POSSIBLE REVERSAL OF DEEP-SEA CIRCULATION 365 



alternative involves a slow growth of the atmosphere also, until it 

 reached a volume similar to the present, when its growth is assumed 

 to have been arrested and thereafter limited by the interplay of 

 opposing agencies. These agencies are thought to have held it ever 

 since within so narrow a range of oscillation as to foster organic 

 evolution. A continuance of the same control offers ground for 

 hope of a perpetuation of conditions congenial to organic and intel- 

 lectual Hfe, through a period to which no definite limits can now be 

 set beyond the presumption that there must ultimately be a limit. 

 The inevitable coohng of a once white-hot earth plays no part in this 

 prognosis. The agencies of atmospheric maintenance and control 

 thus force themselves upon consideration as factors of supreme 

 importance. 



The assigned agencies of atmospheric restraint are molecular 

 velocities, chemical combination, and condensation. By virtue of 

 the first, the lighter constituents are reduced to a minimum and all 

 constituents are restricted within certain large Hmits. By virtue of 

 the second, the chemically active factors are kept down to states of 

 dilution compatible with organic evolution, while the inert elements 

 have probably been permitted to increase steadily. By the third, 

 the excess of water-vapor has been condensed into the ocean, which 

 has probably increased rather than diminished through the ages. 



The postulated agencies of atmospheric supply are accessions 

 from without and emanations from within, of which Vesuvius has just 

 been giving us an impressive illustration. 



To the interplay of these opposing agencies of loss and gain is 

 assigned the maintenance of the requisite narrow range of atmos- 

 pheric constitution, of temperature, and of associated conditions. 

 Under this general resetting of fundamental conceptions, the ques- 

 tion of climatic regulation takes on very concrete aspects and pre- 

 sents specific lines of study. 



Subsidiary to these narrow limitations, recognition of pronounced 

 variations is forced upon us by a growing mass of geologic evidence. 

 Throughout most of the well-known geologic periods, the poleward 

 distribution of life implies warm cHmates, even as high as 70° and 

 80° of latitude. How life of subtropical types could have survived 

 the long polar nights is one of the most obdurate puzzles of the 



