 This process, first studied by Yule, is a "linear" preferential attachment process, since the rate at which genera accrue new species is linear in the number they already have.Linear preferential attachment processes in which the number of urns increases are known to produce a distribution of balls over the urns following the so-called Yule distribution.In the most general form of the process, balls are added to the system at an overall rate of "m" new balls for each new urn.