  The evolving galaxy is considered as a system of baryonic fragments embedded
into the static dark nonbaryonic (DH) and baryonic (BH) halo and subjected to
gravitational and viscous interactions. Although the chemical evolution of each
separate fragment is treated in the frame of one -- zone close box model with
instantaneous recycling, its star formation (SF) activity is a function of mean
local gas density and, therefore, is strongly influenced by other interacting
fragments. In spite of its simplicity this model provides a realistic
description of the process of galaxy formation and evolution over the Hubble
timescale.
