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A Factory increases production by 50%. If a Mfg. Plant is built, the Factory becomes redundant; it no longer increases production and can be sold off. If a Mfg. Plant already exists, a Factory cannot be built.
The development of the Factory evolved from the specialization of labor, where each worker learned only one step in the manufacturing process. When the worker became expert at that one task, the total production of the Factory increased. In Adam Smith's classic example from his book, The Wealth of Nations, a Factory of specialists made many more nails than one where each worker handcrafted nails one at a time. The Factory system was one prerequisite for the Industrial Revolution that soon followed.