b PHYSIOLOGICAL REGULATIONS 



To accomplish this, the organism uses sensory and locomotory 

 (sensorimotor) equipment. There is thus a distinction between 

 the behaviors of alimentation and absorption, which are exchanges ; 

 and the behaviors of making water available (hydrotropism) or 

 absent. The latter is a frequenting of certain environments, a 

 group of external regulations. 



The study of selection among environments is carried out by 

 well-recognized methods. A rat (§43) is compelled to pass certain 

 obstructions to obtain water. Or, a rat is put in an apartment 

 having both moist atmospheres and dry ones ; when poor in body 

 water it shows preference for the moist ones, thereby minimizing 

 its further water loss, while when rich in body water it is more 

 indifferent about moisture. In other words, there is a correlation 

 between the animal's water content and the water content of the 

 environment that it frequents. 



(3) Variability of content. This answers the question, how 

 constant is the content of component J? And, how far does the 

 content change before something is done about it? In a first ap- 

 proximation an investigator might choose a hundred individuals 

 under specified conditions and ascertain the content of J in each. 

 But sooner or later it might be found that the individuals chosen 

 did not belong to a homogeneous population. It seems to me 

 preferable to make, if possible, a hundred determinations of con- 

 tent upon one individual at successive equal intervals of time. All 

 the while the individual studied will be maintaining itself under 

 conditions that the observer believes are adequate, or under those 

 particular conditions that the animal itself frequents. 



The outcome is a succession of values of content. There are 

 various ways of summarizing this succession. If it be shown that 

 the points are random in time, then a distribution of frequencies of 

 content, or its standard deviation, might characterize the vari- 

 ability. Without that demonstration I believe the physiological 

 variations are best accounted by ascertaining the differences be- 

 tween successive points. These differences are what the organism 

 allows itself in the way of fluctuations of body substance. Con- 

 tents outside the range observed simply do not occur spontane- 

 ously ; this organism in this environment sees to it that they do not 

 occur. Such a variability can be measured in respect to most 

 kinds of content, and represents the net outcome of all regulatory 

 processes. 



