FUTURE HABITABILITY OF THE EARTH CHAMBERLIN. 377 



they are not still in function in some degree. And so, far from as- 

 signing a vast atmospheric and oceanic supply at the start and bring- 

 ing to bear on this a progressive depletion all down the ages, the 

 newer view starts with a much more limited supply and rests on 

 means of continued feeding as time goes on, and makes this feeding 

 run hand in hand with the secular losses in more or less equal balance 

 after the initial stages of growth were over. 



The question of the future, under this view, is not how long will 

 the remnant of the original supply last, but rather, how long will 

 the past and present degree of equilibrium between loss and gain 

 remain effective? The equilibrium is held to be oscillatory but the 

 limits of oscillation fall within the limits of the conditions of life. 

 The specific question of the future, so far as our race is concerned, is, 

 how long will such a degree of equilibrium as has prevailed in the 

 past continue to preserve the critical conditions prerequisite to life? 



The question in this aspect turns us quite away from any serious 

 concern respecting original abundance and centers attention on the 

 geologic record itself as an index of past competencies. In particu- 

 lar it turns attention on the agencies of equilibrium to see if there are 

 signs of any fatal weakening of competency. Are the chief agencies 

 which have controlled life conditions for tens of millions of years 

 still in good working order and likely to continue effective for a 

 long era yet to come, or do they show clear signs of declining power 

 portending an early failure? 



Let us enter a little closer into a study of the specific factors on 

 which life depends, though we may not go far. 



The ancient fear that the end of the earth will come by cataclysm 

 is not yet obsolete nor is it theoretically quite impossible, but violent 

 agencies are among the least to be feared. Volcanic or seismic con- 

 vulsions may be imagined to put life in jeopardy as indeed they 

 often actually do locally, but they really offer no serious menace to 

 life in general, and they do not appear ever to have done so in the 

 known ages. The spectacular destructiveness of these boisterous 

 agencies deeply impresses the emotions, but they contribute but an 

 infinitesimal fraction to a sober computation of the effective sources 

 of loss of world life. The real peril, if peril there be to the whole 

 world life, lies in the deadly unbalancing of agencies of the quiet 

 sort. 



The conditions essential to the maintenance of the habitability 

 of the earth are many, but the more critical factors either lie in 

 the atmosphere itself or are intimately associated with it. The fact 

 of keenest interest is the narrowness of range within which the 

 critical conditions are confined. Any of the constituents of the at- 

 mosphere or all of them might easily, it would seem, be too scant or 

 too abundant to be consistent with life as now organized. In a 



