  The linear cosmological perturbation theory of almost homogeneous and
isotropic perfect fluid and scalar field universes is reconsidered and formally
simplified. Using the existence of a covariant conserved quantity on large
perturbation scales, a closed integral expression for comoving energy density
perturbations is obtained for arbitrary equations of state. On this basis we
establish a simple relation between fluid energy density perturbations at
`reentry' into the horizon and the corresponding scalar field quantities at the
first Hubble scale crossing during an early de Sitter phase of a standard
inflationary scenario.
