vi ■ Preface 



not been an easy one to resolve. There is much to be said for the 

 practice of sketching the broad lines of a topic with a few intel- 

 lectually satisfying concepts and not burdening the student im- 

 mediately with exceptions and difficulties. If I have avoided this 

 procedure— and surely the bewildered reader of Chapter Five will 

 agree that I have-it is because 1 am afraid it can be fundamentally 

 misleading. My intention is to introduce the reader to the field and 

 if possible to give him the "feel" of it, bringing him close to the 

 position of the research workers themselves. Since in my opinion 

 science progresses, like all endeavors, by fumbling, backing out of 

 dead ends, and now and then taking a few steps forward, it is often 

 easy to believe in a clear pattern of conceptually clean "break- 

 throughs" after some time has passed, but it is harder to do so as 

 the work becomes more recent. Or, at least, I doubt my own ability 

 at this sort of judgment. The alternative, then, is to stress the phe- 

 nomena, the empirical observations: these are not so likely to be 

 subjectively distorted, and it is these that must be lived with, ex- 

 amined, correlated, and finally understood. 



All this is of course no excuse for a mere random collection of 

 '[acts," and the reader will find nothing of the kind. It is, however, 

 the justification for bringing in exceptions almost simultaneously 

 Avith the tentative rules, for employing an often deliberate vague- 

 ness in terminology— since words used in a systematic, authorita- 

 tive way can often conceal ignorance— and for stressing, above all, 

 the kinds of experiments and results rather than merely the con- 

 cepts they may or may not illustrate. I can think of no better way 

 to convey the extreme openness of the subject, the way in which few 

 if any principles are irrevocably established. It is all a question of 

 how much confusion is necessary to provide a true picture of the 

 present state of things; I have tried to avoid an excess, but not to 

 exclude it entirely. 



A general outline of the way in which I have grouped various 

 topics for consideration is provided by the table of contents, and 

 requires no further comment here. However, some remarks on the 

 bibliography and the manner in which papers are cited may be 

 useful. 



The proportion of general reviews to original experimental 

 articles cited is relatively high, and I have made no attempt to 

 include all the revelant literature. Frequently a paper is considered 



