Mass Degeneracy of the Heavy Mesons

Consequences of the assumption that the $\theta -\tau$ mass degeneracy is not accidental, but follows from some symmetry principle, are discussed. This assumption implies, among other things the existence of doublets of opposite parity for all particles of odd strangeness number, such as ${\Lambda }^0$ and ${\Sigma }^{\pm }$, ${\Sigma }^0$.