Density-Wave Spiral Theories in the 1960s. II
  By the 1970s the spiral subject was in considerable disarray. The semiempirical theory by Lin and Shu was confronted with serious problems. They were put on the defensive over their tightly wrapped steady modes on two principal fronts: from the radial propagation at the group velocity that would tend to wind them almost at the material rate, and from the tendencies of galaxy disks toward a strong global instability that appeared likely to overwhelm them. Of course, one might claim that such threats were imaginary and only of academic interest, on the ground that nature itself had overcome them. One might also be confident that the QSSS hypothesis must be correct as illuminated by the everlasting truth of Hubble's classification. One might even take pride in the fact that a very promising concept developed, although not connected to the wave steadiness, on spiral shocks in interstellar gas and their induced star formation. But such a heuristic approach did not stimulate very strong progress in understanding dynamical principles of the spiral phenomenon; moreover, it often misled, and a rich irony was already that the supposed QSSS favorites M51 and M81 turned out most probably not to be quasi-steady at all. A further irony was the continuing failure of Lin and Shu to account the trailing character of their 'modes', while that was already grasped by their direct 'deductive' opponents. But the greatest irony lay in the fact that the concept later known as swing amplification, worked out by the mid-1960s, was originally denigrated by Lin's camp as relating exclusively to 'material arms', whereas it turned out in the end to be of vital importance to this entire spiral enterprise including the variants of chaotic ragged patterns, tidal transient grand designs and growing or quasi-steady modes. 
I.I. Pasha