Introduction

It is surprising that Hubble (1936, p. 55, pp. 79–81) makes only a few passing references to the fact that early-type (E–S0–Sa) galaxies predominate in rich clusters, whereas the field is dominated by galaxies of late type (Sc–Ir). Spitzer & Baade (1951) were the first to emphasize the physical importance of the fact that the frequency of S0 galaxies is greatest in rich clusters of galaxies. Van den Bergh (1962) subsequently used the difference between the galactic populations in rich clusters and in the field to show that rich clusters must be stable over periods comparable to the age of the Universe. In particular the difference in the galactic populations of clusters and field provided a powerful argument against the tentative speculation by Hubble (1936, p. 81) ‘that the disintegration of clusters may populate the general field.’ The possible physical significance of the relation between galaxy morphology and environmental density was first discussed in great detail by Dressier (1980), who stressed that elliptical galaxies are most frequent in the regions of highest density, whereas late-type spirals predominate in low-density regions. A some-what different approach was taken by Whitmore & Gilmore (1991) who found that galaxy morphology was strongly correlated with distance from the cluster center. It is, of course, difficult to disentangle these effects because local density and distance from the cluster center are closely correlated. Sanromà & Salvador-Solé (1990) found that galaxy morphology does not appear to be affected by sub-clumpings within rich clusters.