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tokens: distribution of these galaxies in -space and compare it with the cumulative probability distribution function for a uniform distribution (i.e. a straight line). The Kologorov–Smirnov (KS) statistic for these distributions is D = 0.128 indicating consistency. Figure 2. The cumulative fraction of where α is the pitch angle is shown, using the central 90 per cent range of pitch angles from Yu Ho ( 2018 ). The dashed line shows a uniform distribution which is what would be expected if the spiral structure winds up at a constant rate (equation 5 ). The KS statistic for the comparison between these two distributions is D = 0.128, confirms a lack of statistical disagreement. As a consistency check, we also show the normalized distribution of pitch angles for different wavebands in Fig. 3 , again using data from Yu Ho ( 2018 ). As would be expected, the distribution of pitch angles are similar for the different wavebands. Within error bars (which we do not show for clarity), the pitch angles again exhibit a uniform distribution in the range for the B, V, R , and 3.6 μm bands. We leave any further studies across different wavelengths or other parameters to future observational studies. Figure 3. The distribution of pitch angles is shown for different wavebands, using the data from Yu Ho ( 2018 ). 4 DISCUSSION Recent work has attempted to correlate measurements of pitch angles with other galaxy properties as a means to distinguish between mechanisms for spiral arm generation. In these observational studies, the pitch angle is usually assumed to have a fixed value independent of radius or time. In particular, Yu et al. ( 2018 ) and Yu Ho (2018 , 2019 ) argue that measurements of pitch angles, and the correlations between these and other galaxy properties, are broadly consistent with the expectations of Lin–Shu modal density wave theory (Section 2.1 ). 3 We find nothing that contradicts this possibility. However, their arguments and analysis are predicated on the basic assumption of Lin–Shu theory, which is that each galaxy contains a spiral pattern whose pattern speed, Ω p , and pitch angle, α, are fixed in time (and space), being just dependent on the various properties of the galaxy, for example on morphology of the galactic rotation curve (fig. 11 in Yu Ho 2019 ) or the galactic shear rate (fig. 12 in Yu Ho 2019 ). Here, we investigate whether an alternative premise, i.e. that the pitch angles of galaxies evolve with time, is consistent with the observed data. Our finding that the distribution of pitch angles () is more or less uniform among their observed galaxies, opens up the possibility that the pitch angles are actually evolving in time, as is found in the simulations (e.g. Grand et al. 2013 ; Pettitt Wadsley 2018 ; Mata-Chávez et al. 2019 ). We note that in the modal density wave picture, this finding would need some other explanation. We argue here that the possibility of evolving pitch angles merits further investigation. Our simple