86 Dr. J. Croll on the Physical Theory 



epoch, and even at a date more recent, permanent ice must 

 have begun to accumulate on lowlands, which could not have 

 been the case had not the ground been previously covered 

 with perpetual snow. 



The only Continental lee on the Globe probably on Loiv- 

 lands. — The only two continents on the globe covered by 

 permanent ice and snow are Greenland and the Antarctic. 

 But are these continents to be regarded as high lands or as 

 low lands ? Mr. Wallace maintains that they are high lands. 

 " It is/' he says, " only where there are lofty mountains or 

 elevated plateaus, as in Greenland &c, that glaciers accom- 

 panied by perpetual snow cover the country. The north 

 polar area is free from any accumulation of permanent ice, 

 excepting the high lands of Greenland and Grinnell Land." 

 And in regard to the Antarctic continent he says, " The much 

 greater quantity of ice at the south pole is undoubtedly due 

 to the presence of a large extent of high land." Were it not 

 for these extensive highlands and lofty mountains, Greenland 

 and the Antarctic regions, according to Mr. Wallace's theory, 

 would be free from permanent snow and ice. He, however, 

 nowhere, so far as I can find, offers any proof for the conclu- 

 sion that those regions possess extensive highlands, elevated 

 plateaus, and lofty mountains sufficient to account for these 

 icy mantles. In the paper just referred to (Phil. Mag. No- 

 vember 1883) I have discussed this subject at considerable 

 length, and have arrived at conclusions diametrically the 

 opposite of those advocated by Mr. Wallace, viz. that Green- 

 land and probably the greater part of the Antarctic regions 

 consist of land probably not much above sea-level, and that 

 the mass of ice under which they are buried must be due to 

 some other cause than elevation of the land. 



Mr. Wallace's Modification of the Theory Examined. 



Mr. Wallace's chief, and, I may say, only real modification 

 of my theory is this. I give it in his own words: — 



" The alternate phases of precession— causing the winter of each 

 hemisphere to be in aphelion and perihelion each 10,500 years — 

 would produce a complete change of climate only where a country 

 was partially snow-clad ; while, whenever a large area became 

 almost wholly buried in snow and ice, as was certainly the case with 

 Northern Europe during the glacial epoch, then the glacial con- 

 ditions would be continued, and perhaps even intensified, when the 

 sun approached nearest to the earth in winter, instead of there 

 being at the time, as Mr. Croll maintains, an almost perpetual 

 spring."— P. 503. 



" When geographical conditions and eccentricity combine to 



