Introduction

Abstract

This is a book about one aspect of the links between the development of animals and their evolutionary history, between their ontogeny and their phylogeny. The very mention of “ontogeny and phylogeny” will remind many biologists of the erudite book of that title by Stephen Jay Gould (1977) or the earlier works Embryology and Evolution and Embryos and Ancestors by Gavin De Beer (1930, 1940), but there is virtually no overlap between these books and the present work. Gould and De Beer both explored the extent to which animals recapitulate their previous evolution during their development and the evolutionary effects of acceleration and retardation of development. I wish to explore an entirely different link between the way many animals develop and the way they have evolved. Evolutionary theory, as it is today generally accepted, postulates that species evolved from other species in monophyletic lines of descent by the accumulation of gradual, heritable changes. I agree that this accounts for much of evolution, but I claim that this theory alone is not adequate to explain why some animals have the embryos and larvae they do and the life histories they do.