Validity of the `Conservation Law' in the Evolution of Cosmological Perturbations

It is shown that the `conservation law' for metric fluctuations with long wavelengths is indeed applicable for growing modes of perturbations, which are of interest in cosmology, in spite of a recent criticism [L. P. Grishchuk, Preprint gr-qc/9511074, now published in N. Sanchez, A. Zichichi (eds.), `String Gravity and Physics at the Planck Scale', Kluwer, Dordrecht, p. 369 (1996)]. This is demonstrated both by general arguments, and also by presenting an explicitly solvable toy model for the evolution of metric perturbations during inflation.