From a consideration of these phenomena, which have not been studied enough, it seems rather certain that acquired periodicity-that is to say, innate-has as an antecedent a dependent periodicity closely associated with the rhythm of the environment, and its autonomy, its independence of the stimulating actions, results from a longer or shorter exercise of this dependent periodicity. The periodicity is simply dependent with the blood-sucking larvae of the Auchmero-myias and the Choeromyias, it seems already acquired with the misanthropic Pagurus of the Gulf of Gascony, and it is complete with the nocturnal moths and the fireflies. There are all kinds of intermediary steps between these two kinds of periodicity.
How can acquired periodicity come from dependent periodicity? Without doubt, on account of physicochemical phenomena which provoke the latter. Bohn attributes in large part the rhythmical movements of Convoluta, Liiiorina, and Pleurosigma "to alterations of desiccation and rehydration" which are produced in their organisms in the periods of flux and reflux. We know, also, that the periodical movements of plants are due to the same causes. On the other hand, the daily rhythms of diurnal or nocturnal insects and of phosphorescent articulates are in close dependence upon the intensity of the light rays, and we must not forget that these count among the most efficient chemical agents. As to the periodic movements of the blood-sucking larvae, they are produced by heat, whose chemical action is not less than that of light.