The ambiguities of Alfred Marshall paralyzed the best brains in the Anglo-Saxon branch of our profession for three decades . . . Because of his unwillingness to make sharp distinctions between perfect and less-than-perfect competition , Marshall managed to set back the clock both on competitive theory and on the theory of monopoly ... Marshall was so afraid of being unrealistic that he merel y ends up being fuzzy and confusing - and confused. I

Samuelson is right , but so is Marshall-who believed that cases in the theory of market structures 'shade into one another by imperceptible degrees' ,2 and that there is more merit in being fuzzily right than in being precisely wrong. In his ambiguous and open-ended way he seems to have treated all four of the principal market structures that figure in contemporary economics textbooks, and it is with these four market structures that we shall be concerned in the four sections of the present chapter.