128 WORLD-POWER AND EVOLUTION 



of the lands. We now know, however, that the land often sank 

 at the very times and places where it would have to rise accord- 

 ing to this supposition. Recently a third hypothesis has received 

 wide acceptance. This is the idea that changes in the amount 

 of carbonic acid gas in the atmosphere cause corresponding 

 changes in atmospheric moisture and thus in the retention of 

 heat by the air. Something of this sort doubtless happens, 

 for carbon is sometimes locked up in beds of coal and limestone, 

 and sometimes set free when the rocks are broken up by weather- 

 ing. The composition of the air, however, must change so slowly 

 that this hypothesis can have little to do with the climatic changes ^ 

 with which we are now concerned. 



The only remaining possibility seems to be that climatic changes 

 are due to variations in the energy received from the sun. This 

 brings us into the midst of a hot dispute. One school of meteor- 

 ologists holds that present variations of climate are purely 

 accidental. A newer and rapidly growing school holds that 

 they are due to the sun. Some members of the second school 

 believe that the entire effect arises from variations in the sun's 

 heat, while a more radical group believe that some other form of 

 energy such as electricity is also concerned. This lattef froint 

 need not now concern us, for we shall base our conclusions on 

 observed facts and not on their interpretation. From the work 

 of a large number of scientists, as well as from his own investiga- 

 tions, it seems to the author that there is no escape from the 

 conclusion that present climatic variations are due to correspond- 

 ing variations in solar energy. It seems equally probable that 

 while the slower climatic pulsations of the past may have been due 

 to changes in the altitude of continents and mountains, the more 

 rapid and marked pulsations were due to the sun. Therefore, the 

 best road to an understanding of the conditions under which man's 

 mind evolved most rapidly would seem to be to inquire into the 

 present relation of the sun and the earth, and to see what would 

 happen if the present effects were magnified. The reader must 



