What if there was no one to offer food to monastics?

This is a questions that often comes up when I visit study groups, particularly when the subject of everyone practicing like a monk/nun comes up. Wouldn't monastics be allowed to farm-just enough for sustenance? It seems extreme that they would just starve because no one fed them. My understanding was that the food offering was mostly established for the lay people to have an opportunity to give and gain merit, but I may be wrong. Is there a canonical reference saying monastics can only eat if given food? What did they do before the order was fully established? What about forest monastics who go of for extended periods alone?