Are disk galaxies older spiral galaxies or vice versa?

Spiral galaxies are subdivided among three classes Sa, Sb, Sc, with a parallel sequence for Barred Spirals SBa, SBb, SBc . ... Originally Astronomers thought that the Hubble sequence might be an evolutionary sequence with galaxies evolving from right-to-left (or vice versa) along the tuning fork. read more

-Sub-divided spiral galaxies into two groups: normal spirals (S) and barred-spirals (SB)-Each type of spiral galaxy is then further subdivided into categories Sa, Sb, and Sc 1. Sa galaxies: the largest nuclear bulges and the most tightly-wound arms. 2, Sb galaxies: middle-size bulges and moderately-wound arms. 3. read more