As a matter of fact, if a process of embryonic development is disturbed, it usually returns to normality some time before reaching the adult condition. Its trajectories, that is to say, converges not merely to the normal end state, but to some earlier point on the path leading towards the steady state. This is well symbolised by the epigenetic landscape. If a ball, running down one of the valleys, were pushed partway up the hillside, it might well reach the valley bottom again before the slope of the valley flattens out as it reaches the adult steady state. Such a system exhibits a tendency towards a certain kind of equilibrium, which is restored after disturbance; but this equilibrium is not centred on a static state but rather on a direction or pathway of change. We might speak of such an equilibrium-property as a condition of ‘homeorhesis’ (ρήω, to flow) on the analogy with the well-known expression homeostasis, which is appropriate when it is an unchanging state which is maintained.