  We introduce a simple model which shows non-trivial self organized critical
properties. The model describes a system of interacting units, modelled by
Polya urns, subject to perturbations and which occasionally break down. Three
equivalent formulations - stochastic, quenched and deterministic - are shown to
reproduce the same dynamics. Among the novel features of the model are a
non-homogeneous stationary state, the presence of a non-stationary critical
phase and non-trivial exponents even in mean field. We discuss simple
interpretations in term of biological evolution and earthquake dynamics and we
report on extensive numerical simulations in dimensions $d=1,2$ as well as in
the random neighbors limit.
