Chapter IX 

 GENERAL FEATURES OF WATER EXCHANGES 



§ 68. The investigation is now provided with an array of ma- 

 terials. These materials offer information, and disclose relations, 

 that seem to explain in part what each kind of organism does with 

 water. Each handles water in a special way, as though there is 

 nothing like it, and as though machinery to handle it is indis- 

 pensable to life. How particular species, and indeed, other living 

 units, adjust and recover customary water contents, seems to have 

 been ascertained. 



The next task is to seek among those materials the constant and 

 the inconstant relations. Instead of letting each species furnish its 

 own story, I now try to find what all species together can tell about 

 how to handle water. Two inclusive questions can be raised. What 

 are the general features of all regulations of water content? And 

 what quantitative factors differentiate the maintenances and com- 

 pensations in one living unit from those in another 1 Each question 

 will be answered by asking a series of subsidiary questions. 



For the present, the relations to be explored are among the four 

 sorts of variables constituting the water-time system. Three of 

 these (load, rate, time) are measured as such, though often rate of 

 exchange equals change of load in a unit of time. The fourth 

 (velocity quotient) is a particular ratio between two others (rate 

 and load). Additional variables that creep in, fall in the quali- 

 tative categories of paths, species, and types of loading. 



Those uniformities to be found among species and parts are 

 similarities of correlations among the four variables; they are to 

 be listed here. Diversities among them are of two sorts ; one is 

 quantitative differences in correlations among the four variables, 

 which will also be sought. Such diversities are supplemented by 

 a second sort, namely, other correlatives of the four variables, some 

 of which will be ascertained later (chapters X to XII), additional 

 data being considered to that end. 



'^ 69. Some limitations 



For comparing the water relations of two organisms of great 

 anatomical diversity, such as a mammal and a worm, several dimen- 



168 



