  Optical surveys for galaxies are biased against the inclusion of low surface
brightness (LSB) galaxies. Disney (1976) suggested that the constancy of disk
central surface brightness noticed by Freeman (1970) was not a physical result,
but instead was an artifact of sample selection. Since LSB galaxies do exist,
the pertinent and still controversial issue is if these newly discovered
galaxies constitute a significant percentage of the general galaxy population.
In this paper, we address this issue by determining the space density of
galaxies as a function of disk central surface brightness. Using the physically
reasonable assumption (which is motivated by the data) that central surface
brightness is independent of disk scale length, we arrive at a distribution
which is roughly flat (i.e. approximately equal numbers of galaxies at each
surface brightness) faintwards of the Freeman (1970) value. Brightwards of
this, we find a sharp decline in the distribution which is analogous to the
turn down in the luminosity function at $L^*$. An intrinsically sharply peaked
``Freeman Law'' distribution can be completely ruled out, and no Gaussian
distribution can fit the data. Low surface brightness galaxies (those with
central surface brightnesses fainter than $22~B\sb$) comprise $\gtrsim 1/2$ the
general galaxy population, so a representative sample of galaxies at $z = 0$
does not really exist at present since past surveys have been insensitive to
this component of the general galaxy population.
