192 PHYSIOLOGICAL KEGULATIONS 



the original. Inevitably, as restoration progresses, the rates of 

 net water exchange decrease until an apparent "resting" state is 

 reached. This state is an equality of gains and of losses, it is a 

 kinetic equilibrium. Hence the maintenance of constancy on the 

 part of the organism as a whole in the midst of flux, is represented 

 in quantitative terms. 



Organisms are continually recovering their water balances; 

 perhaps frogs and earthworms more usually from excesses, mam- 

 mals and land plants more usually from deficits. The distinction 

 between these two tendencies is related to the environment in which 

 the organisms exist. But both groups are equipped equally to 

 meet the reverse situation, in which frogs are kept from water and 

 dogs are drenched with it. Hence, although on which side of water 

 balance the animal remains, is at times a matter of supply from 

 the customary surroundings, each animal is physiologically pre- 

 pared for a wider range of conditions and states. 



Equilibration need not be limited to water content. Instead of 

 correlating rate of exchange with load, it is equally feasible to 

 correlate rate of change of exchange (acceleration) with rate of 

 exchange. In that manner the rate of gain or of loss, total or 

 partitioned, appears as a maintained quantity. Whenever the rate 

 has been large, deceleration intervenes toward its recovery; when 

 small, acceleration intervenes. Such a diagram of second (or even 

 third) derivatives also relates itself to the variabilities of rates of 

 exchange, each more extreme rate of water gain or loss being fol- 

 lowed more often than not by a rate that is more usual (table 12). 



Qualitatively, equilibrations can be classified in a manner that 

 is also not limited by what kinds of organs mediate the exchanges 

 (fig. 112). (a) Some species and most parts have no recognized 

 turnover, depending for recovery from load upon the operation of 

 exchanges that were absent before, (b) In deficits gain may be 

 faster while loss is unchanged; in excesses both may be modified 

 so as to promote net loss, (c) In deficits no modification might 

 occur, rendering recovery unlikely; but that case has not been 

 found, (d) A familiar combination of modifications is shown, the 

 symmetrical analog of case b. (e) In deficits, total loss may in- 

 crease, but not so much as gain does ; allowing possible recovery, 

 but constituting a pattern not yet found. In excesses, total gain 

 increases while loss remains unmodified, thereby adding to load 



