  We use hydrodynamical simulations to investigate the response of
geometrically thin, self-gravitating, singular isothermal disks of gas to
imposed rigidly rotating spiral potentials. By minimizing reflection-induced
feedback from boundaries, and by restricting our attention to models where the
swing parameter $X \sim 10$, we minimize the swing amplification of global
normal modes even in models where Toomre's $Q_g \sim 1-2$ in the gas disk. We
perform two classes of simulations: short-term ones over a few galactic
revolutions where the background spiral forcing is large, and long-term ones
over many galactic revolutions where the spiral forcing is considerably
smaller. In both classes of simulations, the initial response of the gas disk
is smooth and mimics the driving spiral field. At late times, many of the
models evince substructure akin to the so-called branches, spurs, and feathers
observed in real spiral galaxies. We comment on the parts played respectively
by ultraharmonic resonances, reflection off internal features produced by
nonlinear dredging, and local, transient, gravitational instabilites within
spiral arms in the generation of such features. Our simulations reinforce the
idea that spiral structure in the gaseous component becomes increasingly
flocculent and disordered with the passage of time, even when the background
population of old disk stars is a grand-design spiral. We speculate that truly
chaotic behavior arises when many overlapping ultraharmonic resonances develop
in reaction to an imposed spiral forcing that has itself a nonlinear, yet
smooth, wave profile.
