Reflecting upon the significance of his work on evolution, and in particular on his demonstration of the possibility of machines which could build modified copies of themselves, von Neumann said ``It is clear that this is a step in the right direction, but it is also clear that it requires considerable additional analyses and elaborations to become really relevant'' [von Neumann 66] (p.131).
It has long been recognised that chief among these additional analyses and elaborations is the incorporation of the evolutionary process into a broader framework that also considers the properties of the environment. Holland has emphasised that the study of adaptation ``involves the study of both the adaptive systems and its environment. In general terms, it is a study of how systems can generate procedures enabling them to adjust efficiently to their environments'' [Holland 62] (p.299). Moreover, Conrad stresses that ``the characterization of the substrate is of such immense importance for the effectiveness of evolution'' [Conrad 88] (p.304).
Studies of evolution in vitro, such as Orgel's experiments with evolving RNA sequences using the viral enzyme Qβ replicase [Orgel 79], have also demonstrated the need for a better theoretical understanding of these issues. Maynard Smith explains:
``More or less independently of the starting point ... the end point is a rather small molecule, some 200 bases long, with a particular sequence and structure that enable it to be replicated particularly rapidly. In this simple and well-defined system, natural selection does not lead to continuing change, still less to anything that could be recognized as an increase in complexity: it leads to a stable and rather simple end point. This raises the following simple, and I think unanswered, question: What features must be present in a system if it is to lead to indefinitely continuing evolutionary change?'' [Maynard Smith 88] (p.221).
The question raised by Maynard Smith is exactly the one of interest in this section: What sort of system (in terms of individuals, interactions and environments) will give rise to an open-ended evolutionary process?