The subject of this book has not been treated comprehensively before. For many years I have hesitated to attempt a monographic presentation because I felt uneasy about the idea of delineating something I am not sure about myself, and I felt it might be rash to try to integrate what seemed-and largely still seems-to consist of a heap of tessera not fitting together. The conviction won over, however, that the many details accumulated thus far call for some order now, and that an appraisal of where we stand would be appropriate. This book, I hope, can fill a gap barring the way to further progress in the field. It is not only a compilation of details but also an attempt to delineate the first outlines of a picture-blurry, obscure, and controversial as it must appear. The imagination of the research worker will be aroused, I hope, and stimulate further progress. In addition to data widely dispersed in the literature much unpublished material has been included. The limited space available prevented me from giving all the details known. The subject is also linked to many neighboring fields of study, but these relationships and their ramifications could not be discussed at length. This is a minor shortcoming, however, compared with our great ignorance of the subject in general.