Davenport interprets the experiments of Loeb and of Reaumur by saying that the diurnal butterflies are so constructed that they react only to a high luminous intensity, and the nocturnal moths to a feeble intensity, "so that the brilliant light of the sun which makes the butterflies come out causes the retreat of the others." This certainly has some reason and several experiments of Loeb prove it, notably that with a diurnal species, Papilio ma-chaon, which, reposing in a diffused light, takes its flight when exposed to the sun's rays. But the explanation of Davenport does not oppose that of periodicity held by Loeb. In reality, these two theses complement each other. It is because their positive phototropism demands a high luminous intensity that the diurnal butterflies fly during the day and rest at night. It is because they demand more feeble luminosity that the nocturnal species fly at twilight or during the night, remaining quiet during the day. Thus understood, the periodicity shows itself to be a simple manifestation of phototropism with these insects.
But something else can happen, as Loeb and Reaumur have shown, aside from any luminous intervention. That is a periodicity acquired by the organism. This is graven upon the being in the course of generations under the phototropic influence of periodical luminous stimulations, and may show itself to-day without their intervention. It has separated itself from the stimulating actions which have produced it.