x.] MR. DARWIN'S CRITICS. 251 



capable of being expressed in terms of one another. 

 Whether we shall ever be able to express consciousness 

 in foot-pounds, or not, is more than I will venture to 

 say ; but that there is evidence of the existence of some 

 correlation between mechanical motion and conscious- 

 ness, is as plain as anything can be. Suppose the poles 

 of an electric battery to be connected by a platinum 

 wire. A certain intensity of the current gives rise in 

 the mind of a bystander to that state of consciousness we 

 call a "dull red light" a little greater intensity to 

 another which we call a " bright red light ; " increase 

 the intensity, and the light becomes white ; and, finally, 

 it dazzles, and a new state of consciousness arises, which 

 we term pain. Given the same wire and the same 

 nervous apparatus, and the amount of electric force re- 

 quired to give rise to these several states of consciousness 

 will be the same, however often the experiment is re- 

 peated. And as the electric force, the light-waves, and 

 the nerve-vibrations caused by the impact of the light- 

 waves on the retina, are all expressions of the molecular 

 changes which are taking place in the elements of the 

 battery ; so consciousness is, in the same sense, an ex- 

 pression of the molecular changes which take place 

 in that nervous matter, which is the organ of con- 

 sciousness. 



And, since this, and any number of similar examples 

 that may be required, prove that one form of conscious- 

 ness, at any rate, is, in the strictest sense, the expression 

 of molecular change, it really is not worth while to 

 pursue the inquiry, whether a fact so easily established 

 is consistent with any particular system of molecular 

 physics or not. 



Mr. Wallace, in fact, appears to me to have mixed up 

 two very distinct propositions : the one, the indisputable 

 truth that consciousness is correlated with molecular 

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