Time evolution of the bisymmetric fluctuations (Equation B.6) in 12 different N–body realisations of the Mestel disc, with N​ = ​25​ × ​10⁶ particles each, using a running average over 30 dynamical times. The disc is initially stable and slowly relaxes towards an unstable state. Once unstable, the evolution is dominated by an exponentially growing mode before it saturates. The disc’s configuration is illustrated in Figure 2. Interestingly, the dispersion among realisations greatly increases close to the instability. The dashed line at t/t_dyn​ = ​150 is the time at which the changes in the DF are measured in Figure 3.