THE CRITERIA OF LIVINGNESS 87 



food, have time for ageucy, and that we have time for 

 thinking about it all. 



We are familiar with the self-preservative devices and 

 reactions of higher animals and with the self-preservative 

 way in which the various organs of the body work into each 

 other's hands; and it is a remarkable fact that a specific 

 activity in a nervous system may be restored after the de- 

 struction of the particular nerve-elements on which the activ- 

 ity previously depended. This vicarious functioning is all 

 the more remarkable inasmuch as there is not in higher 

 animals any regeneration or replacement of nerve-cells after 

 birth. But deeper than all this is the correlation of chemical 

 processes in the individual units, so that down-breaking leads 

 to up-building, so that up-building makes further down-break- 

 ing possible, the pluses balance the minuses, and the creature 

 goes on. The unicellular organism spends its substance and 

 yet has it, through its fundamental capacity for self-renewal. 

 If the living creature is a machine, it is a self-stoking, self- 

 repairing machine, and it can take a rest betimes. 



Several saving-clauses must be appended: — (1) The or- 

 ganism shows persistent functionality, but it is not known 

 to offer any exception to the law of the conservation of 

 energy. In living it expends energy, and suffers wear and 

 tear; it cannot continue unless it captures more energy and 

 is able to repair its structure. Fatigue and the dying of 

 parts, such as leaves, not to speak of senescence and death 

 itself, show that the fundamental capacity for self-main- 

 tenance is not perfect. But the broad fact is that the capacity 

 has for a variable time a very considerable degree of per- 

 fection. The organism's chemical activities (and repair- 

 processes) are so correlated that it remains for a considerable 

 time a going concern. As we shall afterwards see, some 



