WATER BALANCES AND EXCHANGES 293 



(3) Many diversities are related with body size, age, surface of 

 exchange, possible forces of exchange, structural differentiations, 

 and other features of the units whose content is being equilibrated. 



(4) Though paths of water exchange differ, no one kind of path 

 is regularly (inherently) slower or faster than other kinds, or its 

 exchanges less or more accurately related to water load. 



(5) Species and living units may be arranged in various series, 

 depending, for instance, on velocity quotients in recoveries. For 

 gains the order of rates is : dog, snake, rat, plasma, Phascolosoma, 

 earthworm, man, frog, muscle. For losses : plasma, dog, rat, earth- 

 worm, Phascolosoma, man, frog, muscle, snake (figs. 106 and 101). 



Such series are often qualified by the fact that water excess was 

 produced in each unit (dog, plasma, frog, and muscle) by slightly 

 different procedures (administration by stomach, injection of 

 citrated blood by vein, injection by peritoneum). Further restric- 

 tion in type of load, however, would not permit comparison of whole 

 rabbit with its plasma or its leucocytes ; even injection or previous 

 osmosis does not allow, in diverse units, identity of loading. As in 

 all scientific investigations, tentative comparisons are drawn, pro- 

 visional generalizations are obtained, without assurance that an- 

 other generation of physiologists will find as much satisfaction in 

 these particular relations as in some others. 



The above diversities among species are the materials for study 

 of the comparative physiology of water relations. All the quantita- 

 tive materials of tables 8 to 16 are the outcome of that study, which 

 can scarcely be summarized more briefly. Equilibration diagrams, 

 having a uniform plan, fall into classes according as one or two 

 kinds of exchanges are modified in positive and in negative load 

 (fig. 112). Their quantitative study tells how much modificatioil 

 in each kind occurs, perhaps as evaluated by augmentation ratios. 



Rough comparisons of water exchanges have been made in the 

 past, but those with which I am familiar do not appear justified. 

 ' ' The amphibian is unable to preserve his water content . . . inde- 

 pendent of that of his outer world," says Cannon ('32, p. 283). 

 That is no distinction of amphibia or of aquatic animals, for all spe- 

 cies need access to water at some times. Even amphibia do not wait 

 for water to come, but seek it (Czeloth, '30), specific behaviors being 

 indispensable means of preserving water content. ''The urine out- 

 put of most dogs is less readily increased on water ingestion that 

 is the case with man," imply White and Findley ('37b, p. 747). 



