90 DISCUSSIONS IN CLIMATOLOGY. 



Modification of the Theory Examined. 



Mr. Wallace's chief, and, I may say, only real 

 modification of my theory is this. I give it in his own 

 words : — 



" The alternate phases of precession — causing the winter 

 of each hemisphere to be in aphelion and perihelion each 

 10,500 years — would produce a complete change of climate 

 only where a country was partially snow-clad ; while, 

 whenever a large area became almost wholly buried in snow 

 and ice, as was certainly the case with Northern Europe 

 during the glacial epoch, then the glacial conditions would 

 be continued, and perhaps even intensified, when the sun 

 approached nearest to the earth in winter, instead of there 

 being at the time, as Mr. Croll maintains, an almost 

 perpetual spring." — P. 503. 



" When geographical conditions and eccentricity combine 

 to produce a severe glacial epoch, the changing phases of 

 precession have very little, if any, effect on the character of 

 the climate, as mild or glacial, though it may modify the 

 seasons ; but when the eccentricity becomes moderate and 

 the resulting climate less severe, then the changing phases 

 of precession bring about a considerable alteration and even 

 a partial reversal of the climate." — P. 153. 



Again — " It follows that towards the equatorial limits of 

 a glaciated country alternations of climate may occur during 

 a period of high eccentricity, while near the pole, where the 

 whole country is completely ice-clad, no amelioration may 

 take place. Exactly the same thing will occur inversely 

 with mild Arctic climates." — P. 154. 



I have, on the contrary, maintained that the more 

 severe the glacial condition of the one hemisphere, the 

 warmer and the more equable would necessarily be 

 that of the other ; for the very same combination of 

 causes which would tend to cool the one hemisphere 

 would necessarily tend to warm the other. The process 



