162 
ISLAND LIFE. 
[part I. 
by very high land on which ice may accumulate. With us at 
the present time, on the other hand, geographical conditions 
completely mask and even reverse the influence of excentricity, 
and that of winter in perihelion in the northern, and summer 
in perihelion in the southern hemisphere. In the north we 
have a preponderance of sea within the Arctic circle, and of 
lowlands in the temperate zone. In the south exactly opposite 
conditions prevail, for there we have a preponderance of land 
(and much of it high land) within the Antarctic circle, and of 
sea in the temperate zone. Ice, therefore, accumulates in the 
south, while a thin coating of snow, easily melted in summer, 
is the prevalent feature in the north ; and these contrasts react 
upon climate to such an extent, that in the southern ocean, 
islands in the latitude of Ireland have glaciers descending to 
the level of the sea, and constant snowstorms in the height of 
summer, although the sun is then actually nearer the earth 
than it is during our northern summer ! 
It is evident, therefore, that the phenomena presented by the 
varying polar snows of Mars are in no way opposed to that 
modification of Dr. Croll’s theory of the conditions which 
brought about the glacial epochs of our northern hemisphere, 
which is here advocated ; but are perfectly explicable on the same 
general principles, if we keep in mind the distinction between 
an ice-sheet — which a summer’s sun cannot materially diminish, 
but may even increase by bringing vapour to be condensed into 
snow — and a thin snowy covering which may be annually melted 
and annually renewed, with great rapidity and over large areas. 
Except within the small circles of perpetual polar snow there 
can at the present time be no ice-sheets in Mars; and the 
reason why this permanent snowy area is more extensive around 
the northern than around the southern pole may be partly due 
to higher land at the north, but is perhaps sufficiently explained 
by the diminished power of the summer sun, owing to its greatly 
increased distance at that season in the northern hemisphere, so 
that it is not able to melt so much of the snow which has 
accumulated during the long night of winter. 
