6. CONCLUSIONS

This review has high-lighted the role that interactions and mergers play in driving galaxy evolution. At present we remain challenged to understand the relative importance of weak and strong interactions, the details of bulge formation, the existence of nearly pure-disk galaxies, and the merger rate as a function of redshift. Yet, some firm conclusions have been reached and are as follows:

• Gravitational interactions and mergers are forming and transforming galaxies throughout the observable universe. The vast majority involve gas, dissipation, and enhanced star formation.

• The close link between mergers, ultra-luminous infrared galaxies, and quasars suggests that - like quasar activity - major merging may have peaked around z 2.

• Major disk-disk mergers form elliptical galaxies with kinematic subsystems, bimodal globular-cluster populations, and remnant fine structure. Such mergers occurred relatively early near the centers of rich clusters, but continue to the present time in rich-cluster outskirts, poorer clusters, and the field.

• Minor mergers tend to move disk galaxies toward earlier morphological types, creating kinematic subsystems and some bulges (fraction remains unknown).

• In short, the currently available evidence strongly suggests that Hubble's morphological sequence is mainly a sequence of decreasing merger damage.

Acknowledgements

I gratefully acknowledge research support from the Carnegie Institution of Washington and from the National Science Foundation under Grant AST-99 00742.