40 ANIMAL PHYSIOLOGY. 
THE LAW OF HABIT. 
Every one must have observed in himself and others the 
tendency to fall into set ways of doing certain things, in which 
will and clear purpose do not come prominently into view. 
Further observation shows that the lower animals exhibit this 
tendency, so that, for example, the habits of the horse or the dog 
may be an amusing reflection of those of the master. Trees are 
seen to bend permanently in the direction toward which the 
prevailing winds blow. 
The violin that has experienced the vibrations aroused by 
some master’s hand acquires a potential musical capability not 
possessed by an instrument equally good originally, but the 
molecular movements of which never received such an educa- 
tion. ; 
It appears, then, that underlying what we call habit, there is 
some broad law not confined to living things; indeed, the law of 
habit appears to be closely related to the law of rhythm we 
have already noticed. Certain it is that it is inseparable from 
all biological phenomena, though most manifest in those organ- 
isms provided with a nervous system, and in that system itself. 
What we usually call habit, however expressed, has its physical 
correlation in the nervous system. We may refer to it in this 
connection later: but the subject has relations so numerous and 
fundamental that it seems eminently proper to introduce it at 
this early stage, forming as it does one of those corner-stones of 
the biological building on which the superstructure must rest. 
When we seek to come to a final explanation of habit in this 
case, as in most others, in which the fundamental is involved, 
we are soon brought against a wall over which we are unable 
to climb, and through which no light comes to our intellects. 
We must simply believe, as the result of observation, that it 
is a law of matter, in all the forms manifested to us, to assume 
accustomed modes of behavior, perhaps we may say molecular 
movement, in obedience to inherent tendencies. But, to recog- 
nize this, throws a flood of light on what would be inexplicable, 
even in a minor degree. We can not explain gravitation in it- 
self; but, assuming its universality, replaces chaos by order in 
our speculations on matter. 
Turning to living matter, we look for the origin of habit in 
the apparently universal principle that primary molecular 
movement in one direction renders that movement easier after- 
