XV.] THE LAWS OF HABIT. 179 



touch the tendril with a twig causes it to bend ; but if the 

 twig is at once removed, the tendril soon straightens itself. 

 But the contact of other tendrils of the plant, or the falling 

 of drops of rain, do not produce these effects — proving, 

 apparently, that the tendrils have acquired the habit of 

 disregarding these :^ a wonderful instance of vegetable 

 instinct. 



It is a most important fact that organs increase with Organs 

 exercise, not only iu functional power, but also in size ; ^!°^^,.^" 

 wliile, conversely, organs that are disused, in whole or in 

 part, diminish, not only in functional power, but also in 

 size : and such modifications, like all others, are capable of 

 becoming hereditary. It is difficult to prove that this con- 

 nexion between the habitual exercise of an organ and its 

 magnitude is true of the organs of the nutritive life, because 

 most of them are incapable of any excessive stimulation 

 without producing disease ; but I think the " expansion of 

 the chest " which properly-directed exercise produces, shows 

 it to be true of the lungs. It is well known to be true of Lnnsrs, 

 the muscles; and though the evidence is less direct, I ^^^^rajj, 

 think it is scarcely possible to doubt that it is so of the 

 organs belonging to the nervous system — that the brain, 

 for instance, is increased iu functional power and m size by 

 successive generations of mental cultivation.^ 



It will be noticed by the reader that I have taken the Laws of 

 instances of habit which I have quoted, indifferently from ^^13*0^^ 

 among mental and bodily habits, or, as I prefer to say, both mind 

 from among conscious and unconscious habits; showing*'^ °^' 

 how the same laws of habit govern both the conscious and 

 the unconscious life. 



It is to be observed that the laws of habit do not account Laws of 

 for the origin of every particular habit. This, however, is ^q not 

 not because of any imperfection in our knowledge of the account 



for every 



subject : it is because the laws of habit, by the definition particular 

 of the word, have to do only with the repetition of actions '^'^^^*' 

 and the perpetuation of tendencies ; but they do not neces- 

 sarily throw any light on the cause of the first of a series 



1 Quarterly Journal of Science, April ISuQ'. 

 ^ See Note at end of this chapter. 



n2 



