12 
CURT P. RICHTER 
In order to avoid misunderstanding it may be well to emphasize 
here that this curve is a composite curve made up of the individual 
curves of forty animals. The individual curves of course look rath¬ 
er different. A typical individual curve is shown in figures 17 and 
18 on pages 33 and 34. In these curves the alternation of periods 
of activity and inactivity stands out very clearly. These periods give 
the curves a very ragged and irregular outline. The general form of 
these curves corresponds however to that of the composite curve 
shown in figure 6. During the inactive parts of the curve the 
amount of activity in each activity period is relatively very small 
but the periods are present nevertheless. The discrepancy 
between Szymanski’s results and the results presented in the 
previous chapter on the number of activity periods per day may 
be due to the fact that Szymanski failed to take these small 
periods into account. The importance of these small periods will 
be brought out in another place. 
The activity distribution curve shown in figure 6 was for 
animals 250 days old. The form of this curve becomes very 
much modified in very young animals and as well in very old 
animals. In the former the “hump” of activity begins almost 
immediately after the daily feeding period, figure 7, while in the 
latter its onset is very much delayed. The “hump” of activity 
is most striking in animals about 200 days old. After that age it 
becomes less and less marked until in old age, over two years, 
the curve becomes smoothed out and the hump is obliterated. 
This is shown schematically in figure 8. 
In order to make certain that the shape of the activity dis¬ 
tribution curve shown in figure 6 did not depend on any external 
factors, for instance very slight changes of illumination imper¬ 
ceptible to the human eye due to the daily change of light and 
darkness on the outside of the laboratory, or else that the curve 
did not depend on the general city noises and sounds of the day, 
the time of feeding was changed from 12 o’clock noon to 12 o’clock 
midnight, also to 8 o’clock in the evening. In all cases the shape 
of the curve remained practically the same. This was true also 
when records were taken during constant illumination which 
seems rather conclusive evidence against changes in illumination 
having anything to do with results obtained. 
