This volume is devoted principally to the theme of behavioral develop ment. The study of ontogeny has attracted some of the most bitter and protracted controversies in the whole field of ethology and psychology. This is partly because the arguments have reflected more general and continuing ideological battles about nature and nurture. In the opening essay, Oppenheim shows how these debates have recurred in much the same form over the last century. His chapter also brings out a more worrying feature of such argument. He demonstrates that authors who are well known for their strongly held partisan views have written in much more balanced ways than is usually admitted. Although the ex cluded middle is familiar enough in academic argument, the dynamic tensions actually present in developing systems may be particularly prone to polarize debate about what is actually happening. This point is elegantly explored by Oyama in her essay on her concept of maturation.