VARIABILITIES OF WATER RELATIONS 77 



tion, and intake of water lags behind other means of compensation. 

 In this instance, as in very many others, constancy of all quantities 

 is evidently impossible. I can decide to what quantity the organ- 

 ism is apparently insensitive only in terms of some specified mea- 

 surement. Actually the weight of evidence is that both a and b 

 are partial factors, basal body weight changing in a trend while 

 ± AW oscillates about it. 



In all this there is no implication as to whether the dog is better 

 or worse served by having a small range of water contents, i.e., a 

 stricter constancy of it. There is no evidence that great constancy 

 is more fit, or that infrequent inversions of net exchange are 

 cheaper, or whether some mean between them is optimal. The 

 present concern is to find just what constancy prevails under arbi- 

 trarily chosen conditions, when the state of the dog together with 

 the conditions impinging preserve the content of body water within 

 the fluctuations observed. Insofar as the fluctuations are limited 

 both in amount and in time sequence, their evaluation describes the 

 organism as a preserved unit. For often ^'constancy is in itself 

 evidence that agencies are acting, or ready to act, to maintain this 

 constancy" (Cannon, '32, p. 281). 



The physiological significance of variations in content is, I be- 

 lieve, that the limitation of the variations measures the maintenance 

 of that content. Whenever content tends to change, activities on 

 the part of the organism intervene to oppose the change. If this 

 could be said in mathematical language alone, many possible mis- 

 understandings would be avoided. For, such a form of expression 

 appears to hold fewest connotations and implications. On the 

 other hand, numbers and symbols would convey little idea of an 

 organism's constancy and maintenance, were physiological terms 

 not placed in parallel. 



Other measures of water content might be used in place of (!) 

 body weight, and each would fluctuate when observed at 24-hour 

 intervals. (2) Chemical analysis of a group of dogs, killed one or 

 more every day, would measure content. (3) Metabolic retention, 

 measured as total gain of water minus total loss of water, would be 

 another. (4) Sensible retention (intake of water as such minus 

 output of water as such) could be measured in 24-hour periods. 

 (5) The volume of distribution (see § 58) of a substance such as 

 urea or sulfanilamide could be repeatedly ascertained on one 

 individual. 



