Introduction

Gaseous material is expected to flow into galaxies by the capture of high-velocity clouds (Oort 1966, 1970). Oort estimated that the mass of the Milky Way system might be increasing by ∼1% per Gyr as a result of such inflow. However, tidal interactions between galaxies will result in the return of some interstellar gas in galaxies to intergalactic space (Morris & van den Bergh 1994). In addition, some gas may be ejected from galaxies by supernova-induced fountains (Heiles 1987). Perhaps the best example of inflow into the Galactic Disk is provided by “Complex C” (Wakker & van Woerden 1997). From the nondetection of absorption features in stars of known distance, these authors conclude that D > 2.4 kpc and that Complex C is falling toward us with a velocity > 100 km s-1. From spectroscopic observations of S II absorption lines (which are not affected by depletion onto dust) with the Hubble Space Telescope, Wakker et al. (1999) find that Complex C has a metallicity of 0.094 ± 0.020 times solar. This very low metallicity rules out the possibility that the gas in Complex C was ejected in a Galactic fountain. Furthermore, the data on the age–metallicity relation of the Small Magellanic Cloud (Mighell, Sarajedini & French 1998) show that the gas in Complex C is so metal poor that it could only have been stripped from the SMC ≳5 Gyr ago (i.e., long before the Magellanic Stream was formed).