510 THE POPULAR SCIENCE MONTHLY.
Now, there are no more perfect examples of closed systems whicli are the seat of constant motion than living beings, because life is a type of never-ceasing co-ordinate changes. Can instances be found in living beings ? What we have to look for is a reaction- ary force, which not only opposes the generating stress by setting up one like it and opposite in direction, but, furthermore, as the change progresses it must tend to reduce the initial impulses which created the change, this being what I have ventured to call the new principle.
Let us once more consider the case of muscular fatigue in the light of this idea. The initial cause of muscular contraction is the nervous stimulus sent to the organ. As soon as the muscle contracts, the motion within it generates free acid. This acid, which is therefore of the nature of a reactionary product, reduces the irritability of the fibrillse, but, in addition, it reduces the power of a nerve to transmit and to generate nerve force, so that not only is the mandate traveling along the nerve resisted by the greater sluggishness of the muscle, hut also the nerve force itself, which is the material form taken by the will, is attacked and less- ened in the very place of its origin.
Is this not closely analogous to the cutting down of the electro- motive force of the dynamo by the current which that same force creates ?
Another example, dealing with the chemical rather than the mechanical force of the body, is found in digestion. Hunger is a sensation which is probably the collective cry sent up from all parts of the organism ; but the stomach and certain nerves seem to be its principal seat. The irritability of a hungry man is a well-known phenomenon. The exacerbation of many nervous symptoms due to exhaustion is familiar to physicians. Hunger, then, is an active, not a passive, state, and denotes that certain changes of a positive kind are going on which tend to proceed to the ultimate destruction of the animal if not checked. When food enters the stomach and commences to be digested that organ works harder, but the production of this labor taxes the forces of the body by calling blood away from other organs ; in addition to this, the nutriment given to the nerves stops the wasteful action going on in them. So here, as in the previous cases, the reaction set up cuts down the initial cause, and hunger vanishes.
Many other instances might be drawn from physiology, but, leaving them on one side, I desire to make a few suggestions con- cerning that larger aggregate of life the social state.
The warlike temperament of man has been one of his most prominent characteristics from the earliest times. To live to fight has been the chief aim of most primitive peoples, and has been a leading occupation of all civilized ones. Armies have grown in