WATER BALANCES AND EXCHANGES 297 



etc., are often correlatives. Any of the above theories may be use- 

 ful as working hypotheses ; they have suggested experiments and 

 measurements. But to suppose that a complete and permanent 

 understanding of water regulation will be obtained by pushing the 

 responsibility for regulation of A upon B which in turn needs regu- 

 lation by C, and so ad infinitum, is encouraging an endless game of 

 shuffling the factors. All theories actually take the form of mak- 

 ing variable A dependent upon variable B. Sooner or later some 

 other investigator suggests that B depends on A. Evidently no 

 one theory satisfies either the data or the investigators. Eventually 

 there arise one or more inferences concerning each of the many 

 relations that are likely to be established. 



I know of no theory specifying the forces concerned in water 

 maintenance that applies to all organisms and living units. I share 

 the opinion that forces balance across the boundaries of all living 

 units in stationary states. I too recognize partial similarities be- 

 tween some of the sorts of forces present in living units and those 

 in non-living systems. I see little probability of identifying all the 

 forces present and assigning a proportionate role to each; or of 

 separating forces concerned in water balance from those related 

 to other components. And I see no way of feeling completely 

 satified even though categories are assigned to each force present. 



The theory that all organisms are set to adjust their water con- 

 tents by modifying their rates of water exchanges, appears to me 

 to be as valuable an inference as any. For the cases investigated 

 it has changed from a theory to a rule. The pattern in which the 

 organisms are set, and the quantitative features of their adjust- 

 ments, are revealed by measuring further the modifications of 

 composition, of metabolism, of behavior, and of variability that 

 coincide with diverse water increments of the organism. 



Very possibly there is no ultimate ''determinant" of water con- 

 tent, short of the whole living organism and its environment ; for 

 anything less is a very partial account of the adjustments con- 

 cerned. If so, then whatever all living units have in common, all 

 water regulations also have in common. In other respects very 

 diverse relations may be shown, and no theory general enough to 

 preclude other theories is to be expected. 



<^ 110. Summary 



The course of the investigation may be described as follows. 

 At the start, particular procedures (especially the emphasis on 



