  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.
