  The evolution of a quantity, described by a function of space and time,
relates the first derivative in time of this function to a spatial operator
applied to the function. The initial value of the function at time $t=0$ is
given.
  The fractional extension of this evolution consists of replacing the first
derivative in time by a fractional derivative of order $\alpha$, $0 < \alpha
\le 1$.
  We give a relationship between the solution of the equation of evolution and
the solution of the equation belonging to its fractional extension.
