Developmental structure in brain evolution

First, we clarify the central nature of our argument: our attempt is to app ortion variation in brain size between developmental constraint, system-spe cific change, and "mosaic" change, underlining the unexpectedly large role of developmental constraint, but making no case for exclusivity. We conside r the special cases of unusual hypertrophy of single structures in single s pecies, regressive nervous systems, and the unusually variable cerebellum r aised by the commentators. We defend the description of the cortex (or any developmentally-constrained structure) as a potential spandrel, and weigh t he implications of the spandrel concept for the course of human evolution. The empirical and statistical objections raised in the commentary of Barton are discussed at length. Finally, we catalogue and comment on the suggesti ons of new ways to study brain evolution, and new aspects of brain evolutio n to study.