Xo. 629] BEHAVIOR AND ASSIMILATION 507 



Let us consider, by way of example, a simple case of a 

 system of three phases, namely, ice, water and water- 

 vapor, in equilibrium with respect to temperature and 

 pressure. This system can exist only at 0° C. and at 

 atmospheric pressure. If heat is withdraw^n from the 

 system and the pressure and volume are kept constant, 

 a part of the water freezes to ice and the temperature is 

 maintained by the latent heat of fusion. Since the for- 

 mation of ice would increase the volume and therefore 

 raise the pressure, a certain amount of vapor condenses 

 to water. If heat is added to the system, changes take 

 place in the reverse direction. Similar changes occur 

 when the pressure is altered at constant temperature. In 

 general (Findley, '04, pp. 56, 57), ''so long as the three 

 phases are present, no change in the trin[)eratiii(' or 

 pressure of the system can occur, but o)dii rlKuitit s in (lie 

 relative amounts of the phases; that is to say, the etfect 

 on the system of change in the exlenial eoiiditions is op- 

 posed by the reactions or changes w hieh take ])hK'e within 

 the system (p. 60). If the speeitie \ ohmirs of the phases 

 are know^l and the sign of the heat effects wliicli accom- 

 pany the transformation of one phase 'into the other, it is 

 possible to predict (by means of the theorem of Le Cha- 

 telier) the changes which will be produced in the system 

 by alterations of the pressure and temperature. ... It 

 should be noted that ./// Ilin r /,hasrs ar< iHrnlr..} lu the 

 change." 



behavior of living organisms. The system in eqnililn'ium 

 is the organism. The <'xternal eondition of the system is 

 the environment. The constraint by whieh the e(iuilib- 

 rium is shifted is the stlmnlns. The I'eaetion tliat op- 

 poses the constraint and ])artiall\- annuls its effect is the 

 response of the irrital)h' mechanism. 



A stimulus is generally considrred to be any chan.u-e in 

 the rehition between the organism and a fact(".r of its .-n- 

 vironnn-nt. hut n<. rrs{,ons<> occurs unless the change be 

 .me bv which the eMuilihrium is shifted, to effect wliich it 



