Prof. Clark sits in the “what revolution?” camp among economic historians that try to date the industrial revolution. His data:
Why does this matter? Well, if there was no revolution, only evolution, to modern industrial society, you need an underlying evolutionary mechanism. Clark favors genes-based stories, but most other folks are more comfortable with culture-based stories. In either case, its hard to look at Clark’s data and pick a year before 1900 that would look like a revolution in efficiency.