54 THE FITNESS OF THE ENVIRONMENT 



general laws, are likewise typical or at all 

 events were so at the time of their origin. 



Neither can the change in crust and atmos- 

 phere which time has wrought be wholly 

 unique, though here a possible exception may 

 again arise in the action of life itself. Since 

 we do not at present positively know of the 

 existence of life elsewhere and certainly have 

 no detailed knowledge of its nature, we can- 

 not feel sure that the conversion of atmos- 

 pheric carbonic acid into oxygen and coal is 

 either a universal or a common occurrence. 

 In details of the geological process indeed there 

 may well be marked differences. Probably the 

 greatest variation will occur in the relative 

 duration of conditions like those which we 

 now enjoy on the earth, the length or brevity 

 of the period from the full establishment of 

 the circulation of water by evaporation, cloud 

 formation, rainfall, with the flow of lakes and 

 streams, until its extinction by cold. Thus 

 there is more liability of error in an analyis 

 of the general characteristics of those spon- 

 taneous changes which must occur upon the 

 surface of a body after the formation of a 

 crust than there is in the attempt to dis- 

 cover the general characteristics of stellar 

 evolution. But here again our knowledge is 

 not based upon terrestrial phenomena alone. 



