150 



ISLAND LIFE. 



[part I. 



almost perpetual ?now storms, even at midsummer, such as now 

 prevail in the worst portions of the Southern Ocean. 



But when such w^as the state of the North Atlantic (and, 

 however caused, such must have been its state during the height 

 of the glacial epoch), can we suppose that the mere change from 

 the distant sun in winter and near sun in summer, to the 

 reverse, could bring about any important alteration — the 

 physical and geogi^a'phical causes of glaciation remaining un- 

 changed For, certainly, the less powerful sun of summer, even 

 though lasting somewhat longer, could not do more than the 

 much more powerful sun did during the phase of summer 

 in jperihelion, while during the less severe winters the sun would 

 have far less power than when it was equally near and at a 

 very much greater altitude in summer. It seems to me, 

 therefore, quite certain that whenever extreme glaciation has 

 been brought about by high excentricity combined with favour- 

 able geographical and physical causes (and without this combi- 

 nation it is doubtful whether extreme glaciation would ever 

 occur), then the ice-sheet will not be removed during the alter- 

 nate phases of precession, so long as these geographical and 

 physical causes remain unaltered. It is true that the warm and 

 cold oceanic currents, which are the most important agents in 

 increasing or diminishing glaciation, depend for their strength 

 and efficiency upon the comparative extents of the northern 

 and southern ice-sheets ; but these ice-sheets cannot, I believe, 

 increase or diminish to any important extent unless some 

 geographical or physical change first occurs.^ 



1 In reply to an objection of a somewhat similar nature to this, Dr. 

 Croll has recently stated {Geol, Mag.^ Oct., 1879) that he "has not 

 assumed that the comparative disappearance of the ice on the warm 

 hemisphere during the period of high excentricity is due to any additional 

 heat derived from the sun in consequence of the greater length of the 

 summer," but that "the real and effective cause of the disappearance of 

 the ice was the enormous transference of equatorial heat to temperate and 

 polar regions by means of ocean currents." But this is surely arguing in 

 a circle ; for the ocean currents are mainly due to the difference of tem- 

 perature 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 



