Left panel: distribution of the slope β for the whole sample of galaxies. The red histogram shows the values of β measured from z = 4 to z = 1, the black histogram from z = 0.2 to z = 0. The dashed lines give the medians of the distributions. Right panel: distribution of the slope β for the galaxies of different masses. The red histogram shows the values of β measured from z = 4 to z = 0, the black histogram from z = 0.2 to z = 0.