The Bottom Rung presents an in-depth investigation of a population that is becoming extinct in American society: the black farmer.   Tracing patterns of marriage and childbearing among both whites and blacks during the first decades of this century, Stewart Tolnay pursues questions about how black southern farm families were formed and dissolved, how they educated their children or put them to work in the fields, and how they migrated in search of opportunity. Further, he considers the possible legacy of these experiences for family life in contemporary urban environments.   Making revealing and innovative use of public records from the early part of the twentieth century, Tolnay challenges the widely held idea that southern migrants to northern cities carried with them a dysfunctional family culture. He demonstrates the powerful impact of economic conditions on family life and views patterns of marriage and childbearing as responsive to prevailing social, economic, and political conditions. In a provocative extension of this perspective, Tolnay argues that current high levels of single-parenthood among urban African American families likewise reflect rational responses to the socio-economic environment and government policies.   By placing post-World War II demographic developments in a wider historical perspective, The Bottom Rung sheds new light on recent discussions of the difficulties faced by the modern black urban family. The text is enhanced by Dorothea Lange's and Russell Lee's poignant photographs.