

 history that provided the conditions which (according 

 to the theory) must have meant the repeated, very 

 general origin of life-forms, higher life-forms, register- 

 ing the increased complexity, then, was due to 

 several causes. These are: 



1. The accumulation of organic substances. The 

 great modifications caused by the presence of organic 

 substances are described by geologists and geo- 

 chemists. 



2. Changes in the atmosphere, the great "turbu- 

 lent sea" of gases that lies above the earth. ^^ That 

 the atmosphere has undergone great changes is 

 pointed out by Arrhenius^^ and is fully recognized by 

 all writers on the subject of life on the earth. 



^* See William Ramsay, The Gases of the Atmosphere. 

 " Das Schicksal der Planeten, 51. 



