THE EVOLUTION OF MIND 



molecules can return to their positions. There- 

 fore if the first wave is instantly followed by a 

 second, starting from the same point, the line 

 already traversed will be the line of least resist- 

 ance, even more decidedly than before. The 

 second wave will encounter less resistance than 

 the first wave, because it will find its work of 

 altering the positions of the molecules already 

 partly done for it. Thus, according to the mo- 

 lecular mobility of the matter in question, the 

 transit of succeeding waves, along the line once 

 established, will rapidly become less and less 

 hindered. And the process must go on either 

 until the inertia of the molecules along the 

 transit line opposes a minimum of resistance to 

 the passage of the wave, or even until the en- 

 ergy given out by the molecules in changing 

 position adds to the momentum of the wave. 

 In either case there is established a permanent 

 line of least resistance, along which all subse- 

 quent waves that start from the same point 

 must travel. The most familiar illustration of 

 this process is afforded by the facts of magneti- 

 zation, which show " that the establishment of 

 undulations along certain lines determines their 

 continuance along those lines." ^ The case of 



^ An illustration of this principle is perhaps to be found in 

 the mellowing of old violins. According to Professor Tyndall, 

 * * the very act of playing has a beneficial influence ; appar- 

 ently constraining the molecules of the w^ood, w^hich in the 



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