58 IN THE GRIP OF AN ICE AGE 



the embryonic machinery which must be the cause of 

 all advances. Surroundings only select the variations 

 which are provided by the embryonic machinery. 



These, however, are deeper matters than we can 

 discuss here. The Ice Age may have lasted a quarter 

 of a million years. It put the population of the earth 

 through a sieve, as it were. The great majority of 

 the plants and animals could not stand the test, and 

 they were rejected. A few types struggled through, 

 and they begin the story of higher animal life: the 

 life of the birds and the mammals. But this figure of 

 speech is very faulty in one sense. If the severe 

 conditions of life had been spread all over the earth, 

 it would be a good figure. But they were not, as we 

 saw. There were still warm regions, where modified 

 survivors of the old Golden Age huddled together. If 

 the old warm conditions returned to the earth, they 

 would spread over it once more. This is what 

 happened. The ice-sheet gradually melted away, 

 probably because the land gradually sank once more, 

 and a new and curious chapter of the story of life 

 opened. 



