Summary and Discussion 221 
Thus the powers and limits of rhythmic periodicity are ex- 
ceedingly difficult to define. That such a condition exists, that 
these actions occur regularly in nature, we shall not for a mo- 
ment deny. Yet we find that most of these rhythms are modi- 
fied by changes in related conditions, either within or without 
the organism. Of course, we ultimately arrive at the realization 
that the rhythmic periodicity of any organism is only that crea- 
ture’s share in the ‘‘Harmony of the Universe’’; so long as the 
planets swing in rhythm, with regular seasonal and day-and- 
night changes on our earth, all living matter must of necessity 
move in harmony with these (or other resulting) cycles. Hence 
many kinds of behavior are explained as rhythms, which are 
only adaptations to the rhythm of the whole. Only a small 
beginning has been made thus far in the study of this fascinat- 
ing subject. It can probably never be singled out alone, for it 
is inextricably intertwined with adaptation, habit, sleep, fatigue, 
stimulations, psychogenesis, ete. 
The results of the present study regarding rhythmic period- 
icity are two-fold. First, our findings agree with Davenport’s 
interpretation of earlier experiments when he says that noc- 
turnal moths are so constructed that they react only to feeble 
luminous intensity, whereas diurnal lepidoptera react to rays of 
high intensity. The second is that rhythmic periodicity within 
the organism can rarely be regarded as a causal factor or phe- 
nomenon, but only as relational, showing the interdependence of 
creatures or a creature and its environment. It is usually no 
more than a ecreature’s habitual adjustment to its surroundings, 
but there are no doubt many instances wherein rhythmic action 
is truly an innate character of the tissue, as the pulsations of the 
heart-tissue of the early chick embryo. At the present moment I 
am inclined to say that those actions which require a stimulus to 
start them are only reactions, or adaptations in behavior, while 
those actions which require outside interference to stop them 
or modify them may be truly termed rhythms. 
