108 DISCUSSIONS IN CLIMATOLOGY. 



of causes which will produce an accumulation of ice 

 on either hemisphere when the winter solstice is in 

 aphelion will produce a melting of that ice when the 

 solstice moves round to the perihelion. 



Another Impossible Condition Assumed. — "What 

 we have to consider," says Mr. Wallace, " is whether 

 the alteration from a long cold winter and short hot 

 summer, to a short mild winter and long cool summer, 

 would greatly affect the amount of ice if the ocean- 

 currents remained the same." Here, again, we have 

 an impossible state of things assumed. It is assumed 

 that, notwithstanding the change from an aphelion to 

 to a perihelion winter, the ocean-currents would still 

 remain the same. And it is asked, would the astrono- 

 mical causes in this case remove the glaciation ? I 

 would be disposed to say that they would not. 



" The force of these currents," he adds, " are, it is 

 true, by our hypothesis modified by the increase or 

 diminution of the ice in the two hemispheres alter- 

 nately (they depend for their strength and efficiency 

 upon the comparative extent of the northern and 

 southern ice-sheets), and they then react upon 

 climate ; but they cannot be thus changed till after 

 the ice-accumulation has been considerably affected 

 by other causes." 



What, then, are the other causes which affect the 

 ice-accumulation and thus lead to a change in the 

 ocean-currents? "These ice-sheets cannot, I believe," 

 says Mr. Wallace, "increase or diminish to any im- 

 portant extent unless some geographical or physical 

 change first occurs." The first thing required to affect 

 the ice-accumulation is thus a geographical or a 

 physical change. But we have just seen that the 

 character of the physical causes depends upon the 

 astronomical. A change from a long cold winter and 



