  A cluster sample of 62 galaxies complete to M(B)=-16.5 mag has been observed
at B,R,I,K' bands with imaging detectors. The distribution of exponential disk
central surface brightnesses is found to be bimodal. The bimodality is
particularly significant at K' because obscuration is not a problem and because
the high surface brightness galaxies are redder than the low surface brightness
galaxies so the bifurcation is greater. The bimodality signal is especially
clear when galaxies with close companions are excluded from consideration. High
and low surface brightness pairs with essentially identical luminosities and
maximum rotation characteristics are compared. It is suggested that the high
surface brightness galaxies have self-gravitating disks while the low surface
brightness galaxies are halo dominated at all radii. Evidently the intermediate
surface brightness regime is unstable. If a disk has sufficiently low angular
momentum and it shrinks enough that the disk potential begins to dominate the
halo potential locally, then the disk must secularly evolve to the high surface
brightness state characterized by a flat rotation curve.
