  The conservation laws of the third order quasilinear scalar evolution
equations are considered via differential system and characteristic cohomology.
We find a subspace of 2 forms in the infinite prolonged space in which every
conservation law has a unique representative. The structure of this subspace
naturally gives rise to the convenient notion of the weight of a conservation
law, and we also compute a universal integrability condition to admit any
higher order (weight) conservation law. As an example, the differential system
descrbing the flow of the curve in the plane by the derivative of its curvature
with respect to the arclength is shown to have the KdV property, i.e., an
infinite sequence of conservation laws of distnct weights.
