326 



THE AMERICAN NATURALIST [Vol. X LIT 



being, by the external agent. The effect dies ont shortly 

 after the agent ceases to act. Whether there is a loss of 

 energy or a gain, no further change ensues in the system, 

 unless it is acted upon from the outside. 



Physiological response, on the other hand, though at 

 bottom of the same sort, differs in that the external agent 

 produces an upset that releases energy previously ac- 

 cumulated, so that the effect exceeds that due to the ini- 

 tial energy acting upon the organism. That is why, in- 

 deed, the term stimulus was originally applied to such an 

 agent ; it brings into action energy often vastly greater 

 than its own. In this case there is a run-down of energy. 

 The system contains less than before by as much work as 

 has been done in response to the stimulus. Repeated 

 responses lead to exhaustion of the accumulated energy, 

 when further stimulation is impotent. To explain such a 

 situation Bose's theory of molecular strain is inadequate; 

 we are driven to take account of stored energy, and not 

 a direct supply. For if the depleted system be kept 

 under conditions as nearly uniform as possible, it pro- 

 ceeds to recover energy by the incorporation of new ma- 

 terial with its potential energy. The protoplast assimi- 

 lates food and presently is ready for new response to any 

 stimulus. It is the cyclic character of their energetics 

 that characterizes living things, no less in the phenomena 



Even a casual examination of the various responses of 

 an organism shows clearly that some of them are to be 

 classed with the physical and others with the physiolog- 

 ical, as above denned. If so, it is plain that it is not mere 



matter responds; it is the ability after a loss of energy 

 by response to regain, by the aid of the environment, a 

 condition which makes response possible once more. 

 This is no mere restoration of a molecular equilibrium 

 which has been disturbed or relief from molecular strain ; 

 it involves acquisition of energy. How far this is due to 

 a direct intake of energy and how far to the utilization of 

 potential energy in available foods, our present knowl- 



