MODIFICATION OF THEORY EXAMINED. 109 



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: — "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 continents, and some one or 

 more of these factors must be altered before the ocean- 

 currents towards the North Pole can be increased." * 



One of these factors — change in the form and 

 position of the continents — may be left out of 

 consideration ; for we have no evidence of any 

 such change during the glacial epoch, except one, 

 which, as has been already proved, could have had 

 no effect. We must, therefore, look to a change in 

 "the difference of temperature of the polar and 

 equatorial areas" for any increase in the currents 

 towards the North Pole. And in order to bring about 

 this change, " the only available factor," Mr. Wallace 

 states, " is the Antarctic ice ; if this were largely 

 increased, the northward-flowing currents might be 

 so increased as to melt some of the Arctic ice. But 

 without some geographical change the Antarctic ice 

 could not materially diminish during its winter peri- 

 helion, nor increase to any important extent during 

 the opposite phase. We therefore seem to have no 

 available agency by which to get rid of the ice over a 

 glaciated country, so long as the geographical con- 

 ditions remained unchanged and the eccentricity 

 continued high." 



According to Mr. Wallace, the only available factor 

 to produce a difference of temperature between the 



* "Island Life," p. 150. 



