  A recent paper (Burke, Collins & Mann 2000) presents the analysis of the
K-band Hubble diagram of 76 brightest cluster galaxies (BCGs) in X-ray clusters
and shows that the properties of BCGs depend on the X-ray luminosity (Lx) of
their host clusters. Unfortunately, the low numbers of nearby clusters in this
sample makes it difficult to constrain evolutionary trends. In this letter we
extend the Hubble diagram of Burke et al. (2000) to a total of 155 clusters
using new data on 79 BCGs at z<0.1 from the 2MASS extended source catalogue. We
show that the major division between BCGs in high and low-Lx clusters
disappears at z<0.1, with BCGs having similar absolute magnitudes independent
of the X-ray luminosity of their host clusters. At larger redshifts the K-band
light of BCGs in high-Lx systems is consistent with little or no merging back
to z~0.8, whereas BCGs in the low-Lx systems have a different evolutionary
history, with many increasing their mass by a factor >4 since z~1. This
provides direct evidence of hierarchical merging in a galaxy population.
