I was in error about Jenkin.  Erich Streissler in a  
series of papers, especially one in HOPE in 1990  
has documented the priority of Rau (1841) and other  
German neoclassicals, who also preceded the usually  
cited Gossen in thinking in terms of marginal utility,  
in putting the axes in the Marshallian manner.  Streissler  
also documents that Marshall was fully aware of this  
literature and strongly suggests that is where he got  
the idea from.  I am not sure that I can defend textually  
or from other sources the claim that I have heard that  
Marshall's motive was his focus on the "corn" market,  
which clearly is driven by an exogenous quantity factor.  
  
Barkley Rosser