FUTURE HABITABILITY OP THE EARTH CHAMBERLIN. 383 



At the present time glaciation in the polar regions and on Alpine 

 heights is contemporaneous with desert conditions in extensive belts 

 where the systematic circulation of the atmosphere favors aridity. 

 There are reasons for thinking that in the past glaciations and 

 aridity were related to one another in some similar way, and that 

 they cooperated to give an aspect of marked vicissitude to the 

 climates of certain geological epochs. It is to be observed, however, 

 that the epochs of glaciation now known are fewer than the epochs 

 of aridity, and it is probable that aridity has been a more common 

 phenomenon than glaciation. 



Set over against the adversities of desert and ice there were stages, 

 as already noted, when abundant life, bearing all evidences of a 

 warm-temperate or subtropical habitat, flourished in high latitudes. 

 In Greenland, Spitzbergen, and other Arctic lands — and we have 

 recently learned also in Antarctic lands — are found relics of life 

 not known to be able to live except in a genial climate. These quite 

 clearly point to subtropical conditions at certain former times 

 where only frigidity now reigns. 



In the light of these contrasted states of ice and desert on the 

 one hand and of geniality and moisture on the other, intervening 

 between one another in unexpected latitudes, we seem forced to the 

 view that profound climatic alternations followed one another 

 throughout the whole stretch of known geologic time. These may 

 have been attended by variations in the constitution, as well as the 

 condition of the atmosphere. 



If we turn to the relations which the great waters have borne to 

 the great lands, an analogous series of oscillations is presented; and 

 there is ground to suspect that the oscillations of the climates had 

 some casual connection with the oscillations of the land and sea. At 

 no time since life began is there clear evidence of the absence of land, 

 and certainly at no time is there evidence of the absence of an ocean, 

 whatever theoretical views may be held of the earliest unknown 

 ages. The conviction seems well sustained that the land areas of the 

 Archean and Proterozoic eras were comparable to those of the pres- 

 ent day both in extent and in limitations, in the sense that they were 

 neither universal nor absent in these earliest known times. Follow- 

 ing down the history, the lands seem at times to have been larger and 

 at other times smaller than now. There appears to have been an 

 unceasing contest between the agencies that made for the extension 

 of the land and the agencies that made for the extension of the 

 sea. While each gained temporarily on the other, complete victory 

 never rested with either. From near the beginning of the read- 

 able record there appears to have been an unbroken continuity of 

 land life, and from a like early stage an unbroken continuity of 

 marine life. Probably the history of both goes back thus unbroken 



