256 PHYSIOLOGICAL EEGULATIONS 



to the arbitrary criteria devised and used in the science of dy- 

 namics. Some forces are proportional to rate of movement or 

 exchange, others to acceleration. The assignment of forces to 

 classes may be a source of satisfaction, but there is nothing ulti- 

 mate in the identification. 



A diagram of water equilibration in Arbacia egg or in Phascolo- 

 soma (figs. 91 and 87) virtually relates initial concentration of body 

 with rate of osmosis. That relation is very readily visualized by 

 those accustomed to juxtaposing a force and a flow, perhaps as 

 partial cause and effect. By extension, water equilibrations in dog 

 and earthworm may be viewed as representing a force and a flow ; 

 in them, however, no causal relation between force and flow has 

 been implied. Often people refuse to believe that data go together 

 (are correlated) unless they be shown some "mechanism" connect- 

 ing them, in terms with which they are already familiar. But bio- 

 logical science does not have to wait until categories of forces can 

 be differentiated and until intermediate steps become familiar; a 

 correlation in itself represents biological processes in which much 

 other interest can be found. 



Analyzing further the concept of osmotic pressure, I believe no 

 physicist pretends to have demonstrated a ''mechanism," in his 

 sense, by which osmotic pressure is effective in moving water. The 

 concept is of the same order as that of gravitation; mathematical 

 relations of masses and the distances between them, sometimes 

 loosely termed ''forces of attraction," have been worked out, but 

 no one presumes to identify the invisible threads by which gravity 

 pulls. Osmotic pressure, however, like gravitational pressure, is 

 a familiar phenomenon. This familiarity, coupled with its fertility 

 in predictions, may account for the feeling of satisfaction that 

 comes to those who find mathematical relations in water exchanges 

 of a sort that fit the generalizations derived from phenomena 

 termed osmotic pressure. 



(2) With each volume increment are correlated diverse modi- 

 fications other than those in rates of volume change. Thus, accom- 

 panying the increments of "plasma" volume in hemorrhage and in 

 transfusion (fig. 96), numerous characteristic changes in blood 

 (and other parts of organisms) of concentrations, physical proper- 

 ties, metabolisms, and many others, have been found. Various 

 attempts have been made to arrange those changes known to occur 

 with hemorrhage into "causal" chains. But each arranger pic- 



