This chapter is a revised form of a paper read at the Annual Meetings of the American Sociological Association, August, 1968. It is based on a chapter in Kenneth C. Land, "Explorations in Mathematical Sociology," unpublished Ph.D. dissertation, The University of Texas at Austin, 1969. The encouragement and assistance of Harley L. Browning and the staff of the Population Research Center of The University of Texas at Austin is gratefully acknowledged. Also, Hubert M. Blalock, Jr., and the editors of this volume made valuable comments on earlier versions of the paper. Any remaining errors and inaccuracies are, of course, the sole responsibility of the author.