342 THE VIEWPOINT OF A PHYSIOLOGIST 



to a given environmental change (Prosser, 1955): (a) an animal 

 may alter itself so that it conforms internally with the environ- 

 ment (adjusts) (Fig. 1A), or (b) it may regulate its internal 

 state, and thus maintain relative constancy in an altered environ- 

 ment (Fig. IB). Familiar examples of these two patterns are 

 found in poikilotherms, in which the internal temperature 

 changes in conformity with the environment, and homeotherms, 

 which maintain relatively constant body temperature. The toler- 

 ated range of internal variation of a given character is greater 

 in a conforming than in a regulating animal (Fig. 1A, B). Accli- 

 mation can shift the range of internal variation in a conformer, 

 and it can shift the limits of regulation in a regulator; both pat- 

 terns of response are adaptive in that they permit tolerance of 

 an altered environment. The genotype fixes the point beyond 

 which acclimation is no longer effective. Conformity to the 

 environment and regulation can be analyzed with respect to 

 temperature, osmotic concentration, ionic balance, oxygen partial 

 pressure, and other aspects of the internal state. Some animals 

 may conform on one side of the tolerated range and regulate on 

 the other side; for example, some shore crabs are weak osmoregu- 

 lators in a dilute medium and osmoconformers in more concen- 

 trated media. Different kinds of animals and different populations 

 of the same species may profitably be compared with respect to 

 the pattern of response of internal state. 



Mechanisms and Limits of Regulation. In a regulator any 

 environmental change starts feedback mechanisms which tend 

 to compensate for the change. At some limits, the feedback mech- 

 anisms fail and survival is no longer possible. The mechanisms 

 at the two limits of stress are not necessarily related and may 

 call upon different organ systems. More physiological studies 

 arc concerned with mechanisms of regulation than with any other 

 aspect of the interaction between organism and environment, 

 probably because man is such a good regulator. Yet there are 

 few systematic comparisons of regulating mechanisms in different 

 races and species. 



Recovery from a Deviated Stale. Every animal, whether it is 

 a conformer (adjuster) or regulator, can withstand some brief 



