136 
ISLAND LIFE. 
[part I. 
from the quantity of moisture they bring with them which will 
be condensed into snow by coming into contact with the frozen 
surface. We may therefore expect the transition from perpetual 
snow to a luxuriant arctic vegetation to be very abrupt, depend- 
ing as it must on a few degrees more or less in the summer 
temperature of the air ; and this is quite in accordance with the 
fact of corn ripening by the sides of alpine glaciers. 
Efficiency of Astronomical Causes in producing Glaciation . — 
Having now collected a sufficient body of facts, let us endeavour 
to ascertain what would be the state to which the northern 
hemisphere would be reduced by a high degree of excentricity 
and a winter in aphelion. When the glacial epoch is supposed 
to have been at its maximum, about 210,000 years ago, the 
excentricity was more than three times as great as it is now, 
and, according to Dr. Croll’s calculations, the mid-winter tem- 
perature of the northern hemisphere would have been lowered 
36° F., while the winter half of the year would have been 
twenty-six days longer than the summer half. This would 
bring the January mean temperature of England and Scotland 
almost down to zero or about 30° F. of frost, a winter climate 
corresponding to that of Labrador, or the coast of Greenland on 
the Arctic circle. But we must remember that the summer 
would be just as much hotter than it is now, and the problem 
to be solved is, whether the snow that fell in winter would 
accumulate to such an extent that it would not be melted in 
summer, and so go on increasing year by year till it covered the 
whole of Scotland, Ireland, and Wales, and much of England. 
Dr. Croll and Dr. Geikie answer without hesitation that it 
would. Sir Charles Lyell maintained that it would only do so 
when geographical conditions were favourable ; while the late 
Mr. Belt has argued, that excentricity alone would not produce 
the effect unless aided by increased obliquity of the ecliptic, 
which, by extending the width of the polar regions, would 
increase the duration and severity of the winter to such an 
extent that snow and ice would bo formed in the Arctic and 
Antarctic regions at the same time whether the winter were in 
perihelion or aphelion. 
The problem we have now to solve is a very difficult one, 
