  We have investigated some statistical properties of integrated spectra of
galaxies from Kennicutt (1992a) spectrophotometric atlas. The input for the
analysis are galaxy spectra sampled in 1300 bins between 3750 \AA~ and 6500
\AA. We make use of Principal Component Analysis (PCA) to analyse the
1300-dimensional space spanned by the spectra. Their projection onto the plane
defined by the first two principal components, the principal plane, shows that
normal galaxies are in a quasi-linear sequence that we call spectral sequence.
We show that the spectral sequence is closely related to the Hubble
morphological sequence. These results are robust in the sense that the reality
of the spectral sequence does not depend on data normalization. The existence
of this sequence suggests that a single parameter may describe the spectrum of
normal galaxies. We have investigated this hypothesis with Bruzual & Charlot
(1995) models of spectral evolution. We show that, for single age models (15
Gyr), the spectral sequence can be parametrized by the characteristic star
formation time-scales of the different morphological types. By examining the
projection of evolutionary tracks of normal galaxies onto the principal plane,
we verify that the spectral sequence is also an evolutive sequence, with galaxy
spectra evolving from later to earlier spectral types. Considering the close
correspondence between the spectral and morphological sequences, this lead us
to speculate that galaxies may evolve morphologically along the Hubble
sequence, from Sm/Im to E.

Astrophysics