MENTAL EVOLUTION 125 



tion of man's mind and hence in our own activities at present, we 

 must first find out what kind of climate prevailed in the Glacial 

 Period. Within recent years our ideas of the climate of the past 

 have changed greatly. 



A few generations ago the earth's atmosphere was supposed to 

 have grown gradually clearer, drier, and cooler for millions of r 

 years. Then it was discovered that glaciation again and again 

 recurred in relatively low latitudes. This means that since the 

 earliest geological times there have been repeated periods when 

 the earth's climate has grown cold and then warm again time after 

 time. There have also been prolonged dry periods and wet periods 

 as is suggested in Figure 21. Coincident with the discovery of the 

 complexity of the earth's climatic history went the further dis- 

 covery that minor variations are constantly taking place. Each 

 glacial period is divided into epochs; each epoch into stages; and 

 each stage into minor cycles. This is illustrated by Figure 23, 

 which shows the climatic pulsations of the Glacial Period, and by 

 Figure 24, which shows the minor fluctuations of the last 3,000 

 years. If we knew the past as well as we know the present, we 

 should doubtless find that every glacial period is a time not only ^ 

 of great climatic changes lasting hundreds of thousands of years, 

 but of innumerable shorter, more rapid changes. 



The inquiring mind naturally asks the cause of these changes. 

 Four chief hypotheses have been advanced, but three of them 

 seem incompetent to explain the known facts. One of these is 

 Croll's idea that glaciation arises because once in about 24,000 

 years the earth is farthest from the sun in winter instead of in i 

 summer as is now the case. Alluring as it sounds, this hypothesis 

 has been entirely discarded by geologists. This is partly because 

 the variations in the sun's distance from the earth are apparently 

 not enough to cause the supposed effect. A much stronger 

 argument is that Croll's hypothesis demands a glacial epoch every 

 24,000 years, which is wholly at variance with the facts. A 

 second hypothesis holds that glaciation is due to the uplifting 



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