  We show that the relativistic analogue of the two types of time translation
in a non-relativistic history theory is the existence of two distinct
Poincar\'{e} groups. The `internal' Poincar\'{e} group is analogous to the one
that arises in the standard canonical quantisation scheme; the `external'
Poincar\'{e} group is similar to the group that arises in a Lagrangian
description of the standard theory. In particular, it performs explicit changes
of the spacetime foliation that is implicitly assumed in standard canonical
field theory.
