Dear Nicholas,

In your answer, I do not see the seminal contributions of Dupuit. I 
am sorry to say, it can't be overlooked. Dupuit came up with both 
formal and literary expressions of the diminishing marginal utility. 
Prior to that, he brilliantly discussed the notion of utility. 
Therefore Gossen, Menger, Jevons and Walras were not really the first 
in the 19th century.

Best,

Guy Numa