Of the six variables in the modified Stokes equation, three (g, pw and n) are either constants or are independent variables. The other three (size, density and form resistance) are organismic properties and, as such, are open to adaptation and evolutionary modification through natural selection. It is possible that certain metabolites released into the water by organisms also affect the viscosity of the adjacent medium. The present section looks briefly at the influences of each on sinking behaviour and the extent to which a planktic existence selects for particular adaptive trends.
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