  Based upon a novel paradigm of the galaxy formation under UV background, the
evolutionary bifurcation of pregalactic clouds is confronted with observations
on elliptical and spiral galaxies. The theory predicts that the dichotomy
between the dissipational and dissipationless galaxy formation stems from the
degree of self-shielding from the UV background radiation. This is demonstrated
on a bifurcation diagram of collapse epochs versus masses of pregalactic
clouds. Using the observed properties, the collapse epochs are assessed for
each type of galaxies with attentive mass estimation. By the direct comparison
of the theory with the observations, it turns out that the theoretical
bifurcation branch successfully discriminates between elliptical and spiral
galaxies. This suggests that the UV background radiation could play a profound
role for the differentiation of the galactic morphology into the Hubble
sequence.
