106 DISCUSSIONS IN CLIMATOLOGY. 



in this struggle manage to overpower the physical and 

 produce a melting of the ice ? I unhesitatingly reply, 

 No ; for the physical causes are far more powerful 

 than the astronomical. The astronomical causes, as 

 we have seen, are perfectly unable to produce a glacial 

 state of things without the aid of the physical. How, 

 then, could we expect that they could remove this 

 glacial state if the physical causes were actually 

 working against them ? 



In thus setting the physical causes against the 

 astronomical, Mr. Wallace is basing his argument for 

 the non-disappearance of the snow and ice on a state of 

 things which cannot possibly, under the circumstances, 

 exist. His question, to have consistency, should be 

 this : — When glacial conditions were at their height, 

 &c, can we suppose that the mere change from the 

 distant sun in winter and the near sun in summer, to 

 the reverse, could bring about any important alteration 

 — the geographical causes of glaciation remaining 

 unchanged ? If the question is put thus, and it is the 

 only form in which it can be put to be consistent with 

 the theory which Mr. Wallace himself advocates, then 

 my reply is, That the change from the distant sun in 

 winter and near sun in summer to the near sun in 

 winter and distant sun in summer, aided by the change 

 in the physical causes which this would necessarily 

 bring about, would certainly be sufficient to cause the 

 snow and ice to disappear without any change in the 

 geographical condition of things. The combined influ- 

 ence of the astronomical and physical causes, when the 

 winter solstice is in perihelion, is perfectly sufficient 

 to undo all that they had previously done when the 

 solstice was in aphelion. When the action of the 

 causes is reversed, the effects will be reversed. 



Had the glacial epoch been produced by geographical 



