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I very grateful for helpful comments from colleagues who read all or parts of this manuscript, including Phil Appleton, Mark Bransford, Vassilis Charmandaris, John Scalo, Dave Schramm, and Bev Smith. Thank goodness for the many things you found missing or just plain wrong (and sorry for any I still missed). To the anonymous referee who really took it apart, providing dozens of detailed and expert suggestions. To many "collisional collaborators" and friends- including, C. Heisler, J. Higdon, R. Lavery, P. Marcum, M. Noguchi, E. Skillman, J. van Gorkom, J. Wallin, and the "oculars" (E. Brinks, B. & D. Elmegreen, M. Kaufman, M. Klaric, and M. Thomasson) - for providing so much enlightment on specific aspects of this field. Some support for this work was provided by NSF grant AST 93-19596.

I'm also grateful to the individuals and publishing companies who provided permission to use figures, and in many cases digital versions of those figures and preprints, including: P. Appleton, E. Athanassoula, J. L. Higdon, R. A. Gerber, R. J. Lavery, I. F. Mirabel, P. J. Quinn, D. B. Sanders, B. J. Smith, A. Toomre, I. R. Walker, B. Whitmore, and M. Yun.

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Lab., Caltech, under contract with the National Aeronautics and Space Administration. Images from NOAO and STSci are the copyright of the Association of Universities for Research in Astronomy Inc. (AURA), all rights reserved. This research has made use of NASA's Astrophysics Data System Abstract Service.

I am especially grateful to my longtime collaborator Phil Appleton for providing all of the above, and much encouragement besides. And to my wife, Megan Fairall, for a lot of support over the course of this project.

This paper is dedicated to the memory of David N. Schramm, who conceived of the project a number of years ago, patiently endured the author's slow progress, and provided encouragement through the many interruptions. I am also grateful to Roberta L. Bernstein, David's editorial assistant, for her help, and to M. Kamionkowski for taking over the editorial tasks.

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