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Published online by Cambridge University Press: 12 April 2016
There exist two basically different approaches to the explanation of the structure and the evolution of galaxies. The conventional one is to consider galaxies as isolated self-regulating systems. This approach is, of course, largely justified. To illustrate the essential independence of the properties of galaxies from their intergalactic environment, it is enough to compare the mean densities within galaxies (10-24gcm-3) to the mean cosmological density (≲10-29 gcm-3).