One of the central problems studied by mankind is the problem of the succession of form. If the change of forms were to take place at all times and places according to a single well-defined pattern, the problem would be much easier; we could then set out, once and for all, the necessary order of the change of form in the neighborhood of any point, and this would then be at least an algorithm giving a prediction of phenomena, if not an explanation. In those ambiguous or catastrophic situations where the evolution of phenomena seems ill determined, the observer will try to remove the indeterminacy and thus to predict the future by the use of local models. All models divide naturally into two a priori distinct parts: one kinematic, whose aim is to parameterize the forms or the states of the process under consideration, and the other dynamic, describing the evolution in time of these forms.