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2.3.2. Environmental Effects

The tendency for ellipticals to be found in clusters is a manifestation of the morphology-density relationship, a term coined by Dressler in the first really thorough investigation of the effect [28], although the basic tendency for ellipticals to be found in rich clusters was known to Hubble, and the physical significance of this environmental effect was emphasized by Spitzer & Baade (1951). The issue has resurfaced as point of controversy in recent investigations which seek to determine whether the key correlations are between morphology and local galaxy density, or between morphology and radial distance from the centre of rich clusters [75, 86]. Obviously local galaxy density and cluster-centric radius are strongly correlated, so the distinction can be rather subtle, but resolving this issue is central to understanding whether the distribution of galaxy types is built up gradually from large scale structure by accretion from the field, or is instead primordial in origin (the nature versus nurture controversy).