The AAAS Selected Symposia Series was begun in 1977 to provide a means of more permanently recording and more widely disseminating some of the valuable material which is discussed at the AAAS Annual National Meetings. The volumes in the Series are based on symposia held at theMeetings which address topics of current and continuing significance, both within and among the sciences, and in the areas in which science and technology have an impact on public policy. The Series format is designed to provide for rapid dissemination of information, so the papers are reproduced directly from...