  Many galaxies appear to have taken on their familiar appearance relatively
recently. In the distant Universe, galaxy morphology started to deviate
significantly (and systematically) from that of nearby galaxies at redshifts,
z, as low as z = 0.3. This corresponds to a time ~3.5 Gyr in the past, which is
only ~25% of the present age of the Universe. Beyond z = 0.5 (5 Gyr in the
past) spiral arms are less well-developed and more chaotic, and barred spiral
galaxies may become rarer. By z = 1, around 30% of the galaxy population is
sufficiently peculiar that classification on Hubble's traditional tuning fork
system is meaningless. On the other hand, some characteristics of galaxies do
not seem to have changed much over time. The co-moving space density of
luminous disk galaxies has not changed significantly since z = 1, indicating
that while the general appearance of these objects has continuously changed
with cosmic epoch, their overall numbers have been conserved. Attempts to
explain these results with hierarchical models for the formation of galaxies
have met with mixed success.
