150 
ISLAND LIFE. 
[part I. 
almost perpetual snow storms, even at midsummer, such as now 
prevail in the worst portions of the Southern Ocean. 
But when such was 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 geographical 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 perihelion , 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 
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 
