100 Dr. J. Croll on the Physical Theory 



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 tempe- 

 rature 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 north- 

 ward-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 

 perihelion, 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 conditions remained unchanged and 

 the eccentricity continued high." 



According to Mr. Wallace, the only available factor to 

 produce a difference of temperature between the south-polar 

 area and the equator, so as to increase the north-flowing 

 currents and thus melt the arctic ice, would be an increase of 

 the antarctic ice; but this he considers impossible without 

 some geographical change. Without such a change, the ant- 

 arctic ice, he maintains, would neither be increased nor dimi- 

 nished. Hence it follows that without this change there is, 

 according to Mr. Wallace's theory, no possibility of getting 

 quit of our northern ice during interglacial periods. 



This sweeping conclusion seems to be based on two as- 

 sumptions, both of which appear to me to be erroneous. First y 

 that the "only" factor available is the antarctic ice; and, 

 secondly , that the antarctic ice can neither be increased nor 

 diminished without some geographical change. 



A Geographical Change not necessary in order to remove the 

 Antarctic Ice. — In reference to the first, that the antarctic 

 ice is the " only " available factor, I shall presently show that 

 there are other causes affecting the northward-flowing currents 

 as powerfully as the antarctic ice. As to the second, that the 

 antarctic ice can neither be increased nor diminished mate- 

 rially without some geographical change, this is an assumption 

 based, no doubt, on the opinion which he holds that the ant- 

 arctic ice is due to the elevated nature of that continent. Of 

 course if this opinion be correct, then, without a lowering of 

 the land, the ice can never disappear or be greatly changed in 

 amount by astronomical or physical causes. But from what 

 has already been stated in a former article * in reference to 

 the condition of the Antarctic regions, I think it likely that 



* " The Ice of Greenland and the Antarctic Continent not due to Ele- 

 vation of the Land," Phil. Mag. November 1883. 



