From the Editorial Board. This is the presentation of the author’s talk given at the 10th International Symposium on the Interaction of Leptons and Photons at High Energies (24–29 August 1981, Bonn, Germany); published first in the Proc. of the 1981 Intern. Symp. on Lepton and Photon Interactions at High Energy, 24–29 August, 1981, Bonn, Germany (Ed. W Pfeil) (Bonn: Bonn Univ., Phys. Inst., 1981) p. 1018. The text of the lecture for Uspekhi Fizicheskikh Nauk (Russian version of Physics-Uspekhi journal) was translated from English into Russian by M I Vysotsky and is published here in English (style edited by K Franchuk) by the kind permission of Prof. W. Pfeil (Ed.), Physikalisches Institut, University Bonn.