  The backward evolution approach to modelling galaxy source counts is
re-visited in the wake of the numerous results and revelations from the
Infrared Space Observatory (ISO), the Submillimetre Common User Bolometer Array
(SCUBA) and the detections and measurements of the cosmic extragalactic
background light. Using the framework of the Pearson and Rowan-Robinson (1996)
galaxy evolution model, the observed source counts and background measurements
are used to constrain the evolution in the galaxy population. It is found that
a strong evolution in both density and luminosity of the high luminosity tail
of the IR luminosity function, interpreted as the ultraluminous galaxies
discovered first by IRAS and later elevated in status by SCUBA and ISO, can
account for the source counts from 15um (where it matches the undulation in the
integral counts and hump in the differential counts extremely well), to the
sub-mm region, as well as explain the peak in the cosmic infrared background at
140um. The sub-mm counts are interpreted as the superposition of 2 separate
populations comprising of ULIGs at the brighter sub-mm fluxes and starburst
galaxies at fluxes fainter than 2mJy. In this scenario the high redshift ULIGs
are tenuously interpreted as the progenitors of today's giant elliptical
galaxies.
