The purpose of this book is to elucidate the general principles underlying animal aggression. Aggressive encounters occur even in very simple organisms, and are at the roots of human conflict. A general framework is provided by dividing forms of aggression according to their function - protection of the individual, offspring defence and competition. Within each functional group, the principles underlying the variety of aggressive behaviour in different species, the sexes and different life-cycle stages, are considered, and this is followed by a discussion of the mechanisms through which animals achieve the various functional ends. Aggression is viewed as one possible solution to a recurrent set of problems animals encounter in their natural environments. In any one case, the control and form of aggression are seen as the result of the interaction of past evolutionary environments, developmental influences and current circumstances.