  We test the hypothesis that QSOs are formed via strong tidal interactions or
mergers, initially going through an ultraluminous infrared phase, by looking
for traces to this phase in the host galaxies of QSOs. We present a systematic
spectroscopic and imaging study of a sample of QSOs that may be in a
transitionary stage. By modeling the spectra, we obtain ages for the recent
starburst events in the host galaxies and interacting companions. We have
discussed in detail the first 5 objects in the sample in previous publications;
here we present results for the remaining 4 objects, and discuss the sample as
a whole. We find that all 9 transition QSOs are undergoing tidal interactions
and that 8 are major mergers. Every object also shows strong recent
star-forming activity, and in at least eight cases this activity is directly
related to the tidal interaction. The ages we derive for the starburst
populations range from currently active star formation in some objects, to
post-starburst ages < 300 Myr in others. There is also a clear connection
between interactions, starbursts, and QSO activity. Seven of the QSOs in the
sample are also ULIGs; statistical considerations show that the two phenomena
are necessarily physically related in these objects. Our results imply one of
two scenarios: (1) at least some ULIGs evolve to become classical QSOs, and the
transition stage lasts < 300 Myr, or (2) at least some QSOs are born under the
same conditions as ULIGs, and their lifetime as QSOs lasts < 300 Myr. We
discuss other properties and trends found in the sample, and propose a model
that accounts for all of them, as well as the youth of these systems.
(abridged)
