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