  Gravitational interactions and mergers are shaping and reshaping galaxies
throughout the observable universe. While observations of interacting galaxies
at low redshifts yield detailed information about the processes at work,
observations at high redshifts suggest that interactions and mergers were much
more frequent in the past. Major mergers of nearby disk galaxies form remnants
that share many properties with ellipticals and are, in essence, present-day
protoellipticals. There is also tantalizing evidence that minor mergers of
companions may help build bulges in disk galaxies. Gas plays a crucial role in
such interactions. Because of its dissipative nature, it tends to get crunched
into molecular form, turning into fuel for starbursts and active nuclei.
Besides the evidence for ongoing interactions, signatures of past interactions
and mergers in galaxies abound: tidal tails and ripples, counterrotating disks
and bulges, polar rings, systems of young globular clusters, and aging
starbursts. Galaxy formation and transformation clearly is a prolonged process
occurring to the present time. Overall, the currently available observational
evidence points towards Hubble's morphological sequence being mainly a sequence
of decreasing merger damage.
