i The biological clock ii Why dying is beneficial iii The ageing process of men and women iv Prolonging your life v Limitations of life span vi Modes of development of different species vii A stable life span despite improvements viii Energy consumption ix Fundamental differences in ageing of objects and organisms x Repair of genetic material If a life span is a genetically determined biological characteristic, it is logically necessary to propose the existence of an internal clock, which in some way measures and controls the ageing process and which finally determines death as the last step in a fixed programme.