Mason Gaffney, in describing (accurately) the original (10% of the 
produce of the land) and the current (10% of income), states that 
'the concept has been turned upside down'.  Others might say that it 
has been modernized:  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.

Peter G. Stillman