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