of Secular Changes of Climate. 99 



astronomical, physical, and geographical — is to cool the one 

 hemisphere and to warm the other, to accumulate the ice 

 on the one and remove it from the other. Consequently 

 the same total combination 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, nothwithstanding the change from an aphelion to a peri- 

 helion winter, the ocean-currents would still remain the same. 

 And it is asked, would the astronomical 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 alternately (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, 

 ft increase or diminish to any important 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 cha- 

 racter of the physical causes depends upon the astronomical. 

 A change from a long cold winter and short hot summer to a 

 short mild winter and long cool summer would reverse the 

 operations of the physical causes and lead to a melting of the 

 ice. The physical causes therefore offer no barrier. What 

 more do we still require ? This we have in the following foot- 

 note at page 150 : — " The ocean-currents are mainly due to 

 the difference of temperature of the polar and equatorial areas 

 combined with the peculiar form and position of the conti- 

 nents, and some one or more of these factors must be altered 

 before the ocean-currents towards the North Pole can be in- 

 creased." 



One of these factors — change in the form and position of the 

 continents — may be left out of consideration; for we have no 



H2 



