 Major perturbations such as ether or heat shock eject the developmental pathway from the metaphorical canal, exploring other parts of the epigenetic landscape.Selection in the presence of that perturbation leads to the evolution of a new canal; after the perturbation is discontinued, developmental trajectories continue to follow the canalized pathway.Other evolutionary biologists have agreed that assimilation occurs, but give a different, purely quantitative genetics explanation in terms of Darwin's natural or artificial selection.