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We present high resolution (R = 18,000), high signal-to-noise, 2 micronspectra of 52 infrared-selected Class I and flat-spectrum young stellar objectsin the Taurus-Auriga, $\rho$ Ophiuchi, Serpens, Perseus, and Corona Australisdark clouds. We detect key absorption lines in 41 objects and fit syntheticspectra generated from pre-main sequence models to deduce the effectivetemperatures, surface gravities, near-infrared veilings, rotation velocities,and radial velocities of each of these 41 sources. We find these objects tospan ranges in effective temperature, surface gravity, and stellar luminositywhich appear similar to those of late spectral-type Class II sources andclassical T-Tauri stars. We determine that the mean 2 micron veiling of Class Iand flat-spectrum objects is significantly higher than that of Class II objectsin the same region where both types of objects are extensively observed ($\rho$Oph). We find a significant fraction of our protostellar sample also exhibitsemission lines. Twenty-three objects show H$_2$ emission, usually indicative ofthe presence of energetic outflows. Thirty-four sources show HI Br $\gamma$emission and a number of these exhibit profile asymmetries consistent within-fall. Eight sources show significant $\Delta v = 2$ CO emission suggestiveof emission from a circumstellar disk. Overall, these observations indicatethat Class I and flat-spectrum objects are self-embedded protostars undergoingsignificant mass accretion, although the objects appear to span a broad rangeof mass accretion activity.Comment: LaTeX, 35 pages + 23 figures. Accepted for publication in A

The Physical Natures of Class I and Flat-Spectrum Protostellar Photospheres: A Near-Infrared Spectroscopic Study