Do galaxies form a spectroscopic sequence?
  We identify a spectroscopic sequence of galaxies, analogous to the Hubble
sequence of morphological types, based on the Automatic Spectroscopic K-means
(ASK) classification. Considering galaxy spectra as multidimensional vectors,
the majority of the spectral classes are distributed along a well defined curve
going from the earliest to the latest types, suggesting that the optical
spectra of normal galaxies can be described in terms of a single affine
parameter. Optically-bright active galaxies, however, appear as an independent,
roughly orthogonal branch that intersects the main sequence exactly at the
transition between early and late types.
