THE discovery of different kinds of mesons with rest-masses spreading over a considerable range has brought about a fundamental change in the aspect of the problem of elementary particles. It looks as if there exists a great many, possibly an infinite number of particles with different rest-masses, most of them being unstable, while a few have a long life-time. So long as one had to deal only with a small number of particles, the electron, nucleon (protonneutron), and Yukawa's meson, it was appropriate to treat each of them separately and to describe them by wave equations constructed according to the more or less known properties (rest-mass, spin) of the particles. This procedure has now become inadequate ; one has to face the problem of the interrelation of the particles and in particular of their rest-masses. The safest course to take would, of course, be to wait until enough experimental material has been accumulated for building a reliable theory on it. Yet I think that preliminary work can be done now, namely, a discussion of the type of theory which could deal with the situation.