Despite much recent controversy about the theory of evolution, major changes in our understanding of evolution over the past twenty years have gone virtually unnoticed.^[1] At the heart of Darwin's theory of evolution is an explanation of how plants and animals evolved from earlier forms of life that have long since disappeared; but his theory says nothing about the factors that determine the shape, color, and size of a particular fish, whale, or butterfly. Darwin and his contemporaries realized that understanding the evolution of animal forms and understanding how a fertilized egg develops into a whale, cow, or human being must be deeply connected; but they didn't know how to make the connection.

Surprising discoveries in the 1980s have begun to tell us how an embryo develops into a mature animal, and these discoveries have radically altered our views of evolution and of the relation of human beings to all other animals. The new field of study in which these breakthroughs have been made is called Evo Devo, short for evolution and development, "development" referring to both how an embryo grows and how the newborn infant matures into an adult.

Sean Carroll, author of one of the books under review and a coauthor of another, has made important contributions to the understanding of evolution and development. From DNA to Diversity, written with two other scientists, is the second edition of a book that has become a classic for students of evolution. The title of Carroll's other book, Endless Forms Most Beautiful, comes from the famous final sentence of The Origin of Species: "There is a simple grandeur in this view of life... that from so simple an origin, through the process of gradual selection of infinitesimal changes, endless forms most beautiful and most wonderful have been evolved."

In 1830, nearly thirty years before Darwin published his book, two French naturalists—Georges Cuvier and Étienne Geoffroy St. Hilaire—debated the significance of the anatomical similarities between distantly related animals, such as the flippers of whales and the wings of bats. Cuvier held that form was dictated by function: the bat's wing, needed for flying, had a separate origin from the whale's flipper, needed for swimming.

Geoffroy St. Hilaire opposed this view, arguing that the underlying skeletal similarities pointed to the existence of a common archetype for both flippers and wings. While neither man claimed that animal forms could change over generations, St. Hilaire's archetypal form foreshadowed some recent discoveries about development and evolution. No doubt this debate was in the mind of Charles Darwin as he formulated his theories in an attempt to account for the origins of animal forms.

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