  We discuss the evolution of linear perturbations about a
Friedmann-Robertson-Walker background metric, using only the local conservation
of energy-momentum. We show that on sufficiently large scales the curvature
perturbation on spatial hypersurfaces of uniform-density is conserved when the
non-adiabatic pressure perturbation is negligible. This is the first time that
this result has been demonstrated independently of the gravitational field
equations. A physical picture of long-wavelength perturbations as being
composed of separate Robertson-Walker universes gives a simple understanding of
the possible evolution of the curvature perturbation, in particular clarifying
the conditions under which super-horizon curvature perturbations may vary.
