  We analyze the evolution of energy density fluctuations in cosmological
scenarios with a mixture of cold dark matter and quintessence, in which the
quintessence field is modeled by a constant equation of state. We obtain
analytic expressions for the time evolution of the quintessence perturbations
in models with light fields. The fluctuations behave analogously to a driven
harmonic oscillator, where the driving term arises from the inhomogeneities in
the surrounding cosmological fluid. We demonstrate that the homogeneous
solution, determined by the initial conditions, is completely sub-dominant to
the inhomogeneous solution for physically realistic scenarios. Thus we show
that the cosmic microwave background anisotropy predicted for such models is
highly insensitive to the initial conditions in the quintessence field.
