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the literature much unpublished material has been included.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.