  A general theory of nucleation for colloids and macromolecules in solution is
formulated within the context of fluctuating hydrodynamics. A formalism for the
determination of nucleation pathways is developed and stochastic differential
equations for the evolution of order parameters are given. The conditions under
which the elements of Classical Nucleation Theory are recovered are determined.
The theory provides a justification and extension of more heuristic equilibrium
approaches based solely on the free energy. It is illustrated by application to
the low-concentration/high-concentration transition in globular proteins where
a novel two-step mechanism is identified where the first step involves the
formation of long-wavelength density fluctuations and the second step is the
actual nucleation event occuring within the fluctuation.
