How Do Galaxies Form?

It is currently believed that galaxies were assembled via chaotic hierarchical mergers between massive cold dark matter halos, in which baryonic star forming matter was embedded. One would therefore expect the properties of individual galaxies to be determined by numerous independent factors such as star forming history, merger history, mass, angular momentum, size and environment. It is therefore surprising to find that galaxies actually appear to form an (almost) one parameter family in which galaxy mass is the dominant factor.