280 J. Croll—Examination of Wallace's Modification of the : 
westerly winds, while still further lowering the temperature by — 
the southward diversion of the Gulfstream.’ 
Now if, during the glaciation of the northern hemisphere, 
these mutual reactions produce the opposite effect on the south- 
ern hemisphere, it is evident that they must produce this same 
opposite effect on the northern hemisphere when its winter 
solstice is in perihelion. Their effect then would be to increase — 
the temperature and melt the ice. When the winter solstice is 
moving towards the aphelion, the physical agents begin to act 
and also to react on one another, and this action and reaction 
goes on increasing in intensity till the solstice-point reaches the 
aphelion ; but an exactly similar thing is going on in the other 
hemisphere, only the effects are the reverse. While the actions 
and reactions leading to an accumulation of ice are increasing — 
in intensity, we shall suppose, on the northern hemisphere, the — 
same increase is taking place on the southern hemisphere; but — 
the cold periods the greater would be the amount melted during | 
the warm interglacial periods. . 
Another reason assigned why the ice does not melt.—Mr. Wal- 
lace assigns the following as an additional reason wh the ice 
does not disappear during the interglacial periods when the 
eccentricity is high : : 
