This paper constructs a two-sector unified growth model. Learning-by-doing in agriculture eventually allows the preindustrial economy to leave its Malthusian trap. But entrepreneurs in the manufacturing sector do not attempt invention if not much is known about natural phenomena. This delays the industrial revolution. Since entrepreneurs identify new useful knowledge at all times in a serendipitous way, the industrial revolution is an inevitable outcome. The paper characterizes the equilibrium path from ancient times to the infinite future. According with this characterization, the model economy successfully captures the qualitative aspects of England's unified growth experience.