  We present stacked average far-infrared spectra of a sample of 197 dusty,
star-forming galaxies (DSFGs) at $0.005 < z < 4$ using close to 90% of the
SPIRE Fourier Transform Spectrometer (FTS) extragalactic data archive from the
Herschel Space Observatory based on 3.5 years of science operations. These
spectra explore an observed-frame $\rm 447\,GHz-1568\,GHz$ ($\rm 191\,\mu
m-671\,\mu m$) frequency (wavelength) range allowing us to observe the main
atomic and molecular lines emitted by gas in the interstellar medium. The
sample is sub-divided into five redshift bins at $0.005 < z < 0.05$, $0.05 < z
< 0.2$, $0.2 < z < 0.5$, $0.8 < z <2$, and $2 < z < 4$. To study the dependence
of observed spectral lines on total infrared luminosity, the sources in a
subset of the redshift bins are stacked in luminosity bins. These stacked
spectra are used to determine the average properties of the interstellar medium
and dense molecular gas properties of DSFGs, in particular, the fine-structure
line ([CII] 158 $\mu$m and [OI] 63 $\mu$m) luminosity ratios, and the line to
far-IR luminosity ratios are used to model the gas density and radiation field
strength in the photodissociation regions (PDRs). For the low-redshift sample,
we additionally present the average spectral line energy distributions (SLED)
of CO and $\rm{H_2O}$ rotational transitions and also consider PDR conditions
based on a combination of [CI] 370 $\mu$m and 609 $\mu$m and $\rm CO (7-6)$
lines. For the high-z ($0.8 < z < 4$) sample PDR models suggest a molecular gas
distribution in the presence of a radiation field that is at least a factor of
10$^3$ larger than the Milky-Way and with a neutral gas density of roughly
10$^3$ to 10$^5$ cm$^{-3}$. The corresponding PDR models for the low-z sample
suggest a UV radiation field and gas density comparable to those at high-z.
