CAUSE OF MILD POLAR CLIMATES. 159 



stance which, I think, has misled geologists, and 

 induced them to conclude that because the physical 

 agents brought into operation when the winter solstice 

 was in aphelion, during a high state of eccentricity, 

 failed to produce a well-marked glacial epoch in 

 Tertiary times, consequently the climatic condition of 

 that period was not much affected by eccentricity. 



It would seem to be owing to that peculiar difference 

 between the conditions of the two hemispheres that, 

 even during high eccentricity, the physical agents in 

 operation when the winter solstice was in aphelion 

 were unable to lower the temperature of the northern 

 hemisphere to an extent sufficient to cover high 

 temperate and Arctic regions with permanent ice ; but 

 for this very same reason these agents would be 

 enabled to raise the temperature to an extent 

 exceptionally high when the winter solstice was in 

 perihelion. In other words, this very combination of 

 circumstances, which so much modified the severity 

 of what may be called the Tertiary cold periods, 

 intensified to an exceptionally great extent the 

 warmth and equability of what may be called the 

 Tertiary warm periods. 



