  We investigate a model of disk galaxies whereby viscous evolution of the
gaseous disk drives material inwards to form a proto-bulge. We start from the
standard picture of disk formation through the settling of gas into a dark halo
potential well, with the disk initially coming into centrifugal equilibrium
with detailed conservation of angular momentum. We derive generic analytic
solutions for the disk-halo system after adiabatic compression of the dark
halo, with free choice of the input virialized dark halo density profile and of
the specific angular momentum distribution. We derive limits on the final
density profile of the halo in the central regions. Subsequent viscous
evolution of the disk is modelled by a variation of the specific angular
momentum distribution of the disk, providing analytic solutions to the final
disk structure. The assumption that the viscous evolution timescale and star
formation timescale are similar leads to predictions of the properties of the
stellar components. Focusing on small `exponential' bulges, ones that may be
formed through a disk instability, we investigate the relationship between the
assumed initial conditions, such as halo `formation', or assembly, redshift
$z_f$, spin parameter $\lambda$, baryonic fraction $F$, and final disk
properties such as global star formation timescale, gas fraction, and
bulge-to-disk ratio. We find that the present properties of disks, such as the
scale length, are compatible with a higher initial formation redshift if the
re-distribution by viscous evolution is included than if it is ignored. We also
quantify the dependence of final disk properties on the ratio $F/\lambda$, thus
including the possibility that the baryonic fraction varies from galaxy to
galaxy, as perhaps may be inferred from the observations.
