As time progresses, biology becomes more and more fragmented and specialized and it becomes increasingly difficult to see how all the dis- ! parate facts fit together. It is completely proper that biologists should have sought to reduce complex biological wholes into their parts, and it is natural that studies on the products of this reduction should have diverged from more holistic studies on evolution and ecology. Yet the biological parts, what they do and how they are organized are products of an evolutionary process which fits organisms for life in particular ecological circumstances. Physiology, developmental biology, ecology and evolutionary biology must not be allowed to grow too far apart, therefore, because all these disciplines and the way their subject matters interact are crucial to understanding organisms - and it is this, it seems to me, which is the fundamental goal of the biological sciences. This book has been written in the spirit of unification and synthesis. It is, in a sense, a general biology of the organism - not, however, of organisms as static unchanging systems, but of organisms as dynamic entities which progress through a definite cycle of events from birth to maturity. The central theme, therefore, will be the life cycle, and the book is organized around the three main phases which are characteristic of all life cycles; growth (Part II), reproduction (Part III) and ageing (Part IV).