  Galaxies have clearly evolved since the universe was 1 Gyr old, but methods
to trace and quantify this evolution are still in their infancy. In this paper
I demonstrate that with the careful use of a `physical morphology' it is
possible to determine quantitatively how the process of galaxy evolution is
occurring out to z=4. Using a system of parameters that traces star formation
and galaxy interactions, I show how distinct galaxy populations at high-z can
be identified in deep high-resolution optical and NIR images. These tools are
also used to measure a potential merger fraction of galaxies from 0<z<4. If
these methods are reliable, as is suggested by a local galaxy calibration, the
merger fraction of galaxies scales as (1+z)^{2.1+/-0.5}, peaks near z=2, and
declines thereafter. I also discuss how this system is likely part of the
ultimate physical classification of galaxies.
