  Nowadays, software has become a complex piece of work that may be beyond our
control. Understanding how software evolves over time plays an important role
in controlling software development processes. Recently, a few researchers
found the quantitative evidence of structural duplication in software systems
or web applications, which is similar to the evolutionary trend found in
biological systems. To investigate the principles or rules of software
evolution, we introduce the relevant theories and methods of complex networks
into structural evolution and change of software systems. According to the
results of our experiment on network motifs, we find that the stability of a
motif shows positive correlation with its abundance and a motif with high Z
score tends to have stable structure. These findings imply that the evolution
of software systems is based on functional cloning as well as structural
duplication and tends to be structurally stable. So, the work presented in this
paper will be useful for the analysis of structural changes of software systems
in reverse engineering.
