which, as it is discussed latter, may trigger SC. Self-extinguishing interactions may produce the stability of the original or of a new structure. For example, in the
formation of a star from a molecular cloud, the attractive force produces a contraction, which reduces the distances among the molecules, which increases the
attractions in a self-reinforcing process. Contraction continues, forming a high-den- sity nucleus that accelerates the contraction. In the central part of the cloud, the
velocity and intensity of collisions between the atomic nuclei are enough to start a new process: a thermonuclear reaction with great heat production that stops the
contraction self-stabilising process and leads to the formation of a stable star. In economic systems, there are also many examples of self-reinforcing Arthur, 1990
and of self-extinguishing processes. For example: 1 the regulation of prices by the market through self-extinguishing interactions of demand and supply or through
self-reinforcing interactions among production, employment and consumption, which cause economic booms or slumps. 2 A self-reinforcing process in a
subsystem such as technological innovation that may induce structural changes and instability through the whole economy and may affect the institutional, social
and political environment Pasinetti, 1993.
In the process of assembling, the closed chains of interaction may form integra- tive mechanisms that affect all the parts. These may be as simple as the gravita-
tional field in a cosmic molecular cloud or as complex as the neurological or the hormonal integrative systems in living beings, or the institutions in social systems.
4. Relations between whole and parts: bottom-up and top-down processes
Once a whole is assembled, important cases of endogenous interaction are those in which global mechanisms affect each part or, reciprocally, the parts affect those
global mechanisms. In other words, some news properties or subsystems of the whole may affect parts by restricting or freeing the manifestation of certain
properties or relations between the parts top-down processes. The parts, in turn, may affect the whole bottom-up processes. Each process, or the combination of
both, is a basic mechanism in structural change. There are a lot of examples of this pattern of SC. Some nations may unite bottom-up to form an economic block.
The whole is implemented as a set of rules tariffs, exchange types and production quotas. Changes in the rules, such as decrease of intra-blocks or increase of
inter-blocks tariffs to increase trade and specialisation, may affect the economy of each country top-down process and this in turn affects the economic characteris-
tics of the whole block. The interaction between the whole society that educates and socialises the individuals and the free activity of these, which change the society, is
another example. The evolution and adaptation of the society depend on a balanced interplay between socialisation top-down process and the activities of
individuals bottom-up process. Too strong top-down processes as in repression or enforced unification may paralyse the society; if the bottom-up processes
predominate, disintegration may result.
The followings are special types or aspects of these top-down and bottom-up processes:
1. Qualitative changes induced by variations beyond a threshold. In many processes
a general top-down action, induced by endogenous or exogenous processes, varies steadily in a system built up of uniform parts and produces similar
variations in them. In these cases, it may happen that, if the value of the action crosses a threshold, all the parts undergo changes in their relations, almost at
the same time, producing a spectacular qualitative change in the whole. The classical example is physical phase transition, like the heating of a solid in which
the parts atoms, or ions, or molecules vibrate around rather fixed positions determined by their mutual electromagnetic interactions. If the solid is heated,
the amplitude of the oscillation increases and, beyond a temperature threshold, the parts go out of their limits and wander through the whole space of the body
that has became, rather suddenly, a liquid. This type of change, pointed out by Hegel 1830, is usually referred to as quantitative-qualitative change. There are
a lot of examples of it in natural and artificial systems and they are even mentioned in proverbs and folklore. It is worthwhile noting that, if the system
is heterogeneous, the sudden change is less likely. An example is the heating of a piece of wax, which is a mixture of different substances; in this case, there is
a plastic intermediate state. The influence of diversity is also seen in some social changes. In a more homogeneous society, revolutionary changes are more likely
than in a society with a high degree of social and institutional diversity, which allows evolving SC.
2. Responses to balanced stimuli. There are systems that are complex enough to
