  A full account of galaxy evolution in the context of LCDM cosmology requires
measurements of the average star-formation rate (SFR) and cold gas abundance
across cosmic time. Emission from the CO ladder traces cold gas, and [CII] fine
structure emission at 158 um traces the SFR. Intensity mapping surveys the
cumulative surface brightness of emitting lines as a function of redshift,
rather than individual galaxies. CMB spectral distortion instruments are
sensitive to both the mean and anisotropy of the intensity of redshifted CO and
[CII] emission. Large-scale anisotropy is proportional to the product of the
mean surface brightness and the line luminosity-weighted bias. The bias
provides a connection between galaxy evolution and its cosmological context,
and is a unique asset of intensity mapping. Cross-correlation with galaxy
redshift surveys allows unambiguous measurements of redshifted line brightness
despite residual continuum contamination and interlopers. Measurement of line
brightness through cross-correlation also evades cosmic variance and suggests
new observation strategies. Galactic foreground emission is $\sim 10^3$ times
larger than the expected signals, and this places stringent requirements on
instrument calibration and stability. Under a range of assumptions, a linear
combination of bands cleans continuum contamination sufficiently that residuals
produce a modest penalty over the instrumental noise. For PIXIE, the $2 \sigma$
sensitivity to CO and [CII] emission scales from $\sim 5 \times 10^{-2}$ kJy/sr
at low redshift to ~2 kJy/sr by reionization.
