356 THE AMERICAN NATURALIST [Vol. XLIX 



action and of the phase rule. That changes in the speed 

 of reaction depend upon the concentration of the reacting 

 substances or of the end products of the reaction has 

 been shown in the discussion of the laws of mass action. 

 It is not difficult to see that the respiratory movements 

 owe their origin largely to changes in concentration of 

 carbon dioxide and oxygen, and, since these changes result 

 in a slight change in the concentration of the hydrogen 

 ions it is not difficult to imagine that the law of mass 

 action may be involved in the stimulation of the respira- 

 tory cells in the medulla oblongata. We have given in the 

 third section of this paper two illustrations of conditions 

 coming under the operation of the phase rule. It is true 

 that living matter undoubtedly comprises vastly more 

 complex systems than those described, but that the general 

 principles underlying the reactions are similar in most 

 important respects to the systems employed in labora- 

 tory experiments is scarcely to be doubted. The with- 

 drawal of water from a cell or nerve fiber by osmosis or 

 drying, entailing a quantitative change in the amount of 

 water in the cell, is followed by other changes in the cell 

 which tend to bring about a reestablishment of conditions 

 in accordance with the laws of chemical equilibrium. That 

 a change of phase of some of the components occurs in 

 the process is probable. 



Such influences as drying, applications of heat or me- 

 chanical pressure, whether occurring in the laboratory or 

 in' nature, are known as stimuli, and the changes asso- 

 ciated with their operation in the organism are responses 

 to stimulation. As Jost 47 has pointed out, the formal 

 conditions of existence may act as stimuli to organisms. 

 Although we must admit that a wide gap still exists, it 

 seems to us that the discussion of stimuli and the proc- 

 esses of stimulation in terms of the law of mass action 

 and the phase rule will enable us to meet in some degree, 

 however small, Haldano's objection 1 '" that no causal con- 



47 Jost, "Pflanzenphysiologie," zweite Aufl., p. 618, Jena, 1908. 



48 Haldane, loc. ext., p. 37. 



