PH. XVI.] TEE EVOLUTION OF MIND. 143 



different from what it was a moment ago. The passage of 

 the wave has pushed a linear series of molecules out of 

 position, and a short time must elapse before these molecules 

 can return to their positions. Therefore 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 resistance, 

 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 molecular 

 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 energy 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 re- 

 jistance, along which all subsequent waves that start from the 

 same point must travel. The most familiar illustration of 

 this process is afforded by the facts of magnetization, which 

 show " that the establishment of undulations along certain 

 lines determines their continuance along those lines." ^ The 

 case of liquid matter flowing through solid matter — as when 

 currents of rain-water, percolating through loose soil, gradually 

 break away obstructing particles and excavate small channels 

 which ultimately widen and deepen into river-beds — is a 

 case ill which similar dynamic principles are involved. In 

 all these cases, " if we confine our attention to that part of the 

 motion which escaping transformation continues its course, 



^ An illustration of this principle is perhaps to bo found in the mellowing 

 of old violins. According to Prof. Tyndall, " the very act of playing has a 

 beneficial influence ; apparently constraining the molecules of the wood, 

 which iu the first instance were refractory, to conform at last to tie require- 

 ments of the vibrating strings." O71 Sound, p. 90. As Dr. JIaudsley would 

 Bay, "musical residua" remain in the molecular structure of the wood* 



