Over 100y ago, W Ostwald formulated his rule of stages. He predicted that phase formation processes in complex systems proceed via a discrete series of metastable states, which can be formed in a macroscopic form at given thermodynamic conditions, until finally the most stable phase will be reached. This paper shows that in segregation & crystallisation processes in multicomponent solutions, critical clusters may be formed & evolve via a continuous sequence of states with properties that may differ from the properties of the corresponding macroscopic phases of the equilibrium phase diagram. The pathways of nucleation resemble a scenario similar to spinodal decomposition, that is, the process proceeds via a continuous amplification of density and/or composition differences, which are eventually accompanied by sequential discrete changes of the structure of the system.
