BEHAVIORISTIC STUDY OF THE RAT 
29 
This curve despite its irregularities has in general the same 
shape as the curves obtained by the other two methods. The 
age of maxiipum activity determined in this manner lies near 135 
days, which is about half way between 100 and 160 days, the 
ages of maximum activity determined by the other methods. 
Another aspect of this problem of the relation of activity to 
age was brought out incidentally in connection with the experi¬ 
ments on the revolving drums described above. 
It may be well to emphasize at this point that there is a tend¬ 
ency among workers interested in the activity of animals to 
confuse the activity of an animal due to external stimulation with 
the activity which manifests itself more or less spontaneously 
when the animal is free from all active external stimulation. 
Due to this confusion a number of workers say that the rat is most 
active at the age of thirty to forty days. They base this opinion 
very largely on the fact that every time they enter their labora¬ 
tories they find animals of this age active, while older animals 
remain inactive. They fail to take into consideration that the 
greater activity of the younger animals at these times may be due 
simply to the fact that these animals may be more sensitive to 
any external stimulation, noise made in entering the laboratory, 
and that during the rest of the time when they are not stimulated 
they may remain quite inactive. That is to say that younger 
animals are spontaneously less active than older animals, but 
that they are more sensitive to external stimulation. 
The differences between these two kinds of activity are brought 
out by the records of the activity of six animals of six different 
ages in the revolving drums. These drums both serve to stimu¬ 
late the animals and at the same time to record the activity in 
reaction to the stimulation. The stimulating effect of the drums 
is well known and can easily b£ demonstrated for every movement 
even the slightest destroys the equilibrium and causes counter 
compensatory movements to be elicited. How are animals of 
different ages affected by this stimulation? 
The records of this experiment are shown in figure 15 where the 
abscissae give the days of the experiment, while the ordinates 
give the total number of half revolutions made each day and their 
