XXX11 INTRODUCTION. 



chemical, nervous, sensory, emotional, and intellectual. That these 

 forces are perfectly coordinated that there is some definite relation 

 among them which explains the marvellous dynamic unity of the 

 living organism, does not admit of question. That this relation is 

 of the same nature as that which is found to exist among the 

 purely physical forces, and which is expressed by the term ' Correl- 

 ation,' seems also abundantly evident. From the great complex- 

 ity of the conditions, the same exactness will not, of course, be 

 expected here as in the inorganic field, but this is one of the neces- 

 sary limitations of all physiological and psychological inquiry ; thus 

 qualified the proofs of the correlation of the nervous and mental 

 forces with the physical, are as clear and decisive as those for the 

 physical forces alone. 



If a current of electricity is passed through a small wire it 

 produces heat, while if heat is applied to a certain combination of 

 metals, it reproduces a current of electricity; these forces are, 

 therefore, correlated. A current of electricity passed through a 

 small portion of a motor or sensory nerve will excite the nerve- 

 force in the remainder, while, on the other hand, as is shown in the 

 case of the torpedo, the nerve-force may generate electricity. 

 Nerve-force may produce heat, light, electricity, and, as we con- 

 stantly experience, mechanical power, and these in their turn may 

 also excite nerve-force. This form of energy is therefore clearly 

 entitled to a place in the order of correlated agencies. 



Again, if we take the highest form of mental action, viz. : will- 

 power, we find that while it commands the movements of the sys- 

 tem, it does not act directly upon the muscles, but upon the cerebral 

 hemispheres of the brain. There is a dynamic chain of which 

 voluntary power is but one link. The will is a power which excites 

 nerve-force in the brain, which again excites mechanical power in 

 the muscles. Will-power is therefore correlated with nerve-power 

 in the same manner as the latter is with muscular power. Dr. 

 Carpenter well observes: "It is difficult to see that the dynamical 

 agency which we term will is more removed from nerve-force on 



