A little more on Marshall. Marshall's explanation of adjustment to equilibrium in a competitive market has firms adjusting output until demand price equals supply price. As opposed to Walras' conception of a auction market where price adjustments lead to changes in the quantities demanded and supplied. This does not make a difference if the demand and supply curves have the normal slopes, but it does matter to the stability of various equilibria if the demand curve slopes up or the supply curve down. Malcolm Rutherford