This volume, the proceedings of the third international conference held in Washington, D.C., in December 1963, consists of forty-five papers representing the current status of knowledge and the advances made since publication of the second symposium volume in 1959.Originally published in 1964.A UNC Press Enduring Edition -- UNC Press Enduring Editions use the latest in digital technology to make available again books from our distinguished backlist that were previously out of print. These editions are published unaltered from the original, and are presented in affordable paperback formats, bringing readers both historical and cultural value.