  For supercritical multitype branching processes in continuous time, we
investigate the evolution of types along those lineages that survive up to some
time t. We establish almost-sure convergence theorems for both time and
population averages of ancestral types (conditioned on non-extinction), and
identify the mutation process describing the type evolution along typical
lineages. An important tool is a representation of the family tree in terms of
a suitable size-biased tree with trunk. As a by-product, this representation
allows a `conceptual proof' (in the sense of Kurtz, Lyons, Pemantle, Peres
1997) of the continuous-time version of the Kesten-Stigum theorem.
