434 THE POPULAR SCIENCE MONTHLY. 



of certain outward causes, or India-rubber becomes friable, or plaster 

 " sets." All these changes are rather slow ; the material in question 

 opposes a certain resistance to the modifying cause, which it takes 

 time to overcome, but the gradual yielding whereof often saves the 

 material from being disintegrated altogether. When the structure has 

 yielded, the same inertia becomes a condition of its comparative per- 

 manence in the new form, and of the new habits the body then mani- 

 fests. Plasticity, then, in the wide sense of the word, means the pos- 

 session of a structure weak enough to yield to an influence, but strong 

 enough not to yield all at once. Each relatively stable phase of equi- 

 librium in such a structure is marked by what we may call a new set 

 of habits. Organic matter, especially nervous tissue, seems endowed 

 with a very extraordinary degree of plasticity of this sort ; so that we 

 may without hesitation lay down as our first proposition the following, 

 that the phenomena of habit in living beings are due to the plasticity* 

 of the organic materials of ichich their bodies are composed. 



But the philosophy of habit is thus, in the first instance, a chapter 

 in physics rather than in physiology or psychology. That it is at 

 bottom a physical principle is admitted by all good recent writers on 

 the subject. They call attention to analogues of acquired habits ex- 

 hibited by dead matter. Thus, M. Leon Dumont, whose essay on 

 habit is perhaps the most philosophical account yet published, writes : 

 "Every one knows how a garment, after having been worn a cer- 

 tain time, clings to the shape of the body better than when it was 

 new ; there has been a change in the tissue, and this change is a new 

 habit of cohesion. A lock works better after being used some time ; 

 at the outset more force was required to overcome certain roughnesses 

 in the mechanism. The overcoming of their resistance is a phenome- 

 non of habituation. It costs less trouble to fold a paper when it has 

 been folded already. This saving of trouble is due to the essential 

 nature of habit, which brings it about that, to reproduce the effect, a 

 less amount of outward causality is required. The sounds of a violin 

 improve by use in the hands of an able artist, because the fibers of the 

 wood at last contract habits of vibration conformed to harmonic rela- 

 tions. This is what gives such inestimable value to instruments that 

 have belonged to great masters. Water in flowing hollows out for 

 itself a channel, which grows broader and deeper, and, after having 

 ceased to flow, it resumes, when it flows again, the path traced by 

 itself before. Just so, the impressions of outer objects fashion for 

 themselves in the nervous system more and more appropriate paths, 

 and these vital phenomena recur under similar excitements from with- 

 out, when they have been interrupted a certain time." 



Not in the nervous system alone. A scar anywhere is a locus 

 minoris resistentice, more liable to be abraded, inflamed, to suffer 



* In the sense above explained, which applies to molecular structure as well as to that 

 of grosser parts. 



