The body of evolutionary theory that currently occupies a dominant position in biological thought is neo-Darwinism. While this theory bas considerable explanatory power, it is widely recognized as being incomplete in that it lacks a component dealing with individual development, or ontogeny. This lack is particularly conspicuous in relation to attempts to explain the evolutionary origin of the 35 or so animal body plans, and of the developmental trajectories that generate them. This book examines both the origin of body plans in particular and the evolution of animal development in general. In doing so, it ranges widely, covering topics as diverse as comparative developmental genetics, selection theory and Vendian/Cambrian fossils. Particular emphasis is placed on gene duplication, changes in spatiotemporal gene-expression patterns, internal selection, coevolution of interacting genes, and coadaptation. The book will be of particular interest to researchers and students in evolutionary biology, genetics, palaeontology and developmental biology.