Extract of an email to me today;

According to Sandra Mitchell and her book "Biological Complexity and Integrative Pluralism",(Cambridge University Press, 2003), a complex system can be defined as follows:

"complex systems can be distinguished from simple objects by having multiple parts that  stand in nonsimple relations [...] There is structure or order in the way in which the whole is composed of the parts".

She argues in her book that a form of "integrative pluralism" is the best way
to describe the complex systems in the  biological sciences, which study complex
multi-component and multi-level systems that evolve dynamically over time.

In her attempt to define complexity, she distinguishes between constitutive and dynamic complexity.

Constitutive complexity: "Organisms display complexity of structure, the whole being formed of numerous parts in nonrandom organization

Dynamic complexity:
"Organisms are complex in the process by which they develop from single-celled origins to multicellular adults and by which they evolve from single-celled ancestors to multicellular descendants

Hope this helps
Paul