THE EFFECT OF CLIMATE AND WEATHER 323 



tlon. The gap between the regions that He near the climatic 

 optima and those lying near the cHmatic Hmits becomes 

 steadily greater. 



The only serious objection to such a connection between 

 the distribution of chmate and civilization is found in a 

 comparison of the past with the present. Everyone knows 

 that ancient civilizations reached their height in regions 

 where the chmate is of only medium quahty according to 

 Figure 8. Does not this prove that whatever may be the fact 

 today the chmate of the past cannot have been a main factpr 

 in the distribution of civihzation? This question has been 

 carefully studied but is still in dispute. 



Two points however seem clear. The first is that the 

 chmatic optimum, as has been imphed in previous pages, 

 varies according to people's stage of progress. For unclothed 

 people a higher temperature and a lower degree of vari- 

 abihty are required than for people who wear clothes. The 

 same is true when people without fire are compared with 

 those who have that marvelous means of keeping warm. 

 Houses, stoves, furnaces and various other methods of 

 keeping warm have also tended little by little to lower the 

 optimum temperature and increase the optimum variability. 

 This tendency in itself is enough to account for a considerable 

 part of the shift in the centers of civilization. 



In addition to this a second point needs emphasis before we 

 can understand the relation of climate to civilization. 

 Geologists universally agree that 25,000 or 30,000 years ago 

 great ice sheets covered large sections of North America and 

 Europe where civihzation today stands very high. The 

 climatic change which caused the ice sheets to disappear has 

 taken place irregularly, sometimes proceeding rapidly, then 

 slowly, and even reversing itself. Even during the historic 

 period similar climatic pulsations appear to have taken place 

 on a smaller scale. For centuries the climate has swung in one 

 direction and then for centuries in the other, just as during 

 shorter periods it swings first one way for a few years and 

 then the other. Yet on the whole dry lands like western Asia 

 and the southwestern United States appear to have been 

 somewhat drier during the last one or two thousand years 

 than during the previous period. 



