The writing of this paper was supported by a fellowship from the American Council of Learned Societies and Grant BNS 8405996 from the National Science Foundation. As the final version of this paper was nearing completion, I was shocked and saddened to learn of the death of my dear friend and esteemed colleague, Alfredo Hurtado. Alfredo's personal and intellectual encouragement while we were Visiting Scholars at M.I.T. in 1984-1985 was a major factor in my decision to publish this paper, which I dedicate to his memory. © 1987 D. Reidel Publishing Company.