  The form of the early-type galaxy scaling relation (the Fundamental Plane or
FP) is a direct indicator of the underlying physical origins for the galaxy
sequence. Observed properties of the FP include: (1) the slope increases with
wavelength; (2) the slope deviates from the virial expectation (assuming
homology and constant M/L) at all wavelengths; (3) the intercept evolves
passively with redshift; and (4) the slope decreases slowly with redshift. The
first property implies that stellar populations contribute to the slope of the
FP, the second and fourth properties exclude metallicity effects as the sole
cause of the slope, and the third implies that the stellar content of the
"average" early-type galaxy formed at high redshift. A composite
model--including variations in age and metallicity, as well as a
wavelength-independent effect such as homology breaking--is presented which can
fit all four observed properties. This model implies that the most luminous
early-type galaxies contain the oldest and most metal-rich stars, while the
lowest luminosity galaxies formed the bulk of their stars as recently at z_f~1.
