SOME FORCES OF THE SOCIAL ORGANISM. 507 



case of mechanical stability, whether of a house, a tree, a moun- 

 tain, or a man walking. 



There is another term to be considered in this connection 

 which is used by physicists frequently in a special sense, and that 

 is the word system. An open system is one in which the products 

 of a change do not return into themselves, as, to repeat illustra- 

 tions already given, a waterfall, a fire, or a whirlwind. A closed 

 system, on the other hand, is one in which these products are re- 

 tained, or at least the internal changes of shape or stress do not 

 travel away ; a pendulum might be spoken of as a closed system, 

 because, while gravity causes it to move down to the lowest por- 

 tion of its arc, the motion thus acquired carries it beyond this 

 point and up the other side, thus converting actual into poten- 

 tial energy, and this alternate conversion and reconversion will 

 go on forever in the absence of friction. Some machines may also 

 be considered under this head, as a stationary steam engine. The 

 apparatus is indeed receiving steam at one end and dispensing 

 mechanical power at the other, but on the average these balance, 

 leaving the machine as the seat of many complicated stresses 

 playing back and forth against each other. In this respect the 

 engine is a closed system. 



Now, it may be laid down as a general proposition that self- 

 limited changes occur only in closed systems. Also that any 

 organized structure, and more especially a living animal, may be 

 considered as a closed system ; for, though it is true the animal is 

 dependent on food, and is constantly giving out heat and other 

 forms of energy, still, for any moderate period of time these bal- 

 ance each other, while the organism as a whole is dependent for its 

 integrity upon a constant regulation of its internal states through 

 incessant changes, which, moreover, must be self-regulating in 

 character. Life is but a sequence of these delicately adjusted 

 actions and reactions. Physiology is full of instances of this fact. 

 One illustration may suffice. When a muscle is exercised, a por- 

 tion of it is oxidized or burned. Some of the products of this oxi- 

 dation are acids, but the vitality of a muscle is diminished by the 

 presence^of an acid. The sense of fatigue is the language by 

 which the nerves inform the brain of this muscular state, and the 

 mandate, Let the organ have repose, is only another way of telling 

 the scavengers of the body to go and take that acid away. Here, 

 then, the law is illustrated ; the voluntary change in the muscle, 

 represented by its work, sets up a chemical force which limits and 

 finally stops the change by which it was produced. 



One machine, more than any other I know of, represents the 

 play of self-limited forces admirably. It is the alternating dy- 

 namo for the production of currents of electricity. It has also 

 many of the attributes of an organic being, and I, almost feel like 



