1G2 



ISLAND LIFE. 



[part I. 



by very high land on which ice may accumulate. V/ith 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 pcrilielion 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. 



