SOME SPECULATIONS CONCEKNING REGULATIONS 457 



concerned to adjust the relative rates of a huge number of ex- 

 changes and contents, not in trying to have the biggest of anything. 



The so-called capacities for exchange are far from being fixed. 

 They are one end of a quantitative continuum, ordinarily appear- 

 ing at the tolerated (maximal) load. They as well as the rest of 

 the continuum are modified as the sequels of past influences (ac- 

 climatization, conditioning). Each capacity and provision for 

 modification is itself a component, probably capable of being stud- 

 ied by the methods outlined. 



(3) Limiting factors. It is sometimes supposed that organ- 

 isms would be well off to have every component that may be utilized 

 available to them. But organisms exist by no such ideal of secur- 

 ity; having everything is often itself a limitation. 



Each ''limiting factor," if such there be, presumably corre- 

 sponds to a load in the organism. The response of the organism 

 tends to reduce that load, by conservation through diminished loss 

 and by accretion through selective gain. It looks as though equili- 

 bration is generally set to minimize the limitations imposed by any 

 factor. 



One supposition is that limiting factors always prevail in an 

 organism. Rate of physical exercise is said to be limited some- 

 times by mechanical movement or by cardiac output ; rate of oxygen 

 consumption by enzyme concentration. Limiting factors may be 

 defined to include relations of all sorts among components, and the 

 faith may be held that one of the knoivn components is the one that 

 matters. But when the view is expressed that the limiting factor 

 can be identified, that factor becomes a key process, sometimes a 

 cause. Thus, rats died in an average of X days when deprived of 

 all alimentary materials (Jackson, '25, pp. 98 to 116). When al- 

 lowed water alone, rats died in an average of Y days. Since Y 

 exceeds X, water seemed crucial. This neglects the fact that many 

 other components could be substituted for water, and in many of 

 those instances Z exceeds X. It spoils the implications of the key 

 when almost any component will take its place. There is no demon- 

 stration in those tests that any of the effective components was 

 qualitatively distinctive in controlling survival, so far as I can see. 



In general, a limiting factor is a hypothetical relation that is 

 entertained so long as few components, and none of them quantita- 

 tively, have been studied. The more exhaustive the studies, the less 

 any single relation is likely to be emphasized. 



