Complex systems can evolve from simple systems only if there are stable intermediate forms. The resulting complex forms will naturally be hierarchic. That may explain why hierarchies are so common in the systems nature presents to us. Among all possible complex forms, hierarchies are the only ones that have had the time to evolve.
So the stability of the intermediate form is required to make ot more likely that the largel whole could form. Otherwise it wohuld be less likely for all the pieces to be in the right place at the right time.
