942 



SCIENCE 



[N. S. Vol. XXX. No. 783 



made to grow up thus together from 

 smaller to larger volvimes, without neces- 

 sarily attaining at any stage a very high 

 temperature. The early sources of growth 

 for the atmosphere and the ocean, though 

 reduced in later time, continiied to serve 

 as sources of replenishment when the fa- 

 miliar agencies of loss came into play in 

 the later ages. Thus, far from assigning 

 at the start a vast atmospheric and oceanic 

 supply, and assuming progressive depletion 

 of this with the progress of time, the newer 

 view starts with a minimum supply and 

 rests on means of feeding which are held 

 to run hand in hand with the sources of 

 loss and to more or less completely compen- 

 sate them in a varying way. The question 

 of the future under this view is, therefore, 

 not how long beyond the present day will 

 the original supply last, but rather how 

 long will the oscillating compensation of 

 lass and supply remain effective? Or in 

 other words, how long will the past degree 

 of equilibrium between the opposing agen- 

 cies keep the critical conditions within the 

 limits required by life? This question 

 turns us quite away from any serious de- 

 pendence on the original states and centers 

 attention on the geologic record and on the 

 potency of agencies still in action. Are the 

 chief agencies which have controlled life 

 conditions for tens of millions of years i^ast 

 still in good working order and likely to 

 continue effective for a long era yet to 

 come, or do they show clear signs of de- 

 clining power portending an early failure 1 

 Let us enter a little closer into the consid- 

 eration of the specific factors on which life 

 depends, though time will not permit us to 

 go far. 



The pre-scientific fear that the end of 

 life will come by cataclysm is not yet obso- 

 lete, nor is it theoretically impossible, but 

 violent agencies are among the least to be 

 feared. Life might indeed be imagined to 



be in .jeopardy from volcanic and seismic 

 convulsions but they really offer no serious 

 menace to life in general, and they appear 

 never to have done so in the known ages. 

 The deadliness of these boisterous catas- 

 trophies impresses itself unduly on the 

 emotions. The real peril, if peril there be, 

 lies in the deadly unbalancing of agencies 

 of the quiet sort. 



The conditions essential to the mainten- 

 ance of the habitability of the earth are 

 many, but the more critical factors either 

 lie in the atmosphere itself or are inti- 

 mately associated with it. The point of 

 keenest interest is the narrowness of range 

 to which these mobile factors are confined. 

 The several constituents of the atmosphere 

 might each or all easily be too scant or too 

 abundant. In a peculiar sense is this true 

 of the carbon dioxide which, though one of 

 the least, is preeminently the decisive con- 

 stituent of the atmosphere. A small pro- 

 portion of carbon dioxide is essential to 

 plant life and so to animal life, while a 

 large proportion would be fatal to air- 

 breathing animals. If the three or four 

 hundredths of one per cent, now present 

 were lost, all life would go with it; if it 

 were increased to a few per cent., the 

 higher life would be suppressed or radi- 

 cally changed. And yet, on the one hand, 

 the theoretical sources of supply are abun- 

 dant, while, on the other, the agencies of 

 depletion are efficient and active. There 

 is little escape from the conclusion that 

 ever since the birth of air-breathing life, 

 some 30,000,000 or 40,000,000 years ago, let 

 us say, the interplay of these agencies of 

 supply and depletion has been so balanced 

 that neither fatal excess nor fatal defi- 

 ciency has been permitted to cut short the 

 history of the higher life. 



The dangers of exce-ss or deficiency of 

 the other constituents of the air are indeed 

 less narrow when named in percentages. 



