BEHAVIORISTIC STUDY OF THE RAT 
31 
the more rapid increase and arrival at the maximum must have 
been due to the greater sensitivity of the younger animals to 
external stimulation. The very old animals were apparently 
not stimulated in the least by the drums. Their increase in 
activity from day to day was very gradual and seemed to depend 
almost entirely on the slow progressive adaptation and adjust¬ 
ment of the whole bodies to the drums. 
VI. PERSISTENCE OF RHYTHMS OF ACTIVITY AFTER WITHDRAWAL 
OF RHYTHMIC STIMULUS 
It may be well to add here a preliminary account of two experi¬ 
ments which help to throw further light on the general nature of 
spontaneous activity but from rather a different angle than in 
the previous experiments. Only a preliminary account is offered 
at this time because the experiments were carried out on just a 
few individuals and no opportuniy has presented itself for con¬ 
firming the results on larger groups. 
The first experiment was carried out in connection with the 
work described in Chapter II on the relation of spontaneous 
activity to starvation. In that work the interest was limited 
only to the changes in amount of activity during starvation. In 
the present experiment the interest was focused on the changes in 
the manner of expression of activity changes in the form of the 
activity distribution curve during starvation. It will be recalled 
that when the rat is kept in an environment of constant tempera¬ 
ture and illumination and fed just once per day at a definite time, 
the spontaneous activity is distributed in a regular way with 
relation to the time of the single daily meal. What happens to 
the shape of this curve when the factor upon which it depends 
is removed, that is when the animal is starved? 
Two animals were used for this experiment. They were kept 
in the laboratory under conditions of constant darkness and 
constant temperature for four months preceding the beginning 
of the experiment. During this time they were fed very regularly 
and punctually at a certain time just once per day. Care was 
taken to keep all of the conditions just as constant as possible 
throughout this entire period. 
