  Two large, open source software systems are analyzed from the vantage point
of complex adaptive systems theory. For both systems, the full dependency
graphs are constructed and their properties are shown to be consistent with the
assumption of stochastic growth. In particular, the afferent links are
distributed according to Zipf's law for both systems. Using the Small-World
criterion for directed graphs, it is shown that contrary to claims in the
literature, these software systems do not possess Small-World properties.
Furthermore, it is argued that the Small-World property is not of any
particular advantage in a standard layered architecture. Finally, it is
suggested that the eigenvector centrality can play an important role in
deciding which open source software packages to use in mission critical
applications. This comes about because knowing the absolute number of afferent
links alone is insufficient to decide how important a package is to the system
as a whole, instead the importance of the linking package plays a major role as
well.
