Continental drift is the theory that the Earth's continents have moved over geologic time relative to each other, thus appearing to have 'drifted' across the ocean bed. The speculation that continents might have 'drifted' was first put forward by Abraham Ortelius in The concept was independently and more fully developed by Alfred Wegener in but his theory was rejected by many for lack of any motive mechanism. Arthur Holmes later proposed mantle convection for that mechanism. The idea of continental drift has since been subsumed by the theory of plate tectonics, which explains that the continents move by riding on plates of the Earth's lithosphere.