  The unprecedented resolution of the refurbished Wide Field and Planetary
Camera 2 (WFPC2) on the Hubble Space Telescope (HST) has led to major advances
in our understanding of galaxy formation. The high image quality in the Medium
Deep Survey and Hubble Deep Field has made it possible, for the first time, to
classify faint distant galaxies according to morphological type. These
observations have revealed a large population of galaxies classed as irregulars
or which show signs of recent merger activity. Their abundance rises steeply
with apparent magnitude, providing a likely explanation for the large number of
blue galaxies seen at faint magnitudes. We demonstrate that such a population
arises naturally in a model in which structure forms hierarchically and which
is dynamically dominated by cold dark matter. The number counts of irregular,
spiral and elliptical galaxies as a function of magnitude seen in the HST data
are well reproduced in this model.We present detailed predictions for the
outcome of spectroscopic follow-up observations of the HST surveys. By
measuring the redshift distributions of faint galaxies of different
morphological types, these programmes will provide a test of the hierarchical
galaxy formation paradigm and might distinguish between models with different
cosmological parameters.
