  We consider Saffman-Taylor channel flow without surface tension on a
high-pressure driven interface, but modify the usual infinite-fluid in
infinite-channel configuration. Here we include the treatment of efflux by
considering a finite connected body of fluid in an arbitrarily long channel,
with its second free interface the efflux of this configuration. We show that
there is a uniquely determined translating solution for the driven interface,
which is exactly the 1/2 width S-T solution, following from correct symmetry
for a finite channel flow. We establish that there exist no perturbations about
this solution corresponding to a finger propagating with any other width:
Selection is unique and isolated. The stability of this solution is anomalous,
in that all freely impressible perturbations are stabilities, while unstable
modes request power proportional to their strength from the external agencies
that drive the flow, and so, in principle, are experimentally controllable.
This is very different from the behavior of the usual infinite fluid: The limit
of infinite length is singular, and not that of the literature. We argue that
surface tension on the efflux interface decreases the finger-width by the ratio
of its surface tension to velocity of the high-pressure tip. The perturbation
theory created here to deal with transport between two free boundaries is novel
and dependent upon a remarkable symmetry.
