  We present in this paper a new three-dimensional galaxy classification system
designed to account for the diversity of galaxy properties in the nearby
universe. To construct this system we statistically analyse a sample of >22,000
galaxies at v<15,000 km/s (z<0.05) with Spearman rank and principal component
analyses. Fourteen major galaxy properties are considered, including: Hubble
type, size, colour, surface brightness, magnitude, stellar mass, internal
velocities, HI gas content, and an index that measures dynamical disturbances.
We find, to a high degree, that most galaxy properties are correlated, with in
particular Hubble type, colour, and stellar mass all strongly related. We argue
that this tight 3-way correlation is a result of evolutionary processes that
depend on galaxy mass, as we show that the relation between colour and mass is
independent of Hubble type. Various principal component analyses reveal that
most of the variation in nearby galaxy properties can be accounted for by
eigenvectors dominated by: (i) the scale of a galaxy, such as its stellar mass,
(ii) the spectral type, and (iii) the degree of dynamical disturbances. We
suggest that these three properties: mass, star formation, and
interactions/mergers are the major features that determine a galaxy's physical
state, and should be used to classify galaxies. As shown in Conselice (2003),
these properties are measurable within the CAS (concentration, asymmetry,
clumpiness) structural system, thus providing an efficient mechanism for
classifying galaxies in optical light within a physical meaningful framework.
We furthermore discuss the fraction and number density of galaxies in the
nearby universe as a function of Hubble type, for comparison with higher
redshift populations.
