FUTURE HABITABILITY OF THE EARTH CHAMBERLIN. 381 



early geologists to be the more intrusive. In the early part of the 

 record it seems peculiarly easy to find convincing evidences of stages 

 marked by prevailing humidity, by great uniformity of climate, and 

 by conditions congenial to subtropical life ranging through wide 

 stretches of latitude. If we continued to center attention on these 

 alone, the old view would now, as heretofore, seem to be sustained. 

 But these evidences do not abound at all horizons, and the view is 

 selective. Between these horizons lie the strata that bear evidences 

 of marked aridity as well as those that bear the still more impressive 

 evidences of low temperatures to which we shall turn in a moment. 



Combining the two sets of facts of diverse import, we seem forced 

 to recognize that from the earliest known stages of distinct life record 

 there have been times and places of pronounced aridity much as 

 now, and sometimes even more intense, while at other times and 

 places intervening between these, humidity has prevailed. 



This picture of alternations grows in vividness and strength if we 

 turn from states of atmospheric moisture to states of temperature. 

 The body of scientific men have rarely been more hesitant in accept- 

 ing any interpretation of terrestrial phenomena than that of the 

 glacial invasion of the lowlands of Europe and America in mid- 

 latitudes when that view was first advanced by Louis Agassiz. In 

 the face of the then prevalent view of general warmth as the domi- 

 nant characteristic of all the earlier ages, it seemed beyond belief 

 that great sheets of ice could have crept over large areas of the 

 habitable part of Europe and America even in the geologic stage 

 just preceding our own.' The acceptance of this view was, however, 

 made somewhat less difficult by the belief, also then prevalent, that 

 the earth had greatly cooled down in the progress of the ages, and 

 that concurrent with this the atmosphere had been much depleted 

 by the formation of oxides, carbonates, coal, and carbonaceous mat- 

 ter, and that the ocean had been reduced by hydration and by physi- 

 cal penetration into the earth. By the combined influence of these 

 it was easier to believe that a stage had been reached that made 

 possible an epoch of exceptionally depressed temperature attended 

 by glaciation. These special pleadings were in eminent harmony 

 with the inherited view of a great thermal declension as the master 

 fact of geologic history, and under this influence the ice age came to 

 be generally regarded as but the first episode of a succession of 

 secular winters upon which the earth was entering, a series destined 

 to lead on to the total refrigeration of the earth. This presumption 

 was furthermore abetted by the theory of a cooling sun. The cool- 

 ing and depleting processes were naturally regarded as inevitably pro- 

 gressive, and so the final doom of the earth seemed clearly fore- 

 shadowed in the near future, geologically speaking. 



