84 ORGANISM AND ENVIRONMENT 
Rubner showed that nitrogenous food, fat, and car- 
bohydrate are substituted for one another as material 
for oxidation in exact proportion to the energy which 
they yield in the body. The sum of this energy per 
unit of body weight remains constant during rest, 
whether food is given or withheld. Even when loss of 
heat is prevented as far as possible, the oxidation pro- 
cesses in the body remain sensibly constant in spite of 
prolonged deprivation of food. The diminished oxida- 
tion of nitrogenous material during starvation depends 
simply on the fact that the body stores its energy- 
forming material mainly as fat, and consequently uses 
up mainly fat during starvation. When all the fat 
is exhausted there is again, before death from starva- 
tion, a great increase in the oxidation of nitrogenous 
material. This latter fact adds new emphasis to the 
persistence of the oxidation processes. 
The internal environment which is maintained so 
constant is in reality the expression of a balance be- 
tween activities which disturb and activities which 
restore it. When we speak of “the function” of an 
organ and regard this function as what it does to 
restore the internal environment we are thinking in 
terms of an imperfect and misleading conception of 
what that organ is, and what an organism is: for we 
are thinking -of only one side of its activities to the 
exclusion of others which are just as important. To 
put this-into philosophical language we are thinking 
abstractly, or regarding only a part of the reality we 
are dealing with. We can speak more correctly of 
