"Marshall considered the most important market to be that for bread...  In most agricultural markets one can think of quantity as being the variable that is set more exogenously due to weather variations from year to year, so that is why he put Q on the horizontal axis to determine P."
That's right. I would just add that Marshall had too great an on-field knowledge of production processes and real markets to believe that things might be significantly different for most other commodities. Hence actual production had to be the driver in most markets, at least in the short(est) period. 
Nicola Giocoli
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