This book reflects the personal prejudices I have developed in twenty years of reading the scientific literature. I like monographs; good ones assemble a great deal of information in a logical sequence and in enough detail to enable one to see why current beliefs are held. For this purpose, it is entirely useless to write "as Smith has shown21 ,81,117 •••• " That only means that one must go to the library and turn up Smith's original papers, and one's object in reading a monograph is precisely to avoid that neces­ sity. One needs to know what Smith did and why he thought his observa­ tions proved whatever he claimed. Because life is short, it is impossible to deal with several thousand papers in this way, and the author must there­ fore select a relatively few papers that he regards as crucial. Often, several papers of equal merit might be quoted, and the selection is then arbitrary. I therefore apologize to authorities who do not find their work discussed. Omission does not mean that I thought their work was not valuable; it means only that I preferred to quote twenty references that people might read rather than two thousand that assuredly no one would read. Another strong prejUdice is that the full understanding of present knowledge requires one to know how present views have developed.