This book was undertaken for the purpose of bringing together the widely diverse lines of experimental work and thinking which has been expressed but has often been unheard on the title question. It will be clear to the reader that a critical viewpoint has been maintained in assembling the material of this rapidly expanding area of concern to organic chemists. It should be clear, too, that the authors are not purvey ing a singular viewpoint and do not regard the discussions presented as the ultimate word on the subject. In fact, it should be anticipated that many ofthe viewpoints pre sented...