INERTIA 11 



sonieAvhat different form, and is known as the principle of the 

 necessity of reactions. It is, " Every eheniieal change which is 

 acconiphshed without the ad(iition of external energy necessarily 

 occurs if it is accompanied bij a disengagement of heat.'" This means 

 that, where possible, substances which can react spontaneously do 

 so, if the products of the reaction contain less energy than the 

 reacting substances. In other words, water, if allowed, runs 

 downhill. These two principles are important generalisations for 

 the biologist, but there are many exceptions to them. When a 

 state of strain is made more or less permanent, the organism 

 readjusts itself to meet the strain. That is, the easiest course is 

 not to remove the cause of strain, but to make such an alteration 

 in itself as shall render the external change innocuous. This is the 

 principle underlying the theory of adaptability. A tree arranges 

 its branches so as to offer least resistance to the prevailing wind. 

 Other examples might be drawn from the sciences of physiology, 

 economics, psychology and ethics (Chap. XVII.). 



Physiology. The introduction of an irritating substance into the alimentary 

 canal causes vomiting to remove the cause of irritation, i.e., to relieve strain. 

 Some less exhausting means of relieving strain has to be taken to meet the 

 more or less continuous administration of poison. The cells of the organism 

 so alter as to be immune from such irritation. Mithridates is said to have 

 qualified for the throne of Pontus by the ingestion of all sorts of poisons in 

 his youth. 



Economics. The law of supply and demand, rates of exchange, etc., are 

 merely restatements of this principle of least action. 



Psychology and Ethics. The unjust judge met the early appeals of the 

 widow with a firm refusal. His mind was relieved, his case settled. Because 

 of her very importunity, persistent strain was set up which had to be relieved 

 by reopening the case and giving a just decision. 



Enough has been said to show the possibilities of this deduction 

 from the second law of energetics. The thorough-going mechanist 

 states that this law of least action is the principle governing the 

 action of living as well as dead matter. 



All action, it is said, is a response to stimulus, and is such as 

 wHll most permanently and w4th " least action " relieve the state 

 of strain. The mechanist denies any cause of action but this. 

 What has been taken for the effect of will or instinct is in reality 

 the effect of light, of gravity, of friction, of chemical force, or of 

 some other known or knowable external force. In short, some 

 alteration in an external factor has brought about an instability 

 in the physico-chemical equilibrium of the object or of the 

 organism, and thus a shift in the equilibrium will take place in 

 such a direction as to decrease the magnitude of the alteration 

 which would otherwise occur. The animal, human or otherwise. 



