  A classical result in complex geometry says that the automorphism group of a
manifold of general type is discrete. It is more generally true that there are
only finitely many surjective morphisms between two fixed projective manifolds
of general type. Rigidity of surjective morphisms, and the failure of a
morphism to be rigid have been studied by a number of authors in the past. The
main result of this paper states that surjective morphisms are always rigid,
unless there is a clear geometric reason for it. More precisely, we can say the
following.
  First, deformations of surjective morphisms between normal projective
varieties are unobstructed unless the target variety is covered by rational
curves.
  Second, if the target is not covered by rational curves, then surjective
morphisms are infinitesimally rigid, unless the morphism factors via a variety
with positive-dimensional automorphism group. In this case, the Hom-scheme can
be completely described.
