CHAPTER V 



IN THE GRIP OF AN ICE AGE 



We have already begun to realize what a deep in- 

 fluence changes in the surroundings may have on 

 the earth's inhabitants, although I have been able to 

 give only a few illustrations. The struggle of land 

 and water, which I have described, caused a great 

 number of other new developments. Suppose, for 

 instance, a vast marshy region, that had lain stagnant 

 for hundreds of thousands of years, were tilted and 

 drained, so as to form a great tract of dry country 

 with a few swift-running rivers. The whole animal 

 and plant population must change. The fish and 

 shell-fish, the luscious plants and all that fed on 

 them, down to the microscopic animalcules, the in- 

 sects and amphibians, must give way to new types. 

 If the changes are very gradual, the new types are 

 largely their own descendants. There is time for 

 evolution to adapt them to the new surroundings. 



This was happening in very many parts of the earth 

 during the period we have just considered; and we 

 will glance at one further effect of such changes before 

 we come to the great transformation which crowned 

 them all. It is quite plain that the amphibious 



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