56 IN THE GRIP OF AN ICE AGE 



Ice Age. The bird and the mammal are products of 

 an Ice Age. 



We will try to picture what happened. The ice 

 sheet spread over South Africa and the continent 

 which then spread from Africa to Australia in the 

 east and India in the north-east. Suppose we take 

 the northern part of Africa, which was free from ice. 

 Our African tropics would then be a "temperate 

 zone." Further north there would still, no doubt, 

 be large regions with warm conditions. But the 

 overcrowding would be terrible. How many million 

 ' ' refugees ' ' would there be from the icy continent ? I 

 do not, of course, mean refugees in the modern sense. 

 The change had taken ages. The animals would 

 migrate gradually. But in the height of the Ice Age 

 the warm region would be packed. 



On the fringes of such a region natural selection 

 would be at work. If you take a thousand people at 

 random in London, you find that some stand cold 

 better than others. There are little variations of all 

 kinds in every species. Now the power to stand 

 cold would be an advantage in the conditions we are 

 studying. Such individuals would be more active, and 

 could get food enough in regions where others would 

 starve. These would prosper in the ' ' temperate zone, ' ' 

 which would not be so densely inhabited. The more 

 tender and sluggish among their offspring would 

 perish. The hardier and more active would multiply. 

 The standard of the population would rise. As they 



