20 
CURT P. RICHTER 
behavior side of the problem—the total reactions of the organisms 
to different temperatures—has been neglected. This may be 
brought out by a consideration of the problem of the relation of 
metabolic processes of the body to external temperature. 
Physiologists speak of a critical point of the temperature of the 
air surrounding an organism at which the metabolism of the 
organism is at a minimum. The increase in metabolism below 
this point is brought about by chemical processes, while the 
increase above this point is brought about either by vaso-motor 
or by respiratory changes, or by muscular or glandular changes. 
Nothing is said however regarding the organism’s activity, its 
total reactions to the changes in temperature. Indeed in most 
of the experiments on this problem the animals are confined and 
tied in very small chambers where not even the slightest movement 
is possible. Without this information of the behavior of total 
activity the physiologist’s study of the problem must remain on 
a purely static level. 
IV. THE RELATION OF SPONTANEOUS ACTIVITY TO ILLUMINATION 
In the environment of all animals and at all times there is 
some degree of illumination or else no illumination at all, that is 
complete darkness. It is of greatest importance in studying be¬ 
havior problems to know just how these ever present factors 
affect the activity of organisms. On the basis of the present 
knowledge on this subject animals are classed as being either 
nocturnal or diurnal, according to whether they are more active 
at night or during the day. This classification as it stands 
at present is based not only on the reactions of the animals to 
the light and darkness of the day and night, but also on the reac¬ 
tions of the animals to other factors of their environment, such 
for instance as possibilities of getting food, attacks from other 
animals, etc. This is a decided limitation. There is still a 
further limitation of this classification, in that it places animals 
in either one group or the other and does not tell just to what 
extent the animals are more active in the dark than in the light, 
or the converse. 
