The papers collected in this volume were originally presented at a sym posium held at the University of Pennsylvania in December of 1968. Each of the papers has been revised in light of the discussions that took place during this symposium. None of the papers has appeared in print previously. The extensive bibliography that appears at the end of the volume was originally distributed during the symposium and was revised on the basis of many helpful suggestions made by those who participated. The symposium was made possible by a grant from The National Science Foundation and funds contributed...

The papers collected in this volume were originally presented at a sym posium held at the University of Pennsylvania in December of 1968. Each of the papers has been revised in light of the discussions that took place during this symposium. None of the papers has appeared in print previously. The extensive bibliography that appears at the end of the volume was originally distributed during the symposium and was revised on the basis of many helpful suggestions made by those who participated. The symposium was made possible by a grant from The National Science Foundation and funds contributed...