MODIFICATION OF THEORY EXAMINED. 87 



opinion. A condition of the greatest importance, 

 though one not absolutely necessary to the produc- 

 tion of a glacial epoch, as will presently be shown, is 

 the existence of perpetual snow. The question then 

 is, Could not those physical agencies brought into 

 operation during a high state of eccentricity cover low 

 lands with perpetual snow without the aid of high 

 lands? Mr. Wallace replies, "Perpetual snow nowhere 

 exists on low lands." Supposing this were true (I 

 have endeavoured to show in the last chapter it is 

 not), still it does not follow that perpetual snow may 

 not have existed on low lands, or that, when the 

 present condition of things changes, it may not yet 

 exist. It is not difficult to conceive how, under 

 certain conditions, the snow-line may in some places 

 have been brought to the sea-level. In arctic, or even 

 in subarctic regions, an excessively heavy snowfall, 

 followed by piercingly cold winds from the north, 

 during the whole of the summer months, would keep 

 the snow at a low temperature, and certainly prevent 

 it from disappearing. Keep the surface of the snow 

 at or below the freezing-point, and melting will not 

 take place, no matter how intense the sun's rays may 

 be. A strong wind below the freezing-point will cool 

 the surface of the snow more rapidly than the sun can 

 heat it. Another cause which would tend to keep the 

 snow at a low temperature would be that, along with 

 a cold northerly wind, there is usually a great 

 diminution of aqueous vapour, thus allowing the 

 surface of the snow to radiate its heat more freely 

 into stellar space. For, were it not for the aqueous 

 vapour in the atmosphere, as has been shown in 

 Chapters II. and III., the snow-line, even at the 

 equator, would descend to the sea-level. 



Perhaps it is owing to the warm southerly winds of 



