  We have investigated some statistical properties of integrated spectra of
galaxies with principal component analysis. The projection of the spectra onto
the plane defined by the first two principal components shows that normal
galaxies are in a quasi-linear sequence that we call spectral sequence and is
closely related to the Hubble morphological sequence. 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, we speculate that galaxies
may evolve morphologically along the Hubble sequence, from Sm/Im to E. If this
is the case, the first galaxies were mainly gas rich irregular objects, which
evolved later along the morphological sequence as long as mergers and
interactions increased their masses and developed their spheroidal components.
