  The internal space-time symmetries of relativistic particles are dictated by
Wigner's little groups. The $O(3)$-like little group for a massive particle at
rest and the $E(2)$-like little group of a massless particle are two different
manifestations of the same covariant little group. Likewise, the quark model
and parton pictures are two different manifestations of the one covariant
entity.
