  Using a categorial version of Fra\"iss\'e's theorem due to Droste and
G\"obel, we derive a criterion for a comma-category to have universal
homogeneous objects.
  As a first application we give new existence result for universal structures
and for \omega-categorical universal structures.
  As a second application we characterize the retracts of a large class of
homogeneous structures, extending previous results by Bonato, Deli\'c, Dolinka,
and Kubi\'s.
  As a third application we show for a large class of homogeneous structures
that their polymorphism clone is generated by polymorphisms of bounded arity,
generalizing a classical result by Sierpi\'nski that the clone of all functions
on a given set is generated by its binary part.
  Further we study the cofinality and the Bergman property for clones and we
give sufficient conditions on a homogeneous structure to have a polymorphism
clone that has uncountable cofinality and the Bergman property.
