  Two classes of 1D nonintegrable systems represented by the Fermi-Pasta-Ulam
(FPU) model and the discrete $\phi^4$ model are studied to seek a generic
mechanism of energy transport in microscopic level sustaining macroscopic
behaviors. The results enable us to understand why the class represented by the
$\phi^4$ model has a normal thermal conductivity and the class represented by
the FPU model does not even though the temperature gradient can be established.
