220 Trans. Acad. Sci. of St. Louis 
little further the example given above, the night and day tem- 
perature of the owl has already been cited as an example of the 
action of rhythmic priodicity. Most birds are slightly warmer 
during the day than at night, but the owl’s temperature rises 
during its nocturnal time of activity. In most cases the observer 
neither asks nor answers the question of which is the causal and 
which the resultant character; but, fortunately, in this case 
Elton makes his data significant by telling us that the tempera- 
ture variation of the owl may be reversed by reversing its 
periods of activity and rest. 
Our search for authoritative rhythms, rhythms which dictate 
terms to the organism, continues. The sleep of birds seems 
rhythmic; a rooster makes an excellent alarm-clock, provided 
you are willing to be called at four in the summer and six in 
the winter and allow him an extra crow after a midday eclipse. 
Probably one of the most persistent rhythms we can cite is that 
of the menstrual interval in mammals, yet climate, environment, 
disease and even occupation break the measure of that rhythm. 
But despite the fact that our experiments on these three moths 
show that light of a certain quality is the controlling factor for 
activity, and the habitual or rhythmic program of these crea- 
tures may be caused to vary by manipulation of the light, yet 
we must acknowledge the presence of some undiscovered con- 
trolling factor, or else something ingrained in the organism that 
makes them respond in the open at the period when they do. 
The unanswered question which still baffles us is this: If light 
rays of a certain intensity were the sole cause, why do the moths 
not flock to the females when the sun is at an equal distance 
below the horizon in the west (evening) instead of the east 
(dawn)? It may be that the physicist will be able to tell us 
that the light of dawn comprises rays which are different in 
kind from the rays of numerous artificial lights of the city. It is 
quite possible that the visual organs of these creatures are 
adapted to receive the violet rays or something akin, which our 
eyes do not perceive, and which are not present in the artificial 
lights, but I can hardly imagine that the physicist can tell us 
of differences in the rays of light shooting out from the sun an 
hour below the horizon in the west and an hour before appear- 
ing in the east. We may, with equal fitness, apply this same 
question to the activities of the day-fiying prometheas, which fly 
only in the late afternoon. 
