In 1829, von Baer proposed four principles of development, the first of which states that the embryos of a large group of animals have most in common at early stages of development, the specialised features of different subgroups only emerging later. After the publication of Darwin's The Origin of Species in 1859, the relevance of von Baer's principles to evolution was clear to many biologists, who assessed evolutionary relationships on the basis of similarities and differences between early embryos. The validity of these comparisons, carried out more than a century ago, has been confirmed by recent molecular analyses. These new gene-based comparisons have in turn led to a renewed interest in the scientific literature of the first half of the 20th century, which documented in great detail comparative anatomical studies of living and fossil animals.

The past decade has seen a growing synergy between developmental biology and evolutionary studies. As a result, not only is an evolutionary perspective now integral to developmental studies, but palaeontologists have become deeply interested in embryos. This integration was the subject of the symposium of the Winter Meeting of the Anatomical Society, held in London in January 2001, entitled ‘The Evolution of Developmental Mechanisms’. It was a very stimulating and enjoyable meeting that addressed the theme at a number of levels, ranging from the major evolutionary changes resulting from gene duplication to the fine detail of insect wing vein evolution. These two ends of the spectrum of topics reflect Darwin's perspective on von Baer's principles, i.e. his recognition that homologies between different phyla are to be seen in early embryonic and larval structures, while adaptations that enable an organism to survive in its particular environment are the result of changes acting late in development.

The reviews brought together in this volume represent a thoughtful and thought-provoking synthesis of data collected over many years. They show the rewards of detailed and long-term study of embryos that in some cases are difficult to obtain, but represent living clues to the mechanisms underlying important evolutionary transitions. We feel privileged to have had the responsibility for editing this impressive issue of the Journal of Anatomy, which will, we have no doubt, be a significant landmark in the ongoing story of development and evolution, and will be widely cited.