High-Energy Maxima in theπ−pCross Sections

It is pointed out that the consideration of causality requires that any description of the maximum in the ${\pi }^{-}-p$ total cross section at 800-Mev ${\pi }^{-}$ energy, and the maximum in the ${\pi }^{+}-p$ cross section at 1200-Mev, is necessarily equivalent to a description in terms of resonances and hence in terms of nucleon isobars.