Ostwald's rule remains a long standing guide to crystallisation in polymorphic systems. It is often assumed that this rule has its origins in nucleation processes that favour metastable phases. Here a mathematical formalism is developed which shows that in fact the validity of this rule depends on both the relative nucleation and growth rates of the available forms. The implication of this result for previous nucleation studies is discussed. Surprisingly there is no existing comprehensive data set for both nucleation and growth kinetics of a polymorphic material. Accordingly new data for the α and β forms of p-aminobenzoic acid are reported here. These demonstrate the important contribution of growth to the polymorphic outcome and also suggest molecular scale mechanisms that dominate nucleation. In particular the uniquely rapid crystallisation behaviour observed in aqueous solutions is interpreted in both kinetic and structural terms.