The black hole mass distribution in early-type galaxies: cusps in HST photometry interpreted through adiabatic black hole growth . ... I show that the observed characteristics of these cusps are consistent with the hypothesis that: (i) all early-type galaxies have central BHs that grew adiabatically in homogeneous isothermal cores; and (ii) these `progenitor' cores followed scaling relations similar to those of the fundamental plane. The models studied here are the ones first proposed by Young. ...