Comparison of seemingly different modular units in the mammalian brain raises the possibility of a common mechanism for their formation: the growth of neuropil mediated by trophic interactions. The ongoing postnatal construction of modular circuits according to trophic interplay may in turn account for the remarkable plasticity of the juvenile brain. By the same token, the normal waning of circuit construction during postnatal development may explain the end of critical periods, the diminished ability to recover from injury in older animals, and the decline with increasing age in the ability of mammals to learn complex skills.