Nul values of temperature, illumination, and pressure are those values at which an animal is not stimulated to migrate vertically. A method is described for their determination, and the numerical solutions were calculated for eleven species of oceanic copepods. As with the corresponding
factors, which are the relative sensitivities of the animals to departure from the nuls, there are diurnal rhythms in the nuls; they vary downwards through the population of a species, and they show characteristic differences in shallow- and deep-living species. The fact that the nuls approximate
the environment, and vary downward as it does, supports the validity of the basic mathematical model.
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