  A new selection phenomenon in nonlinear interface dynamics is predicted. A
generic class of exact regular unsteady multi-bubble solutions in a Hele-Shaw
cell is presented. These solutions show that the case where the asymptotic
bubble velocity, $U$, is twice greater than the velocity, $V$, of the uniform
background flow, i.e., $U = 2V$, is the only attractor of the dynamics.
Contrary to common belief, the predicted velocity selection requires neither
surface tension nor other external regularization.
