  A survey of H alpha emission in 320 spiral galaxies in 8 nearby clusters
shows an enhancement of circumnuclear starburst emission with increasingly rich
clusters. These observations provide convincing evidence that spirals have been
transformed into S0s in clusters predominantly by tidal forces, a picture fully
in accord with the most recent numerical simulations of clusters. For the
richest clusters, the enhancement of starburst emission is greater than would
be expected on the basis of increasing galaxy surface density alone, which may
explain the anomalous result for the type-galaxy surface density relation found
for low richness clusters at intermediate redshift.
