136 



ISLAND LIFE. 



[part I, 



from the quantity of moisture they bring with them which will 

 be condensed into snow by coming into contact with the frozen 

 surface. We may therefore expect the transition from perpetual 

 snow to a luxuriant arctic vegetation to be very abrupt, depend- 

 ing as it must on a few degrees more or less in the summer 

 temperature of the air ; and this is quite in accordance with the 

 fact of corn ripening by the sides of alpine glaciers. 



Efficiency of Astronomical Causes in producing Glaciation. — 

 Having now collected a sufficient body of facts, let us endeavour 

 to ascertain what would be the state to which the northern 

 hemisphere would be reduced by a high degree of excentricity 

 and a winter in aphelion. When the glacial epoch is supposed 

 to have been at its maximum, about 210,000 years ago, the 

 excentricity was more than three times as great as it is now,^ 

 and, according to Dr. Croll's calculations, the mid-winter tem- 

 perature of the northern hemisphere would have been lowered 

 36° F., while the winter half of the year would have been 

 twenty-six days longer than the summer half This would 

 bring the January mean temperature of England and Scotland 

 almost down to zero or about 30° F. of frost, a winter climate 

 corresponding to that of Labrador, or the coast of Greenland on 

 the Arctic circle. But we must remember that the summer 

 would be just as much hotter than it is now, and the problem 

 to be solved is, whether the snow that fell in winter would 

 accumulate to such an extent that it would not be melted in 

 summer, and so go on increasing year by year till it covered the 

 whole of Scotland, Ireland, and Wales, and much of England. 

 Dr. Croll and Dr. Geikie answer without hesitation that it 

 would. Sir Charles Lyell maintained that it would only do so 

 when geographical conditions were favourable ; while the late 

 Mr. Belt has argued, that excentricity alone would not produce 

 the effect unless aided by increased obliquity of the ecliptic, 

 which, by extending the width of the polar regions, would 

 increase the duration and severity of the winter to such an 

 extent that snow and ice would be formed in the Arctic and 

 Antarctic regions at the same time whether the winter were in 

 perihelion or aphelion. 



The problem we have now to solve is a very difficult one, 



