INTRODUCTION O 



stancy in so far as the content is juggled by compensations. First 

 gain exceeds loss, then loss exceeds gain, and in diverse degrees. 

 The mean rate of gain is found by measurement to exceed the rate 

 of loss when the content of substance is lower than usual. In other 

 words, the modifications of exchange that are seen when content is 

 disturbed, whereby gain is faster when content is low and loss is 

 faster when content is high, indicate the way in which exchanges 

 are modified so that in the average instance the usual content tends 

 to be restored. 



This relation of exchanges to contents describes how corrections 

 of content occur. It specifies the precise connection between proc- 

 esses of gain and processes of loss, and the actual gradation of each 

 to the content. Corrections of unusual contents may be thought 

 of roughly, though not strictly, as internal regulations. 



Though faster or slower exchanges stand ready to compensate 

 when the organism has gotten into trouble, provisions for keeping 

 out of trouble, for preventing departures from constancy, might 

 save many a need for cure. Those processes of the body and its 

 parts that discourage excesses and deficit of some property J in 

 the routine of life are therefore observable means of preserving 

 constancies. One method of preservation is complete isolation ; a 

 dead animal in a museum might remain constant in that way. 

 Since complete isolation is incompatible with much of living, 

 partial isolation appears more often. To judge when it is present 

 is feasible in two ways. It may be shown that a dog, or some por- 

 tion of its surface, exchanges J more slowly than does a sponge 

 soaked with saline solution and of equal size, shape, temperature, 

 and so forth. Or, it may be shown that modifying or removing 

 the surface hastens the exchanges. In general, the constitution of 

 the organism is such that the environment exchanges with the body 

 more slowly than it would with another constitution believed to be 

 less differentiated. 



(2) Behavior in selecting among environments. Given the 

 physicochemical constitution of the organism, its peculiar gearing 

 to rates of exchange, and its sensitivity to changes of content, still 

 a factor in its preservation is the keeping of itself in environments 

 that are not too hard on it. By selection of surroundings, the 

 organism may obtain the cheapest maintenance if it find a place in 

 which to be temporarily isolated, or in which water or other sub- 

 stance is optimally available. In general the organism seeks some 

 conditions, and avoids others. 



