58 THE FITNESS OF THE ENVIRONMENT 



general process, and meteorological condi- 

 tions on the earth cannot be perfectly 

 unique. 



A number of circumstances, however, cause 

 far greater variations in meteorological pro- 

 ; cesses than in most other phenomena which 

 have yet been discussed. For instance, on 

 small astronomical bodies with weak gravi- 

 tational attractions atmospheres cannot long 

 endure. Like the moon these bodies must 

 gradually lose nearly all their gases to space. 

 Such loss has almost certainly occurred from 

 the earth itself, and probably accounts for 

 the absence of hydrogen and helium from the 

 air. These gases, being very light, are not 

 attracted with sufficient force to the earth, 

 and gradually rise to the upper level of the 

 atmosphere and fly away. Again, in the ab- 

 sence of a near-by sun which steadily provides 

 energy to balance loss by radiation, the period 

 during which water and carbonic acid can 

 remain in an atmosphere must be relatively 

 short. Gradually, but in a time wholly in- 

 considerable in comparison with the duration 

 of the terrestrial atmosphere, the gases sur- 

 rounding bodies so placed must condense and 

 then solidify. Finally, a body which con- 

 stantly turns one face to a sun must slowly 

 condense its whole atmosphere upon its dark, 



