  The evolution of open source software projects in Linux distributions offers
a remarkable example of a growing complex self-organizing adaptive system,
exhibiting Zipf's law over four full decades. We present three tests of the
usually assumed ingredients of stochastic growth models that have been
previously conjectured to be at the origin of Zipf's law: (i) the growth
observed between successive releases of the number of in-directed links of
packages obeys Gibrat's law of proportional growth; (ii) the average growth
increment of the number of in-directed links of packages over a time interval
$\Delta t$ is proportional to $\Delta t$, while its standard deviation is
proportional to $\sqrt{\Delta t}$; (iii) the distribution of the number of
in-directed links of new packages appearing in evolving versions of Debian
Linux distributions has a tail thinner than Zipf's law, with an exponent which
converges to the Zipf's law value 1 as the time $\Delta t$ between releases
increases.
