The prospective reader may well ask about the particular merits of this volume, especially in view of several dozen similar offerings, each with its own excellences, and of the easy availability of symposia, conferences, con- ventional reviews, abstract journals, and serial research reports. In spite of such other attractions, it seems to me that these 34 essays are among the most informative and stimulating which are now available in the areas cov- ered. The editors have been successful in attracting new articles from many of the most prominent investigators now actively working at research...