154 Dynamic Theory. 



of details, and the reaction must be in corresponding detail. There 

 must be repeated blows with the hammer, and the nail must be held in 

 position till started. One hand must hold the nail and the other the 

 hammer. If this is the first mechanical work of the two hands and 

 they are absolutely alike in dexterity, or the want of it, yet the necessity 

 for choice of one of them to hold the hammer and the other the nail is 

 equally absolute. This choice is one of the details of the stimulus and 

 will determine itself possibly by a very small majority one way or the 

 other ; possibly by the fact that one hand is nearer the hammer and tlie 

 other nearer the nail. Which ever gets the hammer is thenceforward 

 the dexter hand and that of the nail the sinister. I say thenceforward, 

 because the moment the first nail is driven the two hands are no longer 

 alike, they have become differentiated. And on a repetition of the 

 stimulus it will find one more ready to respond to the hammer part of it 

 and the other the nail part of it. Where the stimuli are of a simple 

 nature confined to causing such reactions as locomotion merely, or sensa- 

 tion, they affect each side more nearly alike than where the movement 

 in reaction is complicated and detailed. From all which it is easy to 

 see that while the two sides must be developed into a complementary 

 resemblance, j'et they cannot possibly be precisely alike. It being 

 settled that one side shall take precedence of the other, the social habits 

 of man coupled with the economic advantages of uniformity, the choice 

 has generally fallen upon the same one, by chance the right side, 

 although we read of an ancient tribe of barbarians that unanimously 

 gave preference to the left. 



A writer in the American Analyst ascribes the preference given to the 

 right hand to the fact that the heart is situated on the left side, and 

 that the early man, in his combats with his fellows for the purpose 

 chiefly of settling who should be the head of the family and boss of the 

 harem, would instinctively guard that most vulnerable spot with his left 

 arm, which habit would devolve all the active fighting on the right hand and 

 arm. Having become dextrous in fighting it would be also dextrous for 

 work. Such habit would at least be contributory. But our ape ances- 

 tors used their hands to grasp and fight with before the heart had 

 assumed its present exposed position, and we might conjecture that the 

 already dextrous right hand was not passive enough to do its share in 

 guarding the heart, which in consequence shifted over to the shelter of 

 the left, under ^ie law of selection. At any rate there is evidence that 

 the heart has been influenced by the unequal activities of the right and 

 left sides, because while there are usually three cardiac or heart nerves 

 on the right side, called the superior, middle and inferior, there are sel- 

 dom more than tiro on the If ft] viz., the superior and middle, the infe- 

 rior being left out. This want of symmetry must have been brought 



