  The dynamical evolution of galaxies in clusters is modified if dark matter is
self-interacting. Heat conduction from the hot cluster halo leads to
evaporation of the relatively cooler galactic halos. The stellar distribution
would adiabatically expand as it readjusts to the loss of dark matter, reducing
the velocity dispersion and increasing the half-light radius. If the dark
matter content within that radius was f_dm = 25-50% of the total, as indicated
by current observations, the ellipticals in clusters would be offset from the
fundamental plane relation beyond the observational scatter. The requirement
that their halos survive for a Hubble time appears to exclude just that range
of the dark matter cross-section, 0.3 < sigma/m < 10^4 cm^2/g, thought to be
optimal for reducing central halo cusps, unless f_dm < 15%. If the
cross-section is allowed to vary with the relative velocity of dark matter
particles, sigma \propto v^{-2 delta}, a new problem of evaporation of dark
matter arises in the dwarf galaxies with low velocity dispersion. The halos of
large galaxies in clusters and dwarf galaxies in the Local Group can both
survive only if delta < 1.1 or delta > 1.8. In either case the problem of
central density cusps remains.
