One of my favorite coffee-room comments is that we ought to be able to perform a full continuity analysis of the galaxy population, describing all changes in population distributions from one epoch to the next in terms of galaxies that show direct signs of evolving in the interval between the epochs, and relating the abundance per unit quantity x of evolving galaxies to the inverse rate of evolution of quantity x, except where there are sources and sinks. In discussions with Blanton and Moustakas today, I think I figured out the first place to start: The blue galaxy population, where the quantities x will be color, total absolute magnitude, and central stellar density. Because we have star-formation rates (accurately in the centers and inaccurately for the total), we can directly measure some components of the "velocities" of galaxies in the space.
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