Can metastable crystal forms `disappear'?

Woodard & McCrone [J. Appl. Cryst. (1975). 8, 342] claim that the nucleation of stable forms of certain substances may preclude the subsequent preparation of previously known metastable crystalline forms `often even in laboratories many miles away'. However, the literature on these substances does not indicate that any authentic metastable forms have ceased to be obtainable, and we see no reason to believe that the effects of seeding are other than local and temporary.