Focusing on a central problem of the evolution of vertebrae, the author questions whether there is a principle underlying the disparate animal forms. He shows that there is. Following Goethe, he points out that a key to understanding the great variety of vertebrates lies in the plasticity of what he calls the "vertebrate type". He goes on to show how successive classes of vertebrates can be seen as increasingly complete manifestations of the type. Thus the driving forces of evolution are found in the inner lawfulness of living organisms, each of which is itself a manifestation of the type in action.