192 



ISLAND LIFE. 



[part I. 



important alteration of the climates of the temperate and Arctic 

 zones so long as favourable geographical conditions, such as 

 have been now sketched out, render the accumulation of ice 

 impossible. The effect of a high excentricity in producing a 

 glacial epoch was shown to be due to the capacity of snow and 

 ice for storing up cold, and its singular power (when in large 

 masses) of preserving itself unmelted under a hot sun by itself 

 causing the interposition of a protective covering of cloud and 

 vapour. But mobile currents of warm water have no such 

 power of accumulating and storing up heat or cold from one 

 year to another, though they do in a pre-eminent degree possess 

 the power of equalising the temperature of winter and summer 

 and of conveying the superabundant heat of the tropics to 

 ameliorate the rigour of the Arctic winters. However great 

 was the difference between the amount of heat received from the 

 sun in winter and summer in the Arctic zone during a period of 

 high excentricity and winter in aphelion, the inequality would be 

 greatly diminished by the free ingress of warm currents to 

 the polar area ; and if this was sufficient to prevent any 

 accumulation of ice, the summers would be warmed to the full 

 extent of the powers of the sun during the long polar day, 

 w^hich is such as to give the pole at midsummer more heat 

 during the twenty-four hours than the equator receives during 

 its day of twelve hours. The only difference, then, that would 

 be directly produced by the changes of excentricity and pre- 

 cession would be, that the summers would be at one period 

 almost tropical, at the other of a more mild and uniform 

 temperate character; while the winters would be at one time 

 somewhat longer and colder, but never, probably, more severe 

 than they are now in the west of Scotland. 



But though high excentricity would not directly modify the 

 mild climates produced by the state of the northern hemisphere 

 which prevailed during Cretaceous, Eocene, and Miocene times, 

 it might indirectly affect it by increasing the mass of Antarctic 

 ice, and thus increasing the force of the trade-winds and the re- 

 sulting northward-flowing warm currents. Now there are many 

 peculiarities in the distribution of plants and of some groups of 

 animals in the southern hemisphere, which render it almost certain 



