  A four-form gauge flux makes a variable contribution to the cosmological
constant. This has often been assumed to take continuous values, but we argue
that it has a generalized Dirac quantization condition. For a single flux the
steps are much larger than the observational limit, but we show that with
multiple fluxes the allowed values can form a sufficiently dense `discretuum'.
Multiple fluxes generally arise in M theory compactifications on manifolds with
non-trivial three-cycles. In theories with large extra dimensions a few
four-forms suffice; otherwise of order 100 are needed. Starting from generic
initial conditions, the repeated nucleation of membranes dynamically generates
regions with a cosmological constant in the observational range. Entropy and
density perturbations can be produced.
