  The excitation-chain theory of the glass transition, proposed in an earlier
publication, predicts diverging, super-Arrhenius relaxation times and, {\it
via} a similarly diverging length scale, suggests a way of understanding the
relations between dynamic and thermodynamic properties of glass-forming
liquids. I argue here that critically large excitation chains play a role
roughly analogous to that played by critical clusters in the droplet model of
vapor condensation. The chains necessarily induce spatial heterogeneities in
the equilibrium states of glassy systems; and these heterogeneities may be
related to stretched-exponential relaxation. Unlike a first-order condensation
point in a vapor, the glass transition is not a conventional phase
transformation, and may not be a thermodynamic transition at all.
