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THE AMERICAN NATURALIST [Vol. XLII 



Now if these laws are fundamental with all kinds of 

 chemical change they must be at work in the living meta- 

 bolic changes. If the chemical changes associated with 

 protoplasm have any important factor or condition quite 

 different from the state of things which holds when mole- 

 cules react in aqueous solution in a test-tube, then it might 

 happen that the operation of these principles of physical 

 chemistry would be obscured and not very significant, 

 though it is inconceivable that they should be really inop- 

 erative. 



My present intention, then, is to examine the general 

 phenomena of metabolism in an attempt to see whether the 

 operations of these quantitative principles are traceable, 

 and if so how far they are instrumental in giving a clearer 

 insight into vital complexity. 



The Dominance of Irritability ix Physiology. 

 I think that certain manifestations of these principles 

 are indeed quite clear, though not generally recognized, 

 and that this neglect is largely due to the dominance of 

 what our German colleagues call 1 ' Reizphvsiologie"— the 

 notion that every change in which protoplasm takes part 

 is a case of the "reaction" of an "irritable" living sub- 

 stance to a "stimulus." Now this general conception of 

 protoplasmic irritability, of stimuli and reactions was, of 

 course, a splendid advance, the early development and ex- 

 tension of which we owe largely to our veteran physiolo- 

 gist Professor Pf effer, of Leipzig. Great as is the service 

 it has rendered to many departments of botany, yet in one 

 direction, I think, it has overflowed its legitimate bounds 

 and swamped the development of the physical-chemical 

 concepts which I shall indicate later on. The great merit 

 of the "stimulus and reaction" conception is that it sup- 

 plies a very elastic general formula for the sort of causal 

 connection that we find occurring in all departments of bi- 

 ology; a formula which allows the phenomena to be 

 grouped, investigated and formally expounded, whether 

 they be the temporary turgor-movements of "sensitive" 



