n( z) for the combined evolution model, extrapolated to z 4. The circles are the Prochter et al. (2006a) data for W _r >1 Å ( z = 2.6) and W _r >1.5 Å ( z = 4.3). The squares are the results of Jiang et al. (2007). The dotted line is the "no evolution" model, in which the number of absorbers per comoving unit distance is constant.