278 PHYSIOLOGICAL EEGULATIONS 



roles of the former in recovery from water loads is not so easily 

 hypothesized. Each variable also has interrelations with all the 

 others. There is no evidence that an organism is physiologically 

 compartmented to the extent that water exchanges are independent 

 of functions that do not have the name ' ' water ' ' attached to them 

 or implied in them. 



In chapter IX and previous ones I attempted to limit the num- 

 ber of variables considered, to those classified as constituting the 

 time-water system. There, other variables were recognized only 

 in order to label the conditions of measurement, or else to lay them 

 aside for future consideration. In chapters X to XII certain of 

 the other variables are introduced in the form of correlatives of 

 that first system. The study of correlatives may serve to inter- 

 relate many or all the known measurements that are recognizably 

 concerned with water in animals. 



Forces of several physical varieties are undoubtedly concerned 

 in the water exchanges of animals. The difficulties of measuring 

 them with assurance of completeness has so far prevented the con- 

 struction from them of the analog of an equilibration diagram. 

 Grouped factors in water exchange, such as permeability, are 

 within limits useful in representing comparative water exchanges. 

 Body size and age, and their correlatives, serve as backgrounds 

 against which to view the developments of functions of exchange 

 and compensation within the species, and the comparisons among 

 diverse kinds of animals. Factors of both environment {e.g., tem- 

 perature) and heredity distinguish the manners in which mainte- 

 nance and adjustment of water content are achieved. 



