The book is a reflection on patterns of thought, theoretical positions, and research methods in population ecology. It advocates an approach which refrains from attempts at general mechanistic theory building, but which instead tries to explain population phenomena by life history characteristics, physiological and behavioural processes of organisms and to combine these facts in explanatory models. As far as possible the difference between individuals in morphology, physiology and behaviour should be taken into account, so that the connection of population ecology with the science of Neo-Darwinian evolution can be redressed again.