168 ISLAND LIFE part i 



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 

 wliich 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 vajDour 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 dimi- 

 nished power of the summer sun, owing to its greatly 

 increased distance at that season in the northern hemi- 

 sphere, so that it is not able to melt so much of the snow 

 which has accumulated durins: the lonof nio^ht of winter. 



