It's not the Iron Law of Wages, but it was elucidated in the Essay on the External Corn Trade by David Ricardo.  
Ronald Jones discussed labor immobility and this concept in his attempts to expand the Hecksher-Ohlin model.  Its formulator recognized that it did not hold for when all means of production are internationally mobile, while Paul Samuelson argued that it was a social-science construct that was both true and non-trivial.  It's not the Iron Law of Wages, but it was elucidated in the Essay on the External Corn Trade by David Ricardo.  An example of it explains why a man who is the best lawyer and typist in a city is better off only practicing law due to opportunity cost.  For 10 points, identify this concept from economics that describes the benefits of trade specialization, less stringent than absolute advantage.
Ronald Jones discussed labor immobility and this concept in his attempts to expand the Hecksher-Ohlin model.  
{Comparative} Advantage