  We study the evolution of the ICM with a sample of 70 galaxy clusters
spanning 0.18 < z < 1.24. We find that X-ray luminosity and ICM mass at a fixed
temperature evolve with redshift in a manner inconsistent with the standard
self-similar model of cluster formation. Both luminosity and ICM mass evolve
more slowly toward high redshift than the self-similar prediction. We find that
evolution in these two observables can be modeled by a simple evolution in the
cluster gas mass fraction. Excluding cluster cores from measurements results in
evolution more consistent with the self-similar model than when the entire
cluster is used, indicating that the fraction of clusters with cool cores
increases with time, or that cool cores become more developed over time in
those clusters that have them; this is supported by direct study of the
redshift dependence of central surface brightness, which increases in scatter
and magnitude at low redshift. We find that isophotal size-temperature
relations evolve differently according to which isophote is used, indicating
that the central and outer regions of cluster ICM evolve differently. We show
that constraints on the evolution of the gas fraction and isophotal
size-temperature relations constraints can be combined to measure cluster
distances, and thus to constrain cosmological parameters. There are indications
that scaling relation scatter decreases at higher redshift, suggesting that
merging is not the dominant source of cluster structural variation. Our results
provide constraints for simulations attempting to model cluster physics,
indicate some difficulties for cosmological studies that assume constant
cluster gas fractions, and point toward other potentially more robust uses of
clusters for cosmological applications. (Abridged)
