  The existing field theories are based on the properties of closed exterior
forms, which are invariant ones and correspond to conservation laws for
physical fields. Hence, to understand the foundations of field theories and
their unity, one has to know how such closed exterior forms are obtained.
  In the present paper it is shown that closed exterior forms corresponding to
field theories are obtained from the equations modelling conservation
(balance)laws for material media. It has been developed the evolutionary method
that enables one to describe the process of obtaining closed exterior forms.
  The process of obtaining closed exterior forms discloses the mechanism of
evolutionary processes in material media and shows that material media
generate, discretely, the physical structures, from which the physical fields
are formed. This justifies the quantum character of field theories.
  On the other hand, this process demonstrates the connection between field
theories and the equations for material media and points to the fact that the
foundations of field theories must be conditioned by the properties of material
media. It is shown that the external and internal symmetries of field theories
are conditioned by the degrees of freedom of material media. The classification
parameter of physical fields and interactions, that is, the parameter of the
unified field theory, is connected with the number of noncommutative balance
conservation laws for material media.
