EXCURSIONS OF AN EVOLUTIONIST 



the summer temperature be raised like this ? It 

 would not ; and this is because our earth has a 

 means of storing up cold, so to speak, which 

 gives winter the advantage over summer in such 

 a contest. With the mean January temperature 

 of England at 4 F. instead of 39, all the mois- 

 ture which now falls as rain would fall as snow, 

 and would accumulate on the ground. At the 

 coming of summer, all the snow and ice would 

 have to be melted, and it takes a great deal of 

 heat to melt snow and ice. As Mr. Wallace 

 graphically puts it, " to melt a layer of ice only 

 one and a half inch thick would require as much 

 heat as would raise a stratum of air 800 feet 

 thick from the freezing-point to the tropical 

 heat of 88 F. ! " Until the snow is all melted, 

 no amount of solar heat can raise the tempera- 

 ture much above the freezing-point ; and this 

 is the reason why, in regions where much mois- 

 ture is condensed as snow, as in Greenland, and 

 at the summits of the Andes, Alps, and Hima- 

 layas, snow is perpetual. So that, in the case 

 we have supposed, the extra heat received from 

 the sun in the short summer would largely be 

 exhausted in melting the snow, and, instead of 

 raising the mean temperature 60, it is doubtful 

 if it would raise it at all above the point which 

 it attains at the present time. Besides all this, 

 it must be remembered that the rapid melting 

 of great masses of snow produces fog, and thus 

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