  We examine whether early-type galaxies in clusters may have evolved from
later types by the fading of their disks (e.g., as a result of ram-pressure
stripping or strangulation) or by enhancement of the bulge luminosity (e.g.,
due to tidal interactions and mergers). For this purpose, we compare the bulge
and disk luminosities of early- and late-type galaxies and of galaxies at
different radial distances from the cluster center. We find that, in order for
early-type galaxies, including S0s, to have evolved from late-type galaxies,
their bulge luminosities must have been physically enhanced. Disk fading models
cannot explain the differences observed. We then show that galaxy bulges are
systematically brighter at small projected distances from the cluster center,
while disk luminosities are uncorrelated with cluster-centric distance. Our
results suggest that bulge enhancement, not disk fading, distinguishes early
from late types and is thus at least partially responsible for the
morphology-environment relation of bright cluster galaxies.
