MODIFICATION OF THEORY EXAMINED. 121 



formed, while again the snow thus formed chills the 

 air still more and increases the fogs. Again, during 

 the winters of a glacial epoch, the earth would be 

 radiating its heat into space. Had this loss of heat 

 simply lowered the temperature, the lowering of the 

 temperature would have tended to diminish the rate 

 of loss ; but the result is the formation of snow rather 

 than the lowering of the temperature. 



' Further, as snow and ice accumulate on the one 

 hemisphere they diminish on the other. This increases 

 the strength of the trade-winds on the cold hemisphere 

 and weakens those on the warm. The effect of this is 

 to impel the warm water of the tropics more to the 

 warm hemisphere than to the cold. Supposing the 

 northern hemisphere to be the cold one, then, as the 

 snow and ice begin gradually to accumulate, the 

 ocean-currents of that hemisphere, more particularly 

 the Gulf Stream, begin to decrease in volume, while 

 those on the southern or warm hemisphere begin pari 

 passu to increase. This withdrawal of heat from the 

 northern hemisphere favours the accumulation of snow 

 and ice, and as the snow and ice accumulate the ocean- 

 currents decrease. On the other hand, as the ocean- 

 currents diminish, the snow and ice still more 

 accumulate. Thus the two effects, in so far as the 

 accumulation of snow and ice is concerned, mutually 

 strengthen each other.' 



With all this Mr. Wallace seems fully to agree ; for 

 at pp. 137-140 ("Island Life") he gives a very clear 

 statement of the effect of these mutual reactions in the 

 production of glaciation, and says that were it not for 

 them it is probable the astronomical and other causes 

 would not in our latitudes have been sufficient to 

 produce glaciation. In short, he concludes that these 

 reactions " produce a maximum of effect which, with- 



