  We study the general structure of models for structure formation, with
applications to the reverse engineering of the model from observations. Through
a careful accounting of the degrees of freedom in covariant gravitational
instability theory, we show that the evolution of structure is completely
specified by the stress history of the dark sector. The study of smooth,
entropic, sonic, scalar anisotropic, vector anisotropic, and tensor anisotropic
stresses reveals the origin, robustness, and uniqueness of specific model
phenomenology. We construct useful and illustrative analytic solutions that
cover cases with multiple species of differing equations of state relevant to
the current generation of models, especially those with effectively smooth
components. We present a simple case study of models with phenomenologies
similar to that of a LambdaCDM model to highlight reverse-engineering issues. A
critical-density universe dominated by a single type of dark matter with the
appropriate stress history can mimic a LambdaCDM model exactly.
