METAZOAN COMPLEXITY AND EVOLUTION - IS THERE A TREND - PERSPECTIVE

The notion that complexity increases in evolution is widely accepted, but the best-known evidence is highly impressionistic. Here I propose a scheme for understanding complexity that provides a conceptual basis for objective measurement. The scheme also shows complexity to be a b road term covering four independent types. For each type, I describe s ome of the measures that have been devised and review the evidence for trends in the maximum and mean. In metazoans as a whole, there is goo d evidence only for an early Phanerozoic trend, and only in one type o f complexity. For each of the other types, some trends have been docum ented, but only in a small number of metazoan subgroups.