Evolution of nervous systems and psychiatry: consequences of the vertical and horizontal duality of the evolutionary process.

Jacksonian views of brain evolution where new levels "add on" and become higher levels of integration "keeping down" the lower levels are examined. The hierarchical organization is contrasted with modern views of the evolution of nervous systems. These emphasize the "separation or factorization of different aspects of input into distinct processing channels, a factor which appears to be a generalized one in the evolution of brains and a necessary condition to adapt to a varying environment." The advantages of the latter view--vertical and horizontal development--for the interpretation of functional organization of nervous systems are discussed. The view that normal and pathological conditions do not form a continuum, but constitute qualitatively different phenomena, is presented and given support because of recent developments in neuroscience.