  We study the response to an external perturbation of the energy levels of a
disordered metallic particle, by means of the Brownian-motion model introduced
by Dyson in the theory of random matrices, and reproduce the results of a
recent microscopic theory of Altshuler, Simons, and Szafer. This establishes
the validity of Dyson's basic assumption, that parametric correlations in the
energy spectrum are dominated by level repulsion, and therefore solely
dependent on the symmetry of the hamiltonian. ***Submitted to Physica A.****
