  We examine a novel mechanism for structure formation involving initial number
density fluctuations between relativistic species, one of which then undergoes
a temporary downward variation in its equation of state and generates
superhorizon-scale density fluctuations. Isocurvature decaying dark matter
models (iDDM) provide concrete examples. This mechanism solves the
phenomenological problems of traditional isocurvature models, allowing iDDM
models to fit the current CMB and large-scale structure data, while still
providing novel behavior. We characterize the decaying dark matter and its
decay products as a single component of ``generalized dark matter''. This
simplifies calculations in decaying dark matter models and others that utilize
this mechanism for structure formation.
