  Galaxies form and evolve in the context of their local and large-scale
environments. Their baryonic content that we observe with imaging and
spectroscopy is intimately connected to the properties of their dark matter
halos, and to their location in the "cosmic web" of large-scale structure. Very
large spectroscopic surveys of the local universe (e.g., SDSS and GAMA) measure
galaxy positions (location within large-scale structure), statistical
clustering (a direct constraint on dark matter halo masses), and spectral
features (measuring physical conditions of the gas and stars within the
galaxies, as well as internal velocities). Deep surveys with the James Webb
Space Telescope (JWST) will revolutionize spectroscopic measurements of
redshifts and spectral properties for galaxies out to the epoch of
reionization, but with numerical statistics and over cosmic volumes that are
too small to map large-scale structure and to constrain halo properties via
clustering. Here, we consider advances in understanding galaxy evolution that
would be enabled by very large spectroscopic surveys at high redshifts: very
large numbers of galaxies (outstanding statistics) over large co-moving volumes
(large-scale structure on all scales) over broad redshift ranges (evolution
over most of cosmic history). The required observational facility can be
established as part of the probe portfolio by NASA within the next decade.
