  Ultraviolet nebular emission lines are important for understanding the time
evolution and nucleosynthetic origins of their associated elements, but the
underlying trends of their relative abundances are unclear. We present UV
spectroscopy of 20 nearby low-metallicity, high-ionization dwarf galaxies
obtained using the Hubble Space Telescope. Building upon previous studies, we
analyze the C/O relationship for a combined sample of 40 galaxies with
significant detections of the UV O+2/C+2 collisionally-excited lines and
direct-method oxygen abundance measurements. Using new analytic carbon
ionization correction factor relationships, we confirm the flat trend in C/O
versus O/H observed for local metal-poor galaxies. We find an average log(C/O)
= -0.71 with an intrinsic dispersion of {\sigma} = 0.17 dex. The C/N ratio also
appears to be constant at log(C/N) = 0.75, plus significant scatter ({\sigma} =
0.20 dex), with the result that carbon and nitrogen show similar evolutionary
trends. This large and real scatter in C/O over a large range in O/H implies
that measuring the UV C and O emission lines alone does not provide a reliable
indicator of the O/H abundance. By modeling the chemical evolution of C, N, and
O of individual targets, we find that the C/O ratio is very sensitive to both
the detailed star formation history and to supernova feedback. Longer burst
durations and lower star formation efficiencies correspond to low C/O ratios,
while the escape of oxygen atoms in supernovae winds produces decreased
effective oxygen yields and larger C/O ratios. Further, a declining C/O
relationship is seen with increasing baryonic mass due to increasing effective
oxygen yields.
