The concept of isolated galaxy is useful to sort out the intrinsic and extrinsic factors playing a role in the galaxy structure and evolution. Us-ing the Lagrange-Jacoby identity one can order the global energies and powers at play in galaxies determining their equilibrium, respectively their evolution. Some factors appear then obviously as secondary, like magnetic fields, and other as potentially important, like star formation and evolution, or black-holes. It follows that over several Gyr, disks galaxies must be viewed as evolving dissipative structures ruled mainly by a feedback mechanism between stellar activity and disk dynamics. Mass loss from evolved stars appears as sufficiently important over several Gyr to view galaxies, including ellipticals, as essentially dissipative. The old paradigm of conservative dynamics in galaxies seems applicable only over time-scales up to a few Gyr.
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