Peter Stillman writes that 

"in the agricultural societies of old 
(including of 620 AD in Arabia), the produce of the land was a 
surrogate (and easily measurable) indicator of income, and that now, 
in non-agricultural societies, we use income because produce of the 
land is meaningless (or zero) for the many non-agricultural types."

He issues the last line ex cathedra. It is, I believe, seriously wrong and
misleading. I have heavy documentation on hand, and will supply it to anyone
who asks.

Mason Gaffney