ADAPTATION TO ENVTRON-MENT — WARDLAW 391 



A general property of inanimate physical systems is their ten- 

 dency to reach a state of equilibrium, that is, they tend to reach a 

 state in which exchanges of matter and energy between the various 

 parts of the system, and between the system and its surroundings 

 are no longer apparent. It is true that it may be possible to demon- 

 strate that fluctuations in the state of different parts have not entirely 

 ceased in a system which has reached this condition, but these changes 

 which still continue do not lead to any gross or permanent redis- 

 tribution of matter or energy. 



The same property which makes any physical system tend to reach 

 a state of equilibrium, will also resist any agency which tends to 

 disturb this state. If an attempt is made to change such a system 

 in any way the system will react so that the change produced is not 

 as great as it would have been if such a reaction had not taken 

 place. For example, if a volume of gas be heated at constant pres- 

 sure it will expand, and in expanding it will cool, so that the total 

 rise of temperature will not be as great as it would have been had 

 the gas not expanded. This system resists the rise of temperature 

 due to heating. Again, many substances, when they are dissolved 

 in water, cause the temperature of the resulting solution to fall. But 

 these substances are less soluble in the colder water, so that less will 

 dissolve than would have if the temperature had not fallen. The 

 system resists the change of concentration caused by the substance 

 going into solution. This behavior is known as the principle of Le 

 Chatelier. 



The reaction of a living organism to changes of its environment 

 is not, however, limited to that which would take place according 

 to the principle of Le Chatelier. In the first place a living organ- 

 ism is continually expending energy, and so prevents itself from ever 

 attaining a state of equilibrium with its surroundings. Further, it 

 is provided with regulatory mechanisms which not merely resist 

 changes due to alterations of environment, but which are able to 

 neutralize, even to reverse, their effects. To this property of living 

 things Cannon (1929) has applied the term " homeostasis." 



Some of these regulating mechanisms are remarkably efficient. 

 Their object is to maintain unchanged any system of which they 

 are a part. But no such mechanism, however perfect it may be, can 

 render an organism completely independent of external changes. 

 Some response to these changes, however small that response may be, 

 is necessary to set the adjusting mechanism in action, and this 

 mechanism, once set in motion, will not cease to act until the condi- 

 tion aimed at is overshot, no matter how slightly. Such an effect 

 is common to all governing mechanisms. All that they can do is to 

 insure that the variations imjjosed upon the organism by a changing 



