Throughout the book, the analysis tools developed are, from time to time, applied to Adami and colleagues' avida system. The model, derived from Ray's well-known tierra (Ray 1991), simulates the parallel execution and evolution of a population of self-replicating computer programs. The design of avida is not discussed at length in the main text, but rather in the appendix, where the system's User Manual is reprinted. A CD ROM is supplied with the book, which contains versions of avida for Windows 95, Windows NT and Unix systems (but not Macintosh). The CD ROM also contains other supporting material, such as source code for programs to demonstrate aspects of self-organized criticality and percolation theory, as well as an HTML document containing links to sources of further reading on many of the subjects discussed in the book. The latter is a particularly good idea, although, perhaps inevitably, some of the links are now broken (the book, together with the CD, was published in 1998).

In other words, artificial life is distinguished from traditional theoretical biology by its fundamentally synthetic, rather than analytic, approach. I believe that most people would broadly accept this distinction. Although Introduction to Artificial Life describes occasional experiments on avida, the emphasis of the book is clearly on the development of a variety of analytical tools for understanding evolution (both natural and artificial). Of course, the behavior of artificial life simulations needs to be analyzed just as carefully as any other kind of scientific model, and Adami's work represents a valuable and much-needed contribution to such analysis. However, nearly all coverage of the design of avida is confined to the appendix, and even here it is described with little discussion of the theory behind its design. This I consider a great pity, as it is precisely in the process of designing and building such systems that one is faced with a myriad of choices and design decisions, even the smallest of which may turn out to radically alter the resulting behavior of the system. It is in highlighting new questions about the design and construction of systems – questions that simply do not arise when analyzing the behavior of a predefined, given system – that artificial life perhaps has the greatest potential to complement the results of analytical models and thereby contribute to a broader understanding of life.

When Adami does turn to more philosophical considerations of the nature of life, it is clear that he regards information and computation as the critical issues. For example, when discussing the self-replicating programs in tierra and avida, he suggests "because of the remaining computational-universality of their virtual CPUs … we feel compelled to speculate that such constructions constitute minimal living systems" (p.58, original emphasis). To motivate the discussion of information theory, Adami asserts "the discovery of the genetic code cemented the fact that information is the central pillar in any attempt to understand life" (p.59, original emphasis).

Despite an observation in Chapter 1 that "life seems to be a property of a collection of components but not a property of the components themselves" (p.4, original emphasis), which suggests that the organization of organisms is a (or, some would argue, the) defining feature of life, Adami does not expand upon this theme. Throughout the rest of the book he seems happy to regard self-replicating strings (be they computer programs or biomolecules) as "minimal living systems". This lack of consideration of organizational issues is also reflected in the approach to modeling populations of organisms; much of the theory is based on assumptions like the population comprises only a single species (e.g. Chapter 4), or in other cases, that ecological interactions and coevolution between species are negligible (e.g. Chapter 8). Of course, such assumptions are introduced to make the analysis tractable, but I suspect that it is precisely in these questions of organization, both at the level of the individual organism, and at the level of the ecosystem, that living systems may be most appropriately distinguished from non-living. For example, the most interesting (most lifelike?) results to arise from tierra concerned the emergence of ecological relationships, such as parasitism, between the evolving programs (Ray 1991). Models and analyses that concentrate on individual self-replicators evolving in static environments will fail to elucidate the important influences of such ecological interactions.