178 HAEIT AND INTELLIGENCE. [chap. 



The same from the Same cause, is not coufined to mental and volun- 

 the\mcx)n- ^^^T actions, but has its foundation far down in the uncon- 

 scious life, scious life. One instance of this is the well-known fact, 

 Effect of |.|^g^|. ^j^g -power of medicines and stimulants is diminished 



medicines ■■■ 



and stimii- by constant use. Another and very remarkable instance 

 ants. ^£ ^-^^ j^^ lg ^j^g ^^y. ^^ -which the heart responds to a 



tlie lieart stimulus ; such as a blow, or a sudden fright, or an electric 

 under a gi^ock. The first effect of a stimulus on the heart is to 



stinmlns. 



cause a momentary cessation, or at least slackening, of its 



action. If the shock is violent enough, it causes death ; 



but otherwise the effect passes away, and is followed by a 



quickening of the heart's action — the well-known " beating 



of the heart" produced by a shock. If the stimulus is 



repeated, supposing its intensity to be the same, its effect 



will become less with every repetition,^ showing that the 



heart is acquiring the habit of not making any response to 



it — ^just like the sleeper who acquires the habit of making 



no response to the bell. From such instances as these — 



General which are clearly not exceptional, but normal — I think we 



snectTno- ^^^^ infer, not only that, as already stated, organisms are 



passive ini- capable of acquiring a habit of not responding to stimuli, 



jjiessions. ^^^ ^-^^^ ^^^^ they always do form such a habit, unless 



there is some cause to determine them to form the opposite 

 habit ; namely, the habit of making a response. A still 

 more remarkable iiistance of this law, and one where 

 neither voluntary determination nor nervous action of any 

 kind can come into play, is afforded by the motions of 



Instance of those climbing plants which have been already referred to. 



plants.'^" It is stated by Mr. Darwin, that a thread weighing no 

 more than the 32d of a grain, if placed on a tendril of the 

 Passiflora gracilis, will cause it to bend ; and merely to 



1 Claude Bernard, in Revue den Deux Mondes, March 1, 1865. The 

 stimulus used in such exjieriments is that of an electric current sent 

 through the pneumogastric nerve. The heart, in relaxing under a stimu- 

 lus, acts differently from other muscles, which contract under the same ; 

 the arteries, which have a muscular coat, contract under a stimulus, such 

 as drawing the point of a needle over the skin without making a scratch ; 

 but though this effect is opposite in kind to that jiroduced by a stimulus 

 on the heart, yet, like the latter, it is weakened by repetition. (Carpenter's 

 Human Physiology, p. 231.) 



1 



