We saw above that with digital design, care must be taken to prevent switching transients (a feature absent from the designer's model) from affecting the system's overall behaviour. This is done by making sure that one part of the system does not influence the rest until any transient dynamics have died down and a steady state is reached (all transistors stable in the ON or OFF states). Usually, this means that the circuit is broken into modules, the internal transient dynamics of which are hidden from each-other. (Here, the word `module' is used in a general sense to mean a cohesive sub-assembly within a larger system.) The spatial or topological structure of the circuit has thus been constrained to facilitate a design abstraction.
Even if reconfigurable hardware intended for use by digital designers is used (eg. most current FPGAs) then design abstractions, such as the digital model, are not required for intrinsic hardware evolution. Is modularity, then, a designer's constraint that can be abandoned, or is it necessary or useful for all complex systems whether designed or evolved? Are the kinds of modules appropriate for an evolving circuit different from those used by a designer?
These questions are currently difficult to answer fully. Certainly, we have
seen that evolution does not need modular structures to support abstract
designer's models, because intrinsic evolvable hardware (hereafter `intrinsic
EHW') does not use such models. However, the modularity of a system can also
be caused by the nature of the problem-solving or adaptive process that
derived it. Humans typically use some sort of ``divide and conquer'' strategy,
by which the problem is successively decomposed. Whether the decomposition is
a functional one or a behavioural one [17], the final structure
arrived at usually has modules corresponding to that decomposition.
Wagner [18] argues that the evolutionary process also
requires a kind of modularity: that there should be an ``independent genetic
representation of functionally distinct character complexes.'' The idea is
that such a genotype-phenotype mapping prevents small mutational variations
applied at one point from having large-scale ramifications throughout the
whole phenotype, so that parts of it can be improved semi-independently.
However, it is not clear to what extent this consideration necessarily implies
modules in the structure of an EHW circuit, because the ``distinct character
complexes'' of the phenotype are components of the behaviour of the
circuit, not of its physical organisation
. A related
rôle for modules in an evolved circuit was given above
(Section 11) when considering the part which repeated
structures and symmetries can play in the morphogenesis and evolution of
complex systems.
It seems that if modular circuits are desirable in EHW, it is because modularity may aid the evolution of complex systems, rather than because modularity is essential to the operation of a complex electronic circuit. For this reason, the kinds of modules appropriate to EHW will be tuned to the characteristics of the evolutionary process (particularly the genetic encoding and morphogenetic development) and the way in which this interacts with the detailed properties of the particular type of reconfigurable hardware being used. It remains to be seen what such modules may look like, but the important message is that they may be radically different from what is seen in circuits produced by traditional design methods.